[A Structured Process for Prediction of Convection]{.smallcaps}

[Associated with Split Cold Fronts]{.smallcaps}

Steven E. Koch

NOAA Research -- Forecast Systems Laboratory

Jamie D. Mitchem\
 Hazards Research Lab

Department of Geography

University of South Carolina

# 

Revised October 15, 2002

for Nowcast Section *of the Bulletin of the American Meteorological
Society*

[Corresponding author]{.smallcaps}: Steven E. Koch, NOAA Research ---
Forecast Systems Laboratory, 325 Broadway, Boulder, CO, 80305-3328,
Email: koch@fsl.noaa.gov.

A major severe weather event occurred 21--23 January 1999, resulting in
17 deaths, 150 injuries, and more than \$1 billion in damage. This case
far surpassed the records for the most tornadoes in any state on any day
in January. An incredible 56 tornadoes were produced in Arkansas (the
previous record for a single outbreak was 34). Millions of dollars of
damage resulted from a severe squall line spawned by this storm system
as it swept across the Southeast on 23 January. In addition, massive
flooding aggravated by rapid snowmelt affected the Ohio Valley region as
heavy rains fell over frozen soils for most of 22 -- 23 January.

WSR-88D radar, GOES geostationary satellite, and Eta model analyses
reveal that the vast majority of the severe weather occurred in
conjunction with two split cold fronts and their merger in the
mid-troposphere (Fig. 1). A split cold front is characterized by cold,
dry air aloft that advances ahead of the surface cold front by several
hundred kilometers (Browning 1985). For this reason, variables such as
wet bulb potential temperature *θ~w~* or equivalent potential
temperature *θ~e~* are often used to detect the presence of a split cold
front. A cold front aloft (CFA) is quite similar to the split front
model, but according to Hobbs et al. (1990), a pressure trough
(typically a dryline, or "drytrough") replaces the surface cold front,
giving rise to a warm occluded frontal structure. These nonclassical
frontal systems are often prolific producers of heavy precipitation and
severe thunderstorms, since they increase the potential instability and
also provide for the lifting mechanism (the associated frontal
circulation) to release this instability (Locatelli et al. 1995). The
purpose of this brief article is to communicate the fact that the
structure of split cold fronts can be diagnosed and predicted in
real-time by applying a systematic procedure that employs operationally
available data, mesoscale numerical weather prediction (NWP) models, and
special diagnostic tools, as follows:

1.  Is there pronounced cold advection in the middle troposphere (\~700
    hPa) associated with a cloud band or rain band at least 200 km ahead
    of a surface trough or cold front? This feature is often most
    pronounced in the θ~w~ or θ~e~ field from a mesoscale model.

2.  Is there a forecast band of strong upward motion associated with the
    cloud/rainband, and do vertical cross sections indicate that the
    upward motion band is one branch of a frontal transverse circulation
    in the mid-troposphere?

3.  Does GOES water vapor imagery show a pronounced dry wedge (slot) in
    association with strong isentropic descent diagnosed from mesoscale
    NWP model data?

4.  Do WSR-88D radar Velocity-Azimuth Display (VAD) winds show midlevel
    backing (indicative of geostrophic cold advection) above veering
    (warm advection)?

5.  Does application of the thermal wind equation to retrieve
    geostrophic thermal advection from the VAD data (or wind profiler
    data) indicate pronounced cold advection in the 1.5--4.0 km layer?
    Such retrievals *quantify* the cold advection *inferred* from the
    often subtle backing wind patterns and make the advection patterns
    much more discernible to the forecaster.

This systematic procedure is demonstrated with the 21--23 January 1999
severe weather case. Surface frontal systems (Figs. 2b, d, f) included a
stationary or warm front from Missouri eastward to Indiana, a dryline in
eastern Texas and Oklahoma during the early part of this event, and two
surface cold fronts in Texas. The location of these forecast frontal
systems compared very well with what was observed (not shown).
Comparison to the corresponding radar imagery shows that the strong
convection was located several hundred kilometers ahead of the surface
frontal systems throughout this event. Two split cold fronts are
depicted in this figure (actually, the leading one was a CFA during the
time that a dryline was present). The split front analyses were obtained
from a 32-km version of the Eta model forecast *θ~w~* and wind fields at
700 hPa. The leading edge of the strong horizontal gradient of *θ~w~*
defines each split cold front, with the provision that lower values of
*θ~w~* were progressively advancing toward the ridge in *θ~w~*.

Potential instability was generated in a narrow region ahead of the
dryline over Arkansas by 1800 UTC 21 January as the leading split cold
front advected lower *θ~w~* air over higher *θ~w~* air near the surface,
resulting in a sharp increase of Convective Available Potential Energy
(CAPE). This is where and when intense convection, which produced
numerous tornadoes as well as large hail and high winds, rapidly
developed. The southern half of the first split cold front subsequently
crept slowly eastward while the northern half advanced to the northeast.
Meanwhile, a second surge of low *θ~w~* air defining another split cold
front advanced across Texas. This second split cold front began to merge
with the first one by 1200 UTC 22 January (Fig. 2c), and as it did so,
heavy precipitation leading to flash flooding developed along and
northward of the merged frontal system (Fig. 2d, f).

A cross section of front-relative vertical circulation and relative
humidity from Midland, Texas to Macon, Georgia shows two bands of strong
rising motion had developed along the leading edge of each of the split
cold fronts by 1200 UTC 22 January (Fig. 3b). A thermally direct frontal
circulation was associated with active frontogenesis along each split
cold front, with the rising branch of the circulation acting upon very
moist, unstable air. These circulations combined into one large, deep
region of upward motion 12h later as the two split fronts merged (Fig.
3c). Relative humidity was maximized along or just ahead of the split
cold fronts, followed by a surge of dry air. Isentropes sloped upward
behind the analyzed split cold fronts (Fig. 4). Thus, the split cold
fronts were not mere "humidity fronts," but were actually the leading
edge of colder air; nevertheless, the split fronts were most pronounced
in fields of *θ~w~* or *θ~e~*, as is usually the case.

Dry, stratospheric air descended to 700 hPa in association with a deep
tropopause fold behind the second split cold front by 0000 UTC 23
January (Figs. 3c, 4c), accentuating the moisture contrast across the
midtropospheric front. Isentropic analysis showed strong cross-isobar
flow directed toward higher pressure in west Texas, indicating that
rapid descent did indeed occur in association with the second split cold
front. This developing "dry conveyor belt", which is unmistakable in
satellite water vapor imagery at 1200 UTC 23 January (Fig. 5a),
propagated entirely around the upper-tropospheric trough to its eastern
side and interacted with the intense squall line that formed over the
Gulf states. The nature of this interaction consisted of the following
enhancements to the severe convection environment: drying in the
mid-troposphere, a resultant increase in the potential instability
aloft, and a doubling of the vertical wind shear magnitude in the 1000
-- 500 hPa layer from that existing 30h earlier. The resulting squall
line displayed a characteristic leading convective line followed by a
trailing stratiform precipitation region, as seen in the Ft. Rucker,
Alabama radar display (Fig. 5b).

VAD winds from Level II WSR-88D radar data can be used effectively to
detect the presence of split fronts. In each of several detailed case
studies of split cold fronts performed by the lead author over the past
few years, the dry air in the dry conveyor belt has not prevented
determination of winds aloft using the VAD technique. Nevertheless,
other sources of this information are available from the more sensitive
wind profiler systems and GOES water vapor-derived winds in the
mid-troposphere. The VAD Wind Profile (VWP) product, which is a
time-height display of the mean horizontal winds using the VAD
technique, is well suited for detecting temporal changes in the vertical
profile of winds. Geostrophic cold and warm advection can be *inferred*
from the backing and veering of winds with height, but even better is to
*compute* the advection from the VWP layer-mean winds and retrieved
temperature gradients using the thermal wind equation. Analyses
performed as the strong squall line impacted Fort Rucker, Alabama (Fig.
6) clearly show the split cold front. The retrieved cold advection in
the layer from 1.5 to 4.5 km beginning shortly after 1000 UTC occurred
in direct association with a rear inflow jet behind the squall line
(evident as a bulls-eye in the radial velocity isodop display, not
shown). The extended region of most pronounced cold advection and the
associated westerly momentum surge from the leading edge of the split
cold front at 2.0 km at 1030 UTC to the 3.5 km level at 1300 UTC (a
distance of \~150 km) suggests that the descending rear inflow jet was
linked to the synoptic-scale cooling behind the nose of the split cold
front.

In summary, the structure of split cold fronts can be understood in
real-time by synthesizing water vapor channel satellite imagery,
isentropic and cross sectional analysis of mesoscale model fields, and
thermal retrievals and other products obtained from radar VWP data.
Although thermal advections retrieved from the VWP wind data are not
currently available in real time for operational use as a WSR-88D
product field, they can be (and have been) computed offline in real time
in a National Weather Service office with good results (e.g., Koch
2001). Increased use of Doppler radar retrievals in this manner would
aid in forecasting the complex interactions between surface frontal
features and fronts aloft leading to major outbreaks of severe weather
seen in this case and others that have been documented in the
literature.

**FOR FURTHER READING**

Browning, K.A.,1985: Conceptual models of precipitation systems.
*Meteor. Mag*., **114**, 293--319.

Hobbs, P.V., J.D. Locatelli, and J.E. Martin, 1990: Cold fronts aloft
and the forecasting of precipitation and severe weather east of the
Rocky Mountains. *Wea. Forecasting*, **5,** 613--626.

Koch, S. E., 2001: Real-time detection of cold fronts aloft and split
fronts using mesoscale models and WSR-88D radar products. *Wea. and
Forecasting*, **16,** 35--55.

Locatelli, J.D., J.E. Martin, J.A. Castle, and P.V. Hobbs, 1995:
Structure and evolution of winter cyclones in the central United States
and their effects on the distribution of precipitation. Part III: The
development of a squall line associated with weak cold frontogenesis
aloft. *Mon. Wea. Rev.,* **123,** 2641--2662.

Fig. 1. Relation of Eta model forecast split cold fronts at 700 hPa
(open pips) to severe weather reports in 12-h periods ending at a) 0000
UTC 22 January, b) 1200 UTC 22 January, and c) 0000 UTC 23 January 1999.

Fig. 2. (a) Eta model 9-h forecast winds at 0000 UTC 22 January at 700
hPa (full barb, half barb, and flag = 5.0, 2.5, and 25 m s^-1^,
respectively), and wet-bulb potential temperature (1 K isotherm
interval), (b) corresponding composite radar imagery. Panels (c) and (d)
are as in (a) and (b) except at 1200 UTC 22 January, and (e) and (f) are
as in (a) and (b) except at 0000 UTC 23 January. Forecast frontal
features at 700 hPa, including split cold fronts, are shown using open
pips. Surface fronts are denoted with conventional frontal symbols.
Location of cross sections appearing in Figs. 3 and 4 is also depicted.

Fig. 3. Cross sections from Midland, Texas (MAF) to Macon, Georgia (MCN)
of vertical circulation (vectors) relative to motion of split cold
fronts (open pips) and relative humidity (10% intervals, shaded \>70%)
from Eta model forecasts valid at (a) 0000 UTC 22 January, (b) 1200 UTC
22 January, and (c) 0000 UTC 23 January.

Fig. 4. As in Fig. 3, except for isentropes (2K intervals) valid at (a)
0600 UTC 22 January, (b) 1800 UTC 22 January, and (c) 0600 UTC 23
January. Shading represents isentropic potential vorticity values larger
than 1.5 PVU.

Fig. 5. (a) Eta model forecasts of pressure (contoured every 20 hPa) and
winds on the 310K isentropic surface overlaid with satellite water vapor
imagery (black and red = dry, white and blue = moist) for 1200 UTC 23
January, and diagnosed split cold front. (b) WSR-88D radar composite
reflectivity display from Fort Rucker, Alabama (KEOX) at 1208 UTC 23
January.

Fig. 6. (a) VWP display of winds (m s^-1^) from KEOX radar for
0845--1415 UTC 23 January (time axis is flipped to make it easier to
convert time to space, note the length scale), and (b) retrieved
time-height analysis of geostrophic thermal advection (C day^-1^), with
cold advection regions denoted by solid lines (negative values).