Ben C. Bernstein

National Center for Atmospheric Research, Boulder, Colorado

Analysis of the Meteorology Associated

With the 1997 NASA Glenn

Twin Otter Icing Events

NASA/CR—2000-209413

November 2000
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NASA/CR—2000-209413

November 2000

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under SETAR–0088

Ben C. Bernstein

National Center for Atmospheric Research, Boulder, Colorado

Analysis of the Meteorology Associated

With the 1997 NASA Glenn

Twin Otter Icing Events
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A15

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

Available electronically at http://gltrs.grc.nasa.gov/GLTRS

Note that at the time of printing, the NASA Lewis Research Center

was undergoing a name change to the

NASA John H. Glenn Research Center at Lewis Field.

Both names appear in these proceedings.
Foreword

Supercooled Large Droplet (SLD) icing conditions were implicated in at least one recent

aircraft crash, and have been associated with other aircraft incidents. Inflight encounters

with SLD can result in ice accreting on unprotected areas of the wing where it cannot be

removed. Because this ice can adversely affect flight characteristics of some aircraft,

there has been concern about flight safety in these conditions.

The FAA held a conference on in-flight icing in 1996 where the state of knowledge

concerning SLD was explored. One outcome of these meetings was an identified need to

acquire SLD flight research data, particularly in the Great Lakes Region. The flight

research data was needed by the FAA to develop a better understanding of the

meteorological characteristics associated with SLD and facilitate an assessment of

existing aircraft icing certification regulations with respect to SLD.

In response to this need, NASA, the Federal Aviation Administration (FAA), and the

National Center for Atmospheric Research (NCAR) conducted a cooperative icing flight

research program to acquire SLD flight research data. The NASA Glenn Research

Center’s Twin Otter icing research aircraft was flown throughout the Great Lakes region

during the winters of 1996–97 and 1997–98 to acquire SLD icing and meteorological

data.

The NASA Twin Otter was instrumented to measure cloud microphysical properties

(particle size, LWC, temperature, etc), capture images of wing and tail ice accretion, and

then record the resultant effect on aircraft performance due to the ice accretion.

A satellite telephone link enabled the researchers onboard the Twin Otter to communicate

with NCAR meteorologists, who provided real-time guidance into SLD icing conditions.

NCAR meteorologists also provided pre-flight SLD weather forecasts that were used to

plan the research flights, and served as on-board researchers.

This report contains a description of the weather systems and meteorological features

existing at the time SLD icing research flights were conducted during the winter of

1996–97. The information contained in this report complements microphysical data

acquired during the SLD icing research flights, and provides a larger scale meteorological

context for it. This combined set of microphysical and large-scale weather features was

intended to provide a basis for developing a better understanding of SLD and its

formation mechanisms.

This report was prepared by Ben C. Bernstein of NCAR.

NASA/CR—2000-209413 iii
Table of Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Premier Cases

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

January 15, 1997—Freezing rain, freezing drizzle and mixed conditions over Indianapolis, Indiana . . . . . . . . . . . . . . 11

January 24, 1997—Freezing drizzle and mixed conditions over Mansfield and Cleveland, Ohio . . . . . . . . . . . . . . . . . 54

February 4, 1997—Freezing drizzle and mixed conditions over Lake Erie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Remaining Cases

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

January 15, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

Flight #2—Over northeast Indiana and northern Ohio from 1652 to 1842 UTC

January 22, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

Flight #1—Over southeastern Ontario and near Selfridge, MI from 1726 to 1929 UTC

Flight #2—Over Selfridge, MI and southeastern Ontario from 2037 to 2251 UTC

January 23, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

Flight #1—Over the southeast shore of Lake Erie, near Erie and Bradford, PA from 1432 to 1616 UTC

Flight #2—Over Erie, PA and Youngstown, OH from 1709 to 1840 UTC

January 27, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

Flight #1—Over Sandusky, OH from 1330 and 1552 UTC

January 31, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

Flight #1—Between Cleveland, OH and Parkersburg, WV, and over Parkersburg from 1558 to 1815 UTC

Flight #2—Over Parkersburg, WV and enroute to Cleveland, OH from 1928 to 2103 UTC

February 4, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

Flight #1—Over northeast Ohio and northwestern Pennsylvania from 1312 to 1441 UTC

February 5, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Flight #1—Over northeastern Ohio from 1340 to 1607 UTC

March 6, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

Flight #1—Over northeastern Ohio and northwestern Pennsylvania from 1459 to 1634 UTC

Flight #2—Between Erie, PA and Cleveland, OH from 1719 to 1806 UTC

March 11, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

Flight #1—Over northeastern Ohio and northwestern Pennsylvania from 1315 to 1458 UTC

Flight #2—Over northeastern Ohio and northwestern Pennsylvania from 1555 to 1714 UTC

Flight #3—Over Canton-Akron and Cleveland, OH from 1821 to 2006 UTC

March 13, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

Flight #1—Over southeast Ontario, Selfridge AFB and Flint, MI from 1848 to 2036 UTC

Flight #2—Over Flint, Sarnia (Ontario) and southeastern Ontario from 2129 to 2252 UTC

NASA/CR—2000-209413 iv
March 14, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

Flight #1—Enroute to and over Toledo, OH and Fort Wayne, IN from 1744 to 1946 UTC

Flight #2—Over Fort Wayne, IN, Toledo, OH, Detroit, MI, and southeastern Ontario from 2058 to 2250 UTC

March 20, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Flight #1—Over Lake Erie, near Pelee Point, Ontario from 1249 to 1442 UTC

March 25, 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

Flight #1—Over Sandusky, OH from 1355 to 1529 UTC

NASA/CR—2000-209413 v
Overview of Premier Cases

This part of the document contains an analysis of the meteorology associated with the premier icing 

encounters from the January-March 1997 NASA Twin Otter dataset. The purpose of this analysis is to provide

a meteorological context for the aircraft data collected during these flights. For each case, the following data

elements are presented:

1) A detailed discussion of the Twin Otter encounter, including locations, liquid water contents,

temperatures and microphysical makeup of the clouds and precipitation aloft,

2) Upper-air charts, providing hand-analyzed locations of lows, troughs, ridges,

saturated/unsaturated air, temperatures, warm/cold advection, and jet streams,

3) Balloon-borne soundings, providing vertical profiles of temperature, moisture and winds,

4) Infrared satellite data, providing cloud locations and cloud top temperature,

5) 3-hourly surface charts, providing hand-analyzed locations of lows, highs, fronts,

precipitation (including type) and cloud cover,

6) Hourly plots of icing pilot reports, providing the icing intensity, icing type, icing altitudes and

aircraft type,

7) Hourly, regional radar mosaics, providing fine resolution of the locations of precipitation

(including intensity and type), pilot reports of icing (including intensity and type), surface

observations of precipitation type and Twin Otter tracks for a one hour window centered on

the time of the radar data, and

8) Plots of data from individual NEXRAD radars at times and elevation angles that have been

matched to Twin Otter flight locations.

Outages occurred in nearly every dataset at some point. All relevant data that was available is

presented here. All times are in UTC and all heights are in feet above mean sea level (MSL).

Cases included

970115, 970124, and 970204

Description of plots in this document

The following pages provide a complete description of the attributes of the plots that accompany

each case, including the data elements covered and how they have been represented.

NASA/CR—2000-209413 1
Upper-air charts

Analyses are made at 300, 500, 700 and 850 mb. On all charts, trough axes are indicated as

yellow lines, and areas of saturated/near-saturated conditions are shaded green. Saturated/near-saturated

conditions are those where the dew-point depression (temperature minus the dew point; DDP) is < 8 C at

300 mb and < 5 C at 500, 700 and 850 mb. Contours of constant geopotential height (MSL) are indicated as

solid black lines, while contours of constant temperature are indicated as dashed black lines.

On 300 mb charts, jet stream winds are indicated as follows: > 150 knots in purple, > 110 knots in

red, > 90 knots in red-hatching.

On 500, 700 and 850 mb charts, dry areas (DDP > 10 C) are shaded orange, isotherms (lines of

constant temperature) are brown, areas of warm and cold temperature advection (the wind is bringing in

warmer or colder air, respectively) are indicated with red and blue arrows, respectively. The arrows

indicate the direction of the warm or cold advection.

Figure 1 – Example upper-air charts at 300 and 850 mb.

NASA/CR—2000-209413 2
Balloon-borne soundings

On sounding charts, profiles of temperature, dew point and winds are plotted. Solid lines of

constant temperature are skewed, run from the lower left to the upper right, and are given every 10 C.

Altitude is indicated on the left side of the chart in kilometers. Wind barbs are given on the right hand side

of the chart, where a half-barb indicates 5 knots, a full barb indicates 10 knots and a flag indicates 50 knots

of wind speed. The wind barbs indicate the direction that the wind is from.

Figure 2 – Example of balloon-borne sounding data.

NASA/CR—2000-209413 3
Satellite imagery

On visible satellite images, clouds and snow cover show up as white/bright areas, while ground

and water typically show up as black/dark areas. On infrared satellite images, the temperature of the cloud

top, ground or water are indicated. Temperature ranges for each color on infrared imagery are given at the

bottom of each image. Each color represents a 5 C range of temperature values (e.g. –12.5 to –7.5 C), and

the number (e.g. –10) represents the central temperature value for that color.

Figure 3 – Example visible and infrared satellite images.

NASA/CR—2000-209413 4
Surface charts

On surface charts, high pressure centers are indicated with an ‘H’, low pressure centers are

indicated with an ‘L’, and contours of constant pressure are indicated with solid black lines. Fronts are

indicated as heavy black lines with: cold fronts - pointed barbs, warm fronts - rounded barbs, stationary

fronts - pointed barbs on one side and rounded barbs on the other, and occluded fronts - pointed and

rounded barbs on the same side. Trough axes are indicated with dashed lines. Precipitation areas are

shaded as follows: rain/drizzle – green, snow – blue, freezing precipitation (ZL, ZR, IP) – red.

Figure 4 – Example surface chart.

NASA/CR—2000-209413 5
Radar mosaic charts

On regional radar mosaic charts, locations of precipitation with radar reflectivity values of at least

VIP 1 (>18 dBZ) are given. The precipitation is color-coded by precipitation type (freezing precipitation

(ZL, ZR, IP) - red and magenta, rain - green and yellow, snow - blue) and intensity (VIP level 1-6). A color

bar indicating precipitation type and intensity can be found at the bottom of each chart. Pilot reports of

icing are indicated as follows:

ICING TYPES –

R=rime, C=clear/glaze, X=mixed, U=unknown

ICING INTENSITIES –

small font = trace, trace-light or light

medium font = light-moderate or moderate

large font = moderate-severe or severe

NASA Twin Otter tracks are plotted for a one-hour time window centered on the valid time of the

chart. A ‘+’ is plotted at the aircraft’s location every 2 minutes, and an ‘x’ is plotted at the aircraft’s

location every 10 minutes.

Precipitation type reported by surface stations is also plotted in yellow. The symbols for the

precipitation type are given at the bottom-right portion of the chart.

Figure 5 – Example regional radar mosaic chart.

NASA/CR—2000-209413 6
Pilot reports of icing

Pilot reports (PIREPs) of icing are given for a one-hour time window that is indicated at the top of

the chart. Icing PIREP indicators are given at the bottom of the chart. The size and color of the PIREP

indicates its intensity, while the letter plotted indicates its type (R=rime, C=clear/glaze, X=mixed,

U=unknown). The aircraft type is plotted just up and to the right of the R, C, X or U, and the base and top

of the reported icing altitudes are given just below the aircraft type (100’s of feet MSL).

Figure 6 – Example plot of icing pilot reports.

NASA/CR—2000-209413 7
Twin Otter 2D-Grey Probe Imagery

Sample images from the Twin Otter’s OAP 2D-Grey probe are given for each time period of

consistent microphysical conditions for at least 10 seconds.

A table is included for each Twin Otter flight, and it indicates the following information for each

time period: start and end time (hhmmss; UTC), altitude range (feet above mean sea level - MSL), static

temperature range (Celsius), King probe liquid water content range (zero removed; g/m3

), description of 2-

D gray imagery, letter of example 2-D gray probe image, letter of related NEXRAD images, aircraft range,

azimuth and elevation angles from relevant NEXRAD, description of radar reflectivity patterns around the

aircraft, and comments. Xls=crystals, LWC = liquid water content.

Figure 7 – Example 2D-Grey probe images.

NASA/CR—2000-209413 8
Individual NEXRAD radar imagery

Plots from individual NEXRAD radars at times and elevation angles matched to Twin Otter flight

locations are used to provide a direct comparison between flight data and NEXRAD reflectivity. Radar

range rings are typically given for every 20 km, and azimuth lines are plotted every 30 degrees, beginning

at 0 degrees (north). The time and elevation angle are given at the top and a color bar of reflectivity values

is given at the bottom of each plot.

Figure 8 – Example individual NEXRAD radar plot.

NASA/CR—2000-209413 9
Station locations

A map of commonly referenced station locations, as well as the matching 3-letter codes is given

below. This map covers what is referred to in the document as the “forecast area.”

CODE STATION NAME

CLE Cleveland, Ohio

YNG Youngstown, Ohio

CAK Canton-Akron, Ohio

MFD Mansfield, Ohio

TOL Toledo, Ohio

FDY Findlay, Ohio

DAY Dayton, Ohio

CMH Columbus, Ohio

ILN* Wilmington, Ohio

ZZV Zanesville, Ohio

CVG Cincinnati, Ohio

IND Indianapolis, Indiana

FWA Fort Wayne, Indiana

MIE Muncie, Indiana

SBN South Bend, Indiana

GRR Grand Rapids, Michigan

FNT Flint, Michigan

JXN Jackson, Michigan

PHN Selfridge AFB, Michigan

DTW* Detroit, Michigan

YXU London, Ontario

BUF* Buffalo, New York

ERI Erie, Pennsylvania

FKL Franklin, Pennsylvania

PIT* Pittsburgh, Pennsylvania

BFD Bradford, Pennsylvania

PKB Parkersburg, West Virginia

CRW Charleston, West Virginia

* denotes a sounding site

NASA/CR—2000-209413 10
January 15, 1997

Freezing rain, freezing drizzle and mixed

conditions over Indianapolis, Indiana

NASA/CR—2000-209413 11
January 15, 1997 - Flight No. 1

Overview

This case of freezing rain, freezing drizzle and mixed conditions occurred over Indianapolis

between 1310 and 1524 UTC. The synoptic weather scenario over Indiana on January 15 was fairly classic

for freezing rain and ice pellets. It featured a low pressure center approaching from the west, bringing deep,

warm, moist air northward from the Gulf of Mexico to ride over a layer of cold air in place at the surface. In

this document, the case will be discussed beginning at the synoptic scale (upper air charts, satellite data,

surface maps, soundings) to give an overview of the weather, and ending at the mesoscale (radar and

satellite data) and microscale (probe data), where fine details will be highlighted.

Upper air chart analysis-1200 UTC, January 15th

At 1200 UTC, a sharp trough extended from Minnesota to the Texas Panhandle at 300 millibars

(Fig. 1a). Moisture was evident at this level across most of Indiana, Illinois, Wisconsin and Michigan,

while slightly drier air existed over Missouri, Kansas and Oklahoma. A weak ridge was in place over the

Indiana/Ohio border, with dry air to the east of the ridge axis. A jet streak (area of enhanced wind speed

within the jet stream) with wind speeds of 90+ knots existed just ahead of the trough axis. A secondary

circulation in the exit portion of the jet streak may have been the cause of a moist/dry couplet, with the

moist conditions over Lake Michigan and the dry conditions over southern Illinois/Indiana and Kentucky.

Temperatures in the saturated air on the north side of the couplet were -45 to -50 C.

The main trough axis and weak ridge were evident in nearly the same locations at 500 millibars

(Fig. 1b). A very distinct, narrow (200-300 km wide) swath of moisture extended from Oklahoma to Lake

Michigan, with Indianapolis located at the eastern edge of the moisture. The air was much drier to the

southeast and northwest of the moisture swath, with dew point depressions of 18 C at Nashville and 13 C

at Omaha. Temperatures were approximately -22 C over Indiana at 500 millibars. A much broader area of

saturated conditions was in place over most of the Midwest ahead of the main trough at 700 millibars,

which was in nearly the identical location as it was at 300 and 500 millibars (Fig. 1c). Weak, warm

advection was occurring from Louisiana to Indiana, and temperatures were approximately -6 C over

Indianapolis.

NASA/CR—2000-209413 12
At 850 millibars, a weak trough axis extended from eastern Iowa to the Virginia/North Carolina

border. This weak trough, located over Indianapolis, was the upward extension to the developing surface

warm front, to be discussed later. Indianapolis was on the eastern edge of an area of saturated conditions

which covered the High Plains and most of the Midwest (Fig. 1d). Warm advection was very apparent

over Wisconsin, Illinois, Indiana, Ohio, Michigan and states directly to their south, while cold advection

was occurring to the west of the Mississippi River. Temperatures over Indianapolis at this level (~5000

feet) were approximately -1 C.

Upper air chart analysis-0000 UTC, January 16th

By 0000 UTC on 16 January, the main trough progressed eastward to about the Missouri/Illinois

border at all levels (Figs. 2a-d). The saturated conditions at 300 millibars moved eastward to cover all of

the Great Lakes and Northeast, and as far south as Tennessee and North Carolina. Dry conditions prevailed

to the west of the trough. At 500 millibars, the previously narrow swath of moisture had broadened

somewhat, and covered the central Great Lakes and states to their south. Weak warm advection was in

place over the eastern Great Lakes and Northeast. At 700 and 850 millibars, the broader swath of warm

advection and saturated conditions covered most of the area to the east of the Mississippi River (the

approximate location of the main trough), except for some dry air along the eastern seaboard. In general,

the trough, as well as the deep warm advection and the saturated conditions ahead of it all moved eastward

between 1200 UTC on 15 January and 0000 UTC on 16 January, bringing those conditions across the

Indianapolis area just after 1200 UTC on 15 January.

Satellite IR temperature analysis

The infrared satellite image for 1145 UTC (Fig. 3a) roughly reflects the moisture pattern seen in

the 1200 UTC upper-air charts. The coldest cloud top temperatures (CTTs; < -40 C) were evident over

Wisconsin, Illinois, and western Indiana and Michigan, as seen on the 300 and 500 millibar charts.

Relatively warm CTTs (-10 to -20 C) were in place over Ohio and extreme eastern Indiana, representing

the saturated conditions between 700 and 500 millibars. By 1345 UTC (Fig. 3b) the cold cloud shield

progressed northeastward to cover nearly all of Michigan and Indiana. Indianapolis was predominantly

covered with CTTs < -35 C, but close inspection of zoomed infrared imagery (Fig. 3c) reveals breaks in

NASA/CR—2000-209413 13
the upper cloud shield where CTTs were only -20 to -25 C. These pockets of slightly warmer cloud tops

passed over and to the southeast of Indianapolis during the period of the research flight, as seen in close-up

infrared imagery from 1415 to 1645 UTC (Figs. 3d,e). The higher CTTs tend to indicate less efficient

snow formation aloft, causing similar gaps in the precipitation field, as seen in NEXRAD echoes to the

southeast of Indianapolis. Radar data will be discussed in a later section.

Surface map, regional radar and PIREP analysis

A fairly strong (1030 millibar) high-pressure system was centered over eastern Virginia, while a

weak, elongated low pressure system was centered over northern Kansas at 1200 UTC on 15 January (Fig.

4a). This 1008 millibar low was attached by a long trough axis to a stronger (996 millibar) low located

just to the north of the Minnesota/Canada border. The southern low featured two additional trough axes,

one of which extended southeastward to the Texas/Louisiana border, while the other extended eastward

across the southern borders of Illinois and Indiana. The second of these troughs separated relatively cold air

to the north from relatively warm air to the south. At 1200 UTC, snow was occurring predominantly to

the north of the second trough, across northern Illinois, eastern Iowa, and along the Minnesota/Wisconsin

border. Freezing rain was occurring to the south of the trough, across southern Missouri and most of

Arkansas (Figs. 4a, 5a). Fairly cold air was in place in the wake of the high-pressure center, across areas to

the north of the east-west oriented trough. Temperatures in these areas (including Indiana) were in the

teens.

Positions of the main weather features on the surface map did not change much by 1500 UTC, but

the area of freezing rain moved quickly northward into Missouri, extreme southern Illinois and central

Indiana, reaching Indianapolis at approximately 1317 UTC (Figs. 4b, 5b-d). Snow was occurring across

northern Indiana, just to the north of the freezing rain. Data from the research aircraft, which will be

discussed in the mesoscale analysis section, reveal the existence of a layer of air with temperatures above

freezing just below 850 millibars. This was important to the formation of the freezing rain.

Many PIREPs were made in the area of interest. Most were of light intensity and rime or mixed

type, but some moderate rime icing was also reported (Fig. 6). The PIREPs were found both within the

freezing rain layer in the areas of precipitation and at a variety of altitudes along the edges of the

precipitation shield.

NASA/CR—2000-209413 14
Mesoscale and microscale analysis

A detailed analysis of the mesoscale and microscale structure of the clouds sampled on this day is

given in tabular format (see Table 1). Each period of time was chosen based upon apparent consistency in

2d-grey probe imagery for a period of at least ten seconds. The table includes the range of times (UTC),

altitudes (feet MSL), static temperatures (C), liquid water content from the King probe with zero removed

(g/m3), a brief description of the 2d-grey imagery (phase, small/large drops, crystal habit), an assessment of

the radar reflectivity pattern in the location of flight, and comments. A quick overview of the results is

given in the text here. Images from the 2D-grey probe and the Cleveland NEXRAD are given in Figs. 7

and 8.

The aircraft took off from Wright-Patterson Air Force Base (Dayton, Ohio) at approximately 1310

UTC. Between 131028 and 133728 UTC, the aircraft climbed to an altitude of 3930 feet (all altitudes

MSL) in clear air, and flew toward the west-northwest. The eastern end of the precipitation shield was

entered at 133728 UTC, where a mixture of messy ice crystals (irregulars, columns, dendrites), aggregates

and freezing drizzle was observed through ~1350 UTC. The aircraft descended and flew within an altitude

range of ~2400-3000 feet, in a mixture of freezing rain, freezing drizzle, messy crystals and blobs (solid

looking, oddly shaped particles) just beneath the "warm nose" (a layer of air with temperatures above

freezing sandwiched between two layers of subfreezing air) until ~1455 UTC. Snow fell from the

subfreezing layer above, then melted to form rain or drizzle in the warm nose and subsequently fell into the

subfreezing layer below to become freezing rain or freezing drizzle. During a gradual climb between 1454

and 1516 UTC, the aircraft reached the bottom of the warm nose at ~2900 feet, encountered temperatures as

warm as +2 C at ~3200 feet (1456 UTC), and then reached the top of the warm nose at ~5600 feet (1512

UTC). A mixture of rain, drizzle, messy crystals, blobs and aggregates was observed within this layer.

The Twin Otter ascended above the warm nose and flew within the upper subfreezing layer between

~151200 and ~151940. At these altitudes, no freezing rain or rain was observed, indicating that the

freezing rain in the lower subfreezing level was likely to have developed via the melting process described

earlier in this section. Freezing drizzle was observed in the upper subfreezing level, indicating that at least

some of the freezing drizzle in the lower subfreezing layer may have been formed via the collisioncoalescence process. It is possible, however, that some of it developed via small crystals melting and

NASA/CR—2000-209413 15
subsequently supercooling, since some smaller crystals were observed above the warm nose. Large

aggregates and cloud-sized droplets were also found in the upper subfreezing layer.

The aircraft began its final descent from 5900 feet at ~151630 UTC, and landed at Indianapolis at

~152355 UTC. During descent, the aircraft passed through the upper subfreezing layer, the warm nose, and

the lower subfreezing layer, providing a detailed sounding of the "classic" freezing rain process. As seen

earlier, aggregates, messy crystals, some small drops and some freezing drizzle were observed above the

warm nose (4600-5900 feet). Rain, drizzle, messy crystals, aggregates and some cloud-sized drops were

observed within the warm nose (2600-4600 feet), and freezing rain, freezing drizzle, blobs, some messy

crystals, some small drops, and possibly some ice pellets were observed beneath the warm nose (0-2600

feet).

Liquid water contents were quite low throughout the flight, and cloud-sized drops were only

observed during a few brief periods. Conditions were mixed most of the time, but a few brief periods where

water was dominant did occur. These were usually in patches of relatively low reflectivity (< 20 dBZ or

so). A bright band was very evident and the location of this feature corresponded well with that of the layer

of above freezing temperatures observed by the Twin Otter. Reflectivities were typically > 40 dBZ in the

bright band and 15-30 dBZ elsewhere.

NASA/CR—2000-209413 16
970115 - Flight #1

Row Start/end time (UTC) Alt Range (ft MSL) T-static range (C) LWC - KingZR Description of 2D-grey imagery 2D Imgs

1 131028 131138 530 1700 -6.5 0 0 0 None None2 131138 133728 1700 3930 -1 2 0 0 None None3 133728 134828 3830 3930 -3.1 2 0 0 Messy irregs, some col,need,dend,aggs and possible ZL A4 134829 134948 3910 2950 -3 -0.8 0 0.02 Fewer & smaller xls, no aggs, some poss ZL B5 134948 135358 3000 2600 -3 -2.3 0 0 Possible ZL mxd w/irregs, cols& "blobs" C6 135358 135530 2450 2600 -2.7 -3 0 0.01 Aggs/dendr/stell/irreg/blobs with possible ZL D7 135530 135918 2400 2550 -2.3 -3.5 0 0.01 Fewer xls, just col/irreg w/lots of blobs/ZR/ZL, no aggs E8 135918 140118 2400 2800 -2 -2.1 0 0.01 Nearly all ZR & ZL, some blobs, a few irregulars F9 140118 140400 2900 2500 -1.7 -2.5 0 0.01 Blobs, ZR, some ZL, messy small & large crystals G

10 140400 140520 2340 2500 -2.2 -2.5 0 0 Blobs, ZL (no ZR), messy crystals H11 140520 140650 2400 2500 -2.2 -2.4 0 0 Blobs, ZR, ZL, mess big xls, a few messy aggs I12 140650 140728 2400 2500 -2.2 -2.4 0 0 Nearly all ZR & ZL, some blobs J13 140728 141400 2440 2850 -0.8 -2.5 0 0.01 ZR, ZL, some blobs, lg & sm messy xls K14 141400 142000 2850 2950 -1 0 0 0.01 ZR, ZL, some blobs, lg & sm messy xls, a few aggs L15 142000 142230 2850 2950 -0.8 0.4 0 0.01 ZL, aggs, blobs, some ZR and messy xls M16 142230 142710 2850 2950 -0.5 -2 0 0.01 Aggs, ZL, lg & sm messy crystals N17 142710 142820 2850 2950 -0.5 -1.2 0 0.01 ZR, ZL, blobs, lg & sm messy crystals, aggs O18 142820 144020 2850 2950 -1.9 -0.6 0 0.01 Ditto P-Q19 144020 145410 2850 2950 -1.7 0.6 0 0.01 Ditto, plus R & L R-S20 145410 150120 2850 3500 0 2 0 0.01 R, L, blobs, lg & sm messy crystals, aggs T-U21 150120 150212 3500 4400 0.8 1.1 0 0.01 Aggs, some L, big messy crystals, blobs V22 150212 150350 4400 4900 0.1 0.6 0 0.02 Cloud drops, L, aggs, big messy crystals W-X23 150350 150600 4850 4950 0.3 0.5 0 0.01 Aggs, L, messy xls, few plates/stellars/needles/cols Y-Z24 150600 151110 4850 4950 0.2 0.4 0 0.01 Aggs, L, plates/columns/needles, mess crystals AA-BB25 151110 151240 4900 5850 0.3 -0.6 0 0.02 Aggs, L/ZL, messy crystals, some small drops CC26 151240 151630 5830 5950 -0.5 -0.8 0 0.02 Aggs, ZL, messy crystals, some small drops DD27 151630 151840 5900 4600 -0.6 0 0 0.05 Aggs, L/ZL, messy xls, columns, small drops EE28 151840 151940 4600 4100 0 1 0 0.04 Aggs, L, messy xls, some small drops FF29 151940 152100 4100 2600 1.7 0 0 0.01 R, L, some aggregates and messy crystals GG30 152100 152355 2600 600 0 -5.7 0 0.02 ZR, ZL, blobs, few messy xls, some small drops, IP(?) HH-II

NASA/CR—2000-209413 17
970115 - Flight #1

Row Rad Imgs Range-km Azimuth Elev. ang Description of Radar Reflectivity around A/C Comments

1 A 182-190 87-88 0.0-0.3 E of all echo Climbing to first freezing level2 A-E 103-182 65-89 0.3-0.6 Approaching E edge of echo T bouncing around 0 C, flying NW @3800' most of time3 E-G 66-103 58-64 0.6-0.9 Crossing patchy -10 to 25 dBZ echo Warmest @start, cooling as A/C heads SW4 G 61-66 58-58 0.7-0.9 Crossing -5 to 0dBZ patch - patch moving quickly ENE Descending5 H 46-61 54-58 0.6-0.8 Moving SW from 0 dBZ echo toward 25 dBZ pocket "Blobs" are solid looking, oddly shaped particles.6 H 41-46 51-54 0.7-0.8 Crossing narrow pocket of 25-30 dBZ Below the "warm nose"/melting layer7 H-J 28-41 42-51 0.7-1.1 Exited narrow pocket, heading SW across 15-25 dBZ Getting into more ZR8 J-K 21-28 37-42 1.1-1.8 In 15-25 dBZ, just below bband @ 2800 bband=bright band, climb to 2800', ZR > 1mm diam9 K-L 12.0-21.0 19-37 1.8-2.8 In 2-20 dBZ just below bband Descend to 2500 & fewer images @end

10 L-M 10.0-12.0 359-20 2.8-3.2 In pocket of 5-15dBZ early, then 15-25dBZ Very few images, just below and in bottom of bband11 M 10.0-11.0 325-356 3.1-3.4 Flying SW just below bband, 20-25dBZ More images, going to nearly all ZR12 M 11.0-11.0 314-325 2.9-3.1 Ditto, bband slightly lower to SW, A/C reaching base Very few crystals13 M-O 11.0-15.0 218-314 2.1-3.1 In & just below base of bband, 25-40 dBZ Clim to 2900, changing to all ZR/ZL @end14 P-S 7.0-22.0 218-356 1.8-6.1 In bband, 25-40 dBZ Nearly all ZR/ZL first 80 sec, aggs @end15 U 22-35 356-3.0 1.2-1.8 Same, moving NE into higher dBZ last half of period Aggs increasing, ZR/R ending16 T-U 35-47 3.0-12.0 0.9-1.2 Turn SE in 25-45 dBZ, in lower half of bband Lowest T was at at the furthest N point17 V 36-40 12.0-16.0 1.0-1.1 In lower half of bband, 35-45 dBZ Aggs decreasing, ZR starting18 V-Y 12.0-36.0 0.0-37.0 1.1-3.3 Along base of bband, across 20-35 dBZ Aggs off & on, increasing @end19 Z-FF 11.0-18.0 182-360 2.2-3.7 At base bband (20-35dBZ), then in bband (35-40dBZ) More aggs @times, blobs like misshapen drops20 DD-HH 7.0-23.0 46-182 1.8-7.3 In heart of bband (40-45 dBZ) Climbed into warm nose @3400' by 1456Z, leveled21 HH-II 9.0-9.0 28-46 5.7-7.8 Heading NW in area of 40-50 dBZ, upper half of bband Bigger, cleaner looking aggs22 HH-II 9.0-10.0 353-28 7.8-8.5 Just below top of bband @4900', 40-45 dBZ LWC comes and goes quickly23 HH-II 10.0-11.0 314-353 7.4-7.8 Ditto, but 35-40 dBZ Some small drops, heading SW 24 JJ-KK 11.0-20.0 264-314 4.0-7.4 Ditto Xls more discernable at times25 LL-OO 20-23 258-264 3.9-4.4 25-40 dBZ, climbed to top of bband LWC briefly returns26 NN 23-32 247-258 3.0-4.2 Along & just above bband top, 20-30 dBZ Some cleaner xls, climed to top alt of 5950'27 OO-QQ 28-32 233-247 2.6-3.0 Descend to top of bband, 30-40 dBZ Start final descent into IND28 QQ 23-28 230-233 2.5-2.7 Into top part of bband, 30-40 dBZ In the top of the melting layer29 QQ 15-23 228-230 2.4-2.8 Descending through bottom 1/2 of bband, 25-45 dBZ Bottom part of melting layer30 PP-QQ 4.0-15.0 230-244 0.0-2.4 Bottom of and beneath bband in 20-35 dBZ Descending thru supercooled layer, IP observed by crew

NASA/CR—2000-209413 18
Figure 1 – Upper-air charts for 970115, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 19
Figure 1 – Upper-air charts for 970115, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 20
Figure 2 – Upper-air charts for 970116, 0000 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 21
Figure 2 – Upper-air charts for 970116, 0000 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 22
Figure 3 – GOES-8 infrared satellite data for 970115 at a) 1145 UTC.

NASA/CR—2000-209413 23
Figure 3 – GOES-8 infrared satellite data for 970115 at 1345 UTC, for b) continental United States,

and c) Ohio Valley region.

NASA/CR—2000-209413 24
Figure 3 – GOES-8 infrared satellite data for 970115 for Ohio Valley region at d) 1445 and e) 1615

UTC.

NASA/CR—2000-209413 25
Figure 4 – Surface charts for 970115 at a) 1200 and b) 1500 UTC.

NASA/CR—2000-209413 26
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970115, a) 1200

and b) 1300 UTC.

NASA/CR—2000-209413 27
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970115, c) 1400

and d) 1500 UTC.

NASA/CR—2000-209413 28
Figure 6 – Pilot reports of icing for 970115, a) 1200-1259 and b) 1300-1359 UTC.

NASA/CR—2000-209413 29
Figure 6 – Pilot reports of icing for 970115, c) 1400-1459 and d) 1500-1559 UTC.

NASA/CR—2000-209413 30
970115 - Flight #1

ABCDE FGH I

J K LMN O P Q R

S T U V W X Y Z AA

BB CC DD EE FF GG HH II

Figure 7 - 2D-grey probe imagery for flight 1.

NASA/CR—2000-209413 31
Figure 8 – Indianapolis NEXRAD data for 970115, a) 1312 and b) 1318 UTC (0.5 degrees elevation).

NASA/CR—2000-209413 32
Figure 8 – Same, but for c) 1323 (0.5) and d) 1329 UTC (0.5).

NASA/CR—2000-209413 33
Figure 8 – Same, but for e) 1335 (0.5) and f) 1341 UTC (0.5).

NASA/CR—2000-209413 34
Figure 8 – Same, but for g) 1347 (0.5) and h) 1353 UTC (0.5).

NASA/CR—2000-209413 35
Figure 8 – Same, but for i) 1359 (0.5) and j) 1400 UTC (1.5).

NASA/CR—2000-209413 36
Figure 8 – Same, but for k) 1401 (2.4) and l) 1401 UTC (3.3).

NASA/CR—2000-209413 37
Figure 8 – Same, but for m) 1407 (3.3) and n) 1408 UTC (4.3).

NASA/CR—2000-209413 38
Figure 8 – Same, but for o) 1413 (3.3) and p) 1414 UTC (4.3).

NASA/CR—2000-209413 39
Figure 8 – Same, but for q) 1414 (6.0) and r) 1418 UTC (2.4).

NASA/CR—2000-209413 40
Figure 8 – Same, but for s) 1419 (3.3) and t) 1422 UTC (0.5).

NASA/CR—2000-209413 41
Figure 8 – Same, but for u) 1423 (1.5) and v) 1429 UTC (1.5).

NASA/CR—2000-209413 42
Figure 8 – Same, but for w) 1430 (2.4) and x) 1436 UTC (2.4).

NASA/CR—2000-209413 43
Figure 8 – Same, but for y) 1436 (3.3) and z) 1442 UTC (3.3).

NASA/CR—2000-209413 44
Figure 8 – Same, but for aa) 1443 (4.3) and bb) 1449 UTC (4.3).

NASA/CR—2000-209413 45
Figure 8 – Same, but for cc) 1449 (6.0) and dd) 1453 UTC (2.4).

NASA/CR—2000-209413 46
Figure 8 – Same, but for ee) 1454 (3.3) and ff) 1454 UTC (4.3).

NASA/CR—2000-209413 47
Figure 8 – Same, but for gg) 1500 (4.3) and hh) 1501 UTC (6.0).

NASA/CR—2000-209413 48
Figure 8 – Same, but for ii) 1501 (9.9) and jj) 1507 UTC (6.0).

NASA/CR—2000-209413 49
Figure 8 – Same, but for kk) 1507 (9.9) and ll) 1513 UTC (6.0).

NASA/CR—2000-209413 50
Figure 8 – Same, but for mm) 1513 (9.9) and nn) 1517 UTC (3.3).

NASA/CR—2000-209413 51
Figure 8 – Same, but for oo) 1518 (4.3) and pp) 1522 UTC (1.5).

NASA/CR—2000-209413 52
Figure 8 – Same, but for qq) 1523 (2.4).

NASA/CR—2000-209413 53
January 24, 1997

Freezing drizzle and mixed conditions

over Mansfield and Cleveland, Ohio

NASA/CR—2000-209413 54
January 24, 1997 – Flights No. 1 and No. 2

Overview

This case of freezing drizzle and mixed conditions occurred over Youngstown and Canton-Akron

between 1710 and 1918 UTC (flight 1) and over Mansfield and Cleveland between 2015 and 2152 UTC

(flight 2). On this day, a high-pressure area centered over Maine left a shallow layer of cold air in place

over Ohio. A weak, low-pressure area moved northward toward Chicago during the course of the day,

bringing in a deep layer of warm, moist air above the cold pool. Freezing drizzle and mixed conditions

formed above 9,000 feet as this overrunning situation combined with an influx of dry air aloft. In this

document, the case will be discussed beginning at the synoptic scale (upper air charts, satellite data,

surface maps, soundings) to give an overview of the weather, and ending at the mesoscale (radar and

satellite data) and microscale (probe data), where fine details will be highlighted.

Upper air chart and sounding analysis-1200 UTC, January 24th

At 300 millibars, a weak ridge was in place over Cleveland, while a weak trough approached from

the southwest (Fig. 1a). The trough essentially separated relatively dry air to the west from moist air over

the Great Lakes and Upper Mississippi Valley. The exception to this was a pocket of dry air passing just

north of Cleveland, that was collocated with a weak jet maximum of approximately 90 knots. A fairly

similar pattern was evident at 500 millibars, with two weak troughs over the High Plains and saturated

conditions over the Great Lakes and Ohio Valley (Fig. 1b). Warm advection and weak divergence were

evident in the saturated area ahead of a thermal ridge that extended from western North Carolina to Lake

Superior. Some weak, cold advection and dry air from the southwest had reached southern Missouri at this

time, signaling the approach of an area of downward motion at 500 millibars.

The pattern was more complicated at 700 millibars (Fig. 1c). The main trough was farily

pronounced and extended southeastward across central Missouri. A weaker trough positioned across

northern Kentucky was becoming evident just ahead of it. This weaker through (the upward extension of a

surface warm front draped across Tennessee) roughly separated the moist air, which had reached the

Kentucky-Ohio border, from a broad swath of dry air across the Northeast states and from Ohio to

Minnesota. Temperatures in the moist air were between -10 and 0 C. Strong warm advection was

occurring across the Great Lakes states, but the lack of moisture (DPD = 21 C at Detroit, 19 C at Buffalo,

NASA/CR—2000-209413 55
and 11 C at Pittsburgh; DPD = dew point depression) precluded the formation of clouds the Cleveland area

at this altitude and time.

At 850 millibars, the Kentucky trough/warm front was much more evident and another trough

running from Minnesota to Texas became somewhat amplified (Fig. 1d). Strong warm advection was

evident across the eastern third of the U.S., as 20-30 knot south and southwest winds brought warm air

northward ahead of the main trough. Again, very dry conditions existed over the Great Lakes and

Northeast states, while saturated conditions were in place to the south and west. Another weak trough

extended across the northern borders of Ohio, Pennsylvania and New Jersey and was also evident at the

surface, separating very cold air to the north from relatively warmer air to the south. Temperatures were

well above freezing across Ohio at 850 millibars, while they were below freezing at the surface, providing a

classic setup for freezing rain. Sounding data from Wilmington, Pittsburgh, and Detroit indicated that the

freezing rain layer could have been between 1000 and 5000 feet deep (deepest at Detroit) if precipitation was

falling at that time and the temperature structure remained intact (Figs. 2a-c). As moisture moved into the

area over the next 12 hours, freezing rain did develop across Ohio, but the freezing rain layer was shallower

and warmer due to the erosion of the cold surface layer by warm advection. NCAR-CLASS (Cross-chained

Loran Atmospheric Sounding System) soundings taken at Cleveland indicate that the height of the lowest

freezing level (the top of the potential freezing rain layer) dropped from 3150 feet to 2300 feet and the

minimum temperature in the cold surface layer warmed from -3.5 C to -1.8 C between 1422 and 1904 UTC

(Figs. 2d,e).

Upper air chart analysis - 0000 UTC, January 25th

By 0000 UTC, the upper trough at 300 millibars progressed slightly toward the east, while the

dry air that was initially behind it advanced across the Ohio Valley (Fig. 3a). A wedge of dry air advanced

as far as Buffalo in the location of a 130-knot jet streak, which appears to have accelerated the influx of dry

air over Ohio. The same structure was in place at 500 millibars, with very dry air (DDP = 23 C at

Pittsburgh, 20 C at Wilmington) reaching all of Ohio within a local wind speed maximum of ~80 knots

(Fig. 3b). The saturated air across the Ohio Valley at 1200 UTC moved into the Northeast states, and

continued to cover the northern Great Lakes. At 700 millibars, saturated conditions persisted over the

project area except for southwest Ohio, which the dry wedge had reached by this time (Fig. 3c). The

NASA/CR—2000-209413 56
Kentucky trough/warm front at 700 millibars progressed northward to reach Buffalo and northern Michigan.

Warm advection and overrunning were still in place across northeastern Ohio, providing lift there, while

cold advection and dryer air were evident at Wilmington. Temperatures across Ohio were -6 to -3 C at this

level (~10,000 feet).

At 850 millibars, saturated conditions covered nearly all of the eastern half of the country (Fig.

3d). The Kentucky and Lake Erie troughs that were evident in the 1200 UTC analysis merged to form one

upper trough/warm front just north of Lake Erie by 0000 UTC. Warm advection, temperatures in the -10

to 0 C range and saturated conditions between 850 and 650 millibars provided a good environment for the

development of freezing drizzle in this layer. However, the existence of deeper, colder clouds over northeast

Ohio until ~2100 UTC caused the freezing drizzle to be mixed with snow crystals, bringing about mixed

conditions during most of the both flights. As the wedge of dry air moved in from the southwest, the cold

cloud shield over Ohio began to breakup, allowing the SLD to become more prominent. The Twin Otter

encountered a breaks in the cloud decks during flight 1, which kept snow that was falling from the upper

cloud deck from reaching the liquid/SLD cloud deck below, and during flight 2, when the upper cloud deck

was eliminated over Mansfield, thanks to the dry intrusion. Satellite, radar and aircraft data will bear this

out in more detail in later sections.

Satellite IR temperature analysis

Infrared satellite data from 1215 UTC, January 24th reflect the locations of moisture indicated on

the upper air charts, with deep, cold clouds covering much of the eastern _ of the U.S. (Fig. 4a). Ohio,

Indiana and portions of the surrounding states had cloud top temperatures colder than -50 C. Some breaks

in the cold cloud shield were evident over Illinois and Kentucky, while clear skies covered much of the

Arkansas/Texas area, well upstream of Cleveland. This notch of dry air moved swiftly northeastward,

reaching the Kentucky/Indiana border by 1800 UTC, while cold clouds, with some pockets of slightly

warmer/shallower clouds, persisted over Ohio (Figs. 4b-d). These deep, cold clouds were in place during

flight number one, but radar data will provide evidence of gaps in the precipitation shield and breaks

between precipitation layers. These gaps and breaks provided locations where ice crystals from the cold

cloud aloft were not able to survive into the lower cloud deck, and allowed pockets of all-liquid cloud and

freezing drizzle to survive. As the upper level dry slot entered Ohio around 2000 UTC and reached the

NASA/CR—2000-209413 57
Mansfield/Cleveland area by 2100 UTC (Figs. 4e,f), mid-level clouds with cloud top temperatures of -10 to

-25 C were left in place. These clouds provided an icing environment as the deep, cold clouds moved out

of the area. Still, during much of flight 2, cloud top temperatures at the aircraft flight locations were cold

enough (< -15 C) that snow was probably still being produced aloft. Snow production became more

sporadic with time, however, as evidenced by the somewhat spotty nature of the radar echoes (see next

section).

Surface map, radar mosaic and PIREP analysis

The surface map for 1500 UTC (Fig. 5a - 1200 UTC was not available) had a strong (1044

millibars) high-pressure area centered over Maine, with cold, dry air in place to the northeast of Cleveland.

A trough was in place across Lakes Erie and Ontario, as seen at 850 mb. A broad, weak low-pressure area

was centered near Missouri and Arkansas, with a warm front which extended southeastward to the South

Carolina coast and a trough which extended northeastward to the West Virginia/Ohio border. This trough

was a developing warm front, separating cold air over Ohio from warm air over Tennessee and Kentucky,

and became the main warm front by 1800 UTC (Fig. 5b). A swath of rain, drizzle, and freezing rain

preceded the developing warm front, with the freezing rain occurring along the northern edge of the

precipitation shield. Between 15 and 2100 UTC (Figs. 5a-c), a lobe of the low pressure center and the

developing warm front moved northeastward into southern Ohio. Snow, freezing rain, and ice pellets

reached the Cleveland area by 1700 UTC. According to the regional radar mosaic (Fig. 6), the

precipitation shield was fairly solid over the location of flight 1, except that the aircraft approached some

areas of pocketed reflectivity near the southwestern edge of the precipitation shield, near the end of the

flight. It was during this portion of the flight that the Twin Otter sampled areas of all liquid cloud and

freezing drizzle that was not mixed with ice crystals. Just behind the warm front, an area of spotty, light

precipitation, including drizzle, moved into Ohio by 1800 UTC, reaching the Cleveland area by 0000

UTC (Fig. 5d). These breaks in the precipitation shield represent the location of the dry intrusion aloft.

Radar mosaic data from this period indicate the location of the intrusion of dry air aloft, as a major gap in

the reflectivity pattern moved northeastward into central Ohio by 1800 UTC (Fig. 6). PIREPs were

located both inside and outside of reflectivity pockets. Moderate mixed, clear and rime icing was reported

between 8000 and 16,000 feet, while a report of freezing rain and moderate icing was made in the 1800-

NASA/CR—2000-209413 58
2000 foot range over Canton-Akron at 1730 UTC, possibly from the Twin Otter (Figs. 6 and 7). Icing

was evident both in the air above the warm front (above 5000 feet) and in the freezing rain layer close to the

surface, but temperatures were much more favorable for icing above the warm front on this day.

Mesoscale and microscale analysis – Flight 1

A detailed analysis of the mesoscale and micro-scale structure of the clouds sampled on this day is

given in tabular format (see Table 1). Each period of time was chosen based upon apparent consistency in

2d-grey probe imagery for a period of at least ten seconds. The table includes the range of times (UTC),

altitudes (feet MSL), static temperatures (C), liquid water content from the King probe with zero removed

(g/m3), a brief description of the 2d-grey imagery (phase, small/large drops, crystal habit), an assessment of

the radar reflectivity pattern in the location of flight, and comments. A quick overview of the results is

given in the text here. Images from the 2D-grey probe and the Cleveland NEXRAD are given in figures 8

and 9.

Between 170956 and 171310, the aircraft climbed through multiple freezing levels near Cleveland,

encountering temperatures between –0.9 and +3.2 C from the surface to 4300 feet. Warm freezing rain,

rain, drizzle and messy crystals were observed in this layer. Above this upper freezing level, temperatures

fell steadily with height to about –6.5 C at 10,900 feet, and a mixture of ice crystals, small droplets and

possibly some out-of-focus freezing drizzle was observed. Radar reflectivity in these locations was

somewhat pocketed at times and varied between 15 and 30 dBZ or so. A descent to 1300 feet near

Youngstown revealed a very similar structure, but slightly colder temperatures (down to –1.8 C) within the

freezing rain layer below 2700 feet. This layer was too warm to allow significant icing to form on the

aircraft and was not sampled further. Reflectivity was 20-25 dBZ within the ZR layer and 25-40 dBZ in

the bright band caused by the melting of crystals near 2500 feet MSL.

Upon climbing back to 11,000 feet at 1753 UTC, the aircraft continued to encounter a mixture of

crystals and droplets, but more in-focus ZL started to become evident with time. Altitudes near 11,000 feet

continued to be sampled until ~1815, and LWC values did not exceed 0.1 from the beginning of the flight

until that time. The combination of tracking southwest to the reach the southwestern edges of the

precipitation swath and further climbing to 12,000-14,000 feet allowed the Twin Otter to reach a location

which was more dominated by ZL and cloud-sized droplets. While within areas of reflectivity greater than

NASA/CR—2000-209413 59
~5 dBZ, the aircraft encountered mixed conditions and LWC values of less than 0.05. However, when the

Twin Otter reached the southwestern edge of the precipitation shield and began sampling pockets of no

echo (below detectable limits) and reflectivity of –5 to 5 dBZ, consistent areas of in-focus ZL and cloudsized droplets, with LWC up to 0.55, were observed. Some ice crystals (columns and irregulars) were

mixed with the water droplets at times during this period (1828-1849 UTC). LWC values tended to reach

minimum values when the crystals were present. After 1849 UTC, the aircraft descended and headed back

to Cleveland to refuel. Radar imagery during the latter stages of this flight, when the good LWC and ZL

were observed, reveal a break in the precipitation shield (see gap between echo ring near 80 km and solid

echo inside 60 km on Fig. 9pp). This break in the precipitation shield indicates a discontinuity in the

falling precipitation from the cold cloud aloft into the potential liquid and ZL cloud below.

Mesoscale and microscale analysis – Flight 2

A detailed analysis of the mesoscale and micro-scale structure of the clouds sampled on this day is

given in tabular format (see Table 2). A quick overview of the results is given in the text here. Images

from the 2D-grey probe and the Cleveland NEXRAD are given in figures 10 and 11.

Between 201550 and 202205 UTC, the aircraft climbed through multiple freezing levels over

Cleveland, with temperatures fluctuating between -1.2 and +1.5 C, reaching the highest freezing level at

7200 feet MSL. A mixture of rain, drizzle and a few crystals was observed in this range of altitudes, with

cloud drops mixed in above the cloud base (4260 feet MSL). Temperatures decreased gradually with

height to -4.9 C as the aircraft continued to climb to 11,000 feet. Mixed conditions of freezing drizzle,

cloud drops, columns, needles and irregular crystals persisted in this layer, with some variability in the

relative proportions of those particle types. Liquid water contents seemed to vary inversely with crystal

concentrations, reaching maximum values of ~ 0.4 g/m3

. Data from the Cleveland NEXRAD radar

indicated that the aircraft was flying through pocketed precipitation and tended to be passing through or

near patches of 10-20 dBZ reflectivity when crystals were prevalent in the 2D-grey imagery and water

content was low. When the aircraft was within areas of lower reflectivity, droplets were more dominant and

water contents were relatively high. This pattern continued as the aircraft flew northwestward from

Cleveland at altitudes between 9900 and 11,000 feet from 202815 to 204845 UTC. A few periods were

NASA/CR—2000-209413 60
observed where essentially only water drops (cloud and freezing drizzle) were present and water contents

reached 0.3 g/m3, but most of the time, the conditions were mixed in character.

The aircraft flew south toward Mansfield from 202845 to 212920 UTC, remaining at altitudes

between 9000 and 11,000 feet. As described in this synoptic weather section, dry air began to reach the

Mansfield area around this time. This is evident in the radar data as the southwest edge of the precipitation

shield moved northeastward and became more patchy. The aircraft was flying in and out of the patches near

the southwest edge of the precipitation shield in this time frame. Mixed conditions continue to dominate,

but some periods occurred where only water droplets were observed, including an 11 minute period from

211830 to 212920 UTC. Although occasional patches of columns, dendrites and even aggregates were

observed, much of the 2D-grey imagery indicated a mixture of small, out of focus freezing drizzle and cloud

droplets. While the aircraft flew through patchy areas of -5 to 20 dBZ, peak liquid water content values of

up to 0.45 g/m3 were measured along the interfaces between high and low reflectivity areas. Again, greater

amounts of crystals were observed when the aircraft flew within patches of 15+ dBZ, though this pattern

was not always consistent during the portion of the flight near Mansfield.

The aircraft flew back toward Cleveland from 212920 to 215300 UTC, gradually descending from

9800 feet to the surface, continuing in and out of mixed and all-water conditions with widely varying liquid

water contents. Overall, freezing drizzle and cloud droplets were observed during much of the flight,

mostly mixed with crystals of many shapes in sizes. Occasional periods of pure or nearly pure freezing

drizzle and cloud water conditions were sampled lasting as long as 11 minutes.

NASA/CR—2000-209413 61
970124 - Flight #1

Row Start/end time (UTC) Alt Range (ft MSL) T-static range (C) LWC - KingZR Description of 2D-grey imagery 2D Imgs

1 170956 171013 800 1200 2.1 0 0 0 Warm rain to 1mm, some L A2 171013 171051 1200 2000 0 -0.9 0 0 Same, with a few messy crystals and ZR B3 171051 171206 2000 3200 0 3.2 0 0 A few messy crystals at top of layer, some rain C4 171206 171310 3200 4300 2.8 0 0 0.01 Aggs, messy xls, cols D5 171310 171710 4300 8100 0 -4.3 0 0 Aggs, iregs, cols, few plates E6 171710 172130 8100 10900 -4 -6.4 0 0 Ditto, plus some small drops F7 172130 172205 10900 10900 -6.4 -6.6 0.01 0.04 Small drops, some aggs & crystals G8 172205 172355 10800 10900 -6.4 -6.7 0 0.03 Small drops, with more aggs & crystals H,I9 172355 172558 10900 10900 -6.4 -6.6 0.01 0.05 Mostly small drops, small cols, irregs & aggs J

10 172558 173057 10900 4300 -6.4 0 0 0.01 Cols, needles, aggs, poss some oof ZL K11 173057 173525 4300 2600 0 0.6 0 0.06 Aggs, irregs, some poss oof L L12 173525 173610 2600 2300 0.7 0 0 0 L, R, messy aggs and crystals M13 173610 173941 1300 2700 0 -1.8 0 0.01 ZR, ZL, a few messy crystals and blobs N14 173941 174100 2700 3500 0 1.1 0 0.04 R, L, messy aggs & xls, some small drops O15 174100 174135 3500 4200 0.1 0.6 0.02 0.04 More small drops, some messy crystals P16 174135 174238 4200 5100 0.3 0 0 0.02 More aggs, messy xls, possibly some L Q17 174238 174535 5100 7600 0 -3 0 0.01 Aggs, irregs, cols, poss some ZL R18 174535 174910 7600 10700 -3 -5.6 0 0.09 Cols, irregs, small drops, needles, some ZL S19 174910 175250 10700 11000 -5.6 -6.2 0 0 Ditto, but few/no small drops T20 175250 175840 10700 11000 -6.2 -6.7 0 0.01 Irregs, aggs, cols, possible ZL U21 175840 180425 10800 11000 -5.8 -6.2 0 0.01 Cols, irregs, ZL, occasional aggs V22 180425 180740 10800 10900 -5.5 -5.9 0 0.07 Ditto, with small drops and no aggs W

23 180740 181510 10800 11200 -5.3 -6 0 0.01 Cols, irregs, ZL, few/no small drops X24 181510 181620 11200 12000 -6 -7 0 0.04 Ditto, with small drops, more ZL, fewer crystals Y25 181620 181755 11900 12000 -6.9 -7.2 0 0.04 ZL, irregs, small drops Z26 181755 182330 11900 12000 -6.9 -7.5 0.05 0.13 Small drops, ZL, a few irregs AA27 182330 182600 12000 11500 -7 -6 0.02 0.12 ZL, small drops BB28 182600 182640 11500 10900 -6 -5 0.01 0.06 Ditto, with columns CC29 182640 182730 10900 10300 -5 -4 0 0 ZL, columns, needles DD30 182730 182820 10300 9800 -4 -3.2 0 0.2 Small drops, ZL, columns, needles EE31 182820 183629 9900 8000 -3.2 0 0 0.3 Small drops, oof & in-focus ZL, some xls @times FF32 183629 183735 9900 10600 -3 -5 0 0.01 Cols, ZL, few small drops GG33 183735 184030 10600 12100 -4.4 -6.6 0.01 0.55 Small drops, cols & ocnl oof ZL in lower half HH34 184030 184720 12100 14000 -6.6 -10.1 0.02 0.5 Small drops, borderline ZL and some small ZL II35 184720 184911 12500 9800 -7.4 -3.1 0.01 0.5 Ditto, with cols & irregs below 11200 ft JJ36 184911 184945 9800 9200 -3.1 -1.8 0 0.02 Cols, irregs, small ZL KK37 184945 185514 9200 8900 -1.5 -2.5 0 0.2 Small drops, cols, some ZL, few irregs LL38 185514 185635 8900 9000 -2.3 -2.7 0 0 ZL and columns MM39 185635 190000 9000 8000 -2.6 -3 0 0.1 Ditto with cloud drops and irregs NN

NASA/CR—2000-209413 62
970124 - Flight #1

Row Rad Imgs Range-km Azimuth Elev. ang Description of Radar Reflectivity around A/C Comments

1 A-C 1.0-2.0 85-88 0.5-3.0 N/A - inside hole at center of plots None2 D-F 2.0-4.0 88-89 3.0-6.3 Ditto Probably not cold enough for ZR & ZL to stick to AC3 F 4.0-6.0 89-102 6.2-7.0 Bottom of bright band - 40 dBZ Max T=+3.2 at 2700'4 F 6.0-9.0 102-109 7.0-7.5 Climbing through bright band, 40-45 dBZ Top half of warm nose5 H,I 9.0-20.0 109-119 6.5-7.8 Above bright band, 20-30 dBZ Smaller cols, maybe some small drops @end6 H,I 20-34 115-117 5.6-6.9 Moving SE into lower reflect. (25-->15 dBZ) Pockets mostly small drops, fewer/smaller aggs7 H,J 34-36 117 5.3-5.6 ~15-20 dBZ, somewhat variable Possibly a few oof ZL drops8 J 36-44 117-119 4.3-5.3 Ditto, echo tops well above a/c, lower SW Ditto9 J,M 44-53 119-120 3.5-4.3 10-20 dBZ, more crystals at the end Ditto

10 L-O 53-64 120-128 1.2-3.5 In patch of 20-30 dBZ Descent, fewer small drops, fewer aggs @start11 M-P 53-57 128-141 0.6-1.2 20-35 dBZ, getting into melting zone In weak warm nose, weak bright band12 PP,QQ 57-60 141-143 0.5-0.6 In melting zone, ~40 dBZ Ditto13 P,Q 60-70 143-149 0.2-0.5 At or below 0.5 scan, which showed 20-40 dBZ Below weak, poorly defined warm nose14 P,Q 69-70 145-149 0.5-0.7 25-40 dBZ, pocketed In weak warm nose15 Q,R 69-69 143-145 0.7-0.9 10-25 dBZ, bright band not obvious near A/C In weak warm nose16 Q,R 69-69 139-143 0.9-1.2 10-25 dBZ, pocketed Pockets of small drops, in top of warm nose17 Q,S 69-71 129-139 1.2-1.8 10-25 dBZ, crossing pockets Aggs tail off @end of period18 S,S2 71-78 116-129 1.8-2.4 20-30 dBZ on 1.5 scan, 10-20 dBZ on 2.4 scan Ocnl aggs early, small drops ending19 S2,T,T2 78-91 103-116 2.1-2.4 20-25 dBZ on 1.5 scan, 15-20 dBZ on 2.4 scan Few aggs at the end20 S2,T2,T-V 89-97 94-103 2.0-2.1 Within & along edges of 10-25 dBZ More aggs, in-focus ZL at the end21 V 78-89 98-106 2.1-2.4 In 10-25 dBZ ZL now more evident22 X 73-78 106-111 2.4-2.6 In 15-20 dBZ, NW of 25+ dBZ pocket Pockets with small drops23 Y,Z 64-73 111-127 2.6-3.1 Ditto Climbing @ end, some large irregs present24 Z 63-64 127-129 3.1-3.3 0-15 dBZ, nearing upper end of echoes on 3.3 scan Climbing, ZL now larger and dominating25 Z,BB 63-63 129-133 3.3-3.4 Ditto More small drops & less ZL @end26 Z,BB 63-64 133-147 3.3-3.4 Refl. = -5 to 10 dBZ, hitting SW edge/top of echoes Fewer crystals with time27 DD 64-66 147-153 3.1-3.3 Refl. = -5 to 5 dBZ, hitting SW edges on 3.3 scan Descend, few cols @end, top of cloud deck @12k?28 CC,DD 66-67 153-154 2.9-3.1 Ditto on 3.3 scan, 0-10 dBZ on 2.4 scan Small drops ending, cloud base @11.5k?29 CC,DD 67 154-156 2.7-2.9 Ditto No small drops, between decks30 CC,EE 67-68 156-158 2.5-2.7 0-10 dBZ, in solid echo on 2.4 scan LWC coming up, cols disappearing, in lower deck31 EE,FF 68-76 158-173 1.9-2.5 Ditto, approaching SW edge Freezing level at 8k, lower LWC SW of CAK32 FF 76-78 173-174 2.3 Refl. = -5 to 0 dBZ on SW edge Between cloud decks33 FF,GG 78-83 174-178 2.4-2.6 Patchy -5 to 0 dBZ, in hole to the SW at end Peak LWC at 12kft34 GG,HH 66-86 171-181 2.6-3.4 In SW edges of echoes, in a hole/gap for first half Small pockets of cols & irregs early35 II,JJ 58-66 162-171 2.9-3.4 Getting back into SW part of echo, -5 to 5 dBZ Poss break between cloud decks near 12.4k36 II,JJ 56-58 160-162 2.8-2.9 In SW part of echo, up to 10dBZ Another break in cloud decks37 II-MM 41-56 150-160 2.8-3.8 Pockets of -5 to 10 dBZ echoes Pockets of all-small drops and ZL38 LL,MM 37-41 156-160 3.8-4.2 Ditto Cloud break near 9k39 NN-QQ 21-37 150-160 4.2-6.7 Ditto In and out of clouds, double layer evident on high tilts

NASA/CR—2000-209413 63
970124 - Flight #2

Row Start/end time (UTC) Alt Range (ft MSL) T-static range (C) LWC - KingZR Description of 2D-grey imagery 2D Imgs

1 201550 201635 800 1650 1.5 0 N/AN/AWarm rain & drizzle, no ice A2 201635 201710 1650 2400 -0.7 0 0 0.01 R & L, few messy xls B3 201710 201738 2400 2900 0 0.4 0 0.01 R & L, few cols/irregs, one stellar C4 201738 201906 2900 4260 -0.6 0 0 0.01 R & L, few cols/needles, ocnl messy big xls D5 201906 201920 4260 4500 0 -0.7 0 0.02 R & L, few irreg E6 201920 202040 4500 6000 -1.2 0 0 0.1 Mostly bigger cloud drops, ocnl L/R (decr w/time) F7 202040 202205 6000 7200 0 1.1 0 0.1 Mostly cld drops mxd w/poss L, some irr/col/agg G8 202205 202340 7200 8300 0 -1.6 0 0.04 Mostly bigger cld drops mxd w/possL, few lg. irr/col H9 202340 202425 8300 8800 -1.6 -2.4 0.02 0.05 Mix cloud/ZL, xls decreasing in number and size I

10 202425 202520 8800 9500 -2.4 -3.5 0.05 0.2 Mix cloud/ZL, some small xls J11 202520 202630 9500 10200 -3.5 -4.1 0.2 0.43 Ditto, some col/irreg K12 202630 202715 10200 10700 -4.1 -4.3 0 0.25 Mix cloud/poss ZL, more cols/needs L13 202715 202815 10700 11000 -4.3 -4.9 0 0.15 Ditto M14 202815 203155 10000 11000 -4.9 -3.5 0 0.01 Lots of col/ned/irr, few agg mxd w/poss ZL & cld drops N15 203155 203325 10000 10100 -3.4 -3.7 0 0.01 Small/fewer large col/ireg, few aggs mxd w/possZL/cld O16 203325 203345 10000 10050 -3.4 -3.6 0 0.2 Mix cloud/poss ZL w/some col/ireg/aggs P17 203345 203540 9900 10050 -3.6 -3.8 0 0.3 Nearly all water, ocnl col/irreg Q18 203540 204215 10000 10100 -3.7 -5.1 0 0.02 Mix cld/possZL/col/irg/few agg, xls decr. some @ end R19 204215 204330 9950 10050 -5.1 -5.4 0 0.1 Fewer xls (col/irreg), more cloud drops, few aggs S20 204330 204845 10000 10100 -5.3 -5.7 0 0.01 Mix cloud/poss ZL & lots of irreg/col/aggs T21 204825 204935 10050 10300 -5.3 -5.4 0 0.05 Mix cld/ZL, fewer xls, decreasing to almost none U22 204935 205020 10300 10900 -5.4 -5.1 0 0.05 Mix cloud/ZL, ocnl column/irregular V

23 205020 205430 10900 11050 -5.4 -6 0.05 0.15 Mostly cld, some ZL, all water xcpt few xls early W24 205430 210135 10950 11070 -4.9 -5.7 0 0.25 Lots of cols/aggs mxd w/poss ZL & cloud drops X25 210135 210250 10900 11070 -4.8 -5.5 0 0.05 Fewer xls (col/ireg/aggs) mxd w/cloud drops, poss ZL Y26 210250 210635 10500 11050 -4.6 -5.4 0 0.02 Lots of aggs/cols/irregs mxd w/cloud drops & poss ZL Z27 210635 211015 10500 9800 -4.6 -3.3 0 0.1 Fewer xls (aggs/cols/irregs), few dndr mxd w/cld/posZL AA28 211015 211345 9080 9800 -1.9 -3.1 0 0.1 More xls (agg/col/dendr/col/ireg) mxd w/possZL & cld BB29 211345 211635 9100 9200 -1.8 -2 0 0.1 Fewer xls mxd w/ZL & cloud CC30 211635 211830 9050 9200 -1.6 -1.9 0 0.03 More xls mxd w/possZL & cloud DD31 211830 212920 9050 10150 -1.8 -4 0.03 0.45 Mostly cld drops, some ZL, patches of col/dnd/agg EE32 212920 213110 9200 9350 -1.7 -2 0 0.04 Mix of cloud/possZL/xls(cols/irreg/dend(?)) FF33 213110 213800 9150 9800 -0.7 -2.6 0.05 0.23 Mostly cloud & small ZL drops, some col/irreg GG34 213800 213910 9300 9400 -1.9 -2.1 0 0.01 More xls (irreg) mxd w/larger ZL & cloud HH35 213910 214650 6800 9300 0 -2.1 0 0.25 Mostly small ZL & cloud drops w/some irregs/cols II36 214650 215300 650 6800 0 2 0.1 0.3 Bigger L & even R mxd w/cloud drops, no ice JJ

NASA/CR—2000-209413 64
970124 - Flight #2

Row Rad Imgs Range-km Azimuth Elev. ang Description of Radar Reflectivity around A/C Comments

1 A1.0-3.0 82-87 1.8-6.5 25-30dBZ on edge of SW-NE oriented band on 1.5 T falling to freezing level2 B 3.0-4.0 87-89 6.5-8.1 Within ~30dBZ area on 9.9 scan 750m subfreezing layer, minT @ 933mb3 C 4.0-5.0 88-89 8.0-8.3 Ditto Warm layer to +0.4C4 C 4.0-6.0 88-109 8.1-8.3 Ditto Just below cloud base.5 C 4.0-4.0 108-115 8.1-14.3 Ditto Entered cloud base, R & L appear to be decreasing6 C,D 4.0-4.0 115-153 14.3-16.0 Ditto, 2024Z/9.9 scan shows 30+ dBZ moving NE A/C entering 10-20 dBZ7 D 4.0-5.0 153-182 23.5-25.0 In 10-20 dBZ on 9.9 scan Another >0C layer, at the top of the melting zone8 D 5.0-8.0 182-200 18.4-23.5 Ditto, little evidence of bright band Cross highest freezing level, xls decreasing @end9 D 8.0-9.0 200-203 16.3-18.4 In 10-20 dBZ on 9.9, near 20+dBZ echo to NW

10 D 9.0-12.0 203-206 14.0-16.3 Ditto, but echo to N of A/C now on 9.9 scan LWC gradually rising11 D 12.0-14.0 206-209 12.5-14.0 Ditto, double layer seen in lack echo 20-30km on 9.9 LWC > 0.3 most of period12 F 14-16 209-213 12.0-12.5 Ditto LWC decreasing during period13 E,F 16-18 213-215 11.0-12.0 25+dBZ beneath A/C on 6.0 scan More LWC when fewer cols/needles14 E,F 18-29 215-219 6.1-11.0 Poss between layers,clipping 25+dBZ on 6.0 scan Starting SW track on 220 deg radial15 E,G,H 29-33 219-220 5.2-6.1 Exited into speckles of -5 to 10 dBZ to SW on 6.0 scan Mostly < -10 dBZ on 4.3 scan16 G,H 33-34 220-221 5.1-5.2 In -5 to 0 dBZ hole in echo @4.3, outside echo on 6.0 LWC rising (Nevz), King off by ~0.5 suddenly17 G,H 34-37 221-229 4.7-5.1 In hole & along edges of 10 dBZ echo to NW/N LWC=0.3 til 2035, lower LWC when more xls, King OK18 G-J 33-41 230-280 4.3-5.3 Into 5-15dBZ area to N, hitting W edge of echo @end19 J 41-44 280-288 4.0-4.3 N into -10 to 0 dBZ along W edge of echo20 K,L 44-49 288-299 3.6-4.0 N back into 10-25dBZ on 3.3/4.3, hole follows/lags A/C Some big aggs21 L 42-44 293-296 4.0-4.2 Back S into -10 to 5 dbZ hole Begin climb to 11000ft22 M,N 40-42 291-293 4.2-4.7 Into -10 to 0 dBZ on 4.3 scan, < -10 dBZ on 6.0 scan LWC incr/xls decr. @end, hole moved N to meet A/C23 M,N 38-40 274-291 4.8-5.1 Crossing hole, -10dBZ or less on 4.3, none on 6.0 Consistent LWC24 O,P 40-48 251-274 4.0-4.8 SW through 5-25 dBZ S of hole. LWC max N edge echo, all LWC/few xls @start25 R 48-50 249-251 3.9-4.0 In 20-25 dBZ Bright band becoming more evident on 3.3 scan26 R 50-57 241-249 3.2-3.9 Moving SW across widespread 15-25 dBZ echoes Begin descent at end27 R,S 57-65 234-241 2.6-3.2 Crossing subtle pocket of 10-20 dBZ Continue descent28 U 65-73 230-234 2.2-2.6 Clipping pocket of 20-30dBZ on 2.4 scan Descent ends 211135 @9000'29 T,U 73-80 226-230 2.0-2.2 SW into 10-20 dBZ on 1.530 V,W 80-85 225-226 1.9-2.0 Pockets on 1.5 and 2.4 scans Xls decrease at the end31 V-Y 75-89 213-225 1.9-2.1 Into patchy echo on SW edge of shield, -5 to 20 dBZ Peak LWC around edges32 Z,AA 78-79 218-219 2.0-2.1 Away from edges, back in 15-20 dBZ Xls decr. last 30 seconds33 Z-CC 79-84 218-222 1.9-2.1 Near SW edge shield on 2.4, in 10-20 dBZ on 1.5 Pretty steady LWC, lower w/xls present, all water @start34 BB,CC 84-85 222-223 1.9-1.9 Ditto35 BB-EE 40-85 219-223 1.9-2.8 Along SW edge @start, moving NE though 15-30 dBZ Lower LWC when xls present, fairly steady around 0.1536 EE,FF 7.0-40 219-227 2.8-4.0 Through pocket 25-35dBZ on 2.4, part of bright band? Descent to near surface

NASA/CR—2000-209413 65
Figure 1 – Upper-air charts for 970124, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 66
Figure 1 – Upper-air charts for 970124, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 67
Figure 2 – Balloon-borne soundings for 970124, 1200 UTC from a) Wilmington and b) Pittsburgh.

NASA/CR—2000-209413 68
Figure 2 – Balloon-borne soundings for 970124, 1200 UTC from c) Detroit.

NASA/CR—2000-209413 69
Figure 2 – Balloon-borne soundings from Cleveland for d) 1422 and e) 1904 UTC.

NASA/CR—2000-209413 70
Figure 3 – Upper-air charts for 970125, 0000 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 71
Figure 3 – Upper-air charts for 970125, 0000 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 72
Figure 4 – GOES 8 infrared satellite data for a) 1215 and b) 1415 UTC.

NASA/CR—2000-209413 73
Figure 4 – GOES 8 infrared satellite data for c) 1615 and d) 1815 UTC.

NASA/CR—2000-209413 74
Figure 4 – GOES 8 infrared satellite data for e) 2015 and f) 2115 UTC.

NASA/CR—2000-209413 75
Figure 5 – Surface charts for 970124, a) 1500 and b) 1800 UTC.

NASA/CR—2000-209413 76
Figure 5 – Surface charts for c) 970124, 2100 and d) 970125, 0000 UTC.

NASA/CR—2000-209413 77
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970124, a) 1600

and b) 1700 UTC.

NASA/CR—2000-209413 78
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970124, c) 1800

and d) 1900 UTC.

NASA/CR—2000-209413 79
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970124, e) 2000

and f) 2100 UTC.

NASA/CR—2000-209413 80
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for g) 970124, 2200

UTC.

Figure 7 – Pilot reports of icing for 970124, a) 1500-1559 UTC.

NASA/CR—2000-209413 81
Figure 7 – Pilot reports of icing for 970124, b) 1600-1659 and c) 1700-1759 UTC.

NASA/CR—2000-209413 82
Figure 7 – Pilot reports of icing for 970124, d) 1800-1859 and e) 1900-1959 UTC.

NASA/CR—2000-209413 83
Figure 7 – Pilot reports of icing for 970124, f) 2000-2059 and g) 2100-2159 UTC.

NASA/CR—2000-209413 84
A B CDE F G H

K L M N O P Q R

U V W X YZ

EE FF GG HH II

AA

I

JJ

970124 - Flight #1

J

S T

BB CC DD

KK LL MM NN

Figure 8 - 2D-grey probe imagery for flight 1.

NASA/CR—2000-209413 85
Figure 9 – Cleveland NEXRAD data for 970124, a) 1707 (0.5 degrees elevation) and b) 1708 UTC (1.5

degrees elevation).

NASA/CR—2000-209413 86
Figure 9 – Same, but for c) 1709 (2.4) and d) 1710 UTC (3.3).

NASA/CR—2000-209413 87
Figure 9 – Same, but for e) 1710 (4.3) and f) 1711 UTC (6.0).

NASA/CR—2000-209413 88
Figure 9 – Same, but for g) 1716 (4.3) and h) 1717 UTC (6.0).

NASA/CR—2000-209413 89
Figure 9 – Same, but for i) 1721 (3.3) and j) 1722 UTC (4.3).

NASA/CR—2000-209413 90
Figure 9 – Same, but for k) 1726 (1.5) and l) 1727 UTC (2.4).

NASA/CR—2000-209413 91
Figure 9 – Same, but for m) 1727 (3.3) and n) 1730 UTC (0.5).

NASA/CR—2000-209413 92
Figure 9 – Same, but for o) 1731 (1.5) and p) 1736 UTC (0.5).

NASA/CR—2000-209413 93
Figure 9 – Same, but for q) 1742 (0.5) and r) 1743 UTC (1.5).

NASA/CR—2000-209413 94
Figure 4 – GOES 8 infrared satellite data for a) 1215 and b) 1415 UTC. Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970124, c) 1800

and d) 1900 UTC.

Figure 9 – Same, but for s) 1749 (1.5) and s2) 1750 UTC (2.4).

NASA/CR—2000-209413 95
Figure 9 – Same, but for t) 1755 (1.5), and t2) 1756 UTC (2.4).

NASA/CR—2000-209413 96
Figure 9 – Same, but for u) 1801 (1.5) and v) 1802 UTC (2.4).

NASA/CR—2000-209413 97
Figure 9 – Same, but for w) 1807 (1.5) and x) 1808 UTC (2.4).

NASA/CR—2000-209413 98
Figure 9 – Same, but for y) 1813 (2.4) and z) 1814 UTC (3.3).

NASA/CR—2000-209413 99
Figure 9 – Same, but for aa) 1819 (2.4) and bb) 1820 UTC (3.3).

NASA/CR—2000-209413 100
Figure 9 – Same, but for cc) 1825 (2.4) and dd) 1826 UTC (3.3).

NASA/CR—2000-209413 101
Figure 9 – Same, but for ee) 1831 (2.4) and ff) 1837 UTC (2.4).

NASA/CR—2000-209413 102
Figure 9 – Same, but for gg) 1843 (2.4) and hh) 1843 UTC (3.3).

NASA/CR—2000-209413 103
Figure 9 – Same, but for ii) 1849 (2.4) and jj) 1849 UTC (3.3).

NASA/CR—2000-209413 104
Figure 9 – Same, but for kk) 1854 (2.4) and ll) 1855 UTC (3.3).

NASA/CR—2000-209413 105
Figure 9 – Same, but for mm) 1856 (4.3) and nn) 1901 UTC (4.3).

NASA/CR—2000-209413 106
ABCDE FGH

J K LMN O P Q R

S T UVWXY Z

BB CC DD EE FF GG HH II

AA

I

JJ

970124 - Flight #2

Figure 10 - 2D-grey probe imagery for flight 2.

NASA/CR—2000-209413 107
Figure 11 – Cleveland NEXRAD data for 970124, a) 2015 (1.5 degrees elevation) and b) 2018 UTC

(6.0 degrees elevation).

NASA/CR—2000-209413 108
Figure 11 – Same, but for c) 2019 (9.9) and d) 2024 UTC (9.9).

NASA/CR—2000-209413 109
Figure 11 – Same, but for e) 2030 (6.0) and f) 2030 UTC (9.9).

NASA/CR—2000-209413 110
Figure 11 – Same, but for g) 2035 (4.3) and h) 2035 UTC (6.0).

NASA/CR—2000-209413 111
Figure 11 – Same, but for i) 2040 (3.3) and j) 2041 UTC (4.3).

NASA/CR—2000-209413 112
Figure 11 – Same, but for k) 2046 (3.3) and l) 2047 UTC (4.3).

NASA/CR—2000-209413 113
Figure 11 – Same, but for m) 2052 (4.3) and n) 2053 UTC (6.0).

NASA/CR—2000-209413 114
Figure 11 – Same, but for o) 2058 (3.3) and p) 2058 UTC (4.3).

NASA/CR—2000-209413 115
Figure 11 – Same, but for q) 2103 (2.4) and r) 2104 UTC (3.3).

NASA/CR—2000-209413 116
Figure 11 – Same, but for s) 2109 (2.4) and t) 2114 UTC (1.5).

NASA/CR—2000-209413 117
Figure 11 – Same, but for u) 2115 (2.4) and v) 2119 UTC (1.5).

NASA/CR—2000-209413 118
Figure 11 – Same, but for w) 2120 (2.4) and x) 2125 UTC (1.5).

NASA/CR—2000-209413 119
Figure 11 – Same, but for y) 2126 (2.4) and z) 2131 UTC (1.5).

NASA/CR—2000-209413 120
Figure 11 – Same, but for aa) 2132 (2.4) and bb) 2137 UTC (1.5).

NASA/CR—2000-209413 121
Figure 11 – Same, but for cc) 2138 (2.4) and dd) 2143 UTC (1.5).

NASA/CR—2000-209413 122
Figure 11 – Same, but for ee) 2144 (2.4) and ff) 2144 UTC (3.3).

NASA/CR—2000-209413 123
February 4, 1997

Freezing drizzle and mixed conditions

over Lake Erie

NASA/CR—2000-209413 124
February 4, 1997 - Flight No. 2

Overview

This case of freezing drizzle and mixed conditions occurred over central Lake Erie between 1754

and 2005 UTC. The synoptic weather scenario on February 4th was rather complicated. There was a deep,

strong, low pressure system centered to the west of the project area, and a high-pressure center moving off

to the northeast. In this document, the case will be discussed beginning at the synoptic scale (upper air

charts, satellite data, surface maps, soundings) to give an overview of the weather, and ending at the

mesoscale (radar and satellite data) and microscale (probe data), where fine details will be highlighted.

Upper air chart analysis-1200 UTC, February 4th

At 1200 UTC, an elongated, open low center extended from southwestern Iowa to northeastern

Montana at 300 millibars (Fig. 1a). A trough was evident in the height field extending southeast from the

low over Iowa to Mississippi. High altitude moisture was evident along the East Coast and wrapped

westward across the northern Great Lakes to the north of the low center. Dry air was evident to the

southwest, south and southeast of the low. Although two jet maxima were evident across the High Plains

and New England, neither seemed to be directly related to the moisture field at 300 millibars.

At 500 millibars (Fig. 1b), the low over southwestern Iowa became closed, while a trough

extended back toward the second low over Montana. Several other troughs extended from the main low,

most notably one which again extended to the southeast and appeared to be the dividing line between

moist air and warm advection over the eastern states and dry air over the southern plains states. This

trough was mostly evident in the moisture field, but significant height falls were occurring along and just

ahead of the trough (50 decameters at Nashville). Much weaker height falls and rises were evident behind

the trough. Strong height falls in Illinois were mostly due to the approach of the low center, and are not

representative of trough passage. The moisture pattern at 500 millibars is similar to that seen at 300

millibars, with moisture wrapping from the East Coast across the Great Lakes to the north side of the low

center. However, the boundary between moist air and dry air was better defined at 500 millibars and the

moisture plume wrapped in a comma shape around the low pressure center over Iowa. The dry air had

wrapped up into the front side of the low center reaching Illinois and eastern Iowa at 1200 UTC.

NASA/CR—2000-209413 125
The pattern was similar at 700 millibars (Fig. 1c), but the low centered over southwest Iowa had

several closed contours. The dry air from the southwest had only reached central Missouri at this level.

Saturated conditions were now evident across all of Ohio, Indiana and southern Michigan. The trough axis

to the southeast of the low separated the moist air and warm advection to the east from the dry air and cold

advection to the west. At this level, a trough extending across the southern edge of the Great Lakes was

more evident and featured a significant wind shift. Dry air was evident at both 500 and 700 millibars

across eastern Canada, New York and New England, essentially to the northeast of a ridge axis extending

from western New York to Lake Superior. Temperatures across the project area at 700 millibars were

between 0 and -7 C. At 850 millibars (Fig. 1d), the closed low was centered slightly to the east of the

700mb low, with an obvious trough extending across the southern Great Lakes. Dry air was evident to the

north of this trough while saturated conditions existed and to the south of it. Temperatures at this level

were all above freezing and strong warm advection was present across the project area.

Upper air chart analysis-0000 UTC, February 5th

Examination of the upper air charts at 0000 UTC on 5 February 1997 shows that the low center

progressed toward Chicago at all levels (see Figs. 2a-d). The dry air at 300 and 500 millibars progressed

eastward with the system, reaching northern Ohio and western Pennsylvania, while saturated conditions

existed over New England and most of New York. The upper level trough that was initially over the

southern Great Lakes at 700 millibars moved northward into southern Ontario. This trough tilted toward

the north with height, and was the upward extension of a surface trough to be discussed later. The trough

which extends southward from the low center appears to have intensified, as an area of cold advection to the

west caught up with the trough. This helped to develop a north-south oriented cold front from the surface

to 700 millibars by 2100 UTC.

Satellite IR temperature analysis

Infrared satellite imagery from 1509 UTC (earliest time available - Fig. 3a) mimics the

combination of the upper air moisture fields, with deep, cold clouds covering the eastern states and

extending westward to the north of the low center. Dry air ahead of the Iowa low is evident in the warmer

NASA/CR—2000-209413 126
infrared temperatures across Illinois, Indiana, western Ohio, southern Michigan and southern Wisconsin.

The progression of the upper air pattern and its attendant cloud features is clearly seen in the satellite

infrared temperature plots for 1515 to 2045 UTC (Figs. 3a-f). The cold cloud tops (CTT < -40C; CTT =

cloud top temperature) moved eastward with time, with their western edge passing Cleveland (CLE) around

1700 UTC and reaching well into Pennsylvania by 2039 UTC. The passage of this feature was well

correlated with the cessation of the significant precipitation at most surface stations (e.g. CLE at 1729

UTC). Surface analyses at 1800 UTC will show a significant gap in the precipitation field between the

cold cloud shield to the east of CLE and a second cold shield over Indiana associated with the developing

cold front. An area of warmer cloud tops existed between these two areas of deeper, colder clouds. Cloud

top temperatures in these warmer clouds, which were present over Lake Erie at the time of the flight, were

approximately -13 C during the period of flight number 2.

Surface map, upper-air sounding and icing PIREP analysis

Standard surface maps were not available for the period between 0900 and 1500 UTC on February

4th. Analysis from the ETA model (not shown) is used here for surface low positioning at 1200 UTC.

NEXRAD radar mosaic data was not available for February 4th. Thus the mosaic data cannot be discussed

in relation to other features. Data from the Cleveland NEXRAD will be discussed in the mesoscale

analysis section.

A well-organized, low pressure center moved from southeast Iowa toward Chicago between 1200

and 1800 UTC. At 1800 UTC, the 1006 millibar low was centered over Chicago and featured a warm

front which extended across southwestern Ohio to the Tennessee/North Carolina border, and a surface

trough which extended across southern Michigan and Lake Erie (Fig. 4). Ahead of the warm front, freezing

rain and some freezing drizzle was reported across central Michigan, southern Ontario, western New York,

and central Pennsylvania, while rain was occurring in western Pennsylvania. Stations across most of Ohio,

including Cleveland, had been reporting rain in previous hours, but this rain had moved to the east,

leaving the Cleveland area beneath non-precipitating, overcast clouds. Rain ended at Cleveland-Hopkins

airport at 1729 UTC and at Cleveland-Cuyahoga between 1645 and 1745 UTC. The period with no rain

lasted until 1935 UTC, and was associated with the gap in the upper cloud shield, as discussed earlier.

NASA/CR—2000-209413 127
As mentioned earlier, a surface trough extended eastward from the low pressure center, across Lake

Erie, just north of Cleveland. Surface winds to the south of this trough were from the southeast, and

temperatures were in the mid-40s. Stations around Cleveland were reporting overcast skies with cloud

bases of 8000-10,000 feet. To the north of the trough, surface winds were from the east, temperatures were

in the low 30s, ceilings were 1000 feet or less, and a mixture of freezing rain, freezing drizzle, ice pellets,

snow and rain was falling. The surface trough tilted northward with height, as seen on the 850 and 700

millibars charts for 0000 UTC, 5 February. Warm advection and overrunning conditions were evident at

all levels, indicative of a warm frontal structure.

Sounding data from Detroit at 1200 UTC and Buffalo at 0000 UTC (Figs. 5a,b) showed that a

freezing rain temperature structure existed just ahead of the surface trough, with saturated conditions aloft, a

layer of above-freezing air (a "warm nose") near 850 millibars, and a sub freezing layer below. Fluctuations

between precipitation types and intensities across the area appeared to be dependent upon the depth and

strength of the warm nose and the depth of the saturated layer above. In places where deep, efficient clouds

were in place, snow probably existed above the warm nose, fell through it and then into the subfreezing

layer below to form freezing rain and ice pellets. When more shallow, less efficient clouds (warmer cloud

top temperatures) were in place, the lifting from the warmer frontal structure and overrunning produced

supercooled liquid water and SLD aloft. During the first flight on this day, the Twin Otter flew into the

former situation near Cleveland at ~1500 UTC, finding mostly ice crystals and little supercooled liquid

water. During the second flight, the Twin Otter flew into the latter situation, encountering both small and

large supercooled liquid water drops and relatively few ice crystals.

Pilot reports of icing were made across the region during the period of interest. Nearly all of the

PIREPs were made within the areas of warmer cloud tops indicated in the GOES-8 infrared imagery, were

at altitudes between 9000 and 15,000 feet MSL, from light to moderate in intensity and of all icing types

(Fig. 6).

Mesoscale and microscale analysis

A detailed analysis of the mesoscale and micro-scale structure of the clouds sampled on this day is

given in tabular format (see Table 1). Each period of time was chosen based upon apparent consistency in

2d-grey probe imagery for a period of at least ten seconds. For each period, the table includes the range of

NASA/CR—2000-209413 128
times (UTC), altitudes (feet MSL), static temperatures (C), liquid water content from the King probe with

zero removed (g/m3), a brief description of the 2d-grey imagery (phase, small/large drops, crystal habit and

appearance), an assessment of the radar reflectivity pattern in the location of flight, and comments. A quick

overview of the results is given in the text here. Images from the 2D-grey probe and the Cleveland

NEXRAD are given in figures 7 and 8.

Between 175400 and 180830 UTC, the plane climbed from the surface to the freezing level,

crossing an inversion (possibly the warm front) between 2000 and 4400 feet. The freezing level was

encountered at approximately 8900 feet AGL during the ascent. The period of flight from 180830 to

192500 UTC was spent above the freezing level, between 8900 and 13900 feet, where temperatures were

between 0 and -11 C. Liquid water content varied greatly in this range of altitudes, with values from 0 to

0.5 grams per cubic meter. A variety of conditions were sampled, including freezing drizzle, cloud-sized

droplets, and ice crystals (columns, needles, stellars, plates and aggregates). A combination of drops sizes

from ~10 to ~200 microns was most common when areas of only liquid were being sampled, but there were

many periods where mixed conditions (drops mixed with ice crystals) existed. Freezing drizzle and cloudsized droplets were observed both inside and outside of areas of reflectivity detectable by the Cleveland

NEXRAD radar (down to about -10 dBZ, depending upon range). Freezing drizzle was observed through

the entire range of altitudes from 8900 to 13900 feet (just below cloud top), and drizzle was observed at

altitudes beneath the freezing level. The freezing drizzle and warm drizzle (temperature above freezing) were

observed within, beneath and between cloud layers at all locations from Cleveland to 105 km north of this

site (the northern end of the flight track). Thus, the freezing drizzle layer was approximately 5000 feet deep

and extended across at least 100 km in the north-south direction on this day. Note that the aircraft did

sample from just below cloud top down to the freezing level both upon takeoff and when ~105 km to the

north of Cleveland. Freezing drizzle was observed throughout that depth at both locations, giving profiles

of the SLD conditions over Cleveland and near the northern end of the flight path.

Ice crystals were observed outside of reflectivity areas, but their concentrations seemed to be

highest within echoes of at least -5 dBZ. Columns and needles were most commonly observed at altitudes

below approximately 10,000 feet, though they were occasionally seen above this level. At higher altitudes

crystal habits were often difficult to distinguish or were irregular in appearance, but occasional stellar and

dendritic crystals were observed. Some aggregation was evident at times.

NASA/CR—2000-209413 129
970204 - Flight #2

Row Start/end time (UTC) Alt Range (ft MSL) T-static range (C) LWC - KingZR Description of 2D-grey imagery 2D Imgs

1 175400 175600 600 2000 7 3.4 0 0 Warm drizzle (100 to 250 microns) A2 175600 175930 2000 4360 3.4 6.6 0 0 Warm drizzle (L), a few breaks B3 175930 180010 4360 4900 6.6 5.5 0 0 L, a few breaks C4 180010 180300 4900 6000 5.9 3.9 0 0.2 L with cloud drops D5 180300 180830 6000 8900 3.9 0 0 0 Intermittent L & cloud drops E6 180830 180935 8900 9000 0 -1.2 0 0 Mix of poss ZL, columns, needles (?), irregs F7 180935 180952 8900 9000 -1.1 -1.2 0 0.1 A few xls (irreg), mostly cloud drops, some poss ZL G8 180952 181015 8880 8930 -0.9 -1.1 0 0.08 More cols & irregs, mixed w/poss ZL & cloud drops H9 181015 181045 8830 8950 -1 -1.1 0 0.13 Small drops, few poss ZL, some cols, irregs, other ice I

10 181045 181315 8850 11100 -1 -4.7 0 0.1 Ditto J11 181315 181500 11100 12300 -4.7 -7.4 0 0.07 Cloud & ~50 micron oof drops, ocn'l cols, irr, likely ZL K12 181500 181808 12400 12900 -7.6 -8.8 0 0.05 Plenty of ZL, some cloud drops, ocnl ice 181730-45 L13 181808 181910 12800 12900 -8.6 -8.8 0 0.05 Larger ZL (200+ mic), few junky xls M14 181910 182135 12850 13100 -7.8 -8.7 0 0.1 Mix ZL & cloud drops, ocn'l junky xls N15 182135 182420 13200 13900 -8.9 -10.5 0.01 0.25 Mix ZL & cloud drops, sizes fluctuate, no ice O16 182420 182512 13400 13700 -9.4 -10.5 0.1 0.2 Mix ZL & cloud drops, ocn'l xls P17 182512 183135 13400 13800 -9.1 -10.2 0.01 0.4 Mix ZL & cloud drops, size/LWC vary, ocn'l stellars(?) Q18 183135 183202 13600 13700 -9.5 -9.7 0.04 0.06 ZL, cloud drops & more xls, poss ocnl aggs R19 183203 183405 13200 13700 -9.1 -10 0 0.3 Mix ZL & cloud drops, no ice S20 183405 183445 13350 13450 -9.1 -9.1 0.03 0.1 Mix ZL, cloud drops and some stellar (?) and unknown T21 183445 184005 13300 13800 -8.6 -10.1 0 0.5 Mix ZL & cloud drops, no ice U22 184005 184112 13600 13800 -9.7 -10.5 0.01 0.1 Mostly cloud drops, a few oof ZL to 100 microns V23 184112 185310 13500 11300 -5.5 -10.5 0 0.1 Mix ZL & cloud drops, a few xls & aggs W24 185310 185400 11300 9700 -5.5 -2.9 0 0.05 Mix ZL & cloud drops, columns X25 185400 185610 9700 8000 -2.9 0 0 0.15 Small drops w/some ZL & some ice Y26 185610 185830 8000 9250 0 -1.5 0 0.01 A mess! Drops, columns, needles, stellars (?) Z27 185830 190100 9250 11000 -1.5 -4.5 0 0.06 Xls decreasing with height AA28 190100 190310 11000 12400 -4.8 -7.2 0 0.15 Cloud drops & small ZL, occn'l columns BB29 190310 191100 12500 13900 -7.2 -11 0 0.4 Cloud drops & small ZL, no ice CC30 191100 191440 13300 13700 -9.7 -11 0 0.3 Cloud drops & small ZL, occn'l plates starting DD31 191440 191705 13400 12300 -10.6 -7.4 0 0 Fewer images, stellars & plates EE32 191705 191745 12300 11500 -7.4 -6.2 0.01 0.05 Mix ZL & cloud drops, ocn'l column & unknown xls FF33 191745 192420 11500 8450 -6.2 -0.5 0 0.4 Mix ZL & cloud drops w/off/on columns, few aggs GG34 192420 192500 8450 7800 -0.5 0.3 0.1 0.5 Mix L & cloud drops, ocnl columns & unknown HH

NASA/CR—2000-209413 130
970204 - Flight #2

Row Rad Imgs Range-km Azimuth Elev. ang Description of Radar Reflectivity around A/C Comments

1 A,B 0-4.0 78-108 0.0-7.0 No info T falling w/height2 B 4.0-7.0 108-172 7.0-12.3 No info Inversion layer3 B 5.5-6.7 172-184 11.1-12.2 No info Above inversion, enter cloud base @4900'4 B 5.4-7.2 184-273 10.9-17.5 No info In cloud5 C,D 7.2-24 273-295 6.2-12.8 Main area of reflectivity is to the NW In & out of cloud pockets6 C,D 24-29 292-295 5.2-6.2 Clipping SW edge of 5-30 dBZ pocket7 C,D 29-30 294-295 5.0-5.2 Ditto8 C,D 30 295-297 4.8-5.0 Reaching NW part of reflectivity pocket Reflectivity getting patchy9 C,D 32-33 297-300 4.6-4.8 Ditto, -15 to 5 dBZ

10 C,D 27-33 300-318 4.6-7.1 Entering NW portion of pocket, 0-18 dBZ11 E 27-32 318-329 6.9-7.1 In NW portion of pocket, 5-18dBZ, exiting it at end Exiting to NW12 E 32-42 329-352 5.4-6.9 Exited pocket into area of no echo13 G,H 42-46 352-358 4.9-5.4 No echo on 6.0 elev, patchy -10 to 5dBZ on 4.3 elev14 G,H 46-59 358-6.0 4.0-4.9 Along edges of -10 to 5 dBZ echoes Going to lower tilt (4.3), no echo at 6.015 F,G 59-72 6.0-11.0 3.4-4.0 No echo on 4.3, patchy -10 to 0 dBZ on 3.3 elev16 J 72-77 11.0-12.0 3.1-3.4 Patchy -10 to 5 dBZ echo @ 3.3 mostly dissipated Echo dissipated beyond 70km, A/C @72-77 km range17 J 74-82 8.0-15.0 3.0-3.3 At the edge of a pocket of +5 dBZ on 3.3 scan Xls in 5 second patches18 J 80-81 15-16 3.0-3.1 Ditto19 I,J 78-83 13-15 2.8-3.1 Ditto, above and near some 5-20 DBZ on 2.4 scan20 K,L 83-86 13-14 2.8-2.8 No echo on 3.3, in/out of -5 to 5 dBZ on 2.4 scan21 K,L 82-91 8.0-14.0 2.6-3.0 Ditto22 K,L 79-82 8.0-8.0 3.0-3.1 Ditto23 M-P 76-98 8.0-15.0 2.2-3.1 N of solid 5+ dBZ echoes on 2.4, near bits of 0-5 dBZ Begin descent to 8000'

24 Q,R 90-95 13-14 1.8-2.2 N of echoes on 2.4, in patchy -5 to 10dBZ on 1.5 scan25 Q 95-106 14-16 1.3-1.8 In and out of patchy -5 to 10 dBZ Reached freezing level

26 Q 106-110 16-18 1.3-1.5 In 5-15 dBZ on 1.5 scan Begin ascent27 Q,R 99-106 17-18 1.5-2.0 Going to 5-10 dBZ on 1.5 scan, no echo on 2.4 scan28 S 95-99 15-17 2.0-2.3 N edge of patchy -5 to 5 dBZ29 S,T 76-95 10.0-15.0 2.3-3.2 0-15km N of main echoes, approaching them from N30 U,V 77-85 14-25 2.8-3.1 In/out of patchy 5-10 dBZ on 2.4, no echo on 3.3 scan31 U,V 85-90 25-33 2.4-2.8 In hole in echoes, W of patch of 5-10 dBZ on 2.4 scan32 W 88-90 33-34 2.3-2.4 In gap between patches of 5-10 dBZ33 W,Y 70-88 23-34 2.0-2.3 Moving SW though gap, and into 5-25 dBZ at the end Heading south34 X,Y 68-70 21-23 2.0-2.1 Passing thru WE edge of 5-25 dBZ echoes into a hole Remainder of flight at T>0C

NASA/CR—2000-209413 131
Figure 1 – Upper-air charts for 970204, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 132
Figure 1 – Upper-air charts for 970204, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 133
Figure 2 – Upper-air charts for 970205, 0000 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 134
Figure 2 – Upper-air charts for 970205, 0000 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 135
Figure 3 – GOES-8 infrared satellite imagery for 970204, a) 1515 and b) 1615 UTC.

NASA/CR—2000-209413 136
Figure 3 – GOES-8 infrared satellite imagery for 970204, c) 1715 and d) 1815 UTC.

NASA/CR—2000-209413 137
Figure 3 – GOES-8 infrared satellite imagery for 970204, e) 1915 and f) 2045 UTC.

NASA/CR—2000-209413 138
Figure 4 – Surface chart for 970204, 1800 UTC.

NASA/CR—2000-209413 139
Figure 5 – Balloon-borne sounding data for a) 970204, 1200 UTC at Detroit and b) 970205, 0000

UTC at Buffalo.

NASA/CR—2000-209413 140
Figure 6 – Pilot reports of icing for 970204, a) 1600-1659 and b) 1700-1759 UTC.

NASA/CR—2000-209413 141
Figure 6 – Pilot reports of icing for 970204, c) 1800-1859 and d) 2000-2059 UTC. No PIREP data

was available for 1900-1959 UTC.

NASA/CR—2000-209413 142
ABCDE FGH

I J K LMNO P Q

R S T UVWXY

AA BB CC DD EE FF GG HH

Z

970204 - Flight #2

Figure 7 - 2D-grey probe imagery for flight 2.

NASA/CR—2000-209413 143
Figure 8 – Cleveland NEXRAD radar imagery for 970204, a) 1757 (3.3 degree elevation angle) and b)

1758 UTC (6.0 degree elevation).

NASA/CR—2000-209413 144
Figure 8 – Same, but for c) 1810 (4.3) and d) 1810 UTC (6.0).

NASA/CR—2000-209413 145
Figure 8 – Same, but for e) 1816 (6.0) and f) 1821 UTC (3.3).

NASA/CR—2000-209413 146
Figure 8 – Same, but for g) 1821 (4.3) and h) 1822 UTC (6.0).

NASA/CR—2000-209413 147
Figure 8 – Same, but for i) 1826 (2.4) and j) 1827 UTC (3.3).

NASA/CR—2000-209413 148
Figure 8 – Same, but for k) 1838 (2.4) and l) 1838 UTC (3.3).

NASA/CR—2000-209413 149
Figure 8 – Same, but for m) 1844 (2.4) and n) 1844 UTC (3.3).

NASA/CR—2000-209413 150
Figure 8 – Same, but for o) 1849 (2.4) and p) 1850 UTC (3.3).

NASA/CR—2000-209413 151
Figure 8 – Same, but for q) 1854 (1.5) and r) 1855 UTC (2.4).

NASA/CR—2000-209413 152
Figure 8 – Same, but for s) 1907 (2.4) and t) 1908 UTC (3.3).

NASA/CR—2000-209413 153
Figure 8 – Same, but for u) 1913 (2.4) and v) 1913 UTC (3.3).

NASA/CR—2000-209413 154
Figure 8 – Same, but for w) 1919 (2.4) and x) 1924 UTC (1.5).

NASA/CR—2000-209413 155
Figure 8 – Same, but for y) 1925 UTC (2.4).

NASA/CR—2000-209413 156
Overview of Remaining Cases

This part of the document contains a basic analysis of the meteorology associated with the remaining

 icing encounters from the January-March 1997 NASA Twin Otter dataset. The purpose of this analysis is 

to provide a meteorlogical context for the aircraft data collected during these flights. For each case, the

following data elements are presented:

1) A discussion of the Twin Otter encounter, including locations, liquid water contents,

temperatures and microphysical makeup of the clouds and precipitation aloft,

2) Upper-air charts, providing hand-analyzed locations of lows, troughs, ridges,

saturated/unsaturated air, temperatures, warm/cold advection, and jet streams,

3) Balloon-borne soundings, providing vertical profiles of temperature, moisture and winds,

4) Infrared and visible satellite data, providing cloud locations and cloud top temperature,

5) 3-hourly surface charts, providing hand-analyzed locations of lows, highs, fronts,

precipitation (including type) and cloud cover,

6) Hourly, regional radar mosaics, providing fine resolution of the locations of precipitation

(including intensity and type), pilot reports of icing (including intensity and type), surface

observations of precipitation type and Twin Otter tracks for a one hour window centered on

the time of the radar data, and

7) Hourly plots of icing pilot reports, providing the icing intensity, icing type, icing altitudes and

aircraft type.

Outages occurred in nearly every dataset at some point. All relevant data that was available is

presented here. All times are in UTC and all heights are in feet above mean sea level (MSL).

Cases included

970115, 970122, 970123, 970127, 970131, 970204, 970205, 970306, 970311, 970314, 970320, 970325

Description of plots in this document

The following pages provide a complete description of the attributes of the plots which accompany

each case, including the data elements covered and how they have been represented.

NASA/CR—2000-209413 157
Upper-air charts

Analyses are made at 300, 500, 700 and 850 mb. On all charts, trough axes are indicated as

yellow lines, and areas of saturated/near-saturated conditions are shaded green. Saturated/near-saturated

conditions are those where the dew-point depression (temperature minus the dew point; DDP) is < 8 C at

300 mb and < 5 C at 500, 700 and 850 mb. Contours of constant geopotential height (MSL) are indicated as

solid black lines, while contours of constant temperature are indicated as dashed black lines.

On 300 mb charts, jet stream winds are indicated as follows: > 150 knots in purple, > 110 knots in

red, > 90 knots in red-hatching.

On 500, 700 and 850 mb charts, dry areas (DDP > 10 C) are shaded orange, isotherms (lines of

constant temperature) are brown, areas of warm and cold temperature advection (the wind is bringing in

warmer or colder air, respectively) are indicated with red and blue arrows, respectively. The arrows

indicate the direction of the warm or cold advection.

Figure 1 – Example upper-air charts at 300 and 850 mb.

NASA/CR—2000-209413 158
Balloon-borne soundings

On sounding charts, profiles of temperature, dew point and winds are plotted. Solid lines of

constant temperature are skewed, run from the lower left to the upper right, and are given every 10 C.

Altitude is indicated on the left side of the chart in kilometers. Wind barbs are given on the right hand side

of the chart, where a half-barb indicates 5 knots, a full barb indicates 10 knots and a flag indicates 50 knots

of wind speed. The wind barbs indicate the direction that the wind is from.

Figure 2 – Example of balloon-borne sounding data.

NASA/CR—2000-209413 159
Satellite imagery

On visible satellite images, clouds and snow cover show up as white/bright areas, while ground

and water typically show up as black/dark areas. On infrared satellite images, the temperature of the cloud

top, ground or water are indicated. Temperature ranges for each color on infrared imagery are given at the

bottom of each image. Each color represents a 5 C range of temperature values (e.g. –12.5 to –7.5 C), and

the number (e.g. –10) represents the central temperature value for that color.

Figure 3 – Example visible and infrared satellite images.

NASA/CR—2000-209413 160
Surface charts

On surface charts, high pressure centers are indicated with an ‘H’, low pressure centers are

indicated with an ‘L’, and contours of constant pressure are indicated with solid black lines. Fronts are

indicated as heavy black lines with: cold fronts - pointed barbs, warm fronts - rounded barbs, stationary

fronts - pointed barbs on one side and rounded barbs on the other, and occluded fronts - pointed and

rounded barbs on the same side. Trough axes are indicated with dashed lines. Precipitation areas are

shaded as follows: rain/drizzle – green, snow – blue, freezing precipitation (ZL, ZR, IP) – red.

Figure 4 – Example surface chart.

NASA/CR—2000-209413 161
Radar mosaic charts

On regional radar mosaic charts, locations of precipitation with radar reflectivity values of at least

VIP 1 (>18 dBZ) are given. The precipitation is color-coded by precipitation type (freezing precipitation

(ZL, ZR, IP) - red and magenta, rain - green and yellow, snow - blue) and intensity (VIP level 1-6). A color

bar indicating precipitation type and intensity can be found at the bottom of each chart. Pilot reports of

icing are indicated as follows:

ICING TYPES –

R=rime, C=clear/glaze, X=mixed, U=unknown

ICING INTENSITIES –

small font = trace, trace-light or light

medium font = light-moderate or moderate

large font = moderate-severe or severe

NASA Twin Otter tracks are plotted for a one-hour time window centered on the valid time of the

chart. A ‘+’ is plotted at the aircraft’s location every 2 minutes, and an ‘x’ is plotted at the aircraft’s

location every 10 minutes.

Precipitation type reported by surface stations is also plotted in yellow. The symbols for the

precipitation type are given at the bottom-right portion of the chart.

Figure 5 – Example regional radar mosaic chart.

NASA/CR—2000-209413 162
Pilot reports of icing

Pilot reports (PIREPs) of icing are given for a one hour time window which is indicated at the top

of the chart. Icing PIREP indicators are given at the bottom of the chart. The size and color of the PIREP

indicates its intensity, while the letter plotted indicates its type (R=rime, C=clear/glaze, X=mixed,

U=unknown). The aircraft type is plotted just up and to the right of the R, C, X or U, and the base and top

of the reported icing altitudes are given just below the aircraft type (100’s of feet MSL).

Figure 6 – Example plot of icing pilot reports.

NASA/CR—2000-209413 163
Station locations

A map of commonly referenced station locations, as well as the matching 3-letter codes is given

below. This map covers what is referred to in the document as the “forecast area.”

CODE STATION NAME

CLE Cleveland, Ohio

YNG Youngstown, Ohio

CAK Canton-Akron, Ohio

MFD Mansfield, Ohio

TOL Toledo, Ohio

FDY Findlay, Ohio

DAY Dayton, Ohio

CMH Columbus, Ohio

ILN* Wilmington, Ohio

ZZV Zanesville, Ohio

CVG Cincinnati, Ohio

IND Indianapolis, Indiana

FWA Fort Wayne, Indiana

MIE Muncie, Indiana

SBN South Bend, Indiana

GRR Grand Rapids, Michigan

FNT Flint, Michigan

JXN Jackson, Michigan

PHN Selfridge AFB, Michigan

DTW* Detroit, Michigan

YXU London, Ontario

BUF* Buffalo, New York

ERI Erie, Pennsylvania

FKL Franklin, Pennsylvania

PIT* Pittsburgh, Pennsylvania

BFD Bradford, Pennsylvania

PKB Parkersburg, West Virginia

CRW Charleston, West Virginia

* denotes a sounding site

NASA/CR—2000-209413 164
January 15, 1997

Flight #2 – Over northeast Indiana and northern Ohio from 1652 to 1842 UTC. 

Brief overview

Two flights were made on this day. An in depth analysis of the first flight can be found in the 

“premier cases” document. That flight was made into classical freezing rain and freezing drizzle conditions 

in the vicinity of Indianapolis. The second flight was made in similar conditions over Indianapolis, Muncie 

(IN), and Toledo as an area of freezing rain and drizzle moved northward along the Indiana-Ohio border, 

decreased in areal extent, and became both shallower and warmer. Conditions were marginal for ice 

accretion at and above minimum vectoring altitudes (approximately 2500 feet), since the temperature was 

warmer than –3 C there, the base of the warm nose was only 500 feet above, and it had temperatures 

greater than +5 C within it. This provided very little time for the rather warm rain and drizzle drops to 

supercool before being sampled by the Twin Otter. The majority of the second flight was made within this 

very warm mixture of freezing rain and freezing drizzle near 2500 feet, but the warm nose and upper subfreezing layer (above the warm nose, height greater than 6000 feet) were also sampled. As expected, rain 

and drizzle dominated the warm nose, while aggregates and a mixture of crystals were found in the subfreezing layer above. Very little liquid water content was observed during the flight, with maximum values 

of 0.1. Most, if not all, of the freezing rain and freezing drizzle occurred below cloud base. Most of this 

flight took place within or along the northern edge of a SW-NE oriented swath of reflectivity running 

across the southern half of Indiana and the northwestern third of Ohio. 

Relevant weather features

At 1200 UTC, a north-south oriented trough extended from the North Dakota/Minnesota border to 

the Texas Panhandle at 500 mb (Fig. 1). Ahead of the trough, a ribbon of moisture was present from 

Oklahoma to Lake Michigan, then widened to cover all of the Great Lakes. The moisture covered the 

northwestern half of Indiana, while dry air was present to southeast. At 700 mb, moisture was more 

widespread ahead of the trough, covering the forecast area, except for eastern Ohio, southeast Ontario, New 

York, Pennsylvania, and West Virginia. Weak warm advection was present to the south and southwest of 

Indianapolis. At 850 mb, moisture was present to the west of the Ohio-Indiana border and good warm 

advection was present across the forecast area, while a weak trough axis was becoming evident across 

central Indiana. 

Surface maps for 1200 UTC (Fig. 2) also indicate this weak trough/warm front across southern 

Illinois and southern Indiana, extending from a weak, 1008 mb low centered over Kansas and Oklahoma. 

A 1030 mb high was centered over the Virginia-North Carolina coast and was responsible for the cold, dry, 

low-level air in its wake across the forecast area. An area of freezing rain was present across northern 

Arkansas, southeast Missouri and southern Illinois, while snow was falling north of there. The freezing 

NASA/CR—2000-209413 165
rain and snow moved northward into Indiana by 1500 UTC, with Indianapolis located just south of the 

transition between freezing rain and snow (colocated with the leading edge of the warm nose of abovefreezing air aloft). By 1800 UTC, the transition zone was just south of the Indiana-Michigan and OhioMichigan borders, while patchy freezing rain was occurring across northern Indiana. Also, a weak, 1008 

mb low developed in western Tennessee, and the trough axis across southern Indiana became a more welldefined warm front. 

Satellite data from 1145 UTC (Fig. 3) indicates deep, cold cloud covering much of the forecast 

area to the west the Ohio-Indiana border, with some warmer cloud in eastern and southern Indiana and 

across Ohio. Deep cloud continued to consistently cover northern Indiana, but only intermittently cover 

southern and eastern Indiana between 1200 and 1900 UTC. These clouds are an indication that significant 

amounts of ice crystals and aggregates were developing aloft and falling toward the surface. Pockets of dry 

air, present at a variety of altitudes and locations eliminated this precipitation over some areas, and the 

radar mosaic roughly reflects where the precipitation was able to survive to the surface. Between 1300 and 

1800 UTC, the radar data indicate that the precipitation area and classical freezing rain setup moved 

northeast across Indiana into Michigan and northwest Ohio (Fig. 4). The surface subfreezing layer 

weakened with time and fell apart over northwest Ohio and southeast Michigan, eliminating the freezing 

rain layer by 2100 UTC. The erosion of the surface subfreezing layer is quite evident in a Cleveland 

sounding taken at 2006 UTC by the NCAR CLASS van (Fig. 5). 

PIREPS of icing (Fig. 6) up to moderate intensity were found both at low altitudes (in the freezing 

rain layer – 0 to 3000 feet) and in the upper sub-freezing layer (above the warm nose – 6000 to 15000 feet). 

NASA/CR—2000-209413 166
Figure 1 – Upper-air charts for 970115, 1200 UTC at a) 300 and b) 500 mb. 

NASA/CR—2000-209413 167
Figure 1 – Upper-air charts for 970115, 1200 UTC at c) 700 and d) 850 mb. 

NASA/CR—2000-209413 168
Figure 2 – Surface charts for 970115, a) 1200 and b) 1500 UTC. 

NASA/CR—2000-209413 169
Figure 2 – Surface chart for 970115, c) 1800 UTC. 

NASA/CR—2000-209413 170
Figure 3 – Infrared satellite imagery for 970115, a) 1145 and b) 1445 UTC. 

NASA/CR—2000-209413 171
Figure 3 – Infrared satellite imagery for 970115, c) 1615 and d) 1715 UTC. 

NASA/CR—2000-209413 172
Figure 3 – Infrared satellite imagery for 970115, e) 1815 and f) 1915 UTC. 

NASA/CR—2000-209413 173
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970115, a) 1300 

and b) 1400 UTC. 

NASA/CR—2000-209413 174
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970115, c) 1500 

and d) 1600 UTC. 

NASA/CR—2000-209413 175
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970115, e) 1700 

and f) 1800 UTC. 

NASA/CR—2000-209413 176
Figure 5 – Balloon-borne NCAR CLASS sounding data for 970115, 2006 UTC at Cleveland. 

Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970115, g) 1900 

UTC. 

NASA/CR—2000-209413 177
Figure 6 – Pilot reports of icing for 970115, a) 1500-1559 and b) 1700-1759 UTC (1600-1659 UTC 

PIREPs were not available). 

NASA/CR—2000-209413 178
Figure 6 – Pilot reports of icing for 970115, c) 1800-1859 UTC. 

NASA/CR—2000-209413 179
January 22, 1997

Flight #1 - Over southeastern Ontario and near Selfridge, Michigan from 1726 to 1929 UTC. 

Flight #2 - Over Selfridge, Michigan and southeastern Ontario from 2037 to 2251 UTC. 

Brief overview

Two flights were made on this day, both along and within the western edge of a north-south 

oriented band of precipitation, which tracked eastward across southern Ontario, Lake Erie, and Ohio, and 

gradually dissipated between 1800 and 2300 UTC. During the first flight, the Twin Otter encountered 

mostly snow and almost no LWC within the precipitation band, over Lake Erie and southeastern Ontario. 

The cloud broke into two separate decks on its western edge, with snow above approximately 10000 feet, 

and all supercooled liquid water below approximately 8000 feet, with LWC up to 0.25. During the second 

flight, the Twin Otter went southeast from Selfridge, Michigan to Erie, Pennsylvania, catching up to the 

western end of the precipitation. Near Erie, the aircraft encountered LWC values of 0.2-0.3, with peaks up 

to 0.4 at temperatures of –3 to –5 Celsius, and a few brief periods of large drops. 

Relevant whether features

At 1200 UTC, a disorganized trough was in place across the center of the country at 500 and 700 

mb (Fig. 1). Moisture was evident to the east of the trough, across the forecast area, and extended 

southward to the Gulf of Mexico. The exception to this was a swath of dry air moving into Indiana, 

Illinois, and Michigan at 700 mb. At 850 mb, the system was well defined, with a closed low over the 

Upper Peninsula of Michigan and four troughs extending from the low 1) south into Mississippi, 2) 

southwest into Oklahoma, 3) south into Iowa, and 4) west across Minnesota. The leading trough (#1) 

separated dry air and cold advection to the west from saturated air and warm advection to the east. The 

warm advection was associated with southwest flow pushing into a pocket of cool air across an area from 

eastern Michigan to North Carolina. The moisture was quite deep across the area and a swath of rain at the 

surface roughly matched the locations where saturated conditions existed at all levels, from 850 to 500 mb. 

By 0000 UTC, the disorganized trough moved eastward and became oriented from northeast to 

southwest across the northern Great Lakes at 700 and 850 mb (Fig. 2). At 850 mb, trough #1 was only 

evident across central Pennsylvania, where it still separated cold advection to the west from warm 

advection to the east. The cool pocket was no longer evident to the east of the trough and the warm 

advection had weakened markedly. Although moisture was abundant across the forecast area at 850 mb, 

dry air at 700 mb had worked its way into much of Pennsylvania and southern Michigan, as well as across 

Ohio. This dry air helped break up the deeper clouds and precipitation associated with trough #1. An area 

of saturated air and warm advection was in place across northern Michigan and southeastern Ontario, into 

New York at 700 mb. 

NASA/CR—2000-209413 180
The 1200 and 0000 UTC Detroit and Wilmington (OH) soundings (Figs. 3,4) gave a good 

indication of the change in structure across trough #1. Ahead of the trough, deep, saturated conditions were 

present from 850 mb on up (from the surface on up at Detroit). By 0000 UTC, dry air had pushed in aloft, 

leaving only an approximately 500 foot thick cloud near 850 mb, with CTTs near –6 C over Detroit. The 

cloud layer was strongly capped with a 4 C inversion between 840 and 800 mb. Conditions also dried out 

from above at Wilmington, but left an approximately 2000 foot deep, very warm (CTT = + 2 C) cloud 

there, which was capped by a weaker inversion. At both sites, the lapse rates below the inversion became 

increasingly unstable with time, providing a profile for capped conviction. 

Surface maps for 1500 UTC (Fig. 5) show that a 992 mb surface low was roughly co-located with 

the 1200 UTC 850 mb low. Two cold fronts extended southwest from the low, across Illinois to Oklahoma 

and across Wisconsin and Iowa. These cold fronts were the surface reflection of upper-air troughs #2 and 

#3, respectively, while a surface trough located across Wisconsin and Minnesota is the reflection of upperair trough #4. Upper-air trough #1 was weakly evident at the surface in the pressure and wind fields, but 

was mostly marked by the elongated band of rain from Detroit, across eastern Indiana and western Ohio, 

down to Louisiana. This swath of rain was located to the west of the trough axis, but well ahead of the first 

cold front. Overcast conditions were present between the rain swath and the cold front, across southeastern 

Ontario, Michigan, Indiana and Illinois. As the system moved toward the east-northeast over the next 

several hours, the surface trough fell apart and the northern end of the precipitation band became scattered. 

This occurred in response to weakening in trough #1 at 850 mb and the push of dry air into the area at 

700 mb. 

Radar mosaic data for 1800 to 2300 UTC (Fig. 6) showed the eastward progression of the rain and 

its dissipation as it moved across Ohio and Lake Erie. Pilot reports of icing were indicated within and 

around the precipitation band between 8000 and 25000 feet (Fig. 7). Much of the icing reported in the 

vicinity of the flight was moderate rime, but a few reports of moderate mixed and clear icing were given 

near the eastern end of Lake Erie and the western end of Lake Ontario between 7000 and 11000 feet, and 

across Michigan between 4000 and 8000 feet. 

No satellite data was available for this case. 

NASA/CR—2000-209413 181
Figure 1 – Upper air charts for 970122, 1200 UTC at a) 300 and b) 500 mb. 

NASA/CR—2000-209413 182
Figure 1 – Upper air charts for 970122, 1200 UTC at c) 700 and d) 850 mb 

NASA/CR—2000-209413 183
Figure 2 – Upper air charts for 970123, 0000 UTC at a) 300 and b) 500 mb. 

NASA/CR—2000-209413 184
Figure 2 – Upper air charts for 970123, 0000 UTC at c) 700 and d) 850 mb. 

NASA/CR—2000-209413 185
Figure 3 – Balloon-borne soundings for 970122, 1200 UTC at a) Detroit and b) Wilmington. 

NASA/CR—2000-209413 186
Figure 4 – Balloon-borne soundings for 970123, 0000 UTC at a) Detroit and b) Wilmington 

NASA/CR—2000-209413 187
Figure 5 – Surface charts for 970122, a) 1500 and b) 1800 UTC. 

NASA/CR—2000-209413 188
Figure 5 – Surface chart for c) 970123, 0000 UTC. 

NASA/CR—2000-209413 189
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970122, a) 1800 

and b) 1900 UTC. 

NASA/CR—2000-209413 190
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970122, c) 2000 

and d) 2100 UTC. 

NASA/CR—2000-209413 191
Figure 6 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970122, e) 2200 

and f) 2300 UTC. 

NASA/CR—2000-209413 192
Figure 7 – Pilot reports of icing for 970122, a) 1700-1759 and b) 1800-1859 UTC. 

NASA/CR—2000-209413 193
No data for this hour 

Figure 7 – Pilot reports of icing for 970122, c) 1900-1959 (no data) and d) 2000-2059 UTC 

NASA/CR—2000-209413 194
Figure 7 – Pilot reports of icing for 970122, e) 2100-2159 (no data) and f) 2200-2259 UTC. 

NASA/CR—2000-209413 195
January 23, 1997

Flight #1 - Over the southeast shore of Lake Erie, near Erie and Bradford, PA from 1432 to 1616 UTC.

Flight #2 - Over Erie, PA and Youngstown, OH from 1709 to 1840 UTC.

Brief overview

Two flights were made on this day. The initial flight was made in an attempt to capture freezing

drizzle, as it reached Erie and Bradford. The freezing drizzle was occurring at Buffalo along the leading

edge of an area of snow, which was associated with a cold front moving southward across Lakes Erie and

Ontario. During the first flight, the Twin Otter sampled LWC up to 0.2 and areas of out-of-focus droplets,

which bordered on freezing drizzle sizes, along the southeast shore of Lake Erie. The liquid water content

decreased as the aircraft approached Bradford, where the clouds were mostly comprised of ice crystals.

The second flight was essentially a return to Cleveland, but an area of dissipating, pocketed, light

snow was sampled along the way, near Erie and Youngstown. Snow crystals were encountered there, and

were mixed with small water droplets and low LWC at times. King LWC probe problems were noted

during this flight.

Relevant weather features

At 1200 UTC, a trough axis extended south from Hudson’s Bay to near Erie, Pennsylvania, and

dry conditions were evident across the entire forecast area at 500 mb (Fig. 1). At 700 mb, that trough was

oriented from northeast to southwest, along the northern shores of Lakes Erie and Ontario. Cold advection

was evident in a narrow swath behind the trough and the only saturated conditions evident across the

forecast area were located near Detroit, along the trough axis. At 850 mb, the trough axis/cold front had

the same orientation and was draped across the centers of Lakes Erie and Ontario. Cold advection was

again present behind the trough, and weak cold advection was occurring ahead of it. Saturated conditions

were in place across Michigan, Ohio, Pennsylvania, New York and southeast Ontario.

At the surface, the upper-air trough was reflected as a cold front, which stretched along the south

shores of Lakes Erie and Ontario, and reached Youngstown and Bradford by 1500 UTC (Fig. 2). This cold

front extended southwest from a 992 mb low centered in the Canadian Maritimes. A 1037 mb high was

centered to the north of Lake Superior and was pumping cold, dry air southward behind the front.

Temperatures quickly dropped from near 30 F along the front to less than 10 F in southeast Ontario. Snow

was occurring immediately behind the front and overcast conditions were prevalent ahead of it. Freezing

drizzle was observed at Buffalo between 1000 and 1200 UTC along the leading edge of the snow.

According to radar data, the snow appeared to peak in intensity around 1400 UTC (Fig. 3). Moderate snow

was reported at Cleveland during this period, but no further observations of freezing drizzle were made

across the forecast area after 1200 UTC. The snow decreased in intensity and became patchy in nature by

1600 UTC, while the front pushed further southeast to reach Pittsburgh by 1800 UTC.

NASA/CR—2000-209413 196
The 1200 UTC Buffalo sounding (Fig. 4) was taken during the occurrence of freezing drizzle

there, just as the cold front passed. This sounding is somewhat representative of the conditions sampled

during the initial flight along the southeast shore of Lake Erie, though the clouds sampled were somewhat

deeper, with colder cloud top temperatures. Cold, dry air and northwest winds were evident below 950mb

(~2000 feet), while saturated air existed within the frontal zone (~2000 to 4000 feet) and above it to 8000

feet, and very dry conditions existed above that. According to the sounding, cloud top temperatures were

approximately –13 C, with cloud tops near 8000 feet. Cold advection and deeper moisture behind the cold

front in the surface to 10,000 feet layer is likely to have quickly pushed this sounding into the snow

production regime, matching the flight conditions at Bradford. The 1200 UTC Detroit sounding, taken just

behind the cold front, indicated that conditions were colder below ~8500 feet and that deeper saturated

conditions existed just behind the front. Here, the front was evident near 10,000 feet, and a 3 C inversion

was observed near cloud top, with a minimum temperature of –16 C at the base of the inversion.

Twin Otter data taken during initial climbout from Cleveland indicated these colder temperatures

(–17 C at 8600 feet) and mixed conditions aloft, just behind the cold front. However, during the flight

between Cleveland and Erie, all-water conditions were observed between 8200 and 8600 feet. Snow

crystals were more prevalent below these altitudes. The 3 C inversion observed at Detroit was also evident

at cloud top in the Twin Otter data.

There were many pilot reports made between 1300 and 1500 UTC in association with this front

(Fig. 5). Many of the PIREPs made ahead of the snow area indicated moderate and even some severe icing

between 2000 and 9000 feet, most of which was rime or mixed in type. As the precipitation dissipated, the

PIREPs died down, becoming lighter in intensity and nearly all rime in type.

No satellite data was available for this case.

NASA/CR—2000-209413 197
Figure 1 – Upper air charts for 970123, 1200 UTC at a) 300 mb and b) 500 mb.

NASA/CR—2000-209413 198
Figure 1 – Upper air charts for 970123, 1200 UTC at c) 700 mb and d) 850 mb.

NASA/CR—2000-209413 199
Figure 2 – Surface charts for 970123, a) 1500 and b) 1800 UTC.

NASA/CR—2000-209413 200
Figure 3 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970123, a) 1400

and b) 1500 UTC.

NASA/CR—2000-209413 201
Figure 3 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970123, c) 1600

and d) 1700 UTC.

NASA/CR—2000-209413 202
Figure 3 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970123, e) 1800

and f) 1900 UTC.

NASA/CR—2000-209413 203
Figure 4 – Balloon-borne soundings for 970123, 1200 UTC at a) Buffalo and b) Detroit.

NASA/CR—2000-209413 204
Figure 5 – Pilot reports of icing for 970123, a) 1300-1359 and b) 1400-1459 UTC.

NASA/CR—2000-209413 205
Figure 5 – Pilot reports of icing for 970123, c) 1500-1559 and d) 1600-1659 UTC.

NASA/CR—2000-209413 206
Figure 5 – Pilot reports of icing for 970123, e) 1700-1759 and f) 1800-1859 UTC.

NASA/CR—2000-209413 207
January 27, 1997

Flight #1 - Over Sandusky, Ohio from 1330 to 1552 UTC. 

Brief Overview

The Twin Otter made one flight into a stratiform cloud deck between 6000 and 8000 feet over 

Sandusky, Ohio. This cloud maintained a consistent structure during multiple samples, with LWC 

increasing from 0.0 at cloud base to 0.25-0.35 near cloud top, and drop sizes gradually increasing from 

cloud-sized droplets at cloud top to in-focus freezing drizzle within the lowest few hundred feet of the 

cloud. Freezing drizzle was also observed down to 400 feet below cloud base. This cloud was sampled to 

the north of a large swath of precipitation approaching from the south. The precipitation area reached the 

Cleveland area around 1600 UTC, and was first encountered by the Twin Otter at 1540 UTC, when snow 

crystals were observed at around 9200 feet, well above the liquid cloud top. Soon thereafter, the crystals 

were observed down to the top of the liquid cloud, then at altitudes as low as 3500 feet. 

Relevant weather features

At 1200 UTC, a broad, weak, north-south oriented trough extended from North Dakota to New 

Mexico at 500 and 700 mb (Fig. 1). Cold advection was present behind this “main” trough. At 500 mb, 

very dry air was in place across northern Ohio, southeast Michigan, southeast Ontario, New York and 

Pennsylvania. Saturated conditions approached from the southwest, and had already reached southern Ohio 

and Indiana. Temperatures were approximately –21 C in the saturated air at this level, and weak, warm 

advection was present there. 

At 700mb, a secondary trough extended from western Wisconsin to central Illinois, and separated 

dry air to the southwest from saturated air and warm advection to the northeast. The saturated air and 

warm advection covered the entire forecast area and temperatures within it were near –8 C over Cleveland 

and –4 C just upstream at Wilmington, Ohio (ILN). At 850 mb, a pocket of dry air was in place across 

southeast Michigan and northeast Ohio, while saturated conditions were found across the rest of the 

forecast area. Warm advection was present across Michigan and northern Ohio, to the northeast of the 

secondary trough. 

The 1440 UTC Cleveland sounding data from the NCAR Mobile CLASS (Fig. 2) show the 

temperature and moisture structure for the location of the Twin Otter flight. Dry air indicated on the 

850mb chart is quite evident in the sounding from 820 mb (~6000 feet) to the surface, while saturated 

conditions (with respect to liquid) and temperatures between –3 and –6 C were present from 820 mb to 

about 740 mb (~8600 feet). A strong inversion was present from 8600 to 9500 feet. Twin Otter data 

indicated that cloud top was near the base of the inversion. Although saturated conditions appear to have 

existed up to 680mb (~11,000 feet) in the sounding data, this may have been due to wetting of the dew 

point probe, since the Twin Otter data indicated that no cloud was present from approximately 8200 to 

NASA/CR—2000-209413 208
11,700 feet (the maximum altitude sampled). A second sounding taken at 1546 UTC indicated that 

conditions had moistened below cloud base, matching the appearance of snow below 3500 feet at the end 

of the flight (1552 UTC). 

The surface map for 1500 UTC (Fig. 3) shows a weak, 1013 mb low pressure area centered along 

the western end of the Arkansas/Missouri border. A stationary front extended northeast from the low, 

across Kentucky. A strong, 1042 mb surface high was centered just north of Vermont and kept relatively 

cold air in place across the forecast area and the Northeast states. A trough axis was draped across northern 

Ohio, separating southerly winds to the south of it from southeasterly winds to the north. Rain was mostly 

occurring to the north of the stationary front, across the southern two-thirds of Ohio, but some freezing rain 

and ice pellets were observed along the northern edge of the precipitation swath from southwestern Ohio to 

Kansas City, Missouri. Sounding data from Lincoln, Illinois (ILX – see Fig. 2) indicated that the surface 

subfreezing layer in the area of the freezing rain was fairly warm (T > -3C) and too shallow to sample on 

this day. Regional radar data show the northward progression of the precipitation area with time, reaching 

northern Ohio by 1600 UTC (Fig. 4). 

Pilot reports of icing were prevalent within the area of stratiform cloud ahead of the precipitation 

swath between 5000 and 10000 feet (Fig. 5). The reports included all types of icing and had intensities up 

to moderate. 

No satellite data was available for this case. 

NASA/CR—2000-209413 209
Figure 1 – Upper-air charts for 970127, 1200 UTC at a) 500 and b) 700 mb. 

NASA/CR—2000-209413 210
Figure 1 – Upper-air chart for 970127, 1200 UTC at c) 850 mb (300 mb chart was not available). 

NASA/CR—2000-209413 211
Figure 2 – Balloon-borne soundings from Cleveland for 970127, a) 1440 and b) 1546 UTC. 

NASA/CR—2000-209413 212
Figure 2 – Balloon-borne soundings for 970127, 1200 UTC at c) Lincoln. 

Figure 3 – Surface chart for 970127, 1500 UTC. 

NASA/CR—2000-209413 213
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970127, a) 1300 

and b) 1400 UTC. 

NASA/CR—2000-209413 214
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970127, c) 1500 

and d) 1600 UTC. 

NASA/CR—2000-209413 215
Figure 5 – Pilot reports of icing for 970127, a) 1200-1259 and b) 1300-1359 UTC. 

NASA/CR—2000-209413 216
Figure 5 – Pilot reports of icing for 970127, c) 1400-1459 and d) 1500-1559 UTC. 

NASA/CR—2000-209413 217
January 31, 1997

Flight #1 - Between Cleveland and Parkersburg (WV), and over Parkersburg from 1558 to 1815 UTC.

Flight #2 - Over Parkersburg and enroute to Cleveland from 1928 to 2103 UTC.

Brief overview

Two flights were made on this day. Flight #1 was made within and ahead of an area of

precipitation that was moving southeast across Ohio into Pennsylvania and West Virginia. During the first

hour of the flight, the aircraft traversed the precipitation area enroute to Parkersburg. Within the

precipitation area, mostly crystals with LWC = 0.0 and possibly a little freezing drizzle were encountered.

The aircraft began to break out of the precipitation area to the northwest of Parkersburg at approximately

1700 UTC. To the southeast of the precipitation shield, areas of all-water cloud with pockets of in-focus

freezing drizzle and LWC as high as 0.45 were observed between 9600 and 11000 feet. Some pockets of

crystals, including columns and aggregates were occasionally encountered. Irregular layers of cloud were

present below 9600 feet and crystals were more prevalent at those altitudes, down to 4500 feet.

Flight #2 was simply a return flight from Parkersburg to Cleveland. The microphysical probes

were not operable during the first part of this flight, until 1955 UTC, when the aircraft had nearly reached

Canton-Akron. Mostly crystals were encountered during that flight, with some pockets of all-water cloud.

LWC values were as high as 0.3 at 11300 feet, where temperatures were approximately -7 C.

Relevant weather features

At 1200 UTC, two 500 mb troughs extended into the lower 48 from an upper low centered north

of Hudson's Bay (Fig. 1). The eastern trough ran south through central Pennsylvania to the Carolina coast.

A pocket of cold air preceded this trough and warm advection was present behind it, across eastern Ohio,

West Virginia, Pennsylvania, and New York. Saturated conditions covered most of the forecast area,

except Pennsylvania, West Virginia and southeastern Ohio, where dry air was in place. Temperatures were

approximately -18 C in the saturated air over Ohio at this level. At 700 mb, a weak trough was in place

from Detroit to Georgia, and traversed central Ohio. A lobe of saturated air was present ahead of this trough

across western Pennsylvania, eastern Ohio, southeastern Ontario, and Michigan. Warm advection was

evident across the entire forecast area. At 850 mb, the trough axis extended southeast from a low over

Minnesota to western Pennsylvania, then southwest into Georgia. Dry air and warm advection were found

behind this trough, across Ohio, and western West Virginia.

The 1500 UTC surface chart (Fig. 2) shows a 992 mb low centered along the MinnesotaWisconsin border, with a warm front extending southeast to Cleveland. Snow was falling across northern

Ohio, southeastern Ontario, and eastern Michigan, according to both the surface observations and radar

mosaic (Fig. 3). By 1800 UTC, the warm front pushed slightly to the northeast, as the low tracked

eastward to Green Bay, Wisconsin. The precipitation area moved eastward across Ohio to just north of the

Ohio-West Virginia border, covering much of Ohio, northwestern Pennsylvania and western New York.

NASA/CR—2000-209413 218
Twin Otter tracks for 1600-1630 UTC (Fig. 3c) appear to show the aircraft skimming the eastern

edge of the precipitation shield. However, since the radar data was for 1600 UTC and the precipitation

shield was moving quickly to the southeast, the aircraft was actually well within the precipitation until

~1700 UTC, when it broke out of its southeastern edge (Fig. 3d).

Scattered pilot reports of icing were made around and within the precipitation swath between

13000 and 23000 feet (Fig. 4). Icing types were mostly rime, but some mixed reports were received. Icing

intensities were as high as moderate outside and along the edges of the precipitation, but only trace to light

PIREPs were made within it.

Satellite data was not available for this case.

NASA/CR—2000-209413 219
Figure 1 – Upper-air charts for 970131, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 220
Figure 1 – Upper-air charts for 970131, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 221
Figure 2 – Surface charts for 970131, a) 1500 and b) 1800 UTC.

NASA/CR—2000-209413 222
Figure 2 – Surface charts for 970131, c) 2100 UTC.

NASA/CR—2000-209413 223
Figure 3 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970131, a) 1400

and b) 1500 UTC (1500 UTC radar data was not available).

No radar data for this time

NASA/CR—2000-209413 224
Figure 3 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970131, c) 1600

and d) 1700 UTC.

NASA/CR—2000-209413 225
Figure 3 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970131, e) 1800 UTC.

Figure 4 – Pilot reports of icing for 970131, a) 1300-1359 UTC.

NASA/CR—2000-209413 226
Figure 4 – Pilot reports of icing for 970131, b) 1400-1459 and c) 1500-1559 UTC.

NASA/CR—2000-209413 227
Figure 4 – Pilot reports of icing for 970131, d) 1600-1659 and e) 1700-1759 UTC.

NASA/CR—2000-209413 228
Figure 4 – Pilot reports of icing for 970131, f) 1800-1859 UTC (data for 1900-1959 UTC was not

available).

NASA/CR—2000-209413 229
February 4, 1997

Flight #1 – Over northeast Ohio and northwest Pennsylvania from 1312 to 1441 UTC. 

Brief overview

Two flights were made on this day. An in depth analysis of the second flight can be found in the 

“premier cases” document. The first flight was made over northeastern Ohio within a widespread area of 

precipitation and deep, cold cloud. Rain and drizzle were observed below approximately 7000 feet, and the 

melting level was near 7500 feet. Snow was observed above the melting level, and consisted of columns, 

capped columns, plates, needles, irregulars, and aggregates. It is possible that some in-focus and out-offocus freezing drizzle may have also existed above the freezing level. Occasional pockets of LWC up to 

0.1 were observed, but LWC was near 0 during most of the flight. 

Relevant weather features

At 1200 UTC, a closed low was centered over the Iowa-Missouri border at 500 and 700 mb (Fig. 

1). A number of troughs were attached to the low, including one which ran east across the Michigan-Ohio 

border and another which ran essentially north-south from Chicago to New Orleans. This second trough 

separated dry air to the west from saturated air to the east, which nearly all of the forecast area. 

Temperatures in the saturated air over Ohio were approximately –18 C. The moisture was more extensive 

at 700 mb, covering as far west as the Illinois-Missouri border, while dry air was present across northern 

Michigan, southeastern Ontario, New York, and northeastern Pennsylvania, roughly along and northeast of 

a ridge axis. Warm advection was present ahead of the second trough at both 500 and 700 mb. 

The Missouri-Iowa low was stronger at lower altitudes and had several closed contours at 700 and 

850 mb. At 850 mb, saturated conditions and warm advection again covered the forecast area, except for 

dry air across most of Michigan, southeastern Ontario, New York, and northeastern Pennsylvania. This dry 

air was in the wake of a high/ridge centered along the East Coast, with a ridge axis extending northwest 

across New York and southeastern Ontario. Sounding data from Buffalo at 1200 UTC reveal the deep, dry 

air to the northeast of Cleveland, while the Wilmington sounding indicates deep, saturated conditions over 

Ohio (Fig. 2). 

Surface maps were not available from 0900 to 1500 UTC, but the 0600 UTC surface map (Fig. 3) 

shows a weak, disorganized, 1013 mb low sprawled across Missouri and Arkansas, with a warm front 

which extended eastward across Kentucky, and connected to a stationary front in the Carolinas. A strong, 

1039 mb high was centered in southwestern Quebec and nosed southward down the East Coast, to the north 

of the stationary front, as well as westward across the Great Lakes. A broad swath of rain was present along 

and to the north of the warm front, across Kentucky, southern Ohio and southern Indiana. By 1800 UTC, 

the surface low became well organized, depend to 1006 mb and moved northeast to Chicago, dragging the 

warm front with it. The warm front extended southeastward from the low, across southwestern Ohio to the 

NASA/CR—2000-209413 230
western Carolinas. The broad swath of precipitation also moved northeast into extreme eastern Ohio, 

western Pennsylvania, southeastern Ontario, and Michigan by 1800 UTC, crossing Ohio during the period 

of the first flight. Maps of the surface precipitation type for 1400 and 1500 UTC (Fig. 4) confirm that rain 

was occurring across all of Ohio and was moving into Pennsylvania and Michigan. Some freezing rain was 

occurring at the leading edge of the rain swath. The first flight took place within and just ahead of the 

leading portion of this precipitation. 

Satellite data for 1500 UTC (none was available before 1500 UTC) show the area of deep, cold 

cloud to the east of a line from northern Lake Michigan across Toledo to Cincinnati, with shallower, 

warmer (-10 C) cloud across western Ohio (Fig. 5). The freezing drizzle observed during the second flight 

(premier case) occurred with this area of warmer cloud. 

Most of the pilot reports of icing were made within the trailing, warmer cloud to the west (Fig. 6). 

The icing was up to moderate intensity, of all types and at altitudes between 3000 and 16000 feet. 

Radar mosaic data was not available for this case. 

NASA/CR—2000-209413 231
Figure 1 – Upper-air charts for 970204, 1200 UTC at a) 300 and b) 500 mb. 

NASA/CR—2000-209413 232
Figure 1 – Upper-air charts for 970204, 1200 UTC at c) 700 and d) 850 mb. 

NASA/CR—2000-209413 233
Figure 2 – Balloon-borne sounding data for 970204, 1200 UTC at a) Buffalo and b) Wilmington. 

NASA/CR—2000-209413 234
Figure 3 – Surface charts for 970204, a) 0600 and b) 1800 UTC. 

NASA/CR—2000-209413 235
Figure 4 – Observed precipitation type and cloud cover from surface stations for 970204, a) 1400 and 

b) 1500 UTC. 

NASA/CR—2000-209413 236
Figure 5 – GOES-8 a) infrared and b) visible satellite imagery for 970204, 1500 UTC. 

NASA/CR—2000-209413 237
Figure 6 – Pilot reports of icing for 970204, a) 1100-1159 and b) 1200-1259 UTC. 

NASA/CR—2000-209413 238
Figure 6 – Pilot reports of icing for 970204, c) 1300-1359 and d) 1400-1459 UTC. 

NASA/CR—2000-209413 239
Figure 7 – Twin Otter tracks for 970204, a) 1312-1330, b) 1330-1430 and c) 1430-1441 UTC. 

NASA/CR—2000-209413 240
February 5, 1997

Flight #1 - Over northeastern Ohio from 1340 to 1607 UTC.

Brief overview 

One flight was made over the northeastern corner of Ohio, within a widespread, fairly deep,

stratiform cloud deck. Two distinct cloud layers were present. The lower cloud layer extended from

approximately 1300 to 4000 feet, while the upper cloud layer extended from approximately 4500 to 7000

feet. Within both layers, LWC rose from 0.0 to maximum values of 0.5-0.6, and temperatures were

subfreezing. The clouds were primarily comprised of liquid water, in the form of small drops. Some

freezing drizzle was present, especially within and beneath the bases of both cloud decks. The 2-D Grey

probe showed consistent counts in the higher size bins below about 3000 feet. Some in-focus freezing

drizzle was clearly visible in the 2-D Grey imagery near cloud base at 1300 feet at the end of the flight, but

it was mixed with crystals at that time. The clouds were completely made up of water during the first 90

minutes of the flight, but crystals began to show up near Canton-Akron at 6000 feet around 1520 UTC and

were occasionally observed thereafter. It was during the latter part of the flight that several stations around

the region began reporting light rain, snow (including snow grains) and "unknown precipitation", all of

which was light.

Relevant weather features

At 1200 UTC, a weak, closed low was centered over Lake Huron at 500 mb (Fig. 1). A weak

trough extended southwest from the low across northwestern Ohio and cold advection was present behind

it. Dry conditions prevailed across the forecast area at 500 mb, with saturated air in place to the east of the

Appalachians from Virginia northward. At 700 mb, the low, trough axis, and cold advection became more

pronounced, while dry air still covered the forecast area. No significant temperature advection was present

over Ohio at this level. At 850 mb, the low/trough setup remained consistent, except that a weak

secondary trough extended southward across eastern Ohio. Cold advection was present across New York,

Pennsylvania, Ohio, and West Virginia, with the strongest cold advection over the Appalachians and

relatively weak cold advection over Ohio. Saturated air was found across the forecast area, except for over

Michigan. Temperatures were between -5 and -7 C in the saturated air at 850 mb.

Sounding data from Pittsburgh (Fig. 2) show a solid cloud deck from 920 mb (~3000 feet) to 800

mb (~7000 feet), with a 2 C inversion at cloud top, and a cloud top temperature (CTT) of -8 C. Further to

the west at Wilmington, Ohio, a fairly similar cloud extended from 920 mb to 780 mb (~7500 feet), with a

strong, 5 C inversion at cloud top and a colder CTT of -11 C. This difference in the character of the

stratiform deck is also evident in infrared satellite data from 1215 UTC (Fig. 3). This image shows that

CTTs were slightly colder over the southwestern two-thirds of Ohio compared to those over western

Pennsylvania, northern Ohio, and Lake Erie. The 1545 UTC infrared image indicates that these slightly

colder CTTs had spread across the rest of Ohio, western Pennsylvania and southern Lake Erie, while even

NASA/CR—2000-209413 241
colder (near -15 C) CTTs had covered portions of southern Ohio. This change from CTTs which are less

often (-10 C) to more often (-15 C) associated with the formation of ice crystals and light precipitation

across northeastern Ohio nicely matches the change in character of the clouds from all-water to mixed water

and ice crystals, and the development of light precipitation across the area with time. Most of the

precipitation reached the surface in the form of snow, but some which fell as light rain may have formed via

the collision-coalescence process.

Pilot reports of icing were quite numerous between 1200 and 1600 UTC (Fig. 4). Most were up

to moderate intensity, were rime or mixed in type and were at altitudes between 2000 and 8000 feet. Some

clear icing was also reported, including one severe PIREP from an ATR-42 over South Bend, Indiana.

No radar mosaic data was available for this case.

NASA/CR—2000-209413 242
Figure 1 – Upper-air charts for 970205, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 243
Figure 1 – Upper-air charts for 970205, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 244
Figure 2 – Balloon-borne soundings for 970205, 1200 UTC from a) Pittsburgh and b) Wilmington.

NASA/CR—2000-209413 245
Figure 3 – GOES-8 infrared satellite imagery for 970205, a) 1215 and b) 1545 UTC.

NASA/CR—2000-209413 246
Figure 4 – Pilot reports of icing for 970205, a) 1200-1259 and b) 1300-1359 UTC.

NASA/CR—2000-209413 247
Figure 4 – Pilot reports of icing for 970205, c) 1400-1459 and d) 1500-1559 UTC.

NASA/CR—2000-209413 248
Figure 5 – Twin Otter tracks for 970205, a) 1340-1430, b) 1430-1530 and c) 1530-1607 UTC.

NASA/CR—2000-209413 249
March 6, 1997

Flight #1 - Over northeastern Ohio and northwestern Pennsylvania from 1459 to 1634 UTC.

Flight #2 - Between Erie and Cleveland from 1719 to 1806 UTC.

Brief overview 

Two flights were made over the northeastern corner of Ohio and northwestern corner of

Pennsylvania, near Youngstown, Franklin (PA), Erie, and along the southeastern shore of Lake Erie.

These flights were made within and between a series of northwest-southeast oriented lake effect snowbands.

Periods with all-small, supercooled drops with LWC up to 0.5, all ice crystals (pristine, rimed, aggregates)

and mixed conditions were observed as the aircraft traversed the bands. Cloud top heights varied between

4000 and 6000 feet.

Relevant weather features

At 1200 UTC, a 500 mb trough extended southward across southeastern Ontario, western New

York, and Pennsylvania (Fig. 1). Cold advection was associated with both sides of the trough, and was

present across northeastern Ohio and northwestern Pennsylvania. Dry air was present across the forecast

area, except for a lobe of saturated air from western New York northward into Canada. At 700 mb, a weak,

closed low was present just northeast of Lake Ontario. A trough/cold front extended southward from the

low into eastern Pennsylvania (just ahead of the 500 mb trough). Cold advection was again present across

eastern Ohio, Pennsylvania, and New York, and moisture at 700 mb covered northeastern Ohio,

Pennsylvania, and New York. The temperature was approximately -17 C in the saturated air over eastern

Lake Erie at this level. At 850 mb, the low/trough pattern was slightly further east, and cold advection and

moisture covered the entire forecast area. Winds were from the west-northwest at 500, 700 and 850 mb over

and to the north and east of Ohio.

Surface maps for 0900 UTC (the 1200 UTC map was not available) show a 996 mb surface low

over central New York with a cold front running southward along the East Coast (Fig. 2). West-northwest

winds were also present at the surface, indicating that cold air was blowing across Lake Erie into

northeastern Ohio and northwestern Pennsylvania at all levels. Snow was falling in this area, as well as

across western New York, southeastern Ontario, and Michigan. By 1500 UTC, the low tracked to the

coast of Maine, while the snow moved out of northeastern Ohio, and was found only in western

Pennsylvania and western New York.

Sounding data from Pittsburgh, Detroit, and Buffalo (Fig. 3) all indicate west-northwest winds

from the surface to at least 10000 feet. Saturated conditions extended from 2000 feet to 10000 feet, with a

cloud top temperature of -18 C at Pittsburgh, where snow was reported during most of the event. The

same was true at Buffalo, but deeper saturated conditions were present there, while shallower clouds with

CTTs of -10 to -15 C were present over Detroit.

NASA/CR—2000-209413 250
Satellite from 1145 UTC (Fig. 4) show the colder cloud from the north just reaching Buffalo, with

cloud top temperatures colder than -30 C there, while an area of cloud with CTTs of -15 to -20 C was in

place across western Pennsylvania, eastern Ohio, and West Virginia. Pockets of warmer cloud top

temperatures were evident over southeastern Michigan and western Lake Erie. By 1445 UTC, the clouds

exhibited a variety of cloud top temperatures across northeastern Ohio, from -10 to -20 C or so. Breaks in

the clouds were evident in the visible satellite data, in the same locations where the infrared showed

temperatures of near 0 C (the temperature of the surface). There were some hints of northwest to southeast

oriented bands, as well as some cellular nature in the clouds.

Radar mosaic data also hint at a banded structure in the snow across northeastern Ohio and

western Pennsylvania during the period (Fig. 5). This is a common structure in lake effect snow events.

Between 1500 and 1800 UTC, the snow shifted eastward into Pennsylvania, but some snow was still

evident across the northeastern corner of Ohio at 1800 UTC. Throughout the period, pilot reports of icing

were found between 6000 and 7000 feet in the vicinity of the snowbands, but the locations and timing of

the reports were not fine enough to determine their location relative to individual bands (Fig. 6). It is

likely, however, that most of those pilot reports were probably due to ice accreted within pockets of LWC

between and along the edges of the snowbands. Most PIREPs were rime in type and up to moderate in

intensity.

NASA/CR—2000-209413 251
Figure 1 – Upper-air charts for 970306, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 252
Figure 1 – Upper-air charts for 970306, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 253
Figure 2 – Surface charts for 970306, a) 0900 and b) 1500 UTC.

NASA/CR—2000-209413 254
Figure 3 – Balloon-borne soundings for 970306, 1200 UTC at a) Pittsburgh and b) Detroit.

NASA/CR—2000-209413 255
Figure 3 – Balloon-borne soundings for 970306, 1200 UTC at c) Buffalo..

Figure 4 – GOES-8 Infrared satellite image for 970306, a) 1145 UTC.

NASA/CR—2000-209413 256
Figure 4 – GOES-8 b) Infrared and c) visible satellite image for 970306, 1445 UTC.

NASA/CR—2000-209413 257
Figure 4 – GOES-8 d) Infrared and e) visible satellite image for 970306, 1745 UTC.

NASA/CR—2000-209413 258
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970306, a) 1500

and b) 1600 UTC.

NASA/CR—2000-209413 259
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970306, c) 1700

and d) 1800 UTC.

NASA/CR—2000-209413 260
Figure 6 – Pilot reports of icing for 970306, a) 1400-1459 and b) 1500-1559 UTC.

NASA/CR—2000-209413 261
Figure 6 – Pilot reports of icing for 970306, c) 1600-1659 and d) 1700-1759 UTC.

No data for this time

NASA/CR—2000-209413 262
March 11, 1997

Flight #1 - Over northeastern Ohio and northwestern Pennsylvania from 1315 to 1458 UTC.

Flight #2 - Over northeastern Ohio and northwestern Pennsylvania from 1555 to 1714 UTC.

Flight #3 - Over Canton-Akron and Cleveland from 1821 to 2006 UTC.

Brief overview 

 Three flights were made on this day, all behind a cold front which moved south through

Cleveland at 1200 UTC. The first flight was made along the southeastern shore of Lake Erie, between

Cleveland and Erie, to the north of an east-west oriented band of precipitation. To the northeast of

Cleveland, the aircraft encountered a mixture of ice crystals and small water drops with LWC as high as 0.2

near 6800 feet. Some all-water and some mixed conditions were observed between Erie and Youngstown

and Franklin (PA), with temperatures of -3 C and LWC as high as 0.7 at times. A few out-of-focus drops

bordering on freezing drizzle sizes were observed on occasion.

The second flight took off from Erie, where colder conditions had arrived (T = -5.6 at 4000 feet).

Small drop icing with LWC up to 0.5 was found between Erie and Youngstown at 1600 UTC, and very

consistent 0.3-0.4 LWC was observed between Youngstown and Canton-Akron at 4000 feet, where the

temperature was -3 C. Cloud tops were near 4500 feet with a cloud top temperature (CTT) near -3 C.

Some larger drops were evident over Canton-Akron near the surface, where the temperature was +1 C. This

flight took place to the north of precipitation areas that had moved southeast into Pennsylvania.

The third flight took place over Canton-Akron and Cleveland, well to the northwest of all

precipitation areas. Things had cooled even further (T = -6.1 at 4000 feet). LWC was typically 0.1-0.4,

with lots of droplets bordering on freezing drizzle sizes observed near Canton-Akron between 3000 and

4000 feet at temperatures of approximately -4 C.

Relevant weather features

At 1200 UTC, a weak trough extended southward from a low over Hudson's Bay to the

Pennsylvania-Ohio border at 500, 700, and 850 mb (Fig. 1). At 850 mb, the trough clearly continued to

the southwest from Erie to Fort Wayne (IN). Dry conditions existed across most of the forecast area at 500

mb, except for some moisture to the east of the trough axis, where weak cold advection was present. At

700 and 850 mb, moisture was clearly evident along and ahead of the trough axis across northeastern Ohio,

Pennsylvania, New York, and New England. Cold advection was more evident behind the trough at these

levels. Sounding data from Detroit (Fig. 2) show two distinct cloud layers, one from 6000 feet down to

just above the surface (with a minimum temperature of -5 C at 3000 feet), and a second cloud deck from

8500 to 10000 feet (CTT = -13 C). A similar structure was in place at Buffalo, though the top of the lower

cloud deck was at only 4500 feet and the upper cloud deck was quite thin, existed at 8000 feet and had a

warmer CTT of -10 C.

NASA/CR—2000-209413 263
At the surface (see Fig. 3), a weak, cold front extended southwest from a low in central New York,

across northern Ohio and central Indiana, at roughly the same position as the 850 mb trough. Light rain

and snow was indicated along and just behind the front between Cleveland and Toronto (Figs. 3 and 4). A

lobe of high pressure over North Dakota combined with the weak New York low to produce northerly flow

behind the cold front. The front and the precipitation progressed southeast across Ohio and Pennsylvania

by 1500 UTC, as the low intensified slightly and moved to the New England coast and the lobe of high

pressure moved into Minnesota. Winds behind the front became north-northwesterly, blowing across Lake

Erie into northeastern Ohio and western Pennsylvania. With cold advection occurring at 850 mb behind

the cold front and temperatures remaining surely constant at the surface, the atmosphere became less stable

over northeastern Ohio in the lowest 5000 feet during the period from the first to the third flight.

Radar data for 1300-2000 UTC (Fig. 4) show that the precipitation moved southeast into

Pennsylvania and dissipated with time. Twin Otter tracks indicate that the first flight (Figs. 4a-c) was

made within and near the precipitation, matching the mixed conditions observed. Tracks for the second

(Figs. 4d,e) and third (Figs. 4f-h) show that the all-water conditions were observed well to the north of the

precipitation swath.

Icing pilot reports of up to moderate intensity and all types were made around the area during the

three flights (Fig. 5). Most of these PIREPs were for altitudes between 4000 and 8000 feet, and were found

outside of the areas of precipitation. A few were found within or along the edges of the precipitation.

No satellite data was available for this case.

NASA/CR—2000-209413 264
Figure 1 – Upper-air charts for 970311, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 265
Figure 1 – Upper-air charts for 970311, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 266
Figure 2 – Balloon-borne soundings for 970311, 1200 UTC at a) Detroit and b) Buffalo.

NASA/CR—2000-209413 267
Figure 3 – Surface charts for 970311, a) 1200 and b) 1500 UTC.

NASA/CR—2000-209413 268
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970311, a) 1300

and b) 1400 UTC.

NASA/CR—2000-209413 269
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970311, c) 1500

and d) 1600 UTC.

NASA/CR—2000-209413 270
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970311, e) 1700

and f) 1800 UTC.

NASA/CR—2000-209413 271
Figure 4 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970311, g) 1900

and h) 2000 UTC.

NASA/CR—2000-209413 272
Figure 5 – Pilot reports of icing for 970311, a) 1200-1259 and b) 1300-1359 UTC.

NASA/CR—2000-209413 273
Figure 5 – Pilot reports of icing for 970311, c) 1400-1459 and d) 1500-1559 UTC.

NASA/CR—2000-209413 274
Figure 5 – Pilot reports of icing for 970311, e) 1600-1659 and f) 1700-1759 UTC.

No data for this time

NASA/CR—2000-209413 275
Figure 5 – Pilot reports of icing for 970311, g) 1800-1859 and h) 1900-1959 UTC.

NASA/CR—2000-209413 276
March 13, 1997

Flight #1 - Over southeast Ontario, Selfridge AFB and Flint, Michigan from 1848 to 2036 UTC.

Flight #2 - Over Flint, Sarnia (Ontario) and southeastern Ontario from 2129 to 2252 UTC.

Brief overview

Two fights were made on this day. During the first flight, the aircraft flew to Selfridge,

encountering an all-liquid cloud layer between 7500 and 9500 feet. This cloud contained LWC up to 0.25

and some out-of-focus droplets which bordered on freezing drizzle sizes. Between Selfridge and Flint, the

cloud continued to be nearly all-water in composition and extended up to 11800 feet. More borderline

freezing drizzle was encountered, and some heavily rimed crystals/graupel began showing up near the end of

the flight. Ice pellets and graupel were reported at both Selfridge and Sarnia between 2000 and 2200 UTC,

matching these in-flight observations. Similar conditions were observed during the second flight, but with

more mixed conditions, including graupel and stellar crystals. Some periods of all-liquid were still

observed with sporadic LWC values as high as 0.5. Some clean, in-focus freezing drizzle was observed

near Sarnia at 10000-11000 feet and other pockets of out-of-focus near-ZL sized drops were found in that

vicinity.

These flights were made at the southern end of an area of strong convergence and warm advection

that was moving northward through the area. This convergence area was marked by a band of patchy, light

precipitation just north of the flight area at 1800 UTC. The Flint/Selfridge area was located within a break

in the precipitation shield at 1800 and 1900 UTC, but by 2000 UTC, some light snow grains and pellets,

as well as some radar echo began to show up there.

Relevant weather features

At 1200 UTC, a weak trough was in place across the southern Plains at 500 mb (Fig. 1). Weak

warm advection and saturated conditions existed across the Southeast states, ahead of this trough. A swath

of dry air was in place across Illinois, Indiana, and southwestern Ohio, while saturated air existed across

Michigan, northeastern Ohio, and southwestern Pennsylvania at 500 mb. The moisture pattern was very

similar at 700 mb, but shifted a little to the southwest, with saturated conditions in place just south of

Detroit. Warm advection extended further to the north at 700 mb, reaching northern Indiana and central

Ohio, while weak cold advection was evident in northern Michigan. At 850 mb, the cold advection was

stronger and covered all of Michigan, as well as northeastern Ohio, Pennsylvania, and New York. The

cold advection to the north and strong warm advection from the south met in a line across southern

Michigan and northeastern Ohio, which nicely matched the swath of moisture at 700 mb. This swath of

saturated air was still evident over Wisconsin and Michigan at 850 mb, but did not continue to the

southeast across Ohio, as very dry conditions were evident at Pittsburgh and Buffalo.

The 1200 UTC Detroit sounding (Fig. 2) shows a layer of cloudiness to from about 7000 feet on

up, with fairly dry air below. Strong stability was in place within the cloud deck from 7000 to 8500 feet,

NASA/CR—2000-209413 277
then more unstable conditions were present from 8500 to 10000 feet, capped by a 1 C inversion from 10000

to 11500 feet. Ice saturated or nearly-ice saturated conditions were present up to approximately 400 mb,

where temperatures were around -35 C, roughly matching the CTTs indicated on the 1445 infrared image.

By 0000 UTC, the Plains trough strengthened at all levels and a low center developed over

western Minnesota at 700 mb, and over Missouri at 850 mb (Fig. 3). Saturated air from the south pushed

quickly northward and combined with warm advection to eliminate the dry air that was in place across

Indiana and Ohio at these levels earlier. Saturated conditions covered the forecast area at 500, 700, and 850

mb. The area of strongest warm advection and the tightest temperature gradient pushed into northern

Michigan at 700 mb and over lower Michigan at 850 mb, while the cold, which was evident over

Michigan at 1200 UTC, retreated to New York and New England by 0000 UTC.

Satellite imagery from 1445 UTC (Fig. 4) indicate the existence of deep clouds over most of

Michigan and northeastern Ohio, and breaks in these clouds over Ohio and Indiana, matching the moisture

patterns on the 1200 UTC upper air charts. These were slightly north of the location of the dry air at 1200

UTC, due to its northward movement over 3 hours. With time, the breaks became filled in, as deep

moisture continued to move northward. National and regional radar data (Figs. 5, 6) reflect this

progression. The 1300 UTC national radar mosaic shows a swath of snow across central Michigan, a gap

in the precipitation across Ohio, Indiana, and northern Illinois, and an area of rain to the southwest. By

1800 UTC, the entire precipitation pattern shifted to the north and the gap had shrunk. This trend

continued through the period of flight (1900-2300 UTC). Some ice pellets and snow grains were reported at

Selfridge and Sarnia by 2000 UTC, within some patchy reflectivity on the south side of the central

Michigan precipitation swath. Pilot reports of moderate icing occurred between 9000 and 12000 feet, along

the southern edge of the Michigan precipitation swath early on, but became sparse as the deep, cold cloud

shield filled in (Fig. 7).

At the surface, a 1038 mb high that was centered just north of Michigan nosed southeastward into

Pennsylvania at 1500 UTC (Fig. 8). Relatively clear skies were present to the northeast of this ridge axis,

while clouds and precipitation were occurring to the southwest of it. A weak low/trough was strengthening

over eastern Nebraska and a stationary front extended southeast from the low across Missouri, then eastward

across the Tennessee-Kentucky border. Snow and freezing rain were occurring to the north of the low,

across the upper Midwest, with the aforementioned swath of snow protruded southeast from the main snow

area across central Michigan. Rain was occurring to the northeast of another weak low over southwestern

Arkansas, reaching southern Illinois and southwestern Indiana. By 2100 UTC, the Arkansas low

strengthened slightly and began to merge with the Nebraska low over the Arkansas-Missouri region, while

rain from the south moved northeast into western Ohio and nearly all of Indiana.

NASA/CR—2000-209413 278
Figure 1 – Upper-air charts for 970313, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 279
Figure 1 – Upper-air charts for 970313, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 280
Figure 2 – Balloon-borne sounding for 970313, 1200 UTC at Detroit.

NASA/CR—2000-209413 281
Figure 3 – Upper-air charts for 970314, 0000 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 282
Figure 3 – Upper-air charts for 970314, 0000 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 283
Figure 4 – GOES-8 infrared satellite imagery for 970313, a) 1445 and b) 1845 UTC.

NASA/CR—2000-209413 284
Figure 5 – National radar data, surface observations and icing PIREPs for 970313, a) 1300 and b) 1800 UTC.

NASA/CR—2000-209413 285
Figure 6 – Regional radar data, surface observations, icing PIREPs and Twin Otter tracks for 970313, 

a) 1800 and b) 1900 UTC. 

NASA/CR—2000-209413 286
Figure 6 – Regional radar data, surface observations, icing PIREPs and Twin Otter tracks for 970313,

c) 2000 and d) 2100 UTC.

NASA/CR—2000-209413 287
Figure 6 – Regional radar data, surface observations, icing PIREPs and Twin Otter tracks for 970313,

e) 2200 and f) 2300 UTC.

NASA/CR—2000-209413 288
Figure 7 – Pilot reports of icing for 970313, a) 1700-1759 and b) 1800-1859 UTC.

NASA/CR—2000-209413 289
Figure 7 – Pilot reports of icing for 970313, c) 1900-1959 and d) 2000-2059 UTC.

NASA/CR—2000-209413 290
Figure 7 – Pilot reports of icing for 970313, e) 2100-2159 and f) 2200-2259 UTC.

NASA/CR—2000-209413 291
Figure 8 – Surface charts for 970313, a) 1500 and b) 2100 UTC.

NASA/CR—2000-209413 292
March 14, 1997

Flight #1 - En-route to and over Toledo and Fort Wayne from 1744 to 1946 UTC.

Flight #2 - Over Fort Wayne, Toledo, Detroit and southeastern Ontario from 2058 to 2250 UTC.

Brief overview 

Two flights were made on this day. During the first flight, the Twin Otter headed west to meet

up with the leading portion of an area of stratiform cloud that was advancing eastward across Indiana. The

leading portion of this cloudy area had cloud top temperature (CTT) values near -10 C and no precipitation

was observed beneath it, while CTTs < -15 C and light snow were observed to the west of the leading

edge. Consistent LWC of 0.4 and temperatures near -6 C were found between Toledo and Fort Wayne

above 3900 feet, and LWC gradually decreased to 0 at cloud base (1600 feet). A strong, 5 C inversion was

in place from 4800 feet to cloud top at 6800 feet. Some brief pockets of out-of-focus droplets bordering on

freezing drizzle sizes at temperatures of -4 C were found just west of Toledo near 1859 UTC, while

consistent LWC of 0.3-0.5 with T = -7 C was observed after 1900 UTC between Toledo and Fort Wayne

near 4500 feet.

The second flight began at Fort Wayne in a mixture of snow and possibly a little ZL below 4000

feet. A layer of mixed water and some crystals with LWC values of 0.2-0.4 was observed between 4000

and 6000 feet, as the aircraft tracked to the northeast. The development of snow in the Fort Wayne area

matched the advancement of colder air toward the east, cooling the atmosphere at the base of the inversion

from -9 C at 1930 UTC to -12 C at 2100 UTC. This cooling pushed the cloud into a temperature range

where crystals were more likely to form. The change from the obvious appearance of crystals below 4000

feet to relatively few crystals and mostly water drops from 4000 to 6000 feet may be due to the

overwhelming number of small droplets in the 2D-Grey buffer (concentration > 10000) above 4000 feet

compared to the very low concentrations to of snow crystals alone (concentration < 200) below 4000 feet.

The aircraft continued northward into Michigan in an attempt to sample some freezing drizzle reported at

Jackson before snow moved in from the west. However the aircraft could not reach Jackson in time and

only snow was observed there, along the eastern edge of a patch of echo on the radar. Continuing to the

east over Detroit, en-route to Cleveland, the Twin Otter observed supercooled liquid water and some outof-focus, borderline ZL-sized drops to the east of Detroit. Steady 0.2-0.4 LWC, with a little snow and

possibly a little ZL at times, was observed between Detroit and Cleveland at temperatures between -6 and

-11 C in the 4000 to 6000 feet altitude band.

Relevant weather features

At 1200 UTC, a weak, closed low at 700 mb was centered over northern Wisconsin (Fig. 1).

This low featured two troughs, one of which extended southeast into Indiana, then south to the Gulf Coast,

while the other extended southwest across eastern Iowa, then down to Oklahoma. Cold advection was

evident behind both troughs, but was strongest along and behind trough #2. Dry air was in place behind

NASA/CR—2000-209413 293
trough #2 and saturated conditions to were found ahead of it. At 850 mb, a strong closed low was centered

over Michigan and had a strong trough/cold front to which extended south across western Ohio then

southwest to Mississippi. Strong cold advection was evident behind this trough/front from Canada to

Texas, while strong warm advection was evident ahead of it across eastern Michigan, eastern Ohio, New

York, and the mid-Atlantic states. A weak trough/warm front extended east from the low across

southeastern Ontario and northern New York. Saturated conditions were present across the entire forecast

area at this level.

A strong, 994 mb surface low was centered over western Lake Erie at 1500 UTC (Fig. 2). A cold

front running south from the low across central Ohio to the Texas Gulf Coast was the surface reflection of

upper-air trough #1. Rain was occurring along and ahead of the cold front, where deep moisture was

evident at 1200 UTC. Dry air pushing in aloft (see 500 mb chart for 1200 UTC) left only warm, nonprecipitating cloud in place behind the cold front. Light snow was occurring across Illinois and Wisconsin

at 1500 UTC, and gradually moved eastward across Indiana and Michigan by 2100 UTC. The eastward

progression of this snow was clearly reflected in surface observations.

Infrared satellite data from 1145 UTC (Fig. 3) show deep, patchy cloudiness east of a line from

Lake Michigan to New Orleans, and a lack of deep cloud to the west of that line. These cloud locations

roughly match the moisture patterns at 500 mb. A swath of clouds was advancing eastward across Illinois

to around Fort Wayne by 1745 UTC. The location and movement of this cloudy area during this period is

similar to that of trough #2 at 700 mb, but precedes it, and fairly closely matches the advance of a lobe of

moisture and the T = -10 C contour at 850 mb. The –10 C temperature is approximately the same as the

CTT at the top of the inversion at 6000 feet (~820 mb).

For the most part, the advancing snow was too light to be picked up in the radar mosaic data

(Fig. 4). Tracks for flight #1 indicate that it took place ahead of the snow (as reported in surface

observations). Tracks for flight #2 show that the Twin Otter intercepted an area of snow (visible in the

mosaic data) upon entering Michigan, but that the aircraft got ahead of the snow as it flew east over Detroit.

It was at this point that the significant LWC and some freezing drizzle was observed.

Numerous pilot reports of moderate mixed and rime icing were made between 2000 and 6000 feet

in the area of warm cloud top temperatures just ahead of the advancing snow (Fig. 5). This is where the

aircraft spent most of its flight time on this day.

NASA/CR—2000-209413 294
Figure 1 – Upper-air charts for 970314, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 295
Figure 1 – Upper-air charts for 970314, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 296
Figure 2 – Surface charts for 970314, a) 1500 and b) 1800 UTC.

NASA/CR—2000-209413 297
Figure 2 – Surface charts for c) 970314, 2100 UTC.

NASA/CR—2000-209413 298
Figure 3 – Infrared satellite imagery for 970314, a) 1145 and b) 1445 UTC.

NASA/CR—2000-209413 299
Figure 3 – Infrared satellite imagery for 970314, c) 1745 and d) 2045 UTC.

NASA/CR—2000-209413 300
Figure 4 - Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970314, a) 1800

and b) 1900 UTC.

NASA/CR—2000-209413 301
Figure 4 - Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970314, c) 2000

and d) 2100 UTC.

NASA/CR—2000-209413 302
Figure 4 - Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970314, e) 2200

and f) 2300 UTC.

NASA/CR—2000-209413 303
Figure 5 – Pilot reports of icing for 970314, a) 1700-1759 and b) 1800-1859 UTC.

NASA/CR—2000-209413 304
Figure 5 – Pilot reports of icing for 970314, c) 1900-1959 and d) 2000-2059 UTC.

NASA/CR—2000-209413 305
Figure 5 – Pilot reports of icing for 970314, e) 2100-2159 and f) 2200-2259 UTC.

NASA/CR—2000-209413 306
March 20, 1997

Flight #1 - Over Lake Erie, near Pelee Point, Ontario from 1249 to 1442 UTC.

Brief overview 

One flight was made into a stratiform cloud deck over Lake Erie, between Cleveland and Pelee

Point. With the exception of some graupel and heavily rimed crystals that were encountered during initial

climbout, the clouds and precipitation sampled were entirely made up of supercooled liquid water drops,

including freezing drizzle. The cloud were lower and warmer toward the west. On initial climbout from

Cleveland, a 2 C inversion was present at cloud top and LWC values reached a maximum of 0.5 at 7500

feet. At 1300 UTC, cloud tops just northwest of Cleveland were near 8200 feet and the minimum

temperature within the cloud deck was -7 C. By 1400 UTC, the cloud tops near Pelee Point were down to

6600 feet with CTT = -5.2 C. Drop sizes gradually increased with decreasing altitude, going from all

cloud-sized drops above 7000 feet to in-focus freezing drizzle from 6000 to 4000 feet. Similar patterns were

observed throughout the flight, with the largest drops encountered below 6000 feet, where the temperature

was -4 C or warmer. No precipitation was reported at the surface and no radar echo was present in the

mosaic data in the vicinity of the flights. The Twin Otter accreted glaze ice shapes with nodules, horns

and ridges, according to flight notes.

Relevant weather features

At 1200 UTC, a north-south oriented trough extended from a Hudson's Bay low to the west end

of Lake Ontario, then southwest across Lake Erie and central Indiana at 500, 700 and 850 mb (Fig. 1). At

500 mb, deep moisture and light precipitation was in place ahead of the trough, across New York and

Pennsylvania. Dry air was observed west of the Pennsylvania-Ohio order, including over Lake Erie. At

700 mb, the air was saturated across Ohio and Lake Erie, with dry air as nearby as Detroit. At 850 mb,

saturated air was in place across Michigan, Lake Erie, and southeast Ontario, but it was dry across much of

Ohio. Overall, this combination produced a stratiform cloud deck over northern Ohio, Lake Erie, and

southeastern Michigan, with the cloud deck sloping downward and warming toward the west. This cloud

was clearly evident on infrared and visible satellite imagery for 1345 UTC (Fig. 2). CTTs were slightly

colder on the eastern and of Lake Erie, compared to the western end. Sounding data for 1200 UTC from

Detroit (Fig. 3) show dry air down to 780 mb (~7000 feet) and a cloud deck from approximately 4000 to

7000 feet with CTT of -6 C. This structure was very similar to that observed by the Twin Otter.

The earliest surface map available for this case was for 1800 UTC (3 hours after the flight was

completed; Fig. 4). It shows a weak cold front draped across the southern borders of Indiana, Ohio, and

Pennsylvania, and a trough running from southwest to northeast across eastern Ohio. Northwest winds

were occurring across Ohio, Michigan, and southeast Ontario, bringing air across Lakes Erie and Huron.

The 1200 UTC Detroit sounding showed that this northwest flow extended through a very deep layer, from

the surface to the tropopause.

NASA/CR—2000-209413 307
Radar data and Twin Otter tracks for 1300-1500 UTC (Fig. 5) indicate that this flight took place

in a location where no precipitation was reaching the ground.

Numerous pilot reports of light to moderate mixed, rimed, and clear (light only) icing were made

between 3000 and 8000 feet within this stratiform cloud deck (Fig. 6). These observations match the

glaze/mixed agreements and significant performance hits experienced by the Twin Otter.

NASA/CR—2000-209413 308
Figure 1 – Upper-air charts for 970320, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 309
Figure 1 – Upper-air charts for 970320, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 310
Figure 2 – a) Infrared and b) visible satellite imagery for 970320, 1345 UTC.

NASA/CR—2000-209413 311
Figure 4 – Surface chart for 970320, 1800 UTC.

Figure 3 – Balloon-borne sounding data for 970320, 1200 UTC from Detroit.

NASA/CR—2000-209413 312
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970320, a) 1300

and b) 1400 UTC.

NASA/CR—2000-209413 313
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970320, c) 1500

UTC.

NASA/CR—2000-209413 314
Figure 6 – Pilot reports of icing for 970320, a) 1100-1159 and b) 1200-1259 UTC.

NASA/CR—2000-209413 315
Figure 6 – Pilot reports of icing for 970320, c) 1300-1359 and d) 1400-1459 UTC.

NASA/CR—2000-209413 316
March 25, 1997

Flight #1 - Over Sandusky, Ohio from 1355 to 1529 UTC.

Brief overview 

One flight was made near Sandusky, Ohio into clouds on the south side of a northward moving

area of precipitation. Deep clouds associated with the precipitation were thinning out on their southern

end, with pockets of CTTs as warm as -10 to -15 C. During this flight, the 2D-Grey probe had a great

deal of noise, and it was difficult to discern what was being sampled. The aircraft encountered LWC of 0.1-

0.3, with pockets of LWC as high as 0.5. Cloud base was near 8300 feet, while cloud tops were quite

variable and as high as 15000 feet in places. Breaks were occasionally found within the cloud decks. The

1D-260X probe data showed that particles larger than 100 microns in diameter were present during portions

of the flight, but the nature of these particles is unknown due to the 2D-Grey problems.

Relevant weather features

At 1200 UTC, a sharp, northeast-southwest oriented trough was in place from Minnesota to New

Mexico at 500 mb (Fig. 1). A second trough ran northeast to southwest from southern Wisconsin to

southwestern Missouri. A ribbon of saturated air was in place ahead of trough #2, from Texas to Michigan

and western Ohio, while dry air was found from Louisiana to Kentucky, and over an area from the

Pennsylvania-Ohio border to Quebec. At 700 mb, a closed low was located over northeastern Minnesota,

with a trough/cold front extending south into Iowa, then southwest to New Mexico. Cold advection was

occurring behind the trough, while warm advection was occurring ahead of it, northeast of a line from Green

Bay to Pittsburgh. Again, saturated air was in place ahead of the trough, covering most of the forecast area,

but dry air from the southwest had already reached central Ohio. At 850 mb, the trough/cold front was

slightly further east and the moisture was mostly found behind it, except for a swath across Michigan, Lake

Erie and southeastern Ontario. Dry air was in place ahead of the trough and reached as far north as central

Ohio. Warm advection was occurring ahead of the trough across the forecast area. Soundings taken at

1200 UTC at Detroit and Wilmington (Ohio) show the transition from deep, saturated conditions (Detroit)

to rather dry conditions with a thin layer of residual moisture near 700 mb (10000 feet; Wilmington) across

the warm front (Fig. 2).

At 1200 UTC, an elongated surface low was in place from the Upper Peninsula of Michigan to

Chicago (Fig. 3). A cold front extended southwest from the Chicago low to Texas, and featured a narrow

band of light rain and thundershowers along it. A warm front extended southeast from the Chicago low

across southwestern Ohio then southward into Georgia. High pressure (1036 mb) anchored to the east of

New Jersey left cold air in place across the Great Lakes, mid-Atlantic and New England states, which warm

air from the south was trying to push out. Light rain showers and even some thunder (at Mansfield) were

reported across northern Ohio and southern Michigan, while light snow was occurring further to the north.

NASA/CR—2000-209413 317
By 1500 UTC, the warm front moved northeast to the southwest end of Lake Erie, and a broad swath of

rain was in place to the north of Ohio. Overcast skies were reported just behind the warm front in

northwestern Ohio and northeastern Indiana.

Satellite data from 1145 UTC (Fig. 4) reveal deep, cold cloud along the trough/cold front, as well

as across northern Indiana, northern Ohio, Michigan, southeastern Ontario, and Pennsylvania, closely

matching the moisture patterns on the upper-air charts. Areas of warmer CTTs and breaks in the deeper

clouds across northern Ohio mark the intrusion of dry air and the pocketed nature of the moisture there.

This scenario continues to be evident in the 1445 UTC infrared image, which shows larger areas of CTT

warmer than -25 C over northern Ohio.

Radar mosaic data (Fig. 5) match this pattern quite well, with precipitation across northern Ohio

at 1300 UTC moving northward into Michigan and southeastern Ontario by 1500 UTC, leaving nonprecipitating cloud behind over the southwestern end of Lake Erie by 1500 UTC. Twin Otter tracks show

that the aircraft initially sampled the southern portion of the precipitation swath, then areas to the south of

it. Scattered icing PIREPs of all types and up to moderate intensity were made between 10000 and 15000

feet along the edges of and within holes in the precipitation shield (Fig. 6).

NASA/CR—2000-209413 318
Figure 1 – Upper-air charts for 970325, 1200 UTC at a) 300 and b) 500 mb.

NASA/CR—2000-209413 319
Figure 1 – Upper-air charts for 970325, 1200 UTC at c) 700 and d) 850 mb.

NASA/CR—2000-209413 320
Figure 2 – Balloon-borne sounding data for 970325, 1200 UTC at a) Detroit and b) Wilmington.

NASA/CR—2000-209413 321
Figure 3 – Surface charts for 970325, a) 1200 and b) 1500 UTC.

NASA/CR—2000-209413 322
Figure 4 – Infrared satellite data for 970325, a) 1145 and b) 1445 UTC.

NASA/CR—2000-209413 323
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970325, a) 1300

and b) 1400 UTC.

NASA/CR—2000-209413 324
Figure 5 – Radar data, surface observations, icing PIREPs and Twin Otter tracks for 970325, c) 1500

and d) 1600 UTC.

NASA/CR—2000-209413 325
Figure 6 – Pilot reports of icing for 970325, a) 1200-1259 and b) 1300-1359 UTC.

NASA/CR—2000-209413 326
Figure 6 – Pilot reports of icing for 970325, c) 1400-1459 and d) 1500-1559 UTC.

NASA/CR—2000-209413 327
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Ice formation; Aircraft icing; Cloud glaciation

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National Center for Atmospheric Research

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Ben C. Bernstein

Analysis of the Meteorology Associated With the 1997 NASA Glenn

Twin Otter Icing Events

Project Manager, Dean R. Miller, Turbomachinery and Propulsion Systems Division, NASA Glenn Research Center,

organization code 5840, 216–433–5349.

This part of the document contains an analysis of the meteorology associated with the premier icing encounters from the JanuaryMarch 1997 NASA Twin Otter dataset. The purpose of this analysis is to provide a meteorological context for the aircraft data collected

during these flights. For each case, the following data elements are presented: (1) A detailed discussion of the Twin Otter encounter,

including locations, liquid water contents, temperatures and microphysical makeup of the clouds and precipitation aloft, (2) Upper-air

charts, providing hand-analyzed locations of lows, troughs, ridges, saturated/unsaturated air, temperatures, warm/cold advection, and jet

streams, (3) Balloon-borne soundings, providing vertical profiles of temperature, moisture and winds, (4) Infrared satellite data,

providing cloud locations and cloud top temperature, (5) 3-hourly surface charts, providing hand-analyzed locations of lows, highs,

fronts, precipitation (including type) and cloud cover, (6) Hourly plots of icing pilot reports, providing the icing intensity, icing type,

icing altitudes and aircraft type, (7) Hourly, regional radar mosaics, providing fine resolution of the locations of precipitation (including

intensity and type), pilot reports of icing (including intensity and type), surface observations of precipitation type and Twin Otter tracks

for a one hour window centered on the time of the radar data, and (8) Plots of data from individual NEXRAD radars at times and

elevation angles that have been matched to Twin Otter flight locations. Outages occurred in nearly every dataset at some point. All

relevant data that was available is presented here. All times are in UTC and all heights are in feet above mean sea level (MSL).

Available electronically at http://gltrs.grc.nasa.gov/GLTRS