PRECIPITATION FACTORS LEADING TO

ARC CLOUD FORMATION

Final Report

October 1985 - April 1987

{NAS_-CR-1802_) PREClEI_A_IC_ _AC_ORS

IFADI_G TO AaC CICOD _'C_A_I(_ _inal

_e_ort, Oct. 19E5 - All. 1$£7 (_exas AgE

Cniv.) 227 p CSCL 04B

G3/47

N87-16S46

ZHRU

N87-18949

Unc.las

43773

Principal Investigator: K. C. Brundidge

Texas A&M University

College Station, Texas 77843-3146

Prepared for George C. Marshall Space Flight Center

under Grant NAG8-043
PREFACEANDACKNOWLEGEMENTS

This report represents the combined efforts of three graduate

students and the Principal Investigator. Accordingly, it is in three

sections with separate indexing for each section.

The section titles are as follows:

I. Factors Leading to the Formation of Arc Cloud Complexes.

2. Convective Cell Development and Propagation in a Mesoscale

Complex.

3. Nighttime Atmospheric Stability Changes and their Effects on

the Temporal Intensity of a Mesoscale Convective Complex.

Section I represents the main thrust of the effort as stated in the

contract. The others developed as we went along. Section 3 is an

incomplete study at the time of this report but was to the stage that

somepreliminary results could be given.

The authors are grateful for the sponsorship of this work by the

National Aeronautics and Space Administration under Grant NAG8-043.

Special thanks is given to Dr. JamesArnold of the Atmospheric Sciences

Division, Space Science Laboratory of the Marshall Space Flight Center

who guided the Principal Investigator into a study of mesoscale convective systems during the brief period he spent with the Atmospheric

Science Division. Gratitude also is expressed to Mr. Paul Meyer for his

devoted efforts to procure synoptic data and satellite imagery used in

these studies and for his assistance in the use of the MclDASsystem _t

the Marshall Space Flight Center.
N87:18947

FACTORS LEADING TO THE FORMATION OF ARC CLOUD COMPLEXES

Mark John Welshinger

and j

Kenneth C. Brundidge

Texas A&M University

College Station, Texas

Prepared for George C. Marshall Space Flight Center

under Contract NAG8-043
ABSTRACT

Factors leading to the formation of arc cloud complexes

A total of 12 mesoscale convective systems (MCSs) were investigated. The duration of the gust front, produced by each MCS, was used to

classify the MCSs. Category 1MCSs were defined as ones that produced a

gust front and the gust front lasted for more than 6 h. There were

seven category 1MCSs in the sample. Category 2 MCSs were defined as

ones that produced a gust front and the gust front lasted for 6 h or

less. There were four category 2 MCSs. The MCS of Case 12 was not

categorized because the precipitation characteristics were similar to a

squall line, rather than an MCS. All of the category 1MCSs produced

arc cloud complexes (ACCs), while only one of the category 2 MCSs produced an ACC. To determine if there were any differences in the characteristics between the MCSs of the two categories, composite analyses

were accomplished. The analyses showed that there were significant differences in the characteristics of category 1 and 2 MCSs. Category 1

MCSs, on average, had higher thunderstorm heights, greater precipitation

intensities, colder cloud top temperatures and produced larger magnitudes of surface divergence than category 2 MCSs. It was deduced that

the more pronounced characteristics of category 1MCSs played an important role in the conversion of an MCS to an ACC.
iii

ACKNOWLEDGMENTS

The authors thank Dr. Aylmer H. Thompson, whose helpful comments

aided tremendously in the preparation of this report. Mrs. Jacquelyn V.

Strong typed the manuscript.

The National Aeronautics and Space Administration sponsored this

research through Contract NAG8-043 under the auspices of the Atmospheric

Sciences Division, Space Sciences Laboratory, Marshall Space Flight

Center. Special thanks are extended to Mr. Paul Meyer for his assistance in procuring satellite images and other data through the McIDAS

system at the Marshall Space Flight Center.
iv

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS.......................... iii

TABLE OF CONTENTS......................... iv

LIST OF TABLES .......................... v

LIST OF FIGURES .......................... vi

CHAPTER I, INTRODUCTION ..................... 1

CHAPTER II. RESEARCH PROCEDURES ................. I0

Products Used in the Study .................. I0

Overview ........................... 18

CHAPTER III. DIVISION OF MCSs INTO CATEGORIES .......... 22

Category 1MCSs ....................... 22

Case 2: 9 May 1981 ................... 22

Case 3: 27 May 1981 ................... 30

Case 4: 17 May 1982 .................. 33

Case 7: 26 June 1982 .................. 43

Case 8: 27 June 1982 .................. 48

Case I0: 20 May 1983 .................. 51

Case II: II June 1983 ................. 56

Category 2 MCSs ....................... 58

Case I: I0/II April 1981 ................ 62

Case 5: 19 May 1982 .................. 66

Case 6: I0/II June 1982 ................ 72

Case 9: 28/29 June 1982 ................ 75

Case 12: 14 June 1983 .................... 84

Significant Events Associated with Category 1 and 2 MCSs. 86

CHAPTER IV. COMPOSITE ANALYSES.................. 90

Method of the Composite Analyses ............... 90

Results of the Composite Analyses .............. 91

CHAPTER V. SUMMARY........................ I00

REFERENCES ............................ 107
LIST OF TABLES

Table

1 Mesoscale convective complex (MCC) definition ........

2 Mesoscale convective systems (MCSs) included in study..

3 Characteristics associated with the MCC of Case 2 (8/9 May

1981) ............................

4 Characteristics associated with the MCC of Case 3 (27 May

1981) ............................

5 Characteristics associated with the MCC of Case 4 (16/17

May 1982) ..........................

6 Characteristics associated with the MCS of Case 7 (26 June

1982 ) ................... .........

7 Characteristics associated with the MCC of Case 8 (27 June

1982 ) ............................

8 Characteristics associated with the MCC of Case I0 (20 May

1983) ............................

9 Characteristics associated with the MCC of Case II (II June

1983 ) ............................

I0 Characteristics associated with the MCC of Case 1 (I0/II

April 1981) .........................

II Characteristics associated with the MCS of Case 5 (19 May

1982 ) ............................

12 Characteristics associated with the MCC of Case 6 (II June

1982 ) ............................

13 Characteristics associated with the MCC of Case 9 (28/29

June 1982) .........................

14 Summary of significant events associated with category 1

and 2 MCSs .........................

Page

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88
vi

LIST OF FIGURES

Figure

1 Relationship betweenmaximumintensity of rain and the

maximumintensity of associated surface divergence .....

2 Location of the climatological raingauge network in Texas .

3 Location of the climatological raingauge network in

Louisiana .........................

4 Location of the climatological ralngauge network in New

Mexico...........................

5 Location of the climatological ralngauge network in

Mississippi ........................

6 Location of the climatological ralngauge network in

Arkansas..........................

7 Location of the climatological ralngauge network in

Kansas...........................

8 Location of the climatological ralngauge network in

Missouri ..........................

9 Location of the climatological ralngauge network in

Colorado..........................

I0 Location of the climatological ralngauge network in

Oklahoma..........................

II 50 kPa height (dm) and temperature (°C_ fields for 1200

GMT9 May 1981.......................

12 Radar summarychart for 0235 GMT9 May1981 ........

13 Surface oe (°C) field for 0300 GMT9 May 1981 .......

14 Surface pressure field for I000 GMT9 May 1981.......

15 GOESvisible image for 1300GMT9 May 1981.........

16 50 kPa height (dm) and temperature (°C) fields for 0000

GMT27 May 1981 ......................

17 Surface 0_ (°C) field overlaid upon GOESimage (MB

enhancement)for 0900 GMT27 May 1981 ...........

18 GOESvisible image for 1500 GMT27 May1981 ........

Page

8

II

12

13

14

15

16

16

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23

24

25

27

28

31

32

34
vii

LIST OFFIGURES(Continued)

Figure

19 50 kPa height (dm) and temperature (°C) fields for 1200

GMT16 May1982 ......................

20 GOESIR image with MBenhancementfor 0415 GMT17 May 1982.

21 GOESIR image with MBenhancementfor I000 GMT17 May 1982.

22 Surface pressure field for I000 GMT17 May 1982 ......

23 GOESvisible image for 1832GMT17 May 1982 ........

24 Surface pressure field for 1200 GMT26 June 1982......

25 GOESvisible image for 1832GMT26 June 1982........

26 GOESIR image with MBenhancementfor 0700 GMT27 June

1982............................

27 Surface pressure field for 1200 GMT27 June 1982......

28 50 kPa height (dm) and temperatures (°C) fields for 0000

GMT20 May 1983 ......................

29 GOESvisible image for 1300GMT20 May 1983 ........

30 50 kPa height (dm) and temperature (°C) fields for 0000

GMTII June 1983......................

31 GOES'visible image for 1300GMTII June 1983........

32 Surface divergence field overlaid upon GOESIR image for

0300 GMTII April 1981...................

33 70 kPa height (dm) and temperature (°C) fields for 0000

GMTII April 1981 .....................

34 Surface pressure field for 0300 GMTII April 1981 .....

35 GOESIR image for 0800 GMT19 May 1982...........

36 50 kPa height (dm) and temperature (°C) fields for 0000

GMT19 May 1982 ......................

37 50 kPa height (dm) and temperature (%) fields for 0000

GMTII June 1982......................

Page

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40

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45

46

49

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59

60

63

64

65

68

7O

73
viii

LIST OF FIGURES (Continued)

Figure

38 Surface divergence field overlaid upon GOES IR image for

0700 GMT II June 1982 ...................

39 Surface pressure field for 0500 GMT II June 1982 ......

40 50 kPa height (dm) and temperature (°C) fields for 0000

GMT 29 June 1982 ......................

41 Surface pressure field for 0000 GMT 29 June 1982 ......

42 GOES IR image for 0430 GMT 29 June 1982 ..........

43 Surface pressure field for 0500 GMT 29 June 1982 ......

44 Radar summary chart for I035 GMT 9 May 1981 ........

45 Radar summary chart for I035 GMT 14 June 1983 .......

46 Composite of the maximum hourly thunderstorm tops of

category l (thin line) MCSs and category 2 (thick line)

MCSs ....................... ......

47 Composite of the maximum hourly surface divergence of

category l (thin line) MCSs and category 2 (thick line)

MCSs ............................

48 Composite of the sum of the three largest precipitation

rates of category l (thin line) MCSs and category 2 (.thick

line) MCSs .........................

49 Composite of the maximum hourly point precipitation rates

of category l (thin line) MCSs and category 2 (thick line)

MCSs ............................

50 Composite of the coldest hourly cloud top temperatures for

category l (thin line) MCSs and category 2 (thick line)

MCSs ............................

51 Composite of the maximum hourly radar reflectivity intensity of category l (thin line) MCSs and category 2

(thick line) MCSs .....................

Page

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99

I04
CHAPTERI

INTRODUCTION

Since the launch of the first operational weather satellite in

February 1966 (Schnapf, 1982), satellite observations and interpretation

have becomeincreasingly important in the areas of weather research and

operational forecasting. Onereason is that geostationary satellite

imagery is the only meteorological observing tool that can follow the

evolution of clouds from the synoptic scale down to the cumulus scale.

Therefore, it can depict atmospheric activity which is up to two orders

of magnitude smaller than can be resolved by conventional meteorological

observations. This unique ability of the satellite provides the meteorologist a mechanismto infer weather events downto the mesoscale.

A well-known phenomenonwhich occurs as a thunderstorm propagates

past a station is the rise and oscillation of the surface pressure.

Downdrafts descending from thunderstorm cells are the source of the air

forming the cold domeover the ground, which leads to the rise in surface pressure (Byers and Braham,1949; Fujita, 1959, 1963). If the

downdrafts of cold air are strong enough and last for a sufficient time,

an organized high pressure system develops at the surface. This high

pressure system is called the "mesohigh."

Purdom(1973), using satellite imagery, showedthat the leading

edge of a mesohigh appeared as an arc-shaped line of convective clouds

moving out in all directions from a dissipating thunderstorm area. The

The citations on the following pages follow the style of the

Journal of the Atmospheric Sciences.
2

arc-shaped cloud line, normally composed of cumulus, cumulus congestus,

or cumulonimbus clouds, has been named the "arc cloud" (Purdom, 1973,

1976, 1979). As the leading edge of the cold air spreads outward from

the storm area, it undercuts and lifts the warmer, moister air ahead of

it and the arc cloud is formed.

In many ways, the structure of the leading edge of the cold air

associated with the arc cloud (also referred to as the mesoscale outflow

boundary) resembles a true cold front. Byers and Braham (1948) remarked

that the "air mass discontinuity acts as a miniature cold front and that

the movement of this pseudofront has a marked influence on the initial

formation of new cells." Normally, abrupt changes in wind direction and

speed, called the gust front, rapid cooling and rising pressure, sometimes called the density surge line (Sinclair and Purdom, 1982, 1984),

pressure surge line (Fujita, 1955), or pressure jump line (Tepper,

1950a), accompany the passage of the outflow boundary.

Large, long-lived convective weather systems are most commonly referred to as mesoscale convective systems (MCSs). Maddox (1980} has described a specific type of MCS which he has named the "mesoscale convective complex." The definition is presented in Table I. The definition

is based on the areal extent and duration of specific cloud top temperatures observed in enhanced Geostationary Operational Environmental

Satellite (GOES) infrared (IR) imagery.

The MCC, an organized meso-: scale convective system (Orlanski,

1975), is nocturnal in nature and occurs most frequently during the warm

season months, March through September (Maddox, 1980; Maddox et al.,

1982; Rodgers et al., 1983; Rodgers et al., 1985). Maddox surmises that
Table I. Mesoscale convective complex (MCC) definition. The definition is based upon analysis of enhanced IR satellite imagery (after

Maddox, 1980).

Physical Characteristics

Size: A. Cloud shield with IR temperature less than or equal to

-32°C must have an area greater than or equal to

lO0,O00 km2.

B. Interior cold cloud region with temperature less than or

equal to -52°C must have an area greater than or equal to

50,000 km2 .

Initiate: Size definitions A and B first satisfied.

Duration: Size definitions A and B must be met for a period greater

than or equal to 6 h.

Maximum

extent:

Contiguous cold cloud shield (IR temperature less than or

equal to -32°C) reaches maximum size.

Shape: Eccentricity (minor axis/major axis) greater than or equal to

0.7 at time of maximum extent.

Terminate: Size definitions A and B no longer satisfied.
during the genesis stage MCCs entrain mid-level, potentially cool environmental air, which produces strong, evaporationally-driven downdrafts, surface mesohighs and outflow boundaries. Similar results were

obtained by Zipser (1969) in his study of a tropical convective system

which developed during the Line Islands Experiment. Zipser concluded

that the evaporationally-driven downdrafts had mid-tropospheric origins,

somewhere between 60 kPa and 40 kPa, where environmental air of equivalent potential temperature similar to that at the surface was located.

In the development stage of an MCC, a merging of the mesohighs of

individual thunderstorms produces a single large mesohigh. Vertical

motions along the forward edge of the spreading, cooler air generate the

arc cloud. However, the arc cloud is on a larger time and space scale

than the gust front for an individual thunderstorm. The large mesohigh

and surroundingarc cloud have been collectively termed an arc cloud

complex (ACC) by Brundidge (1983).

The dissipation stage of the MCC is characterized by a rapid demise

of the intense convection. One reason Maddox (]980) gives for the dissipation is that "the cold air dome beneath the system may become so intense that the surface convergence zone moves away from the region of

mean mesoscale ascent into a region of mid- and upper-level sibsidence."

ACCs can last for many hours. Even after the demise of the MCC (or

more generally the MCS), the cool air and outflow boundary may still be

evident. As Maddox (1980) states, "Although the MCC rapidly loses its

meso-_ scale organization, the cool air and outflow boundary at the surface may persist for many hours." Sinclair and purdom (1982} believe

that the regeneration of weak precipitation along portions of the arc,
coupled with sub-cloud evaporation is one explanation for an arc cloud

line being able to maintain its identity as a single feature for several

hours.

Becauseof the long life-span of someACCs,a chain of storm events

often occurs, leading to the formation of other MCSs. Miller (1984), in

a case study of an ACC, found convection along the arc cloud at different times during its life, which eventually resulted in the formation

of an MCSmuchlarger than the ACC. Bartels (1983), in a study of a

dual MCC,showedthat an outflow boundary associated with the western

MCCwas related to an MCCtype system that developed over Oklahomaduring the following afternoon.

Fritsch et al. (1981) showedthat MCCsproduce widespread regions

of measureable rainfall and that they account for a significant portion

of growing-season rainfall over muchof the United States' corn and

wheat belts. Maddox(1980) stated that manyMCCsproduce locally intense rainfalls and flash flooding. In Bartels' (1983) study, rains

from the western MCCresulted in flash flooding. Evacuations were required in Springfield, MOand Joplin, MO, as somehomeshad water 3 ft

deep. Since ACCscan develop into newMCCs,the ACCcan be related to

the occurrence of flash floods.

ACCsare often the forerunners of severe weather. Tepper (1950b)

reported a case of tornado formation at the intersection of two pressure

jump lines. Fujita (1963) and Miller (1972) stress the importance of

intersecting boundaries, stating that these areas are favored locations

for severe weather development. Using satellite imagery, Purdom(1974)

showedthat the intersection of an arc cloud with other existing
6

boundaries locates an area for new convection to develop. If atmospheric conditions are favorable, severe weather often forms at the point of

intersection.

As well as the increased threat of severe weather, the ACCis a

dangerous source of potential aircraft accidents (Greene, 1977). In an

observational study of outflow boundaries, Goff (1976) found a wide

variation between one gust front case and another. However, the updrafts along the gust front were generally concentrated in a region a

-I few kilometers wide, with vertical motions as high as 6 to 7 m s

Sinclair and Purdom(1984) found similar vertical motions during aircraft penetrations of the density surge line (DSL). They found upward

vertical motion, with peaks as high as I0 m s-I, at the DSLinterface

region. Between2 and 7 km behind the DSL, a distinct transition was

found from upward to downwardvertical motions. Vertical motions in the

sinking air ranged from -3 to -3 m s-I. Therefore, if an aircraft were

on final approach and encountered an outflow boundary, the extreme

change in vertical motions over such a short distance across the DSL

might lead to an aircraft accident (Sinclair and Purdom, 1984).

In summary,the importance of ACCsis that they can develop into

new MCSs,they can be associated with the initiation of severe weather

and their associated wind shear can lead to potential aircraft mishaps.

The study of ACCsmust include detailed studies of the entity which produces them, namely, MCSs,if our ability to forecast their occurrence is

to improve. It is well-known that someMCSsproduce well-defined,

extremely-potent ACCs,whils others do not. What characteristics are

different in MCSsthat produce ACCscomparedto the MCSsthat yield no
ACC?

Byers and Braham(1948) showedthat rain at the surface was in the

downdraft area of thunderstorm cells. They also showedthat the maximum

intensity of rain at the surface and the maximumintensity of associated

surface divergence have a very high correlation (see Fig. I). That is,

an area of heavy rain at the surface coincides with an area of strong

divergence in the surface winds. The divergence is caused by the downdrafts spreading out across the ground.

In a case study of three ACCs,Brundidge (1984) hypothesized that

the appearance of the arc cloud is a direct result of heavy rainfall on

the south side of an MCS. This agrees with the findings of Byers and

Braham(1948). Fromtheir findings, it logically follows that heavy

precipitation can create strong downdrafts and subsequently a strong

gust front and mesohigh. The presence of the gust front and mesohigh,

in turn, is revealed in the magnitude of the surface divergence and the

pattern of the surface divergence field. Therefore, heavy precipitation

on the south side of an MCScan create strong downdrafts, which lead to

a strong gust front and mesohigh, and eventually an arc cloud forms

along the leading edge of the gust front.

Woodleyet al. (1972), in their study of rainfall estimation from

satellite cloud photographs, discovered several important facts about

rain clouds. Twofacts pertinent to this study are:

I. Clouds with cold tops in the IR imagery produce more rainfall

than those with warmer tops.

2. The highest and coldest clouds form where the thunderstorms are

most vigorous and the rain heaviest.
8

28

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Z

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8

4

0

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D .I0 .ZO ._0 .40 .,_0 .60 .6_

RATE OF RAINFALL-INCHES PER 5 MINUTES

Fig. I. Relationship between maximum intensity of rain and the

maximum intensity of associated surface divergence. The line through

the scatter of points is the linear regression line for the distribution. The correlation coefficient is 0.98 (after Byers and Braham,

1949).
Gagin et al. (1985) attempted to provide a comprehensive formulation of the dependence between the depth of the convective cells and

their gross precipitation features. In their study, they concluded that

taller cells produce larger total rain volumes by virtue of their greater rainfall intensities, their longer duration as precipitating entities

and their larger precipitating areas.

Based on the studies cited above, it appears that precipitation

patterns and rates, cloud top temperatures, thunderstorm heights and

surface divergence patterns are interrelated and seem to play an important role for the conversion of an MCS to an ACC. It has been the purpose of this study to compare these features between MCSs that become

ACCs and ones that do not. Research of this type will enable us to

understand better the occurrence of ACCs and may provide the signatures

manifested in satellite and radar observations which would permit forecasting the development of significant arc clouds.
I0

CHAPTERII

RESEARCHPROCEDURES

Products Used in the Study

Several products and sources of information were used to aid in

this investigation of arc cloud complexes. These were as follows:

Io The records of hourly precipitation from the climatological

networks of raingauges obtained from the National Climatic Data Center

(NCDC)located in Asheville, North Carolina. Figs. 2-10 show the locations by states of the various raingauges used in this study. The data

received from the NCDCwere plotted and analyzed on the state mapsto

determine patterns and rates of rainfall.

2. Sectional surface mapscorresponding to the time and location

of the MCSswere plotted and analyzed. Patterns in the temperature,

pressure and wind fields helped ascertain the existence and strength of

gust fronts.

3. Hourly radar summarycharts from the National Weather Service

(NWS)were used to provide comparisons of the precipitation pattern at

the surface amongthe various MCSs. They also were used to obtain the

maximumthunderstorm height, within the MCS,which occurred at the radar

observation time (normally 35 min past each hour).

4. GOESimagery was used for:

a. An aid in determining the existence of an arc cloud.

b. Determining the duration of the arc cloud, if one existed.

This aided in the classification of the MCS. The classifications of the MCSswill be discussed later.
II

Fig. 2. Location of the climatological raingauge network in Texas.
12

Fig. 3. Location of the climatological raingauge network in

Louisiana.
13

Fig. 4. Location of the climatological raingauge network in New

Mexico.
14

.\

Fig. 5. Location of the climatological raingauge network in

Mississippi.
15

o

Fig. 6. Location of the climatological raingauge network in

Arkansas.
16

Fig. 7. Location of the climatological raingauge network in Kansas.

Fig. 8. Location of the climatological raingauge network in

Missouri.

17

Fig. 9. Location of the climatological rain.qauge network in

Colorado.

Fig. I0.

Oklahoma.

Location of the climatological raingauge network in
18

5. The Man-Computer Interactive Data Access System (MclDAS) of the

Atmospheric Sciences Division of the Systems Dynamic Laboratory,

Marshall Space Flight Center, provided the following products:

a. Hourly surface divergence patterns. I

b. Hourly contoured analysis of the cloud top temperatures as

determined from the GOES IR imagery. I

c. Hourly surface equivalent potential temperature analysis. I

d. Additional satellite images.

e. A computer printout, on an hourly basis, of the size of the

MCS as it appears in the satellite IR imagery. The size is

referenced to the number of pixels that are contained within four specific temperature ranges: -32°C to -98°C; -52°C

to -98°C; -58°C to -98°C; -62°C to -98°C.

Overview

A total of 12 cases was chosen for this study. Three publications

were primarily used in selecting the various cases (Maddox et al., 1982;

Rodgers et al., 1983, 1985). These articles were annual summaries of

the times and locations of MCCs that occurred during 1981, 1982 and

1983. The MCSs that were chosen were studied by Brundidge (1983, 1984)

and Miller (1984) and were known to produce ACCs. The cases that were

finally included in this study are listed in Table 2. Also, dates and

details of the life cycles of each event are listed. Of the cases

selected, most of the MCSs originate or terminate in Texas. This

IThe analysis and contours by the McIDAS are computer products obtained objectively by use of the Barnes (1973) interpolation scheme.
19

Table 2. Mesoscale convective systems (MCSs)included in study.

Initiate and terminate times are after Maddoxet al., 1982; Rodgerset

al., 1983, 1985.

Case Date number

Time

(GMT)

Maximum Initiate Terminate extent**

l lO/ll Apr 81 2315/I0 0300/ll 0531/II

2 9 May81 0115/09 0500/09 I015/09

3 27 May81 0515/27 I000/27 1400/27

4 17 May82 0030/17 0400/17 0730/17

5 19 May82 * 0900/19 *

6 lO/ll Jun 82 2245/I0 0700/ll 1530/II

7 26 Jun 82 * II00/26 *

8 27 Jun 82 0800/27 0900/27 1400/27

9 28/29 Jun 82 2230/28 0200/29 0430/29

lO 20 May 83 0130/20 0800/20 1300/20

II II Jun 83 0530/II 0900/ll 1200/ll

12 14 Jun 83 0130/14 0600/14 1600/14

*These cases were not MCCs;therefore, initiate and terminate times

were not available.

**The times listed are the maximumextent of the area, as depicted

on IR satellite imagery, with temperatures ±-62°C. This information

was determined by the use of McIDAS.
20

stipulation was employed becauseof the denser climatological rainguage

network in Texas, comparedto the other states (see Figs. 2-I0).

Eachcase was carefully analyzed to determine if a gust front existed. Hourly surface maps, hourly surface divergence patterns provided

by the McIDASand satellite pictures, both visual and IR, were used to

determine the existence and duration of a gust front.

The various MCSswere then categorized as follows:

I. MCSproduces gust front and the gust front persists more than

6 h. This is referred to as a category l MCS.

2. MCSproduces gust front and the gust front persists for 6 h or

less. This is referred to as a category 2 MCS.

After the cases were categorized, each case was further analyzed

for the following characteristics associated with the MCS:

I. Maximumhourly surface divergence.

2. Coldest hourly cloud top temperature.

3. Maximumhourly point precipitation rates.

4. Sumof the three largest hourly precipitation rates. An explanation of this analysis procedure is as follows. Assumean MCSproduced

the following rainfall rates, for a particular hour, at six climatological rainguage sites: 0.8, 2.2, 1.5, 0.3, 1.7 and 2.0 in h-I The sum

of the three largest precipitation rates, for this hour, would be 2.2 +

2.0 + 1.7 = 5.9 in h-I. By using this analysis technique, an areal

coverage of the heavy precipitation is inferred, rather than just at a

single point.

5. Maximum hourly thunderstorm height.

Finally, composite analyses of the five characteristics were
21

accomplished for each category. The composites were used to determine

significant differences between the categories. The differences suggested some possible causative factors that aid in the formation of ACCs.
22

CHAPTER I II

DIVISION OF MCSs INTO CATEGORIES

Category 1MCSs

Category 1MCSs are defined as follows: The MCS produces a gust

front and the gust front persists more than 6 h. Seven MCSs are classified in this category. The seven MCSs are from Cases 2, 3, 4, 7, 8, I0

and II. A brief discussion of the synoptic situation and storm characteristics associated with each category 1MCS follows.

Case 2: 9 May 1981

During the late afternoon of 8 May 1981, a cluster of thunderstorms

developed over north central Texas. By 0115 GMT 9 May 1981, the cluster

of storms had conglomerated and an MCC was initiated. Fig. II shows the

50 kPa analysis for 1200 GMT 9 May 1981. A short-wave trough extended

through the central United States. This flow pattern had contributed to

the production of the MCC. The precipitation pattern at 0235 GMT is

depicted in the NWS radar summary chart (Fig. 12). The large precipitation area in north central Texas, associated with the MCC, developed on

the north side of a warm front. The warm front was evident in the 0300

GMT surface ee analysis, which was obtained by use of the MclDAS. The

tight gradient of the ee isolines in northern Texas, indicating the

presence of the warm front, is shown in Fig. 13.

By 0500 GMT, a mesohigh and strong gust front first appeared in the

surface pressure analysis (not shownl. The gust front was located in

northeastern Texas and was moving southeast. As it passed Shreveport,
"LD

P
ORIGINAL PAG__ ;S

OF POOR QUALrTY

23

\

Fig. II. 50 kPa height (dm) and temperature (°C) fields for 1200

GMT 9 May 1981. Height contours (solid lines) are every 60 m; isotherms

(dashed lines) are every 5°C.
ORmGI_._LPA_£ rS

OF POOR QUALiTy

24

R

I P.,II,,' I'Kql

TR_ TJt_

_,_ - .

._,_-_'_ _. _ . .

Fig. 12. Radar summary chart for 0235 GMT 9 May 1981. Shading indicates echo areas. Contours at echo intensities I, 3, and 5; echo

heights are in hundreds of feet; cell movement given at end of arrows in

knots; area and line movement given by pennant with full barb : I0 kt.
ORIG,IffitL PAGE |S

OF. pOOR QUALI'I"Y

25

cO

e,-,-

>,,

o

o

o

s.

o

4-

or4-

¢_)

0

ci)

4-

car)

r--

&

or--
26

LA (SHV), a peak wind of 35 kt was recorded (0723 GMT). The strong,

gusty wind was one of the indicators that the gust front was quite

powerful. Even near termination of the MCC, at lO00 GMT, a strong mesohigh and gust front were still evident in the surface pressure analysis

(see Fig. 14). Since the pressures to the rear of the gust front were

higher than the environmental pressures ahead of it, the gust front

would be expected to remain quite active (Miller, 1972). In fact, the

1300 GMT visible satellite image, shown in Fig. 15, still showed evidence of the gust front and mesohigh. Convection had formed along the

leading edge of the gust front in southern Louisiana, while subsidence

associated with the mesohigh had created partial clearing in central

Louisiana.

The characteristics associated with this MCC were very pronounced

and are shown in Table 3. In the few hours after the initiation of the

MCC (the initiation of the MCC was Oil5 GMT), the maximum hourly

thunderstorm heights were quite impressive. The heights from 0135 GMT

to 0535 GMT, were 60,000, 57,000, 55,000, 55,000, and 53,000 ft, respectively. The hourly surface divergence remained large after the maximum

extent of the MCC. In fact, the largest magnitude of the surface divergence was reached at 0800 GMT (3 x lO-5 s-l), 3 h after the maximum

extent of the MCC. The maximum sum of the three largest hourly point

precipitation rates was 5.96 in h-I. Similarily, the largest hourly

maximum point precipitation rate was 2.2 in h"I. Of particular interest, is the fact that the sum of the three largest hourly point precipitation rates, stayed above 1.5 in h-I for eight consecutive hours.

The coldest hourly cloud top temperatures, during the few hours after
ORIG1NAL PAGE IS

OF POOR QUALITY

I

0

_I0

Oo

o

l)I

oo

OO"

0"

t'--

Z7
ORIGINAL PACE 55 

OF POOR QUALITY 

28 

0 

0 

m 

L 

0 

cc 

m 

W 

0 

c3 
29

Table 3. Characteristics associated with the MCC of Case 2 (8/9 May

1981). The maximum thunderstorm heights were determined at approximately

35 min past each hour. Dashes indicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) height

(x 100 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x lO-s s -I) rates (in h-I) rate (in h-I) (°C)

2300 ....

540

0000 - 0.66 0.40 -

55O

0100 - 2.50 1.30 -

600

0200 l 5.00 1.50 -63

570

0300 l 3.05 1.25 -63

550

0400 l 5.96 2.20 -63

550

0500 l 2.10 1.10 -68

530

0600 2 2.40 0.90 -68

520

0700 3 2.40 1.40 -63

470

0800 3 2.45 1.45 -63

480

0900 2 1.30 . 0.70 -58

420

1000 2 0.73 0.58 -

400

ll00 - 0.30 0.40 -

1200 - 0.10 0.10 -

1300 - 0.00 0.00 -
3O

MCC initiation, were quite cold, as was expected in view of the great

heights of the thunderstorms.

Case 3: 27 May 1981

By 0515 GMT 27 May 1981, an MCS over southeastern Oklahoma had

reached the critical size criteria of the MCC definition. The 50 kPa

pattern associated with this MCC is reproduced in Fig. 16. Notice the

trough-ridge-trough configuration; with ridging located over the area of

the MCC. This pattern was quite different from the previous case. However, at the surface, conditions were very similar to that of Case 2.

Although the NWS's surface analysis (not shown) did not depict a frontal

boundary near the MCC, the surface Be analysis for 0900 GMT (Fig. 17)

indicated a distinct contrast of air masses. This contrast of air

masses was also evident prior to the MCC formation, indicating that it

was not an MCC-produced phenomenon. The ambient surface air temperature

and surface dew point temperature for two stations, Enid, OK (END) and

Waco, TX (ACT), are used to illustrate the contrast of air masses.

Enid's ambient surface air temperature and surface dew point temperature

for 0900 GMT were IO°C and 13°C, respectively, while at the same time,

Waco's were 24°C and 22°C. A definite contrast of air masses existed

across the MCC.

About 2 h after the initiation of the MCC, a mesohigh and gust

front formed near the Oklahoma/Texas borders and subsequently moved

rapidly southeast. Once again, the strength of the gust front was revealed by the strong and gusty winds which it produced. At 0935 GMT, as

the gust front passed Dallas, TX (DAL), a peak wind of 45 kt was
OF POOR QUALITY

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(2,)

'-c"

e-,-

co

o_

e,,,-

tO

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OJ

0

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o

4-- ,----,

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4-.) S.-

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cc--

°_.,.

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Lt. _

e31
32 

ORlGlNAL PAGE CS 

OF POOR QUALITY 
33

recorded.

Upon close inspection of the first visible satellite pictures for

the day (1200 to 1300 GMT), an arc cloud, separated from the parent

storm, was perceived. Fig. 18 shows the visible satellite picture for

1500 GMT. By this time, the arc cloud had nearly reached the Gulf coast

of eastern Texas. After this time, the arc cloud weakened and was no

longer discernable in the satellite images after 1800 GMT.

Characteristics associated with the MCC are given in Table 4. Most

of the characteristics were not as pronounced as those of the MCC in

Case 2. However, they were significantly stronger than the characteristics of the MCSs classified in category 2, as will be shown later. Two

characteristics of the MCC for this case are worth emphasizing. Even

after the termination of the MCC (1400 GMT), the maximum surface divergence increased to 3 x I0 -s s "I (at 1400 and 1500 GMT), compared to 2 x

10 -5 s -I of the previous 6 h. Also, notice that the sum of the three

largest precipitation rates stayed at or above 1.5 in h "I for 6 h. It

may be recalled that for Case 2, this condition existed for 8 h.

Case 4: 17 May 1982

A storm area formed in Kansas and western Oklahoma on 16 May 1982.

This storm area developed on the east side of a stationary front as

analyzed by the NWS (not shown). The front extended from South Dakota

to the Texas panhandle and beyond to the southeast corner of New Mexico.

Fig. 19 shows the 50 kPa analysis for 1200 GMT 16 May 1982. The trough

located over Arizona and New Mexico moved eastward and initiated the

formation of the storm area. By 0030 GMT 17 May 1982 the storm area was
34 

ORlGlNhL PAGE IS 

OF POOR QUALITY 

h 

N 

L 

0 

rc 
35

Table 4. Characteristics associated with the MCC of Case 3 (27 May

1981). The maximum thunderstorm heights were determined at approximately

35 min past each hour. Dashes indicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) thunderstorm

(x lO0 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

diverqence precipitation precipitation temperature

(x lO-_s -I) rates (in h-I) rate (in h-I) (°C)

0400 - 0.60 0.50 -

520

0500 - l.O0 0.50 -

55O

0600 3 l.O0 0.70 -

580

0700 3 0.73 0.43 -73

560

0800 2 1.59 1.30 -68

520

0900 2 1.50 0.60 -73

550

lO00 2 2.70 0.99 -68

480

llO0 2 2.93 1.13 -68

470

1200 2 2.50 0.90 -68

320

130O 2 1.61 l.Ol -

250

1400 3 0.60 0.20 -

160

1500 3 0.60 0.20 -

n

1600 - O.lO O.lO -
ORIGINAL PAGE IS

OF POOR OUALITY

36

Fig. 19. 50 kPa height (dm) and temperature (°C) fields for

1200 GMT 16 May 1982. Height contours (solid lines) are every

60 m; isotherms (dashed lines) are every 5°C.
37

large enough to be classified as an MCC. Fig. 20 shows the MCCat 0415

GMT,near the time of maximumextent. About 2 h earlier, at 0200 GMT,a

mesohigh and gust front formed. The gust front extended from Chanute,

KS (CNU)to OklahomaCity, OK(OKC)to Lubbock, TX (LBB).

The gust front associated with this MCCwas very strong, producing

gusty winds and severe weather. At 0300 GMT,along the leading edge of

the gust front, Fort Sill, OK (FSI) reported a tornado to the west. The

following stations reported wind gusts greater than 34 kt as the gust

front passed: OklahomaCity, OK(0233 GMT,35 kt); Tulsa, OK(0436 GMT,

35 kt); Wichita Falls, TX (0506 GMT,37 kt); McAlester, OK (0640 GMT,

36 kt); Dallas, TX (0830 GMT,35 kt).

Fig. 21 shows the GOESIR image, with the MBenhancementcurve, for

I000 GMT. The thunderstorms had weakenedconsiderably and the MCChad

already terminated. At this time, however, the gust front and large

mesohigh were still very pronounced, as depicted in the surface analysis

shown in Fig. 22. The mesohighencompassedsoutheast Oklahomaand most

of northern Texas. It was about this time that an arc cloud in the IR

imagery first becamediscernible.

Eventually, a massive ACCformed. Fig. 23 shows the arc cloud,

located in Texas, at 1832 GMT. The arrows point to its leading edge.

The arc cloud finally lost its identity in the satellite images around

2200 GMT.

Table 5 shows the characteristics associated with this MCCo The

maximumthunderstorm height of 65,000 ft occurred at 0135 GMT. This

correlated quite well with the maximumpoint rainfall rate, which

occurred during the hour ending at 0200 GMT. The behavior of the values
38 

h 

? 

L 

r 

aJ 

E 

aJ 

V 

S 

0 

N 

0, 

LL 

.I- 
1 

39 

h 

7 

c 

L 

0 

IC 
oGAG

oCS_

OF POG_ _._JALJ'TY

16, o

q87

!

I

4O

Fig. 22. Surface pressure field for I000 GMT 17 May 1982. Altimeter setting contours are every 0.03 in Hg. The gust front, located

in northeastern Texas and western Arkansas, is denoted by a cold front

symbol.
ORIGIN7& PAGE E3 

OF POOR QUALITY 

L 

0 

cc 

aJ 

91 

a 

E 

.r 
42

Table 5. Characteristics associated with the MCC of Case 4 (16/17

May 1982). The maximum thunderstorm heights were determined at approximately 35 min past each hour. Dashes indicate that the parameter was not

determined.

Maximum

Time thunderstorm

(GMT) height

(x 100 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x 10-5 s -I) rates Cin h-I) rate (in h-I) (°C)

2300 - 2.20 1.70 -

590

0000 - 3.83 1.80 -

620

0100 l 3.29 1.76 -73

650

0200 2 2.80 2.30 -73

530

0300 4 2.90 1.40 -68

510

0400 2 3.20 l.lO -68

520

0500 2 2.74 0.94 -68

550

0600 2 3.58 1.50 -63

580

0700 3 2.50 1.10 -63

630

0800 4 3.00 1.10 -

420

0900 4 1.52 0.60 -

410

1000 4 1.40 0.70 -

400

ll00 3 1.78 0.90 -

390

1200 4 2.73 1.63 -

1300 5 1.52 0.60 -

1400 4 1.39 0.75 -
43

of the sums of the three largest hourly rainfall rates was similar to

the previous cases. The sum of the rainfall rates stayed above 1.5 in

h -I for II straight hours, compared to 8 and 6 h for Cases 2 and 3,

respectively. Very cold cloud top temperatures were associated with the

early stages of the MCC, with temperatures as low as -73°C. As expected, the large mesohigh, created by the downdrafts of the thunderstorms,

was associated with strong surface divergence. The maximum of 5 x I0 -s

-I

s occurred at 1300 GMT.

Of the 12 cases included in this study, the MCC of this case produced the largest ACC.

Case 7: 26 June 1982

During the afternoon of 25 June 1982, convection developed to the

lee of the Rockies and moved eastward. During the evening hours an MCS

developed over the western plains stretching from the Texas panhandle to

southwestern South Dakota. Wetzel et al. (1983) described a similar

sequence of events. They found that afternoon orogenic thunderstorms,

such as those that develop along the foothills of the Rockies, often

move eastward and provide the beginnings of significant Plains mesosystems.

The synoptic setting leading to the formation of the MCS was described by Miller (1984). At 50 and 30 kPa, the MCS developed near the

inflection point downstream of a short-wave trough and in an area of

cold advection. The flow at both 30 and 20 kPa was diffluent over the

MCS, indicating the liklihood of convergence below. At both 70 and 85

kPa there was slight warm advection near the MCS. Thus, the troposphere
44

was undergoing destabilization due to differential advection.

Two gust fronts were produced by the MCS. The first gust front

formed around 0600 GMT 26 June 1982 and dissipated by 1500 GMT. The

second gust front was first detected in the surface analysis at II00 GMT

and dissipated around 2200 GMT. Fig. 24 is the surface analysis for

1200 GMT and shows both gust fronts. The gust fronts are denoted by

cold front symbols.

Both gust fronts moved to the southeast and eventually produced arc

clouds. Evidence of the first arc cloud could first be detected in the

IR satellite imagery at 0830 GMT. The second arc cloud was first visible in the satellite images at 1430 GMT and eventually grew into a large

ACC. Fig. 25 shows the second arc cloud, extending from central

Oklahoma to north central Texas and then southwestward to the southeast

portions of New Mexico, at 1832 GRIT.

Table 6 shows the characteristics associated with this MCS. The

maximum hourly thunderstorm heights oscillated, corresponding to the

formation of the two arc clouds. The heights fell drastically, from

54,000 ft to 41,000 ft, between 0735 GMT and 0935 GMT, bracketing the

time the first arc cloud appeared. Similarily, between 1335 GMT and

1535 GMT, the heights fell from 48,000 ft to 34,000 ft, again around the

time of arc cloud formation. The coldest cloud top temperatures oscillated in a similar fashion. The sum of the three largest precipitation

rates stayed above 1.5 in h"I from 0300 to 0800 GMT, a total of 6 h.

The only exception to this was the hour ending at 0600 GMT, where it

appeared likely that the heavy precipitation fell between the climatological raingauge sites. After this period of heavy precipitation, the
ORIGINAL:P_,_';"_'_'; ;3'

OF POOR QUALITY

o_ 0_5" o_Z ooC/ 00(o

0/2

O0

Ol

005 000

oog (

C

ooo

000

45

qq7

o

ql

SEI

26(

O_¥D

o(

qFig. 24. Surface pressure field for 1200 GMT 26 June 1982.

Altimeter setting contours are every 0.03 in Hg. The gust fronts

are denoted by cold front symbols.
N 

03 

cn 

v) 

N 
47

Table 6. Characteristics associated with the MCS of Case 7 (26 June

1982). The maximum thunderstorm heights were determined at approximately

35 min past each hour. Dashes indicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) height

(x lO0 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x lO-5 s-I) rates (in h-I) rate (in h-I) (°C)

0000 - 1.40 0.90 -

OlO0 - 0.50 0.20 -

0200 - 0.50 0.20 -

0300 - 1.88 0.80 -

0400 - 1.54 0.61 -

550

0500 - 1.50 0.50 -

600

0600 - 0.70 0.44 -

540

0700 - 2.00 l.lO -

540

0800 -5 1.51 0.90 -68

480

0900 2 1.26 0.60 -63

410

lO00 l 1.36 0.60 -63

400

llO0 l 1.25 0.60 -68

400

1200 l 1.94 1.25 -68

480

1300 3 0.81 0.50 -68

480

1400 3 0.80 0.60 -68

450

1500 4 1.63 0.90 -63

340

160O - 0.50 0.20 -

1700 - 0.30 O.lO -
48

first arc cloud formed. Another peak in the sum of the three largest

precipitation rates occurred the hour ending at 1500 GMT (].63 in h-l).

This seemed to correspond with the formation of the second arc cloud.

Case 8: 27 June 1982

During the afternoon of 26 June 1982, the large arc cloud discussed

in the previous case intersected with an east-west oriented line of

cumulus and formed an MCS over western Texas. The MCS moved southeast,

reached its most intense stage around 0330 GMT 27 June 1982 and slowly

dissipated thereafter. Only a weak gust front and mesohigh were produced by the MCS.

Another MCS developed during the afternoon of 26 June 1982. This

MCS developed to the lee of the Rockies in northeast New Mexico and

essentially followed the same movement as the first MCS. Fig. 26 shows

both MCSs at 0700 GMT 27 June 1982. The eastern MCS was dissipating,

while the western MCS was still intensifying. By 0800 GMT, the western

MCS was large enough to be classified as an MCC.

The western MCC produced a much stronger gust front and mesohigh

compared to the eastern MCS. This was clearly evident in the surface

pressure field for 1200 GMT, presented in Fig. 27. The remains of the

mesohigh produced by the eastern MCS were still apparent and were located southeast of Austin, TX (AUS). The central pressure of this mesohigh

was approximately 29.97 in of mercury. The mesohigh produced by the

western MCC had a central pressure of about 30.13 in of mercury. The

strength of the western mesohigh and gust front was reflected in San

Angelo's (SJT) winds, which gusted to 45 kt during the passage of the
ORIGINAL PAGE TS 

Of POOR QUALITY 

49 

c 

Fig. 26. GOES IR image with MB enhancement for 0700 GMT 2? June 

1982. 
5O

ORIGINAL PAGE "_S

OF POOR QUAUTY

m

O"

O"

o

0"

0"

e_ 0

o 0 ao 0 o"

"_ -,- O0

O" O" ql_

o" o- 0"

I

cO

4-)

_d

o

_ E

E

°_-- _

_ e4-

°.._

I1.1 e'_

ee--

(].1 e-

_-r"

t-'-

I_ *r'-

c_

o
51

gust front (0958). An arc cloud, associated with the gust front and

separated from the main MCS cloud shield, first became evident in the

satellite images at 1200 GMT.

The characteristics of the western MCC are similar to the previous

category l MCSs and are displayed in Table 7. Dramatic characteristic

changes occurred around the time of arc cloud formation. The maximum

hourly thunderstorm heights dropped from 48,000 ft at I135 GMT to 30,000

ft at 1335 GMT, the sum of the three largest precipitation rates fell

from 3.72 in h-I at llO0 GMT to 0.9 in h-I at 1300 GMT, the maximum

point precipitation rate reduced from 1.5 to 0.5 in h -_ and the coldest

cloud top temperatures warmed from -73 to -58°C. Two other interesting

features are seen in the characteristics associated with the MCC. The

magnitude of the maximum surface divergence remained large after the

maximum extent of the MCC (0900 GMT). Also, the sum of the three largest precipitation rates stayed above 1.5 in h-1 for 7 h, excluding the

hour ending at 0800 GMT, when it appeared the heavy precipitation fell

between reporting climatological raingauge sites. Therefore, heavy precipitation fell over a significant area for a long time, preceding the

formation of the arc cloud.

Case lO: 20 May 1983

On the afternoon and evening of 19 May 1983, an enormous convective

storm system evolved from the merger of several smaller MCSs. The

initial thunderstorms developed around 2000 GMT 19 May 1983, just to the

east of a north-south dry line, which was located in western Texas.

Another almost equally vigorous system developed 2 h later in northeast
52

Table 7. Characteristics associated with the MCCof Case 8 (27 June

1982). The maximumthunderstorm heights were determined at approximately

35 min past each hour. Dashesindicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) height

(x I00 ft)

Maximum Sumof the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x lO-s s-I) rates (in h-I) rate (in h-1) (°C)

0000 - 0.80 0.80 -

OlO0 - 0.90 0.50 -

500

0200 - O.lO O.lO -

500

0300 - 0.20 O.lO -

470

0400 - 0.20 O.lO -

490

0500 - O.Ol O.Ol -

540

0600 3 2,30 1.60 -73

520

0700 4 2.00 l.O0 -68

520

0800 5 0.85 0.50 -73

500

0900 3 1.85 0.90 -73

500

lO00 2 2.49 1.30 -73

500

llO0 3 3.72 1.50 -73

480

1200 2 1.69 0.90 -68

330

1300 3 0.90 0.50 -58

300

1400 4 0.60 0.20 -

260

1500 4 0.30 O.lO -

450

1600 - 0.32 0.20 -

490

1700 ....

530
53

Texas in the vicinity of a weak warm front. By 0130 GMT 20 May 1983,

the two MCSs merged into an MCC. The 50 kPa analysis for 0000 GMT 20

May 1983 is shown in Fig. 28. This flow configuration, termed the "New

Orleans Type" (Belville and Stewart, 1983), consisted of a deep trough

and closed circulation over the southwest United States. This pattern

is often responsible for early spring heavy rainfall events in the

southeast (Belville and Stewart, 1983).

By 0400 GMT, a mesohigh and gust front, located in southeast Texas,

were evident in the surface analysis (not shown). The gust front was an

important factor influencing the National Severe Storms Forecast Center

to issue Tornado Watch Number 179 at 0458 GMT. The watch bulletin stated that an outflow boundary, located from Lufkin, TX, west-southwest to

south of San Antonio, TX, would be the focus for continued development

of very strong thunderstorms. As the gust front interacted with the

very unstable airmass, numerous reports of severe weather were reported.

Two examples were wind gusts of 53 kt at San Antonio at 0453 GMT and a

confirmed tornado at Houston's Intercontinental Airport at 0720 GMT.

After 0900 GMT, the southern portion of the gust front moved into the

northwest Gulf of Mexico. However, the northern portion was still discernable in the surface analysis as it passed stations in Louisiana.

For instance, Lake Charles reported a wind gust of 40 kt at 0928 GMT and

Lafayette's winds changed from calm at II00 GMT to a westerly component

of I0 kt at 1200 GMT.

At 1200 GMT an arc cloud was discernable in the visible satellite

image. Fig. 29 shows the arc cloud, at 1300 GMT, extending from southeastern Louisiana into the northwest Gulf of Mexico. At this time, the
ORIG'i'NAL P_G;_ '_

OF POOR QUALITy

54

I

4.O2

\

Fig. 28. 50 kPa height (dm) and temperatures (°C) fields

for 0000 GMT 20 May 1983. Height contours (solid lines) are

every 60 m; isotherms (dashed lines) are every 5°C.
0ftiGt"AL PAGE IS 

OF POOR QUALITY 

55 

'I ' 'I 
56

NWS's surface analysis depicted a cold front extending from the

Tennessee Valley southwestward to southeast Texas (not shown). Normally, pronounced clearing occurs behind arc clouds. The lack of clearing

in southeast Texas (see Fig. 29) can be partially explained by the

presence of the existing cold front.

The characteristics associated with the MCC are displayed in Table

8. The extreme intensity of the MCC is obvious. The highest reported

thunderstorm height of 66,000 ft was reached at 0635 GMT. The largest

point precipitation rate and the maximum sum of the three largest hourly

precipitation rates, of 2.1 in h-I and 4.10 in h-I, respectively, occurred at 0700 GMT, just before the time of maximum extent of the storm.

Also, the sum of the three largest hourly precipitation rates stayed

above 1.5 in h-I for 5 straight hours. The only exception to this

occurred the hour ending at 0600 GMT, when it again appeared that the

heavy precipitation fell between the climatological raingauge sites.

This was determined by following the movement of the coldest cloud tops

on satellite images and by keeping continuity of the most intense precipitation. The coldest cloud top temperatures of -78°C occurred at

both 0700 and 0800 GMT. It should be noted that the decrease of the

maximum hourly surface divergence after lO00 GMT, was probably due to

the lack of wind reports behind the gust front as it moved into the Gulf

of Mexico.

Case ll: II June 1983

On the evening of lO June 1983, thunderstorms formed over eastern

New Mexico and western Texas ahead of a short-wave trough in the
57

Table 8. Characteristics associated with the MCCof Case I0 (20 May

1983). The maximumthunderstorm heights were determined at approximately

35 min past each hour. Dashesindicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) height

(x 100 ft)

Maximum Sumof the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x I0 -s s"I) rates (in h-I) rate (in h-I) (°C)

0000 2 - - -

0100 4 - - -63

D

0200 2 - - -63

0300 2 0.40 0.40 -63

610

0400 2 0.70 0.40 -68

630

0500 2 2.65 1.00 -73

640

0600 l 1.20 0.80 -73

660

0700 0 4.10 2.10 -78

550

0800 l 2.74 1.20 -78

520

0900 l 2.16 0.92 -73

540

1000 2 0.99 0.43 -73

420

ll00 0 0.87 0.40 -68

330

1200 0 0.15 0.10 -63

350

1300 0 0.21 0.10 -58

350

1400 - 0.40 0.20 -

320

1500 - 0.71 0.40 -

1600 - 0.06 0.06 -
58

mid-troposphere. At 0000 GMT II June 1983, the short-wave trough was

evident in the 50 kPa analysis and extended from western Kansas to

southwestern New Mexico (Fig. 30). At the surface, the storms developed

to the east of a dry line and in an area of strong convergence, with

magnitudes as large as -3 x I0 -s s -I, as determined by the MclDAS. The

thunderstorms expanded and at 0530 GMT an MCC was initiated.

At 0800 GMT, a gust front and mesohigh developed. As the gust

front passed Wichita Falls, TX, a wind gust of 44 kt was reported.

By 1300 GMT, an arc cloud was evident in the visible satellite

image (Fig. 31). The arc cloud extended from southeastern Oklahoma to

central Texas. At 1600 GMT, the gust front and mesohigh were still discernible in the surface analysis and the arc cloud was clearly evident

in the satellite images (not shown).

Table 9 shows the characteristics associated with the MCC. Although the characteristics were not as pronounced as some of the previous category 1MCSs, heavy precipitation fell for a significant time.

The sum of the three largest hourly precipitation rates stayed above 1.5

in h -I for five consecutive hours.

Category 2 MCSs

Category 2 MCSs are defined as follows: The MCS produces a gust

front and the gust front persists for 6 h or less. Four MCSs are

classified in this category. The four MCSs are from Cases I, 5, 6 and

9. A brief discussion of the synoptic situation and storm characteristics associated with each category 2 MCS follows.
ORIGINAL PAGE IS

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59

!

!

I

÷02

'2

Fig. 30. 50 kPa height (dm) and temperature (%) fields

for 0000 GMT II June 1983. Height contours (solid lines) are

every 60 m; isotherms (dashed lines) are every 5%.
ORIGINAL PAGE IS 

OF POOR QUALITV 

60 

F 

F 

L 

0 

0 

cr) 

L 

0 

cc 
61

Table 9. Characteristics associated with the MCC of Case II (II

June 1983). The maximum thunderstorm heights were determined at approximately 35 min past each hour. Dashes indicate that the parameter was not

determined.

Maximum

Time thunderstorm

(GMT) height

(x lO0 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x lO -s s-I) rates (in h -I) rate (in h -I) (°C)

0200 - 0.16 O.lO -

0300 - 1.12 0.50 -

0400 -3 0.67 0.40 -58

500

0500 -3 l.lO 0.60 -58

510

0600 -l 2.30 0.90 -63

520

0700 -3 1.80 0.70 -63

510

0800 -2 1.60 0.60 -63

450

0900 3 2.00 0.90 -68

440

lO00 l 1.55 0.60 -63

390

llO0 2 1.08 0.50 -58

320

1200 2 0.88 0.44 -58

310

1300 - O.Ol O.Ol -

300

1400 - 0.30 O.lO -

300
62

Case I: I0/II April 1981

On I0 April 1981 an MCC developed over the Texas panhandle during

the late afternoon. It developed along a stationary front, which extended from southeastern Minnesota to north central Kansas and then

southward to the Texas panhandle. Convection occurred all along this

front (see Fig. 32). The MCC developed in response to a short wave in

the mid-troposphere. Of course, a number of other conditions must have

co-existed to support MCC development, such as lower tropospheric warm

advection, ample moisture and convective instability (Maddox, 1981).

Fig. 33 shows the 70 kPa analysis for 0000 GMT II Apt 1981. The short

wave trough line is clearly evident, located over the western portion of

the Texas panhandle.

By 0200 GMT II Apr 1981, an organized surface high pressure system

and gust front, produced by the downdrafts of the thunderstorms associated with the MCC, first appeared. Fig. 34 shows the surface analysis

for 0300 GMT. The gust front, denoted by the cold front symbols, was

located from just west of Oklahoma City, OK (OKC), to Fort Sill, OK

(FSl) and southwestward to just south of Childress, TX (CDS). Note the

west wind at Hobart, OK (HBR) and the southeastern winds at Amarillo, TX

(AMA) and Dalhart, TX (DHT). These diverging winds are a typical pattern associated with the presence of the mesohigh and gust front. Fig.

32 shows the surface divergence field overlaid upon the GOES IR imagery,

both for 0300 GMT. A well-organized divergence pattern, with magnitudes

up to 4 x I0 -s s -I, was centered over the northeastern Texas panhandle.

The MCC terminated at 0531GMT (see Table 2, page 19}. The gust

front quickly lost its identity and by 0700 GMT was no longer evident in
63 

ORlc'!NuS\L PAGE IS 

Of POOR QUALITY 

Fig. 32. Surface divergence field overlgid rpon GOES IR image for 

0300 GMT 11 April 1981. Values are times 10' s- and is01 ines are 

drawn at an increment of 1 x s-'. 
64

+0

4-OO

11.137_ \

Fig. 33. 70 kPa height (dm) and temperature (°C) fields

for 0000 GMT II April 1981. Height contours (solid lines) are

every 30 m; isotherms (dashed lines) are every 5°C.
65

0988

/

001

qel# 8%7

c/gfl.

997

Fig. 34.

\

\

l

l

00o

timeter setting contours are every 0.03 in Hg.

denoted by a cold front symbol.

003 oo& 009

Surface pressure field for 0300 GMT II April 1981.

The gust front is

000

DUA

0

PNX

0 Ps

oo3

,OOb

CL

,o0(}

A1-
66

the surface analysis (not shown). Therefore, the gust front lasted for

5 h, from 0200 to 0700 GMT. No arc cloud was ever apparent in the

satellite images.

Table lO shows specific characteristics associated with this MCC.

The maximum thunderstorm height of 56,000 ft occurred at 0435 GMT.

After the termination of the MCC, as expected, the heights quickly lowered. The hourly surface divergence reached its maximum at 0200 and

0300 GMT, near the time of maximum extent (see Table 2) of the system,

as measured by the number of pixels in the IR satellite imagery contained by the -62°C isotherm. The surface divergence slowly weakened thereafter, which indicated the gust front was also weakening. The maximum

sum of the three largest hourly precipitation rates and the largest

point precipitation rate were 1.5 in h-I and l.O in h-I, respectively.

These rainfall rates are rather light compared to the MCSs classified "in

category I. The coldest hourly cloud top temperatures followed a similar pattern as the maximum hourly thunderstorm tops; a rapid warming of

the temperatures, which corresponds to a lowering of the thunderstorm

heights, occurred shortly after termination of the MCC.

Case 5: 19 May 1982

During the nighttime hours of 19 May 1982, two MCSs developed over

western Kansas and western Oklahoma. The first MCS began forming around

0030 GMT and subsequently dissipated around 0800 GMT. The second MCS,

the one of most importance for this study, formed around 0800 GMT, in

western Oklahoma and dissipated by 1300 GMT. Fig. 35 shows the GOES IR

image at 0800 GMT. The first MCS, now dissipated, was located in
67

Table I0. Characteristics associated with the MCCof Case 1 (I0/II

April 1981). The maximumthunderstorm heights were determined at

approximately 35 min past each hour. Dashes indicate that the parameter

was not determined.

Maximum

Time thunderstorm

(GMT) height

(x lO0 ft)

Maximum Sumof the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x I0 -s s-I) rates (in h-I) rate (in h-I) (°C)

2300 - 0.59 0.40 -

510

0000 - 0.50 0.20 -63

5OO

0100 2 0.00 0.00 -63

460

0200 4 0.30 0.20 -63

510

0300 4 1.40 0.80 -63

490

0400 3 1.50 l.O0 -63

560

0500 2 1.17 0.50 -63

530

0600 2 0.62 0.40 -58

500

0700 2 0.30 O.lO -

430

0800 2 0.50 0.30 -

300

0900 l 0.13 O.lO -

u

lO00 l O.lO O.lO -
68 

ORIGINhL PAGE t3 

OF POOR QUALITY 

0 

0 

co 

0 

L 

0 

rc 

v, 

w 

0 

c3 
69

central Kansas. The second MCS, in the development stage, was located

in western Oklahoma.

The mid-tropospheric flow pattern, associated with the development

of the MCSs, is depicted in the 50 kPa analysis, Fig. 36. A deepening,

negatively tilted trough, encompassed the western United States. However, the axis of a weak ridge was located just to the west of the

development region. There was no indication in the flow, over the

development region, of a weak embedded trough, which normally accompanies the formation of MCSs. This might have been one factor which led

to the short duration of the MCSs.

A weak mesohigh and gust front, associated with the second MCS,

appeared over western Oklahoma at I000 GMT. By II00 GMT, evidence of a

small arc cloud could be seen in the GOES IR image. However, the gust

front, mesohigh and arc Cloud were undetectable in the surface analysis

and satellite images after 1600 GMT.

Brundidge (1983) also studied this case. He gave three reasons for

the short life and relative unimportance of this arc cloud:

I. The cold-air layer at the ground was very shallow, amounting to

only about 200 m in depth.

2. The wind in the shallow layer had no northerly component;

therefore, the outflow was not undercutting and lifting the warm, moister, surface air in northern Texas to maintain convection.

3. It appeared that the storm was simultaneously raining itself

out and drawing drier air into the system, thus reducing the energy

supply.

Table II shows the characteristics associated with primarily the
ORIGINAL PA_E _

OF. POOR QUALITY

70

/

!

+O4 ",

+04 \

Fig. 36. 50 kPa height (dm) and temperature (°C) fields for

0000 GMT 19 May 1982. Height contours (solid lines) are every 60 m;

isotherms (dashed lines) are every 5°C.
71

Table II. Characteristics associated with the MCS of Case 5 (19 May

1982). The maximum thunderstorm heights were determined at approximately

35 min past each hour. Dashes indicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) height

(x I00 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitatioD temperature

(x I0 -s s -I) rates (in h-1) rate (in h-I} (°C)

0300 - 0.08 0.05 -

490

0400 3 0.08 0.08 -63

460

0500 3 0.12 0.09 -63

420

0600 1 0.32 0.17 -63

49O

0700 1 0.52 0.32 -63

490

0800 1 0.23 0.20 -63

45O

0900 -I 1.19 I.I0 -63

420

I000 0 1.05 0.60 -63

38O

II00 0 1.50 1.00 -63

400

1200 1 0.30 0.I0 -58

35O

1300 - 0.70 0.50 -

310

1400 - 0,I0 0.I0 -

290
72

second MCS. However, any characteristics given before 0800 GMTwere

associated with the first MCS. Similar to Case I, the characteristics

were very weak. For the second MCS,the maximumthunderstorm height was

only 45,000 ft and the maximumsurface divergence was only 1 x I0 -s s-I

The maximumsumof the three largest hourly rainfall rates was 1.5 in

h-I, the sameas Case I. Also, the largest hourly point precipitation

rate was only I.I in h-l, comparedto 1.0 in h-z for Case I. Lastly,

the coldest cloud top temperature was only -63°C.

Case 6: I0/II June 1982

A very large MCCformed over western Texas on I0 June 1982. At 50

kPa, near zonal flow existed over the development area at 0000 GMTII

June 1982 (Fig. 37). At the sametime, very strong warmadvection was

present at both 85 and 70 kPa (not shown). Maddox(1983), in view of

the quasi-geostrophic equations (in particular the tendency and omega

equations), noted that the MCCdevelops in a region of upward vertical

motion and that the upwardmotion "is primarily a reflection of strong

low-level warmadvection rather than of strong differential PVA."

The lifetime of the MCC,initiation to termination, was 2245 GMT

I0 June 1982 to 1530 GMTII June 1982. Analysis of the cloud top

temperatures, provided by the MclDASand the plotted precipitation

rates, revealed that the MCCactually consisted of two separate MCSs.

The first MCSdeveloped in the Texas panhandle and movedinto southwest

Oklahoma. The most intense phaseof the MCSlasted from approximately

0300 to 0800 GMT. Fig. 38 showsthe surface divergence overlaid upon

the GOESIR imagery with MBenhancement, both for 0700 GMT. According
73

Fig. 37. 50 kPa height (dm) and temperature (°C) fields

for 0000 GMTII June 1982. Height contours (solid lines) are

every 60 m; isotherms (dashedlines) are every 5°C.
74 

aJ 

S 

3 

3 

Ou, 

01 

hO 

OlaJ -0 vv) 

s -r 

a, 

LS 

LLv) 

a, 

3 
75

to the MB enhancement curve, the large black area in the Texas panhandle

and in western Oklahoma, corresponds to cloud top temperatures between

-58°C and -62°C. The repeat gray area and the whitish area, enclosed by

the black area, represent the coldest cloud top temperatures. Therefore, the maximum thunderstorm tops associated with the first MCS were

located in the eastern portions of the Texas panhandle and in southwestern Oklahoma. An organized mesohigh and gust front, produced by

this MCS, appeared in the surface analysis at 0500 GMT (Fig. 39). After

the demise of the MCS, the mesohigh and gust front weakened. By I000

GMT, the mesohigh and gust front were ill-defined in the surface analysis. The second MCS developed in northeastern Oklahoma, around the time

the first MCS was terminating. The mesohigh and gust front associated

with the second MCS were much weaker than those produced by the first

MCS. No arc cloud was produced by either MCS.

The characteristics of the first MCS are displayed in Table 12.

The sum of the three largest hourly precipitation rates stayed above 1.5

in h-I for only 3 h. Of the two precious MCSs classified in category 2,

this sum stayed at or above 1.5 in h"I for only 1 h. Another interesting feature is exhibited in the trend of the maximum hourly surface divergence. The magnitudes of the divergence were strong, as high as 4 x

I0 -s s -I, before the demise of the mesohigh and gust front. However,

after I000 GMT, the divergence weakened, which corresponded to the weakening of the mesohigh and gust front.

Case 9: 28/29 June 1982

A short-lived MCC developed over northeastern Texas during the
76

oo5 OO& OOq 0o9

qq,t

?qy qq7

006

003

000

Fig. 39. Surface pressure field for 0500 GMT II June 1982. Altimeter setting contours are every 0.03 in Hg. The gust front is denoted

by a cold front symbol,
77

Table 12. Characteristics associated with the MCC of Case 6 (II June

1982). The maximum thunderstorm heights were determined at approximately

35 min past each hour. Dashes indicate that the parameter was not determined.

Maximum

Time thunderstorm

(GMT) height

(x lO0 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x lO-s s -1) rates (in h-I) rate (in h-1) (°C)

0100 - 1.40 1.00 -

0200 - 0.40 0.20 -

520

0300 - 1.20 1,00 -

530

0400 2 1.32 0.70 -73

550

0500 4 1.99 0.90 -73

540

0600 3 2.59 1.30 -73

550

0700 2 1.83 0.70 -73

500

0800 3 0.91 0.31 -73

460

0900 4 1.40 0,70 -68

48O

I000 3 1.40 0.70 -68

410

II00 1 0.57 0.20 -

340

1200 2 0.22 0.I0 -

380

1300 1 0.29 0.I0 -

4OO
78

afternoon of 28 June 1982. Fig. 40 depicts the 50 kPa analysis for 0000

GMT 29 June 1982. This was near the initiation time of the MCC. An

upper level trough was associated with the MCC and a very pronounced

ridge was situated to the west. At 85 kPa, warm air advection (i.e.,

quasi-geostrophically forced upward motion) was evident over eastern

Texas (not shown). Absolute moisture contents within the air mass feeding the MCC were high. For instance, at 85 kPa, the moisture content

for Victoria, TX (VCT) was II g kg-I at 0000 GMT 29 June 1982. Maddox

(1982), in a composite study of lO MCCs, described a similar synoptic

setting for the development of MCCs.

At 0000 GMT 29 June 1982, a gust front and mesohigh appeared in the

surface analysis (Fig. 41). The MCC terminated at 0430 GMT (see Fig.

42). By 0500 GMT the gust front and mesohigh had virtually disappeared

in the surface analysis (Fig. 43). The weakness of the gust front, during its lifetime, was verified by the lack of strong winds as it passed

reporting weather stations. The strongest wind gust was only 15 kt,

which occurred at both Lufkin, TX (LFKI and College Station, TX (CLL).

No arc cloud was produced by this MCC.

The characteristics associated with the MCC are depicted in Table

13. The maximum hourly thunderstorm heights were impressive, reaching

58,000 ft at 0135 GMT. However, the heights quickly lowered after 0235

GMT. The maximum surface divergence reached magnitudes of 3 x lO-5 s-I

at 0400 and 0500 GMT and then rapidly weakened. The sum of the three

largest hourly precipitation rates and the maximum hourly point precipitation rates were quite large before OlO0 GMT, but dramatically dropped

thereafter. AS the precipitation rates decreased, the coldest cloud top
79

I

5#r 12 ÷Q3

_2 X ÷02

\

F -o \

Fig. 40. 50 kPa height (dm) and temperature (°C) fields for 0000

GMT 29 June 1982. Height contours (solid lines) are every 60 m; isotherms (dashed lines) are every 5°C.
ORIGINAL PAGE _,S

OF POORQUALITY

8O

cO

,g

8

o I'-"

.-_

P- u

I,- o

_ °_-

_ m

_ o e-

&o.
ORIGINAL PAGE E3 

W POOR QUALITY 

81 

v, 

w 

0 

L3 
ORIGINAL PAGE IS

OF POOR QUALITY

82

tO

i/'l

!,,-

o

co

o

01=-

o

_J

4J

o

.1==.

i==- •

*_r--

0

• r::

CO

cO 0

n_

O_.-

oJ 0

I--

0

0

0 0

_- 0

0 _"

0

n_

_==

.1=,=

4- *1._

cO 0

0

O0

• e'-

°_,- 0

LI-

>
83

Table 13. Characteristics associated with the MCC of Case 9 (28/29

June 1982). The maximum thunderstorm heights were determined at approximately 35 min past each hour. Dashes indicate that the parameter was not

determined.

Maximum

Time thunderstorm

(GMT) height

(x lO0 ft)

Maximum Sum of the Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x lO-s s-I) rates (in h-I) rate (in h -I) (°C)

2100 - 2.72 1.62 -

510

2200 -2 2.46 0.90 -73

5OO

2300 0 1.80 l.O0 -73

570

0000 l 0.99 0.40 -68

550

OlO0 0 1.60 1.30 -73

580

0200 l 0.60 0.40 -68

550

0300 2 0.50 0.30 -63

450

0400 3 0.88 0.48 -58

420

0500 3 0.23 0.17 -53

340

0600 l 0.40 0.20 -

360

0700 -l 0.20 O.lO -

0800 - O.lO O.lO -

0900 - O.lO O.lO -
84

temperatures warmed, which agrees with the findings of Gagin et al.

(I 985).

Case 12: 14 June 1983

During the afternoon of 13 June 1983, convection formed along a

strong cold front, which was located through the central United States.

An MCC developed near the southern portions of the cold front. By 0600

GMT 14 June 1983 it had reached its maximum extent. At this time, the

MCC engulfed north central Texas and most of Oklahoma.

The precipitation pattern produced by the MCC was not typical of

most MCCs. Heavy precipitation associated with the MCC fell exclusively

along the cold front. Close inspection of the plotted hourly precipitation revealed that no precipitation fell along the southern edge of the

MCC cloud shield. This was rather unusual, as many instances of an arcshaped band of heavy precipitation along the southern portions of MCCs

have been reported (Maddox, 1980, 1981; Leary and Rappaport, 1982;

Howard and Maddox, 1983; and Bartels, 1983). Bartels (1983) stated,

"Throughout the MCC life-cycle the most intense rainfall occurred near

its southern periphery." Maddox (1980) argues that the most intense

convective elements occur along the convergence zone produced by the

interaction of the outflow boundary and the low-level inflow. This convergence zone occurs along the southern portion of the MCC. An example

of this arc-shaped precipitation band, produced by the MCC from Case 2,

is shown in Fig. 44. The arc-shaped band of heavy precipitation was

located from eastern Texas to central Louisiana. Also, notice the circular shape in the overall precipitation pattern that was produced by
85

NE

@

NE

NE

o I

NE T_ '

__ NE

Fig. 44. Radar summary chart for 1035 GMT 9 May 1981. Shading

indicated echo areas. Contours at echo intensities I, 3, and 5; echo

heights are in hundreds of feet; cell movement given at end of arrows

in knots; area and line movement given by pennant with full barb =

I0 kt.
86

the MCC of Case 2. This is typical of MCCs because of their circular

shape, imposed by the MCC definition (refer to Table l, page 3). In

contrast, the precipitation pattern produced by the MCC of this case was

markedly different. Fig. 45 displays this pattern and also shows the

approximate location of the MCC cloud shield (scalloped area). There

was no arc-shaped line of heavy precipitation on the southern periphery

of the MCC. In fact, there was no precipitation falling there at all.

Also, the overall precipitation pattern was much more elongated, like

that produced by squall lines.

In summary, the precipitation pattern produced by the MCC of this

case resembled that of a squall line, rather than an MCC. Therefore,

this MCC was not classified in category l or 2 and was not used in the

composite analyses (see next section).

Significant Events Associated with Category l and 2 MCSs

Table 14 summarizes the times that the following events occurred

for both category l and 2 MCSs:

Maximum extent of the MCS.

First appearance of the gust front and mesohigh in the surface

II

2.

analysis.

3.

images.

First appearance, if any, of an arc cloud in the satellite

In most cases, the first appearance of an arc cloud could only

be determined by visible satellite images. It is extremely difficult to

detect their existence in the IR images because arc clouds are normally

composed of low, warm clouds. Therefore, the actual time of arc cloud

formation could have been earlier than the time indicated in Table 14.
87

NE

I NE

1

Fig. 45. Radar summary chart for 1035 GMT 14 June 1983. Shading

indicates echo areas. Contours at echo intensities I, 3, and 5; echo

heights are in hundreds of feet; cell movement given at end of arrows

in knots; area and line movement given by pennant with full barb =

I0 kt. The scalloped area represents the approximate location of the

MCCcloud shield at I000 GMT 14 June 1983.
88

Table 14. Summary of significant events associated with category 1

and 2 MCSs. Time of MCS maximum extent, gust front/mesohigh first

appearance in hourly surface analyses and arc cloud first appearance and

termination, as determined by satellite imagery are listed.

Arc Arc

Case Maximum Gust front/ cloud cloud

extent mesohigh

began ended

number (GMT) (GMT) (GMT) (GMT)

Category l MCSs

2 0500 0500 1200 1800

3 lO00 0700 1230 1800

4 0400 0200 lO00 2200

7 llO0 llO0 1430 2200

8 0900 0800 1200 2100

lO 0800 0400 1200 1800

II 0900 0800 1200 1800

Category 2 MCSs

l 0300 0200 none none

5 0900 lO00 llO0 1600

6 0700 0500 none none

9 0200 0000 none none
89

4. The arc cloud, if any, becameindistinguishable in the satellite images.

As seen in Table 14, all category l MCSsproduced arc clouds.

These MCSs,by definition, produced gust fronts that persisted for more

than 6 h. In contrast, only one of the category 2 MCSsproduced an arc

cloud. These MCSs,by definition, produced gust fronts of shorter

duration, lasting 6 h or less.
9O

CHAPTER IV

COMPOSITE ANALYSES

To determine any differences between the MCSs of the two categories, composite analyses of the five following characteristics were

accomplished:

I. Maximum hourly thunderstorm height.

2. Maximum hourly surface divergence.

3. Sum of the three largest precipitation rates.

4. Maximum hourly point precipitation rate.

5. Coldest hourly cloud top temperature.

Method of the Composite Analyses

The maximum extent of each MCS was determined by the MclDAS (see

Table 2, page 19). The time of maximum extent, as defined in this research, occurred when the area, as depicted in IR satellite imagery,

with temperatures less than or equal to -62°C, reached its maximum size.

For the composite analyses, the time of maximum extent was referred

to as the zero (0) hour or zero (0) reference time. That is, this time

was used as a reference time for all MCSs and determined when all the

MCSs were at the same stage in their life-history. The hours before the

maximum extent were defined as minus (-) hours and the hours after the

maximum extent were defined as plus (+) hours. For example, the maximum

extent time of the MCC for Case 1 was 0300 GMT II April 1981. Therefore, 0300 GMT was referred to as the zero (01 reference hour. 0400,

0500 and 0600 GMT were referred to as +I, +2 and +3 hours, respectively.
91

The MCScharacteristic, "maximumhourly point precipitation," will

be used as an example to demonstrate the method employed to accomplish

the composite analyses. Assumethe MCSsof Cases 2, 3, 4, 7, 8, I0 and

II (category 1MCSs) had maximumpoint precipitation rates of I.I0,

1.00, 0.90, 0.50, 0.50, 1.50 and 1.50 in h-I, respectively, at hour +I

(I h after maximumextent). The composite analysis, for this hour,

would be the average of the precipitation rates (I.00 in h-l). This

procedure was applied for several hours before and after the maximum

extent.

Results of the Composite Analyses

Composite analyses of the five characteristics were accomplished

for the MCSsof category I. The results of the composite analyses are

displ.ayed in the following five figures (thin solid lines). Similarly,

composite analyses of the five characteristics were accomplished for the

MCSsof category 2. These results are also shownin the following five

figures (thick solid lines).

Fig. 46 graphically displays the results of the composite analysis

for the maximumhourly thunderstorm heights. In the hours before the

maximumextent time, category 1MCSs, on average, had higher maximum

thunderstorm heights, comparedto the MCSsof category 2. This was especially true from -2 to -6 h (2 to 6 h before the time of maximumextent). This is the critical time period for the formation of the gust

front, as the gust front and mesohigh usually form within the few hours

before the maximumextent time of the MCS(see Table 14, page 88}. In

the hours after maximumextent, the differences in the maximum
92

575-

550-

525-

M

R

X

500-

T

H

U

N 475.

D

E

R

5

T 450-

0

R

M

H 425-

E

I

G

H

T 400-

X

I 375-

0

0

F

T 35:3-

325-

30_- I

I ......... I ......... _ ......... I ......... I ........ ; ........ I ....... I ................ I ......... I ......... I ........ ]

-8 -7 -6 -5 -4 -3 -2 -1 0 I 2 3 4

NUMBER OF HOURS FROM MC5 MAXIMUM EXTENT

Fig. 46. Composite of the maximum hourly thunderstorm tops of

category l (thin line) MCSs and category 2 (thick line) MCSs. Minus (-)

hours are before MCS maximum extent and plus (+) hours are after MCS

maximum extent.
93

thunderstorm heights of category 1 and 2 MCSs became negligible.

Fig. 47 depicts the results of the composite analysis of the maximum hourly surface divergence. The most striking result from the analysis occurred from hours +2 to +5. For category 2 MCSs, the divergence

decreased from 2.25 x 10 -5 s -I at hour +2 to 1.00 x I0 -s s -I at hour +5.

The opposite occurred for category 1 MCSs. The divergence increased

from 2.43 x I0 -s s -I at hour +2 to 3.40 x I0 -s s-1 at hour +5. The increasing magnitude of the maximum surface divergence associated with

category 1MCSs, from hours +2 to +5, may be misleading. A possible explanation of the increase, which occurred after MCS maximum extent, is

as follows. During this period, the gust front for category 1MCSs was

still well-defined and continued to propagate away from its initial

formative location, thus affecting an increasing number of weather

stations. Therefore, at hour +5, the surface divergence associated with

the gust front/mesohigh was calculated from a greater number of reliable

winds, compared to the calculation at hour +2. This could have led to

an apparent increase in the magnitude of the surface divergence, as calculated by the MclDAS. Divergence values as large as I0 "4 s-1 have been

measured in a mesonetwork of instruments (Ulanski and Garstang, 1978).

The most significant result of the composite analysis is that at hour

+5, the magnitude of the maximum surface divergence associated with

category 1MCSs was much larger than the magnitude of the maximum surface divergence associated with category 2 MCSs.

Fig. 48 depicts the composite analysis of the sum of the three

largest precipitation rates. There was a distinct difference between

the precipitation rates of the two categories. With the exception of
94

).56)-

3.25.

3.00.

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R

X 2.75.

S

U

R 2.5D.

F

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E 2.25-

D

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N

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E

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1 .00.

0.75.

O .50-

0.25.

0 .DO.

I

I

I

I

I

!

-2 -l

I I .... I

I

NUMBER OF HOURS FROM MC5 M_XIMUM EXTENT

Fig. 47. Composite of the maximum hourly surface divergence of

category l (thin line) MCSs and category 2 (thick line) MCSs. Minus (-)

hours are before MCS maximum extent and plus (+) hours are after MCS

maximum extent. Divergence values have units of lO-s s -I.
95

2 "_5-

2 50"

5

U 2 25-

M

O

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2 OD3

I

N O 75-

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E

R

0 50:

H

R

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I

I

I

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-4 -3 -2 -I 0 ] 2 3 4 5 6 ?

NUMBER OF HOURS FROM MCS MCXIMUM ZXTENFig. 48. Composite of the sum of the three largest precipitation

rates of category l (thin line) MCSs and category 2 (thick line) MCSs.

Minus (-) hours are before MCS maximum extent and plus (+) hours are

after MCS maximum extent.
96

hour -5, the sum of the three largest precipitation rates was significantly higher for category 1 MCSs. In fact, in the 8 h of heaviest

precipitation, from -4 to +3, the average sum of the three largest precipitation rates of category 1MCSs was more than double that of category 2 MCSs (2.13 in h-I to 1.03 in h-l). This would imply that category 1MCSs produced significantly higher rainfall rates, over an area,

than the MCSs of category 2.

The intense precipitation associated with category 1MCSs persisted

for a longer period, compared to the heaviest precipitation associated

with the MCSs of category 2. The sum of the three largest precipitation

rates, produced by the MCSs of Cases 2, 3, 4, 7, 8, I0 and II (category

1MCSs), stayed above 1.5 in h-I for 8, 6, II, 6, 7, 5 and 5 h, respectively. However, the sum of the three largest precipitation rates, produced by the MCSs of Cases I, 5, 6 and 9 (category 2 MCSs), stayed above

1.5 in h-I for I, I, 3 and 3 h, respectively. Thus, the sum of the

three largest precipitation rates stayed above 1.5 in h-I for an average

of almost 7 h for category 1 MCSs and only 2 h for category 2 MCSs.

Therefore, category 1MCSs produced more intense precipitation rates

over an area, for a longer period, compared to category 2 MCSs.

Fig. 49 shows the results of the composite analysis for the maximum

hourly point precipitation rates. This analysis verifies that category

1MCSs produced, on average, more intense rainfall rates than category 2

MCSs. As seen in Fig. 49, the higher precipitation rates of category 1

MCSs were maintained throughout the entire time period of the composite

analysis. In the 8 h of heaviest precipitation, from -4 to +3, the

average maximum point precipitation rate of category 1MCSs was almost
97

R

A

I

N O.6-

!

N 0.4-

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E

R

0.3-

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R

0.]-

I

0

"07," , I , , ,

-5 -4 -3 -2 -1 0 ! 2 3 4 5 6 7

NUMgER OF HOUR5 --ROM MC5 M_XIMUM EXTENT

Fig. 49. Composite of the maximum hourly point precipitation rates

of category 1 (thin line) MCSs and category 2 (thick line) MCSs. Minus

(-) hours are before MCS maximum extent and plus (+) hours are after MCS

maximum extent.
98

double that of category 2 MCSs (I.01 in h -I to 0.57 in h-l).

Fig. 50 represents the results of the composite analysis for the

coldest hourly cloud top temperatures. Category l MCSs, on average, had

colder cloud top temperatures, compared to the MCSs of category 2.
99

C

0 68-

L

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E

S

T 6"7.

C

L

0

U 66-

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-[

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P 64.

E

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6D

I

I I _ I I I

-3 -2 -I 0 1 2 3

NUMBER OF HOURS FRDM MCS MRXIMUM EXTENT

Fig. 50. Composite of the coldest hourly cloud top temperatures

for category l (thin line) MCSs and category 2 (thick line) MCSs. Minus

(-) hours are before MCS maximum extent and plus (+) hours are after MCS

maximum extent.
I00

CHAPTER V

SUMMARY

Category 1 and 2 MCSs have significant differences in their characteristics. This contention is supported by the results of the composite

analyses. It appears these differences aid in producing stronger,

longer-lasting gust fronts in category 1MCSs. In turn, since all category 1MCSs produced arc clouds and only one category 2 MCS produced an

arc cloud (see Table 14, page 88), the strength and duration of the gust

front seems to be an important factor in whether an arc cloud forms.

A summary of the differences in the category 1 and 2 MCSs follows:

I. Category 1MCSs, on average, produce more intense precipitation

rates than category 2 MCSs. In the hours of heaviest precipitation, the

average sum of the three largest precipitation rates of category 1MCSs

is more than double that of category 2 MCSs. For this study, this

average sum for category 1 and 2 MCSs was 2.13 in h"I to 1.03 in h-I,

respectively. During this same period, the average maximum point precipitation rate of category 1MCSs is nearly double that of category 2

MCSs. For this study, the average maximum point precipitation rate for

category 1 and 2 MCSs was 1.01 in h-I to 0.57 in h-I, respectively.

Also, the intense precipitation associated with category 1MCSs persists

for a longer period, compared to the heaviest precipitation associated

with the MCSs of category 2. For instance, the sum of the three largest

precipitation rates stayed above 1.5 in h -I for an average of almost 7 h

for category 1 MCSs and only 2 h for category 2 MCSs.

2. The maximum hourly surface divergence, on average, associated
101

with category 1MCSs, is stronger than the divergence associated with

category 2 MCSs, for the hours +2 to +5.

3. In the hours from -2 to -6, the critical hours for gust front

formation, category l MCSs, on average, have higher maximum thunderstorm

heights than category 2 MCSs, as determined at 35 min past each hour.

During this period, the average maximum thunderstorm heights for category l and 2 MCSs are 55,000 and 51,000 ft, respectively.

4. The coldest hourly cloud top temperatures of category l MCSs,

on average, are colder than those in category 2.

The gust front, on average, formed approximately 1.5 h prior to the

MCS maximum extent, as determined from Table 14. Using this information, coupled with the results shown in Fig. 46-50, for category l MCSs,

the following becomes apparent:

I. The gust front, on average, forms approximately 2 h after the

maximum thunderstorm heights are reached (see Fig. 46).

2. The time of formation of the gust front corresponds to the

period of maximum precipitation intensity (see Figs. 48-49).

3. The formation of the gust front takes place during a period of

increasing magnitude of the surface divergence (see Fig. 47). The increasing surface divergence corresponds to the period of maximum precipitation intensity (see Figs. 48-49). This correlation (see Fig. l)

agrees with the findings of Byers and Braham (1949).

4. The formation of the gust front corresponds to a decrease in

the coldest cloud top temperatures (see Fig. 50).

The arc cloud, on average, formed approximately 4 h after MCS maximum extent, as determined from Table 14. As mentioned earlier, the
102

appearance of the arc cloud was determined, in most cases, by inspection

of the visible satellite images. Undoubtedly, some of the arc clouds

formed during the period when only IR satellite images were available.

During this period the arc cloud would be undetectable, in most cases,

because arc clouds normally consist of low, warm clouds. Taking this

into account, the arc cloud, on average, probably formed approximately

3 h after MCS maximum extent. Using this information, coupled with the

results shown in Figs. 46-50, for category 1MCSs, the following becomes

apparent:

I. The arc cloud forms during a period of rapidly falling thunderstorm heights (see Fig. 46).

2. The arc cloud forms as the precipitation intensity dramatically

lowers (see Figs. 48-49).

3. The arc cloud forms during a period when the magnitude of the

maximum surface divergence remains large (see Fig. 47).

4. The arc cloud forms during a period of rapid warming of the

cloud top temperatures (see Fig. 50).

Using the radar summary charts from the NWS, a composite analysis

of the maximum hourly radar reflectivity intensity for category 1 and 2

MCSs was accomplished. The determination of intensity was done by an

intensity processor known as the Digital Video Integrator and Processor

(D/VIP). Contours on the radar summary charts correspond to the I, 3

and 5 D/VIP levels. D/VIP level 1 corresponds to "light" precipitation,

D/VIP level 3 corresponds to "heavy" precipitation and D/VIP level 5

corresponds to "intense" precipitation (U.S. Department of Commerce,

1980). This analysis was done because the results could be used by
I03

operational meteorologists, since this information is readilyavailable.

Fig. 51 depicts the results of the composite analysis of the maximum hourly radar reflectivity intensity. From hours -5 to -2, both

category l and 2 MCSs produced D/VIP level 5 radar reflectivities. However, from hours -2 to +l, the maximum reflectivities produced by category 2 MCSs dropped below D/VIP level 4, on average, while category l

MCSs maintained D/VIP level 5.

Using the above information, the following criteria are given as

guidance, to the operational meteorologist, in forecasting the occurrence of gust fronts and arc clouds:

I. The gust front, on average, forms approximately 1.5 h before

maximum extent of the MCS.

2. If the gust front persists for more than 6 h, it is likely an

arc cloud will form. The arc cloud, on average, forms approximately 3 h

after maximum extent of the MCS.

. The arc cloud forms while the thunderstorm heights are rapidly

fal Iing.

4. The arc cloud forms while the coldest cloud top temperatures

are rapidly warming.

5. Lastly, it seems likely that an arc cloud will form if the

maximum D/VIP level remains at level 5 through hour +l (l h after MCS

maximum extent). Caution should be used when using this criterion

alone, as the MCS of Case l, which did not produce an arc cloud, produced D/VIP level 5 reflectivities through hour +3. However, using this

criterion with the other guidelines, should prove more useful. For example, even though the MCS of Case l produced D/VIP level 5
104

5.0 =

4.8-

M

A

x 4.6-

R

n

D

R 4.4-

R

R

E

F 4.2-

L

E

[

T

I 4.0.

V

I

T

Y

3.8-

D

/

V 3.6-

I

P

L

E 3.4-

v

E

L

3-2-

3.0'

I

I

I

I

I

I

i

-S -4 -3 -2 -I 0 1 2 3 4

NUMBER OF HOURS FROM MCS MRXIMUM EXTENT

Fig. 51. Composite of the maximum hourly radar reflectivity intensity of category 1 (thin line) MCSs and category 2 (thick line) MCSs.

Minus (-) hours are before MCS maximum extent and plus (+) hours are

after MCS maximum extent.
105

reflectivities through hour +3, the gust front only lasted for 5 h,

therefore, it is unlikely an arc cloud will form.
106

REFERENCES

Barnes, S. L., 1973: Mesoscale objective map analysis using weighted

time-series observations. NOAA Tech. Memo. ERL NSSL-62, NTIS COM73-I0781, 60 pp.

Bartels, D. L., 1983: Internal structure and evolution of a dual mesoscale convective complex. Preprints, 5th Conf. on Hydrometeorology, Tulsa, OK, Amer. Meteor. Soc., 97-102.

Belville, J. D., and N. O. Stewart, 1983: Extreme rainfall events in

Louisiana: The "New Orleans Type." Preprints, 5th Conf. on Hxdrometeorology, Tulsa, OK, Amer. Meteor. Soc., 284-290.

Brundidge, K. C., 1983: Investigation of the arc cloud complex. Research Reports - 1983 NASA/ASEE Summer Faculty Fellowship Program,

George C. Marshall Space Flight Center, Huntsville, AL, 23 pp.

, 1984: Precipitation factors leading to arc cloud formation.

NASA Proposal, Texas A&M University, College Station, TX, 14 pp.

Byers, H. R., and R. R. Braham, Jr., 1948: Thunderstorm structure and

circulation. J. Meteor., 5, 71-86.

, and , 1949: The Thunderstorm. U.S. Dept. of Commerce,

Weather Bureau, Washington, D. C., 282 pp.

Fritsch, J. M., R. A. Maddox and A. B. Barnston, 1981: The character of

mesoscale convective complex precipitation and its contribution to

warm season rainfall in the U.S. Preprints, 4th Conf. on Hydrometeorology, Reno, NV, Amer. Meteor. Soc., 94-99._

Fujita, T., 1955: Result of detailed synoptic studies of squall lines.

Tellus, 7, 405-436.

, 1959: Precipitation and cold air production in mesoscale

thunderstorm systems. J. Meteor., I_66,454-466.

, 1963: Analytical mesometeorology: A review. Meteor. Monogr.,

-5, 77-122.

Gagin, A., D. Rosenfeld and R. E. Lopez, 1985: The relationship between

height and precipitation characteristics of summertime convection

cells in south Florida. J. Atmos. Sci., 4__22,84-94.

Goff, R. C., 1976: Vertical structure of thunderstorm outflows. Mon.

Wea. Rev., I04, 1429-1440.

Greene, G. E., 1977: Wind shear characterization. U.S. Fed. Aviation

Admin., Washington, D.C., Report No. FAA-RD-77-33, 120 pp.
107

Howard, K., and R. A. Maddox, 1983: Precipitation characteristics of

two mesoscale convective systems. Preprints, 5th Conf. on Hydrometeorology, Tulsa, OK, Amer. Meteor. Soc.

Leary, C. A., and E. N. Rappaport, 1982: Internal structure of a mesoscale convective complex. Preprints, 21st Conf. Radar Meteor.,

Edmonton, Alberta, Canada, Amer. Meteor. Soc., 70-77.

Maddox, R. A., 1980: Mesoscale convective complexes. Bull. Amer.

Soc., 61, 1374-1387.

, 1981: The structure and lifecycle of midlatitude mesoscale convective complexes. Ph.D. dissertaion, Colorado State University,

171 pp.

, 1982: Forecasting mesoscale convective complexes over the

central United States. Preprints, 12th Conf. Severe Local Storms,

San Antonio, TX, Amer. Meteor. Soc., 180-183.

, 1983: Large-scale meteorological conditions associated with midlatitude, mesoscale convective complexes. Mon. Wea. Rev., Ill,

1475-1493.

, D. M. Rodgers and K. W. Howard, 1982: Mesoscale convective complexes over the United States during 1981. Mon. Wea. Rev., llO,

1501-1514.

Miller, R. C., 1972: Notes on Analysis and Severe-Storm Forecasting

Procedures of the Air Force Global Weather Service, Tech. Rep. 200

(Rev.), Air Weather Service, Scott Air Force Base, IL, 190 pp.

Miller, R. L., 1984: The arc cloud complex: A case study. Masters

Thesis, Texas A&M Univ., College Station, TX, lOl pp.

Orlanski, I., 1975: A rational subdivision of scales of atmospheric

processes. Bull. Amer. Meteor. Soc., 56, 527-530.

Purdom, J. F. W., 1973: Picture of the month: Mesohigh and satellite

imagery. Mon. Wea. Rev., lOl, 180-181.

,.1974: Satellite imagery applied to the mesoscale surface analysls and forecast. Preprints, 5th Conf. Weather Forecastin 9 and

Analysis, St. Louis, MO, Amer. Meteor. Soc., 63-68.

, 1976: Some uses of high-resolution GOES imagery in the mesoscale

forecasting of convection and its behavior. Mon. Wea. Rev., I04,

1474-1483.
108

Purdom, J. F. W., 1979: The developmentand evolution of deep convection. Preprints, llth Conf. Severe Local Storms, Kansas City, MO,

Amer. Meteor. Soc., 143-150.

Rodgers, D. M., K. W. Howard and E. C. Johnston, 1983: Mesoscale convective complexes over the United States during 1982. Mon. Wea.

Rev., Ill, 2363-2369.

, M. J. Magnano and J. H. Arns, 1985: Mesoscale convective complexes over the United States during 1983.. Mon. Wea. Rev., ll3,

888-901.

Schnapf, A., 1982: The development of the TIROS global environmental

satellite system. NASA Conf. Pub. 2227, Meteorological SatellitesPast, Present, and Future, 20th Aerospace Sciences Meeting, AIAA,

Orlando, FL, Jan., 1982, 7-16.

Sinclair, P. C., and J. F. W. Purdom, 1982: Integration of research

aircraft data and 3 minute interval GOES data to study the genesis

and development of deep convective storms. Preprints, 12th Conf.

Severe Local Storms, San Antonio, TX, Amer. Meteor. Soc., 269-271.

, and , 1984: Aircraft penetrations of arc cloud lines.

Preprints, Satellite Meteorology/Remote Sensin 9 and Applications,

Clearwater Beach, FL, Amer. Meteor. Soc., 160-163.

Tepper, M., 1950a: A proposed mechanism of squall lines: The pressure

jump line. J. Meteor., _, 21-29.

, 1950b: On the origins of tornadoes.

3_]_I,311-314.

Bull. Amer. Meteor. Soc.,

Ulanski, S. L., and M. Garstang, 1978: The role of surface divergence

and vorticity in the life cycle of convective rainfall. Part I:

Observation and analysis. J. Atmos. Sci., 35, I047-I062.

U. S. Department of Commerce, 1980: National Weather Service Radar Code

User's Guide, NOAA, National Weather Service publication, Silver

Spring, Maryland, 179 pp.

Wetzel, P. J., W. R. Cotton and R. L. McAnelly, 1983: A long-lived

mesoscale convective complex. Part I: The mountain generated component. Mon, Wea. Rev., ll__]_l, 1893-1918.

Woodley, W. L., B. Sancho and A. H. Miller, 1972: Rainfall Estimation

from Satellite Cloud Photographs. NOAA Tech. Memo. ERL OD-ll, 43

PP.

Zipser, E. J., 1969: The role of organized unsaturated convective downdrafts in the structure and rapid decay of an equitorial disturbance. J. Ap__pl.Meteor., 8, 799-814.
h/87- 18948

CONVECTIVE CELL DEVELOPMENT AND PROPAGATION

IN A MESOSCALE CONVECTIVE COMPLEX

Yoo-Shin Ahn

and

Kenneth C Brundldge

Texas A & M University

College Station, Texas

Prepared for George C. Marshall Space Flight Center

under Contract NAG 8-043
ii

ABSTRACT

A case study was made of the mesoscale convective complex (MCC)

which occurred over southern Oklahoma and northern Texas on 27 May 1981.

This storm moved in an eastsoutheasterly direction and during much of

its lifetime was observable by radars at Oklahoma City, OK. and

Stephenville, TX. It was found that the direction of cell (VIP level 3

or more reflectivity) propagation was somewhat erratic but approximately

the same as the system (VIP level i reflectivity) movement and the

ambient wind. New cells developed along and behind the gust front

making it appear that once the MCC is initiated, a synergistic

relationship exists between the gust front and the MCC. The

relationship between rainfall patterns and amounts and the infrared (IR)

temperature field in the satellite imagery were examined. The 210°K

isotherm of GOES IR imagery was found to encompass the rain area of the

storm. The heaviest rainfall was in the vicinity of the VIP level 3

cells and mostly contained within the 205°K isotherm of GOES IR imagery.
iii

ACKNOWLEDGEMENTS

The authors are grateful the support for this work provided by the

George C. Marshall Space Flight Center under Grant NAG. 8-043.
iv

ABSTRACT--

TABLE OF CONTENTS

Page

li

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

I,

II.

III.

IV.

V.

VI.

Vll.

Vlll.

INTRODUCTION

RESEARCH PROCEDURES

SYNOPTIC CONDITIONS AND STORM HISTORY ...........

STORM MOTION

NEW CELL DEVELOPMENT ..................

RADAR DATA DETAILS

COMBINATION OF SATELLITE IMAGERY AND RADAR OBSERVATION

CONCLUSIONS AND RECOMMENDATIONS

REFERENCES

ill

iv

v

vl

1

7

12

19

30

33

43

50

53
v

TABLE

1

2

3

LIST OF TABLES

Mesoscale convective complex (MCC) definition

Description of manually digitized radar (MDR) code,

Characteristics associated with the MCC of 27 May 1981

Page

3

8

18
vl

FIGURE

i

2

3

4

5

6

7

8a

8b

LIST OF FIGURES

Schematic models of multi-cell storm propagation

The climatological network of raingauges in Texas

The climatological network of raingauges in OklahomaSynoptic analysis at 70 kPa for 0000 GMT, 27 May 1981

Synoptic analysis at 50 kPa for 0000 GMT, 27 May 1981

Surface synoptic analysis at 1400 GMT. 27 May 1981

GOES visible image for 1500 GMT, 27 May 1981

VIP level I cell outlines at hourly intervals

from 0000 to 0800 GMT .................

VIP level i cell outlines at hourly intervals

from 0901 to 1558 GMT ..........

9a VIP level 3 cell outlines at 20 mln intervals

from 0120 to 0700 GMT--

9b VIP level 3 cell outlines at 20 mln intervals

from 0721 to 1240 GMT ........

i0 Hodographs at 1200 GMT for OKC (solid) and SEP

(dashed)

ii

12

13

Page

5

9

i0

13

14

16

17

20

21

23

24

26

VIP level i cell centrold positions at hourly

intervals.from 0000 to 1558 GMT .........

Gust front and VIP level 3 cell positions at hourly

intervals.

28

31

Combined VIP level I and level 3 radar echoes at

hourly intervals from 0100 to 1558 GMT ...................... 34
vii

14 Simultaneous views of satellite imagery and radar

echoes at 0700 GMT ........

15 Simultaneous views of satellite imagery and radar

echoes at 0800 GMT .........

16 Simultaneous views of satellite imagery and radar

echoes at 0900 GMT .............

17 Simultaneous views of satellite imagery and radar

echoes at i000 GMT ...........

18 Simultaneous views of satellite imagery and radar

echoes at 1200 _T ........

44

46

47

48

49
i

I.INTRODUCTION

For many years, meteorologists had believed that thunderstorms

occurred in two basic forms: air mass storms represented by isolated

cells and squall lines involving manycells organized into a line

However, as pointed out by Purdom(1979), the advent of the

Geostationary Operational Environmental Satellite (GOES)has enabled

meteorologists generally to see a variety of atmospheric events not

distinguishable by ordinary synoptic observations. In particular, a

third modeof thunderstorm activity has been recently discovered. It

has been found that many summertimestorms are organized aggregations of

thunderstorm cells which Maddox(1980b) has called mesoscale convective

complexes (MCCs).

Through examination of satellite imagery it has been found that

thirty or more MCCsmayoccur in the United States during the months

March through September(Maddox, 1980b; Maddoxet al._ 1982; Merritt and

Fritsch, 1984; Rodgerset al., 1983; Rodgers et al., 1985). These

systems may produce severe weather such as hail, tornadoes, damaging

winds and flash flooding (Bosart and Sanders, 1981); however, they also

have been found to be the main source of summertimerainfall over the

crop lands of the mid-west (Fritsch et al., 1981).

MCCs have a distinctive appearance in satellite imagery. They

appear as a nearly circular mass of cloud covering an area of i00,000 km 2

or more and at a temperature less than or equal to -32°C. The complete

definition of an MCC is presented in Table I. Whereas they may start in
2

the afternoon or evening MCCs primarily are nocturnal systems which

reach maximum intensity sometime after midnight and may not dissipate

until after sunrise (Maddox, 1980a). Frequently during the dissipating

stage, the MCC produces an arc cloud (Purdom, 1973, 1976; Gurka, 1976)

which moves outward from the MCC and may play a role in new storm

development the following day.

The problem of understanding the movement and internal dynamics of

thunderstorms and thunderstorm systems has been the subject of much

research of the years Byers and Braham (1949) found that small radar

echoes moved with the cloud layer-mean wind. However, Zehr and Purdom

(1982) did not find this wind to be a reliable indicator of storm

motion. Newton and Fankhauser (1975) stuied storm motlon as related to

storm size They found that small- to medium-sized storms generally

move to the left of the mean vector wind in the troposphere while large

storms with diameters of about 20-30 km generally move to the right of

the mean wind.

Observational studies which have been made of multi-cell systems

(Fujita and Brown, 1958; Heymsfleld and Schotz, 1985; Marwitz, 1972)

have shown that such systems simultaneously move and maintain themselves

by a process called "discrete propagation". In this process, new cells

develop on the right flank of the storm area and move toward the left

flank where they dissipate. This produces a complex of cells which as a

whole moves to the right of the individual cell motion. Figure i shows

three distinct propagation models as described by Marwitz (1972) for

three different multi-cell storm systems. In Fig. la, the individual
3

TABLEi. Mesoscale Convective Complex (MCC) definition.

The definition is based upon analysis of enhanced IR

satellite imagery (after Maddox, 1980b).

Physical Characteristics

Size : A: Cloud shield with IR temperature _ -32°C must

have an area _ i00,000 km 2

B Interior cold cloud region with temperature

-52°C must have an area _ 50,000 km2

Initiate: Size definitions A and B are first satisfied

Duration: Size definitions A and B must be met for a period

6 h

Maximum

extent:

Contiguous cold cloud shield (IR temperature _ -32°C)

reaches maximum size

Shape : Eccentricity (minor axis/major axis)_ 0.7 at time

of maximum extent

Terminate: Size definitions A and B are no longer satisfied
whole cells, represented by ovals, propagate continuously in the

direction of the meanwind coupled with discrete propagation to the

right of the meanwind. The cells in Fig. ib propagate both

continuously and discretely to the right of the meanwind, which

provides a large angular departure of storm motion from the meanwind.

The third possibility in Fig. ic is one in which continuous propagation

is to the left of the meanwind but the offsetting discrete propagation

to the right results in overall storm motion in the samedirection as

the meanwind. Numerical simulation studies such as those by Miller

(1978) and Wilhelmson and Chen(1982) also show the tendency for

discrete propagation to control storm motion. As pointed out by

Wilhelmson and Chen, how the individual cells moverelative to the mean

vector wind in the cloud layer bears on the rate at which moisture can

be supplied to them for continued growth.

The studies of multi-cell systems cited above did not involve the

use of satellite imagery. Therefore, it is not clear to what extent the

results may be applicable to MCCswhich, as noted before are defined on

the basis of satellite imagery using criteria developed by Maddox

(1980b). MCCsare large enoughin scale to generally encompassseveral

observing sites at any given time; however, the internal processes are

sub-synoptlc and therefore not easily determined It is safe to say

that these processes are poorly understood. Several questions maybe

asked:

a. Howare MCCsmaintained during the nighttime hours when no

surface heating is occurring to destabilize the boundary layer?
5 

a 

STORM MOTION 

b 

‘\TOW 

- MOTfON 

C 

Fig. 1. Schematic models of multi-cell storm propagation (from Marwitz, 

1972). I 
6

b. Do new cells develop on the interior of an MCC in a random

fashion at the intersections of outflow boundaries, as occurs with air

mass storms? Or is cell development related to the meso-_ scale outflow

boundary of the system as a whole?

c. How do the movement of individual cells and the movement of the

system as a whole relate to one another and to the mean cloud-layer

wind?

At present, answers to these questions can only be inferred from

inadequate observations. Maddox and Howard (1983) have urged the use of

satellite imagery and radar observations in combination to obtain a

better understanding of the precipitation structure of mlddle-latltude

mesosystems, to improve understanding of the life cycle of MCCs as an

aid to forecasting them, and to provide better estimates of rainfall

patterns and amounts based on cloud-top IR temperature structure The

study described here was this approach to focus on the problem areas

described above in a case study of the MCC which occurred over southern

Oklahoma and northern Texas on 27 May 1981. This storm moved in a

southeasterly direction and during much of its lifetime was observed

simultaneously by radars at Oklahoma City (OKC), OK. and Stephenville

(SEP), TX.
II. RESEARCHPROCEDURES

The data used and the procedures followed in this study are as

follows.

a Radar films for 27 May 1981 for the OKC and SEP radar sites

were obtained from the National Climatic Data Center, Asheville, North

Carolina. These films, with frames at one minute intervals, were

examined to determine the history of cell development and movement. For

this purpose a "cell" was defined to be a reflectlvity region at a Video

Integrator Processor (VIP) level of 3 or more. The system as a whole

was defined as all the area enclosed by the VlP level i reflectivlty

value. The relatlonships between the manually digitized radar (MDR)

code, which is used for the hourly radar summary maps provided by the

Natlonal Weather Service (NWS), VIP levels, and storm intensity levels

are shown in Table 2

b. The rawinsonde soundings at OKC and SEP at 1200 GMT were used

to determine the mean wind structure during the mature stages the storm.

c The records of hourly precipitation from the climatological

networks of ralngauges were obtained from the National Climatic Data

Center. Figures 2 and 3 show the locations of the recording ralngauges

in Texas and Oklahoma, respectively. These data were plotted and

analyzed on the state maps.

d. Sectlonal surface maps corresponding to the time and location

of the MCC were plotted and analyzed. Microbarograph traces and Weather
TABLE2. Description of Manually Digitized Radar (MDR)Code

(After Foster and Reap 1973).

Maximum Coverage Maximum

Code No. Observed In Box Rainfall

VIP Values Rate (in./hr.)

Intensity

Category

0 No Echoes

i i Any Vip 1 •1

2 2 --_50% of VIP 2 .i- .5

3 2 > 50% of VIP 2 .5-1.0

4 3 __ 50% of VIP 3 1.0-2.0

5 3 > 50% of VIP 3 1.0-2.0

6 4 _ 50% of VIP 3 and 4 1.0-2.0

7 4 > 50% of VIP 3 and 4 1.0-2.0

8 5 or 6 __ 50% of VIP 3,4,5, 2.0

and 6

9 5 or 6 > 50% of VIP 3,4,5, 2.0

and 6

Weak

Moderate

Moderate

Strong

Strong

Very Strong

Very Strong

Intense or

Extreme

Intense or

Ex treme
9

Fig. 2. The climatological network of rainguages in Texas•
i0

!

Fig. 3. The climatological network of rainguages in Oklahoma•
ii

Bureau/Army/Navy (WBAN)records from stations in Oklahoma and northern

Texas obtained from the National Climatic Data Center were utilized in

this analysis. Gust front position and propagation were more accurately

determined with this information and related to radar echo structure and

precipitation patterns.

e The Man-Computer Interactive Data Access System (MclDAS) of the

Atmospheric Sciences Division of the Systems Dynamic Laboratory,

National Aeronautics and Space Administration Marshall Space Flight

Center, was used to obtain hourly, contoured analyses of the cloud top

temperature as determined from the GOES IR imagery. This information

was combined with the raingauge data.
12

III. SYNOPTICONDITIONSANDSTORM HISTORY

The MCC of 27 May 1981 started with isolated thunderstorms over the

Texas and Oklahoma panhandles on the afternoon of 26 May. These storms

grew in size and intensity and amalgamated. During the evening of the

26th (0100 to 0340 GMT, 27 May) there were two reports of funnel clouds

and four reports of tornadoes preceeded by golfball-sized hail

associated with this storm. These reports came from Briscoe, Motley and

Cottle Counties in northwestern Texas very near the Oklahoma border.

Shortly before midnight (0515 GMT, 27 May 1981) the storm system

met the criteria for an MCC stated in Table I. By this time the center

of the storm lay roughly on the Red River between OKC and SEP. The

maximum extent of the MCC, defined in terms of the area enclosed by the

210 K isotherm depicted in the IR satellite imagery was reached at i000

GMT on 27 May Reports of hail came from the northern Texas Archer and

Tarrant Counties at 0900 GMT and 1055 GMT, respectively. A windstorm

was reported in Fannin County at 0935 GMT.

The termination of the storm as an MCC came at 1400 GMT in the

northeastern corner of Texas; however, the system continued to produce

precipitation until 1600 GMT.

Figures 4 and 5 show the 70 kPa and 50 kPa analyses for 0000 GMT,

27 May 1981. There is evidence, particularly at 70 kPa of a weak,

short-wave trough moving through the long-wave ridge over the area in

which the MCC developed. Gurka (1976) found that many summertime,

mesoscale systems producing gust fronts observable as arc clouds in
\

ORIGINAL PAGE 15

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satellite imagery formed in short-wave troughs imbedded in northwesterly

flow aloft. The low level warm advection indicated in Fig. 4 may have

contributed to the initiation of the MCC. A mesoscale warm pocket is

also seen over the area in the 50 kPa analysis.

At 0600 GMT, a mesohigh and strong gust front first appeared in the

surface pressure analysis. The gust front, which will be examined

later, was located across the Oklahoma/Texas border and was moving

southeastward. As it passed Dallas (DAL), TX., a peak wind of 45 kt was

one of the indicators that the gust front was quite powerful. Even near

termination of the MCC, at 1400 GMT, a strong mesohigh and gust front

were still evident in the surface pressure analysis (see Fig 6).

The GOES visible image for 1500 GMT is shown in Fig. 7. Only

wide-spread layer-cloud precipitation was occurring at this time. The

arc cloud produced by this MCC can be seen in Fig. 7 to be approaching

the Gulf Coast of eastern Texas. The arc cloud finally vanished from

the satellite images after 1800 GMT.

Other characteristics of this MCC as determined by Welshinger

(1985) are shown in Table 3.
\

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18

TABLE 3. Characteristics associated with the MCC of 27 May 1981.

The maximum thunderstorm heights were determined at approximately 35 min

past each hour. Dashes indicate that the parameter was not determined

(Welshinger, 1985).

Time Maximum

(GMT) thunderstorm

height

(x I00 ft)

Maximum Sum of _he Maximum Coldest

surface three largest point cloud top

divergence precipitation precipitation temperature

(x 10-5 s) rates (in/h) rate (in/h) (°C)

0400 - 0.60 0.50

520

0500 - 1.00 0.50

550

0600 3 1.00 0.70

580

0700 3 0.73 0.43

560

0800 2 1.59 1.30

520

0900 2 1.50 0.60

550

1000 2 2.70 0.99

480

1100 2 2.93 1 13

470

1200 2 2.50 0.90

320

1300 2 1.61 1 01

25O

1400 3 0.60 0.20

160

1500 3 0.60 0.20

m

1600 - 0.10 0.10

-73

-68

-73

-68

-68

-68
19

IV. STORM MOTION

Figures 8a and b show time sections of the radar echoes at VIP

level i covering the period from 0000 to 1558 GMT 27 May 1981 at I h

intervals. This is a composite representation using data obtained

simultaneously at OKC and SEP. Due to the elevation angle and the

position of the storm between the two radars, data from the southern

part and northern part were attenuated for OKC and SEP, respectively

Thus, compositing was necessary in order to get the complete picture.

The displacement of the echoes seen in the figure are in part real and

in part due to the analysis technique. The radar echoes at 60 mln

intervals were plotted with respect to the radar station located on the

upper sloping line which was oriented from 295 to 115 ° . After the

echoes at a given time were plotted, the station was shifted 20 n mi

along this llne, and the procedure was then repeated. The station

positions and the echoes were labeled with letters related to the time

of the observation, as indicated in the figure. Sloping solid straight

lines were then drawn connecting individual cells and conglomerations of

cells in time. The hatching of the system at 1458 and 1558 GMT was done

simply for better identification because it was slow and very small at

these times. It must be stated that not all the VIP level 1 echoes that

existed at each of the times given in Fig. 8a and b are shown. Only

those associated with the main storm area and which existed for at least

1 h were considered.

The time sequence of zhe echoes reveals interesting features that
20 

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0000 mmba 

0000 

a 

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0) 

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03 

M 

rl 

CLl 

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rl 

2 

2 

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rl 

CLl 
22

might not be detected with a casual study of the individual echo

photographs. It may be noted how the VIP I echoes grew in size and

merged as the time approached 0515 GMT, the time at which the storm

qualified as an MCC. Examination of the figure also indicates the

movement of the VIP level i echoes varied somewhat from time to time

both in direction and speed. However, overall the movement was in the

same direction as that chosen for the displacement of the radars, from

295 a at a speed of ii ms-_

Time sections of the VIP level 3 or more radar reflectivity,

covering the period from 0120 to 1240 GMT 27 May 1981 are shown in Fig.

9a and b. The radar echoes at 20 min intervals were plotted by again

using a moving coordinate system with a 20 n mi displacement. The

station and the echoes were labeled with letters related to the time of

the observation. Somewhat erratic cell motion is seen in this figure in

that some of these cells moved to left of the storm motion which others

moved to the right. Whereas in general, the VIP 3 cells moved at about

the same speed as the overall storm (VIP level I) some cells (hatched

in Fig. 9b) which developed starting a 0821GMT moved at 20 m_L When

they first appeared, they were located on the west side of the system.

By 1240 GMT, they were almost at the middle, southern part of the

system.

A careful examination of Fig. 9 also reveals that the appearance

of new cells, or vanishing of old cells, did not take place in a

consistent fashion. For example, new cells appeared on the north side

of the developing system at 0341 GMT, 0400 GMT and 0440 GMT. Some of
oogo g 0 0 0

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25

these eventually dissipated. Likewise, new cells appeared on the south

side of the system at 0520 GMT, 0600 GMT and 0620 GMT Again, some of

these cells dissipated. New cell development also took place to the

rear of the primary convective line, starting around 0821 GMT. Thus a

consistent pattern of cell development on one side of the storm and

dissipation on the other side, such as found by Marwitz (1972) did not

occur in this case.

The mean wind flow in the vicinity of storm was determined from

1200 GMT rawinsonde measurements at OKC and SEP. This observation time

was chosen because it was closest to the time of maximum extent of the

MCC, i000 GMT.

Figure i0 is a hodograph plot of the winds at OKC and SEP at the

85, 70, 50, 30 and 20 kPa levels. The average propagation vector of the

storm (and the VIP level 3 cells), from 295 at Ii ms", also is

indicated. This speed is moderate compared with some reported in the

literature (Maddox and Howard, 1983).

The mean cloud -layer wind vector and mass-weighted mean wind

vector also are shown in Fig. i0. The mean cloud-layer wind was

calculated by first determining the mean vector wind for each station

over the layer from 85 to 20 kPa, and then averaging the resulting two

vectors. The mass-welghted wind was calculated in the same way but

using the relationship

wherep is density and the summations extended from 85 to 20 kPa. It is
_00000

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27

seen that the mass-weighted vector has the same direction as the mean

cloud-layer wind vector but is smaller in magnitude. On the other hand,

the mean cloud layer wind matches very well with the storm vector,

allowing for errors in wind observations and determination of echo

displacements. Zehr and Purdom (1982) found that the direction of the

mean cloud layer wind is usually very similar to the 50 kPa wind. It

can be seen that this match is close for SEP but not as good at OKC.

Likewise, the i km-50 kPa shear vector found as an indicator of the MCC

motion by McAnelly and Cotton (1986)does not work well for this MCC.

The observed winds at OKC and SEP in comparison with the storm

motion show that the relative motion of the air at and below 70 kPa was

into the storm from the front and right flanks; this continuous supply

of low-level moist air no doubt contributed to the maintenance of the

storm. At and above 50 kPa, dryer environmental air was entering the

system from the left and rear flanks.

Figure ii again shows the VlP level i echoes; however, this

representation differs from that shown in Fig. 8a and b in several

ways First, in this representation, the echoes have not been

translated relative to the radar sites, thus their true positions as a

function of time relative to 0KC and SEP are shown. Secondly, the

letters in Fig. Ii show the instantaneous position of the echo centers;

therefore, the confusing overlapping of echo areas appearing in Fig. 8a

and b is eliminated. Finally, Fig. ii shows all VIP level i echoes,

regardless of lifetime or position, at the given times Thus, echoes

outside the main storm area are shown, e. g. note the echoes at 1202
28

a

m

io

J j.

J

.SEP

A 0000 J 0901

B 0100 K !001

C 0200 L !!00

D 0300 M 1202

E 0400 N 1300

F 0500 0 1400

G 0600 P 1458

H 0700 G. 1558

I 0759

oOKC

/k\l k

Jk

Fig. ii. VIP level i cell centroid positions at hourly intervals from

0000 to 1558 GMT. Variations in propagation direction through the lifetime

of the storm are evident.
29

GMT appearing to the north and west of OKC.

This figure shows somewhat more clearly how the tracks of the

echoes varied in direction through the lifetime of the storm. About

0901GMT, new VIP level i echoes developed to the south of the main

storm area and moved toward the northeast. In all likelihood, these

echoes were related to warmer clouds and their motion was a result of

the low-level flow.
3O

V. NEW CELL DEVELOPMENT

!

One objective of the study was to examine how new cells (VIP level

3 or more) developed in relation to the gust front of the system as a

whole. Figure 12 shows the trajectories of selected VIP level 3 cell

centers in conjunction with the positions of the gust front in time.

Many of the cells shown in Fig. 9 are not included here. The gust

front positions were determined from the combination of WBAN logs and

microbarograph traces from selected stations coupled with sectional

surface observations.

The simultaneous positions of the cells and the gust front in time

are labeled with letters at approximately 1 h intervals, as in previous

figures. The gust front was not identifiable until after the storm had

become an MCC. Thereafter, it was found on the eastern, southern, and

western extremities of the VIP level 1 echo field- more will be said

about this relationship later. Figure 12 again shows the great

variability in the trajectories of the cells Furthermore, it shows

that new cells developed along and behind the gust front, and either

dissipated quickly or translated toward the southeast along with the

gust front for several hours before dissipating. In some instances,

merging of cells occurred.

/

One interesting phenomenon which can be seen to some extent in Fig.

12 is the tendency of the region of greatest reflectlvlty to appear as a

llne oriented in a somewhat south to north direction in the early llfe

of the MCC. Note the cells labeled A at 0600 GMT. In this case, as
Fig. 12. Gust front and level 3 cell positions at hourly intervals.
32

time went on, older cells progressed southeastward while new cells

developed behind the less-mobile, western side of the gust front- see

the cells labeled D at 0901 GMT. Consequently, the maximum reflectivity

became oriented along a more west-east line. This phenomenon also has

been observed by Howard and Maddox (1983) and Barrels and Rockwood

(1983) In the first case, the western end of the line eventually

developed into a second MCC. After the time of the maximum extent of

the MCC at i000 GMT, it is seen in Fig. 12 that the gust front

accelerated and no new cell development occurred after 1202 GMT.

Although it cannot be ascertained with certainty with these

observation, it appears likely that the gust front plays a significant

role in the development of new VIP level 3 (or more) cells, as suggested

by Maddox (1980b). At some time it moves outward so far from the

primary convective activity that the associated low-level horizontal

convergence no longer can maintain the storm.
33

VI. RADAR DATA DETAILS

The echoes seen in Fig. 13 at 1 h intervals from 0100 to 1558 GMT

illustrate the important evolutionary features of the MCC- Outer solid

lines denote the VIP level i systems, and hatched regions denote the

cells with VIP level 3 or more reflectlvlty. The blackened region at

0200 GMT and later times denotes no radar echo as a consequence of

attenuation. At 0300 GMT the precipitation amount was 0.3 in at

McLean, TX. These precipitation values represent accumulated amounts

over the previous hour from the ciimatological networks of ralngauges.

The dot in the figure shows both the location of the precipitation and

the decimal point. The storm was growing and at 0400 _ moderate

rainfall had occurred at Altus, OK. Over the hour from 0300 to 0400 GMT

this incipient MCC produced funnel clouds and tornadoes in Texas.

The storm intensified rapidly and met MCC criteria by 0515 GMT.

The expansion in area of the system is apparent from comparison of Figs.

13 e and f. Moderate rainfall occurred during the initiation time. The

northeast to southwest oriented llne of cells (VIP level 3 or more

reflectivlty) continued to move eastward for several hours (Fig.

13d-g), with the northern cells moving somewhat faster on convergent

tracks. About three quarters of an hour after the initiation of the

MCC, a mesohigh and gust front formed across the Oklahoma/Texas border

and subsequently moved southeastward. As the MCC intensified, scattered

rainfall occurred, and moderate to heavy rainfall was reported in the

vicinity of the VIP level 3 cells. During the merging of the discrete
(A)

...... ---o--z/....

I

I

I

I

I

(B)

(C)

I--.....

I

I

!

I

!

!

0

I

I

I

I

I

I

0

0300Z

.OKC

%

(D)

34

0200Z

040-OZ--

Fig. 13. Combined level i and level 3 radar echoes at hourly intervals

from 0100 to 1558 GMT. Outer contours represent VIP level 1 echoes;

level 3 echoes are hatched. One-hour accumulated rainfall amounts from

,recording rainguages are shown. Starting with 0600 GMT, the gust front is

denoted by cold front sysmbols. Blackened areas indicate radar attenuation.

I
I N0o _; c.0o *:,.J I!

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0,. W

W

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Fig. 13. (Continued)
38

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-SEP

Q

Fig. 13. (Continued)
39

(K) I IOOZ

C)

"OKC

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Fig. 13. (Continued)
4O

(L) 1202Z

- ---""l

-OKC

,!

.li

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-i

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•4 .! .I

Fig. 13. (continued)
41

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cM) o _ 1300Z (N) 1400Z

............ _ .o.o !

o ! , ,

_'_''-'__"'__& _

/ _ " " >I\

•SEP ,_.-"

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(O) -oKc il 458 Z

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t %.p/ % I

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Fig. 13. (Continued)
42

cells into a larger area of widespread intense reflectivity at

Gordonville, TX., rainfall of 1.3 in was recorded (see Fig. 13h). In

Figs. 13f-i new cells can be seen to have developed right along the

gust front. The MCC rapidly developed after 0800 GMT and the clockwise

rotation of the axis of maximum reflectivity, which was mentioned

before, can be seen to have taken place between 0600 and 0901 GMT. Very

high reflectivity (more than VIP level 3) was observed at 0901GMT. The

strength of the gust front at the same time was revealed by the strong

and gusty winds which it produced. At 0935 GMT, as the gust front

passed Dallas, TX. a peak wind of 45 kt was recorded.

Figure 13j shows the system at the time of maximum intensity.

Although the system was fully developed, the reflectivity was somewhat

weaker and there were fewer VIP level 3 cells. Nevertheless, heavy

rainfall occurred in the vicinity of the VIP level 3 cells. After ii00

GMT, the gust front moved away from the storm and the system began to

dissipate. The dashed boundary on the east side of the wide-spread area

of rain in Fig. 131 had to be estimated because of attenuation of both

radar beams.

By 1300 GMT, all VIP level 3 or more cells had vanished; however,

during the previous hour strong rainfall had occurred in the amount of

1.01 in at Ferris, TX , and 1.42 in at Denison Dam, TX. These heavy

rainfalls were produced by the fading VIP level 3 cells at 1202 GMT.

Near the termination time of the system, rainfall was wide-spread and

light rainfall continued for another 2 h thereafter.
43

VII. COMBINATION OF SATELLITE IMAGERY AND RADAR OBSERVATION

Figures 14 through 18 represent comparisons of satellite imagery

with VIP level i and 3 radar patterns at a number of times through the

lifetime of the storm of 27 May 1981. Included in these figures are

contours of cloud-top temperatures determined from the GOES IR imagery

through the use of the MclDAS system at the Marshall Space Flight

Center. The reader may wish to compare these figures with Figs.

13g-131 which show the same radar patterns coupled with the gust front

positions.

Figure 14 shows the situation at 0700 GMT. On the right-hand side

of the figure, the outer llne shows the overall cloud shape, as provided

by the satellite image on the left-hand side of the figure, The inner,

thicker lines depict the cloud top equivalent blackbody temperatures of

240 °, 210 °, and 205°K.

The satellite image displays a large, nearly circular shield of

cloud, as required in Maddox'$ definition (1980) for an MCC, with

coldest tops occurring over the west central portion of the system

(compare with Fig. 13g). The radar depiction on the right- hand side

of Fig. 14 indicates that the VIP level I rainfall at this time was

fairly well contained within the 210°K or -63°C isotherm of the storm

canopy About half of the VIP level 3 echo fell within the 205°K

isotherm. Also, further comparison with Fig. 13g shows that the gust

front position in Fig. 13g falls between the 210 and 240°K isotherms

shown in Fig. 14. Gurka (1976) found that the gust front generally is
44 

U 

m 

rla 

aJE 

ale+ 
45

located very close to the strongest gradient in IR temperature on the

eastern and southern sides of a mesoscale storm

Figures 15 through 18 in general show the same features as noted in

Fig. 14. Figures 15 and 16 show good agreement between the 210°K

isotherm, the boundary of the VIP level i reflectivlty region, and the

position of the gust front. There is some tendency for the VIP level 3

area to be enclosed by the 205°K isotherm.

At I001GMT (Fig. 17), the bands across the satellite image

represent radiance data that were lost in transmission from the

satellite. This is the time of greatest extent of the storm as measured

by the area enclosed by the 210°K isotherm. It is seen that a small

appendage of warm-cloud precipitation has developed on the south side of

the system; otherwise, the relationships between the temperature values

and echo intensities noted earlier still prevail. At this time the gust

front on the western side of the storm is moving away from the

precipitation area.

By 1202 GMT (Fig. 18) the storm is halfway in time between its

time of maximum extent and its termination (the time when it ceased to

qualify as an MCC). Figures 18 and 131 show that the gust front has now

moved outside the 210°K isotherm on the east side of the system and even

outside the satellite image on the west side. The good agreement

between the 210°K isotherm and the VIP level I reflectivlty region also

has deteriorated somewhat.
ORIGINAL PAGE 15 

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VIII. CONCLUSIONS AND RECOMMENDATIONS

The MCC of 27 May 1981 developed over southwestern Oklahoma and

northern Texas and moved in a eastsoutheasterly direction. A synthesis

of radar, satellite, sounding and surface data was used to determine the

relationships between system and cell motions relative to the mean cloud

layer wind, the relationship between cell development and the gust

front, and the relationship between the rainfall patterns and amounts

and cloud-top IR temperature structure.

Findings regarding the development and propagation of cells in an

MCC are as follows:

i) The direction of cell (VIP level 3 or more reflectivity)

propagation was found to be approximately the same as the system (VIP

level I reflectivity) movement and the ambient wind.

2) Level 3 (or more) cells developed and dissipated on both the

north and south ends of the line of maximum reflectivity Thus, the

system, as denoted by level i reflectivity, grew in size with time.

Also, new cells developed to the rear of the line of maximum

reflectivity as older cells continued their eastsoutheastward

propagation. This led to a clockwise rotation of the line of maximum

reflectivity. The resulting pattern of system and cell motion was not

consistent with any of the models described by Marwitz (1972) for

multi-cell systems.

3) Prior to its becoming an MCC the axis of maximum reflectivity

was located near the center of the level i reflectivity. After MCC
51

initiation, this axis was observed to moveto a position Just behind the

leading edge of the precipitation area. This agrees with results found

by Leary and Rappaport (1983) using the combination of satellite imagery

and radar echoes for an MCCin southwestern Texas. A gust front became

identifiable in the surface synoptic data shortly after the initiation

of the MCC. Its posltion in time was found to match well with the

boundary of the precipitation area. Level 3 cells were found to form

along or just inside the gust front and movewith it. Later, as the

gust front movedaway from the precipitation area, further cell

development ceased. These results imply that a synergistic relationship

may exist between the system and the gust front in which the downdrafts

and outflow from the MCCmaintain the gust front, which in turn provides

the convergence and updrafts to produce the cells.

Only a small portion of the storm area as seen in satellite imagery

represents rainfall of sufficient intensity to be detected by radar. It

was found that the 210°K isotherm in the IR satellite imagery tends to

encompass this VIP level i region. Furthermore, the 205°K isotherm

tends to surround a large portion of the heavier rainfall area as

denoted by level 3 cells.

These generalizations are drawn from only one MCC and thus may not

be representative of MCCs in different synoptic conditions, in different

geographical regions, or earlier or later in the convective season.

It is obvious that a better, if not complete, understanding of the

internal structure and dynamics of middle-latitude mesoscale systems can

only be gained through an elaborate and specialized field study.
52

Fortunately, plans for such a study have been formulated (Zipser, 1984)

and a prellmlnary_ geographically- limited study has been carried out

(Cunning, 1986) to test instrumentation and logistics. Until this field

study can be accomplished, it is recommended that studies along the

lines carried out here be continued. If the results found here can be

confirmed, some understanding of MCCs will have been gained which may

serve useful in the final planning of a large-scale field program.
53

REFERENCES

Bartels, D. L., and A. A. Rockwood, 1983: Internal structure and evolution

of a dual mesoscale convective complex. Preprints 5th Conf.

Hydrometeorology, Tulsa, OK., Amer. Meteor. Soc._ 97-102.

Bosart, L. F., and F. Sanders, 1981: The Johnstown Flood of July 1977: A

long-llved convective system. J. Atmos. Sol., 38, 1616-1642.

Byers, H. R., and R. R. Braham, Jr., 1949: The Thunderstorm. U. S. Dept.

of Commerce Weather Bureau, Washington, D C._ 282pp.

Cunning, J. B., 1986: The Oklahoma-Kansas preliminary regional experiment

for STORM-Central. Bull. Amer. Meteor. Soc., 67, 1478-1486.

Foster, D. S., and R. M. Reap, 1973: Archiving of manually digitized

radar data. Techniques Development Laboratory Office Note 73-6,

National Weather Service, Silver Springs, Md., 12pp.

Fritsch, J. M., R. A. Maddox and A. G. Barnston, 1981: The character of

mesoscale convective complex precipitation and its contribution to

warm season rainfall in the United States. Preprints 4th Conf.

Hydrometeorology, Reno, NV., Amer. Meteor. Soc., 94-99.

Fujita, T., and H. A. Brown, 1958: A study of mesosystems and their radar

echoes. Bull. Amer. Meteor. Soc., 39, 538-554.

Gurka, J. J._ 1976: Satellite and surface observations of strong wind

zones accompanying thunderstorms. Mon. Wea. Rev., 104, 1484-1493.

Heymsfield, G, M,_ and S. Schotz, 1985: Structure and evolution of severe

squall llne over Oklahoma. Mon. Wea. Rev., I13, 1563-1589.

Howard, K, W._ and R. A. Maddox, 1983: Precipitation characteristics of
54

two mesoscale convective systems. Preprints 5th Conf. Hydrologyj

Tulsa, OK., Amer. Meteor. Soc., llpp.

Leary, C. A., and E. N. Rappaport, 1983: Internal structure of a

mesoscale convective complex, Preprints 21st Conf. Radar Meteorology,

Edmonton, Amer. Meteor. Soc.j 70-77.

Maddox, R. A., 1980a; A satellite based study of mldlatitude, mesoscale

convective complexes. Preprints 8th Conf. Weather Forecasting and

Analysis, Denver, CO., Amer. Meteor. Soc., 329-338.

---, 1980b: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61,

1374-1387.

---, D. M. Rodgers and K. W. Howard, 1982: Mesoscale convective complexes

over the United States during 1981. Mon Wea. Rev., IiO, 1501-1514.

---, and K. W. Howard, 1983" Simultaneous radar and satellite depictions

of mesoscale convective systems. Preprlnts 5th Conf. Hydrometeorology,

Tulsa, OK., Amer. Meteor. Soc., 10pp.

McAnelly, R. L., and W. R. Cotton, 1986: MJeso-@-scale characteristics of

an episode of meso-_-scale convective complexes. Mon. Wea. Rev._

114, 1740-1770.

Marwitz, J. D., 1972: The structure and motion of severe hailstorms. Part

II: Multi-cell storms. J. Appl. Meteor., II, 180-188.

Merritt, J. H., and J. M. Fritsch, 1984: Convective complexes. Earth and

Mineral Sciences, 54, 1-4.

Miller, M. J.j 1978: The Hsmpstead storm: A numerical simulation of a

quasl-statlonary cumulonimbus system. _uart. J. Ro 7. Meteor. Soc.,

104, 413-427.
55

Newton, C. W., and J. C. Fankhauser, 1975: Movementand propagation of

multlcellular convective storms. Pageogh.> i13_ 747-764.

Purdom, J. F. W., 1973: Meso-highs and satellite imagery. Mon. Wea. Rev.,

i01, 180-181.

---, 1976: Some uses of hlgh-resolution GOES imagery in the mesoscale

forecasting of convection and its behavior. Mon. Wea. Rev., 104,

1474-1483.

---, 1979: The development and evolution of deep convection. Preprints

llth Conf. Severe Storms, Kansas City, MO., Amer. Meteor. Soc.,

143-150.

Rodgers, D. M., K. W, Howard and E. C. Johnston, 1983: Mesoscale convective

complexes over the United States during 1982. Mon. Wea. Rev., iii,

2363-2369.

---, M. J. Magnano and J. H. Arns, 1985: Mesoscale convective complexes

over the United States during 1983. Mon. Wea. Rev., ii_33,888-901.

Welshlnger, M. J., 1985_ Factors leading to the formation of arc cloud

complexes. Masters Thesis, Texas A & M Univ., College Station, TX.,

117 pp.

Wilhelmson, R. B., and C. S. Chen 1982: A simulation of the development

of successive cells along a cold outflow boundary. J. Atmos. Sci.,

39, 1466-1483.

Zehr, R. M. and James F. W. Purdom, 1982: Examples of a wide variety of

thunderstorm propagation mechanisms. Preprints 12th Conf. Severe Storms,

San Antonlo, TX., Amer. Meteor. Soc., 499-502.

Zlpser, E. J., 1984: The national STORM program STORM-Central phase
56

preliminary program design. Prepared by the National Center for

Atmospheric Research for the team for STORM-Central,Boulder, CO

80303.
.N87-18949

NIGHTTIME ATMOSPHERIC STABILITY CHANGES

AND THEIR EFFECTS ON THE TEMPORAL INTENSITY

OF A MESOSCALE CONVECTIVE COMPLEX

Jeffrey S. Hovis

and

Kenneth C. Brundidge

Texas A&M University

College Station, Texas

Prepared for George C. Marshall Space Flight Center

under Grant NAG8-043
ABSTRACT

Nighttime Atmospheric Stability Changes and their Effects on the

Temporal Intensity of a Mesoscale Convective Complex.

A method of interpolating atmospheric soundings while reducing the

errors associated with simple time interpolation was developed. The purpose

of this was to provide a means to determine atmospheric stability at times

between standard soundings and to relate changes in stability to intensity

changes in an MCC. Four MCC cases were chosen for study with this method

with four stability indices being included.

At this timej the study of all four cases is incomplete and only the

preliminary results for Case i are described. The discussion centers on

three aspects for each stability parameter examined: the stability field in

the vicinity of the storm and its changes in structure and magnitude during

the lifetime of the storm, the average stability within the storm boundary

as a function of time and its relation to storm intensity, and the apparant

flux of stability parameter into the storm as a consequence of low-level

storm relative flow. It was found that the results differed among the four

stability parameters, sometimes in a conflicting fashion. Thus, an

interpolation of how the storm intensity is related to the changing

environmental stability depends upon the particular index utilized. Some

explanation for this problem is offered.
iii

ACKNOWLEDGEMENTS

The work reported upon here was in part supported by the National

Aeronautics and SpaceAdministration under the auspices of the Atmospheric

Sciences Division, Space Sciences Laboratory, Marshall Space Flight Center

under Grant NAG8-043.
iv

TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

I. INTRODUCTION

II. PROCEDURE

III. RESULTS

IV. TENTATIVE CONCLUSIONS

V. REFERENCES

iii

iv

v

vi

1

5

8

37

39
v

LIST OF TABLES

Table

i

2

5

i0

Absolute error analysis of the original data set for

the two interpolation methods for temperature at 0600 GMT - -

Least squares coefficients for temperature and mixing

ratio to be used to interpolate the 0600 GMT soundings

Absolute error analysis of the original data set for

the two interpolation methods for temperature at 0600 GMT - -

Absolute error analysis of the two interpolation methods for

temperature at 0300 GMT

Absolute error analysis of the two interpolation methods for

temperature at 0600 GMT

Absolute error analysis of the two interpolation methods for

temperature at 0900 GMT

Absolute error analysis of the two interpolation methods for

mixing ratio at 0300 GMT

Absolute error analysis of the two interpolation methods for

mixing ratio at 0600 GMT

Absolute error analysis of the two interpolation methods for

mixing ratio at 0900 GMT

Mesoscale Convective Storm Systems included in study

Page

i0

Ii

13

14

15

16

17

18

19
vl

LIST OFFIGURES

Figure

la

ib

ic

2a

2b

2c

3a

3b

3c

4a

4b

4c

Modified K index field for 0000 GMT ii April 1981

Same as Flg. la except for 0300 GMT

Same as Fig. la except for 0600 GMT

Modified Total-Totals index field for 0000 GMT

ii April 1981

Same as Flg. 2a except for 0300 GMT

Same as Fig. 2a except for 0300 GMT

Showalter index field for 0000 GMT ii April 1981

Same as Flg. 3a except for 0300 GMT

Same as Fig. 3a except for 0600 GMT

Potential Instability Index field for 0000 GMT

ii April 1981

Same as Fig. 4a except for 0600 GMT

Same as Fig. 4a except for 0600 GMT

Page

22

23

24

26

27

28

30

31

32

33

34

35
I. INTRODUCTION

Mesoscale Convective Complexes (MCCs) have recently been receiving

considerable attention from the meteorological research community. The

research on MCCs seems to have blossomed since these long-lived thunderstorm

systems were defined by Maddox (1980) on the basis of their appearance in

satellite imagery coupled with cloud top temperatures of their convective

elements.

MCCs are important features in the weather of the midwestern section of

the United States. Fritsch et al. (1981) found that these storms were

responsible for a large percentage of the rainfall in this area. Several

researchers (Maddox et al., 1982; Rodgers et al., 1983) have found that they

also produce a significant amount of severe weather including flash floods,

tornadoes, high winds, and hail, which cause many deaths, injuries, and

significant property damage.

Four groups of researchers (Maddox, 1981; Maddox et al., 1982; Rodgers

et al., 1983; Rodgers et al., 1985) have studied the life cycle of these

storms by using the criteria set up by Maddox (1980). The first storms were

found to form, on average, in the midafternoon about 2100 GMT. The storm

was found to build in size with time and usually to meet the definition of

an MCC at approximately 0300 GMT, which corresponds to the nighttime hours

in the midwest. The maximum extent and intensity were reached in the early

morning hours at about 0800 GMT. The time of 1230 GMT was the approximate

time of termination, in the sense that the storm no longer met the

definition of an MCC. Thus, while the average life span of an MCC as such
was about 9 h, convective storm activity associated with the system could

last much longer.

Several researchers have shown that upper air observations at 12-h

intervals are inadequate for the study of short-llved mesoscale processes.

House (1960) concluded that the upper air network with an average spacing

between observations of 220 n mi and time spacing of 12 h was insufficient

to-resolve the mesoscale features that led to the formation, movement, and

dissipation of squall lines. Kreitzburg and Brown (1970) showed the

mesoscale features within the synoptic scale flow could be linked to the

variability of the weather. When looking at a continental occlusion with

rawinsonde observations at 90-min intervals they found synoptic-scale

features, while having continuity of days in time and around I000 mi in

space, contained mesoscale features with time continuity of a few hours and

space continuity of a few hundred miles.

Wilson and Scogglns (1976) studied the atmosphere using rawinsonde

observations taken during AVE (Atmospheric Variability Experiment) II. They

found that between 30-60% of the total change in atmospheric variables

observed over the 12-h period occur within a 3-h interval. In several

cases the 3-h changes were larger than those measured over 12 h. As a

matter of fact, large changes of vertical motion and potential instability

can lead to a change in convective activity by a factor of more than seven

over that found in the 12-h data.
Dupuls and Scogglns (1979) used a linear time interpolation scheme to

estimate the structure of the atmosphere over the regular 12-h interval

between rawinsonde observations. These interpolated values were compared

with the actual 3-h observations. They found that the magnitudes of the

non-linear changes were largest in areas of convection. The instability

increased non-llnearly in areas of convection as did low-level convergence

and upward vertical motion. This was determined to be important in the

release of the potential instability. They concluded that linear time

interpolation was inadequate in defining variability over time of

atmospheric parameters.

Since vortlclty and atmospheric stability are important parameters in

convection, Read and Scogglns (1977) used 3-h rawlnsonde data from AVE IV to

compare the 3-h changes of these parameters to those measured over 12 h for

the same time period. Instability was found to be greatest at 3 h prior to

thunderstorm development. This would hardly be seen with conventional 12-h

observations. Changes in the magnitudes of terms in the vortlclty equation

also were found to be larger in the 3-h data than that found in the 12-h

data. They concluded that the changes in the vorticity, stability, and

intensity of the convection over the 3-h periods indicated that studies of

convective activity could not be adequately done using 12-h observations.

Vertical motion variability between 3-h and 12-h rawinsonde data has

been studied by Overall and Scogglns (1975). Vertical motion had been

determined to be important in enhancing and maintaining convective activity

by Byers (1944). So, unseen changes in vertical motion could lead to

unexplained occurrences of convection. Overall and Scoggins found that
4

changes in vertical motion for 3-h data could be as large as four times that

found when looking at 12-h observations. They also found that the number of

mesoscale features that could be analyzed increased as the interval between

rawinsonde observations decreased from 12 to 3 h.

Since the MCC is predominantly a nocturnal event, daytime heating does

not play an important role in the maintaining of the MCC intensity. So,

some other important feature, such as atmospheric stability aloft, must play

a significant role in controlling the intensity of the MCC during the

nighttime. Considering all this information, research was performed with

the following objectives in mind.

(i) to develop a method of interpolating atmospheric soundings

to 500 mb between the standard observation times, and

(2) to use these interpolated soundings to examine how the

changing stability of the atmosphere in the environment of a Mesoscale

Convective Complex (MCC) is related to its temporal intensity changes.
5

II. PROCEDURE

In order to obtain reasonably accurate interpolated atmospheric

soundings, the following was done.

TwoAVE-SESAME(Atmospheric Variability Experiment- Severe

Environmental Storms and Mesoscale Experiment) data sets were obtained in

which radiosonde observations were taken at 3-h intervals for 00 through 12

GMT.

Initially, the data sets were analyzed to separate them into three

different groups (storm, non-storm, and all stations). This separation was

done by utilizing radar summarymapsfor the periods involved. If a station

had rain or a thunderstorm located within approximately 50 km of it during

the period of 2235 GMT from the previous day to 1235 GMT of the day of

interest, the station was classified as a storm case. Otherwise, it was a

non-storm case. Once this was done, the following interpolation routines

were applied to each of these new data sets.

Linear interpolation in time at 25 mb intervals up to 500 mb, not

including the surface level, was then performed between the O0 and the

following 12 GMT observations to obtain values of temperature and mixing

ratio at 03, 06, and 09 GMT. These interpolated values were compared to the

actual measured values in order to identify a level of least average error

for each of the times.
Vertical interpolation in pressure was then performed using the values

at the levels of least average error and the observed values at the surface.

This process was carried out from the surface to the 500 mb level at 25 mb

intervals also.

In order to incorporate both interpolation methods into one general

method, the values obtained from the two interpolations (time and pressure)

at each reported time and every level were then matched with the actual

measured values to get a least squares fit to the observed data. Three sets

of coefficients (storm, non-storm, and all stations) were obtained for use

in the resulting interpolation equations. An estimate of the accuracy of

the resulting interpolation relationships for temperature and mixing ratio

was then obtained by applying them to another AVE-SESAME data set. Also,

the results were then examined to see if there is any significant benefit in

having separate interpolation equations for storm and non-storm location.

After the interpolation relationships had been determined, they were

applied to data sets from areas around MCCs that had previously been

identified by Welshinger (1985). Each data set included all upper air

stations with available data that were within several hundred kilometers of

the location of the MCC.

In order for a station to be used, upper air data, including mandatory

and significant levels up to 500 mb, had to be available. Also, the station

had of have surface data for 03, 06, and 09 GMT for the time period of

interest, including temperature, dewpoint temperature, and sea level

pressure or altimeter setting.
7

Various stability indices were then calculated and plotted on maps

showing the air flow in and around the MCC. These indices, including the K

index, Total-Totals, CAPE (Convective Available Potential Energy), and the

maximum rate of decrease of equivalent potential temperature over a 25 mb

layer from the surface to 700 mb (PII- Potential Instability Index), showed

the stability of the environmental air feeding the storm and how it changed

through the lifetime of the MCC.

The intensity of the MCC as a function of time was indicated by the

area enclosed by given values of cloud-top temperatures obtained through

satellite measurements. This was in keeping with the criteria established

by Maddox (1980).
8

III. RESULTS

Interpolation Procedure

In an attempt to reduce the errors associated with linear time

interpolation, interpolation with respect to pressure was also utilized.

This procedure would allow changes at the surface to influence the final

soundings. Table i shows an example of the errors associated with time

interpolation and interpolation with respect to pressure. In the lowest

layers, the errors associated with each method are similar. But above that

level, time interpolation is much better.

In order to combine the two procedures and reduce the errors to a

minimum, a least-squares fit was applied to the interpolated values at

various pressure levels and various times. An example of the coefficients

for the interpolation equations that resulted after the least-squares fit

was applied is given in Table 2. An example of the actual interpolation

equation is

Q = a0+a I Qt+a2Qp

where Qt and Qp represent the values of the given quantity obtained from

time interpolation and pressure interpolation, respectively. Once these

coefficients were determined, the interpolation relationships were tested on

the original data set to see how much improvement over linear time

interpolation was found. Table 3 gives a comparison of the errors resulting

from simple time interpolation in comparison to the errors resulting from

the new method. For this purpose_ the absolute values of the errors on
9

Table I. Absolute error analysis of the original data set

for the two interpolation methods for temperature

at 0600 GMT. All values are in C'.

Time Pressure

interpolation interpolation

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

i000 0.8 0.56 0.4 0.34

975 1.0 0.65 1.2 1.09

950 1.3 1.05 1.3 1.54

925 1.5 1.24 1.5 1.72

900 1.6 1.65 2.1 2.23

875 1.6 1.86 2.5 2.57

850 1.6 1.68 3.1 2.81

825 1.6 1.52 3.4 2.94

800 1.8 1.56 4.0 3.05

775 1.9 1.68 5.0 2.92

750 1.7 1.30 6.0 2.86

725 1.6 1.08 6.1 2.70

700 1.2 0.93 5.8 2.51

675 i.i 0.91 5.4 2.35

650 0.9 0.74 5.0 1.93

625 0.9 0.74 4.3 1.64

600 1.0 0.97 3.4 1.53

575 i.i 1.01 2.5 1.24

550 1.0 0.80 1.6 0.98

525 1.0 0.89 1.0 0.89

500 i.i 0.91 1.4 1.23
i0

TABLE2. Least squares coefficients for temperature and mixing ratio

to be used to interpolate the 0600 GMT soundings.

Temperature Mixing ratio

Pres

(rob) ao a, a_ ao a, a_

I000 14.34 0.84 -0.51 2.10 0.72 0.18

975 -0.41 0.71 0.31 -0.52 0.49 0.58

950 0 . 43 0 . 56 O. 43 O. 48 0 • 18 0 . 82

925 0.47 0.46 0.55 0.21 0.24 0.80

900 0 • 92 O. 55 O. 43 -0.42 O. 64 0 • 44

875 1.51 0.55 0.39 0.00 0.76 0.28

850 0.86 0.73 0.23 1.05 0.94 -0.04

825 0.11 0.85 0.15 0.71 0.72 0.22

800 0.54 0.81 0.20 1.52 0.69 0.11

775 0.13 0.88 0.24 2.49 0.78 -0.28

750 0.17 0.95 0.21 3.32 0.74 -0 48

725 0.48 0.96 0.19 4.21 0.54 -0.63

700 0.56 0.96 0.17 3.70 0.52 -0.60

675 0.53 i .00 0.07 3.23 0.60 -0.67

650 0.83 0.94 0.ii 2.23 0.80 -0.61

625 1.15 0.85 0.19 1.12 0.88 -0.42

600 i.ii 0.80 0.24 0.96 0.79 -0.43

575 1.68 0.50 0.55 0.30 0.61 -0.08

550 0.74 0.66 0.35 0.20 -1.75 2.22

525 0.35 1.97 -0.96 0.24 0.42 0.20

500 0 •15 0.96 0.05 0 •13 0.69 0.05
ii

Table 3. Absolute error analysis of the original data set

of the two interpolation methods for temperature

at 0600 GMT. All values are in C °.

Time New

interpolation method

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

I000 0.8 0.56 0.i 0.ii

975 1.0 0.65 0.8 0.56

950 1.3 1.05 i.i 0.98

925 1.5 1.24 1.3 1.07

900 1.6 1.65 1.6 1.33

875 1.6 1.86 1.6 1.43

850 1.6 1.68 1.6 1.54

825 1.6 1.52 1.5 1.22

800 1.8 1.56 1.7 1.36

775 1.9 1.68 1.8 1.35

750 1.7 1.30 1.5 1.07

725 1.6 1.08 1.3 1.04

700 1.2 0.93 1.0 0.90

675 I.i 0.91 1.0 0.87

650 0.9 0.74 0.8 0.65

625 0.9 0.74 0.7 0.67

600 1.0 0.97 0.9 0.88

575 I.i 1.01 1.0 0.87

550 1.0 0.80 0.9 0.81

525 1.0 0.89 1.0 0.87

500 i.i 0.91 1.0 0.92
12

each pressure surface were averaged. The reduction in error associated with

"the new method is significant. If one considers an algebraic average on

each pressure surface, the resulting errors are approximately zero (not

shownhere). This is what we would expect after applying a least-squares

fit to the data. However, this does not quarantee the efficacy of the

procedure when applied to independent data. Therefore, as a test of the

procedure, it was applied to a third AVE-SESAMEcase. For the newmethod to

be considered good, the errors associated with this new case should be

small. Also, the standard deviations should be small. In Tables 4-6, the

errors in temperature associated with simple interpolation in time and the

new technique are compared. For this purpose, there has been no separation

into storm vs non-storm soundings.

At 03 GMT (Table 4), it can be seen that between i000 and approximately

900 mb, the new method appears to be better than linear time interpolation.

Above that level, the two methods have similar errors, with neither

providing a distinct advantage. Similar results are seen in Tables 5 and 6.

The results for the mixing ratio (Table 7-9) are quite similar to those

for the temperature calculation. Again, the new method shows some advantage

in the boundary layer where the influence of values at the surface is the

greatest. However, above the boundary level, simple interpolation in time

gives somewhat smaller errors.

The conclusion reached from these comparisons was that the smaller

errors provided by the interpolation relationships (Table 2) in the boundary

layer was of greater significance for the purpose of this study than the

slight advantage gained by using linear time interpolation in the upper
13

Table 4. Absolute error analysis of the two interpolation

methods for temperature at 0300 GMT. All values are

in C ° .

Time New

interpolation method

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

i000 1.2 0.86 0.5 0.28

975 0.8 0.47 0.7 0.56

950 1.7 2.67 1.5 2.05

925 2.0 2.48 1.7 1.75

900 1.6 1.59 1.5 1.47

875 1.3 0.92 1.2 1.09

850 1.2 0.97 1.3 1.07

825 1.3 0.81 1.3 0.96

800 1.0 0.65 1.0 0.73

775 0.9 0.62 1.2 0.81

750 0.9 0.59 0.9 0.60

725 0.8 0.58 0.8 0.53

700 0.8 0.59 0.8 0.60

675 0.8 0.78 0.9 0.79

650 0.8 0.76 0.8 0.78

625 0.7 0.60 0.8 0.70

600 0.8 0.68 0.8 0.70

575 0.9 0.77 1.0 0.80

550 i.i 1.00 1.2 1.01

525 i .0 O. 92 i. 0 0.86

500 1.0 0.92 0.9 0.88
14

Table 5. Absolute error analysis of the two interpolation

methods for temperature at 0600 GMT.All values are

in C°.

Time New

interpolation method

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

I000 i.i 0.63 0.9 0.49

975 2.0 1.81 1.6 1.19

950 2.3 2+19 1.9 1.38

925 2.4 2.21 2.2 1.51

900 1.8 1.81 1.8 1.44

875 1.7 1.42 1.7 1.47

850 1.2 1.13 1.3 1.30

825 1.3 0.92 1.5 1.15

800 1.3 0.97 1.3 i.I0

775 1.3 1.06 1.3 0.98

750 1.2 1.00 1.0 0.81

725 1.0 0.70 0.9 0.74

700 0.8 0.63 0.9 0.67

675 0.8 0.73 0.8 0.77

650 0.7 0.66 0.8 0.60

625 0.8 0.57 0.7 0.55

600 0.8 0.55 0.7 0.48

575 0.9 0.75 0.9 0.77

550 0.9 0.92 0.9 0.96

525 0.9 0.95 0.9 1.01

500 1.0 1.16 1.0 1.16
15

Table 6. Absolute error analysis of the two interpolation

methods for temperature at 0900 GMT. All values are

in C ° .

Time New

interpolat ion method

Pres Mean Standard Mean Standard

(rob) errors deviations errors deviations

I000 0.7 0.69 0.7 0.63

975 1.7 1.46 1.6 1.53

950 2.1 1.62 1.8 1.14

925 2.2 1.91 1.9 1.32

900 1.8 1.44 1.8 1.42

875 1.4 1.47 1.4 1.46

850 I.i 0.96 i.i 0.99

825 1.3 1.04 i .3 i .09

800 0.9 0.78 1.0 0.75

775 0.9 0.83 0.9 0,75

750 0.9 0.80 0 9 0.80

725 0.8 0.73 0.9 0.70

700 0.9 0.69 0.9 0.71

675 0.7 0.72 0.8 0.81

650 0.7 0.59 0.8 0.62

625 0.6 0.57 0.7 0.62

600 O. 7 O. 57 O. 8 O. 57

575 0.7 0.52 0.7 0.47

550 0.8 0.54 0.9 0.56

525 0.7 0.58 0.9 0.71

500 0.8 0.76 0.9 0.84
16

Table 7. Absolute error analysis of the two interpolation

methods for mixing ratio at 0300 GMT. All values are

in g/kg.

Time New

interpolation method

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

i000 1.6 1.07 0.7 0.66

975 1.2 0.98 i.I 0.88

950 I 2 1.13 I.i 0.90

925 1.3 1.19 1.3 1.06

900 i.i i.i0 1.3 1.09

875 1.2 1.21 1.5 i.I0

850 1.5 1.40 1.7 1.54

825 1.5 1.35 1.6 1.36

800 1.5 1.34 1.5 1.25

775 1.2 1.22 1.2 i.i0

750 i.i 1.26 i. i 0.96

725 i.i 1.27 1.3 1.44

700 1.2 1.36 1.6 1.38

675 0.9 1.02 1.3 0.96

650 0.7 0.79 1.0 0.78

625 0.6 0.61 1.0 0.73

600 0.6 0.47 0.6 0.56

575 0.6 0.58 0.7 0.64

550 0.6 0.57 0.6 0.60

525 0.4 0.37 0.5 0.43

500 0.3 0.27 0.5 0.40
17

Table 8. Absolute error analysis of the two interpolation

methods for mixing ratio at 0600 GMT.All values are

in g/kg.

Time New

interpolation method

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

i000 i .4 0.63 0.6 0.37

975 1.3 1.18 0.8 0.87

950 1.0 0.84 0.9 i. i0

925 1.4 1.23 1.3 1.50

900 1.5 1.27 1.4 1.39

875 1.7 1.30 1.7 1.22

850 1.8 1.71 1.9 1.59

825 1.5 1.20 1.5 1.28

800 1.6 1.66 1.6 1.29

775 1.4 1.44 1.3 1.03

750 1.5 1.51 i .5 1.19

725 1.2 1.07 1.6 1.03

700 1.2 1.07 1.7 1.00

675 1.0 0.95 1.4 0.85

650 0.8 0.80 i.I 0.69

625 0.8 0.73 0.8 0.70

600 0.6 0.57 0.8 0.64

575 0.6 0.41 0.7 0.53

550 0.5 0.42 0.7 0.54

525 0.5 0.41 0.5 0.50

500 0.5 0.38 0.5 0.41
18

Table 9. Absolute error analysis of the two interpolation

methods for mixing ratio at 0900 GMT.All values are

in g/kg.

Tline New

interpolation method

Pres Mean Standard Mean Standard

(mb) errors deviations errors deviations

I000

975

950

925

900

875

850

825

800

775

750

725

700

675

65O

625

600

575

550

525

5OO

0.9 0.86 0.5 0.47

1.2 0.99 0.9 0.51

0.9 1.06 0.7 0.47

1.4 1.80 1.2 1.94

1.3 1.67 1.2 1.87

1.4 1.08 1.4 1.20

1.6 1.68 1.8 1.53

1.7 2.00 1.9 1.82

1.3 1.84 1.5 1.69

1.2 1.45 1.4 1.25

1.0 0.92 i.I 0.82

0.9 0.75 1.2 0.73

1.1 0.98 1.3 0.88

0.7 0.70 I.i 0.74

0.8 0.73 i.i 0.64

0.8 0.82 1.0 0.79

0.7 0.67 0.9 0.78

0.6 0.50 0.8 0.61

0.5 0.49 0.6 0.51

0.4 0.44 0.5 0.45

0.4 0.34 0.5 0.36
19

levels. Also when comparing the errors associated with the newmethod

applied to separated storm and non-storm cases, there was no distinct

advantage found to separation of the cases over performing the interpolation

without regard to storm or non-storm sounding. Consequently the combined

interpolation technique without separating the soundings was used in the

remainder of this study.

Stability Indices

The four MCCcases for the study are listed in Table i0. In these four

cases, on average, nine rawinsonde stations were included in the study of

each case. Case I contained seven stations which was the smallest number

while Case 4 had 13 stations.

Table I0. Mesoscale Convective Storm Systems (MCSs)included in study.

Initiate and terminate times are after Maddoxet al. (1982),

Rodgers et al. (1983, 1985).

Case Date

number Initiate Max imum Terminate

extent *

i i0/ii Apt 81 2315/10 0300/11 0531/11

2 9 May 81 0115/09 0500/09 1015/09

3 17 May 82 0030/17 0400/17 0730/17

4 i0/ii Jun 82 2245/11 0700/11 1530/11

*The times listed are the maximum extent of the area, as depicted

on IR satellite imagery, with temperatures < -62 C.
20

After the interpolated soundings were obtained, a variety of stability

indices were examined. Thesewere the K, modified K, Total-Totals, modified

Total-Totals, CAPE,and Showalter indices along with the maximumrate of

decrease of equivalent potential temperature over a 25 mb layer from the

surface to 700 mb (PII- Potential Instability Index). The difference

between the modified and the regular versions of the K and Total-Totals

indices is that in the modified versions, the moisture was averaged between

the surface and 850 mb for determination of a dewpoint temperature to be

used at 850 mb. In the standard versions, the observed 850 mb dewpoint is

used. The justification for this approach is that in manyof the soundings,

the moisture was confined to the layer below 850 mb. So, the value of the

standard index could be misleading in terms of its accounting for lower

tropospheric moisture.

After calculating, plotting and examining the fields of these indices,

the standard versions of the K and Total-Totals indices along with the CAPE

were discarded. The reason for dropping the K and Total-Totals indices was

that the fields of these indices were similar in nature to the fields

plotted from the modified versions Also, it was felt that that the

modified versions were more representative of the actual stability of the

atmosphere. The CAPEindex, which is a measure of the positive area of the

sounding when plotted on a Skew-T, Log-P diagram, was small or even zero

throughout the plotted fields. This result is not surprising since the

stabilizing effect of nocturnal, radiative cooling was present in the

boundary layer.
21

The Environmental Airflow

To show the airflow in the atmosphere around the MCC, the winds at

approximately i km above the surface at O0 and 12 GMT were obtained from the

rawinsondes. This level was chosen because it is thought to be near the top

of the boundary layer and therefore indicates the flow, free of friction,

whose stability may be affecting the storm. To get the winds at 03> 60, and

09 GMT, linear time interpolation between the 00 and 12 GMT wind values was

used. The movement of each storm was determined from enhanced infrared,

satellite images. This motion vector was then subtracted from the i km

winds to get the storm-relative wind vectors.

Results for Case 1

The analyses of the four cases listed in Table i0 are incomplete at this

time. Some of the results for Case I are available and are discussed below.

Figures la,b,and c deal with the field of modified K index at 00, 03,

and 06 GMT, respectively. It can be seen in Table I0 that these times

correspond to 45 min after MCC initiation, the time of maximum extent, and

29 min after MCC termination, respectively. Termination refers only to the

time that the storm ceased to qualify as an MCC, and not to its actual

demis e.

In Fig. la, the storm position is indicated by the heavy lines

indicating the IR equivalent black-body temperature field at the top of the

storm. The storm-relative velocity vectors in knots are shown for selected

stations and the thin lines represent the modified K values. Figures Ib and

ic show the same quantities. Several features can be observed by a
22

Fig. la. Modified K index field for 0000 GMTii April 1981.
23

Fig. lb. Same as Fig. la except for 0300 GMT.
24

\

Fig. Ic. Same as Fig. la except for 0600 G_f.
25

comparison of these three figures. First, it is noted that throughout this

6-h period, there is an axis or ridge of larger modified K values to the

east of the storm. However, as time goes on, there is a generalized

decrease of modified K values across the entire area. This reflects the

stabilizing influence of the nocturnal cooling. The relative flow at 00 and

03 GMT indicated an apparent advection of less stable air into the storm

area which would seem to explain why the storm reached its maximum extent at

03 GMT. However, a visual examination of the modified K values contained

within the 240°K isotherm at each time reveals that the approximate average

value went from about 30 at O0 GMT to 27 at 03 GMT and then to about 24.5 at

06 GMT. Thus, it would appear that the radiation effect dominated over the

advection effect so that even as the storm was approaching maximum

intensity, it was self-destructing in the sense that it was drawing in

more-stable air.

Figures 2a b,and c show the situation for the stability field based on

the modified Total-Totals index. These figures indicate an altogether

different situation than seen in Figures la-c. In these figures the most

unstable region is to the northwestern side of the storm at 00 and 03 GMT

and to the northeast and southwest of the storm at 06 GMT. The relative

flow throughout the time period indicates an apparent advection of more

stable air into the storm area. A general diminishing of stability across

the area occurs only in the period from 03 to 06 GMT. The most unstable

approximate storm average value corresponds to the time of maximum extent at

03 GMT. So in this case, the modified Total-Totals field did not
26

Fig. 2a. Modified Totals-Totals index field for 0000 GMT

ii April 1981.
27

Fig. 2b. Sameas Fig. 2a except for 0300 GMT.
28

Q

¢ \

Fig. 2c. Same as Fig. 2a except for 0600 GMT.
29

immediately indicate the stabilizing effect of nocturnal cooling.

The Showalter index stability fields at 00 through 06 GMTare shownin

Figures 3a,b_and c, respectively. These figures show a similar situation to

that shown in Figures 2a-c. The most unstable regions from 00 to 06 GMT are

located to the north and south of the storm. The relative flow at 00 GMT

indicates that the apparent advection at this time is small but is probably

moving slightly less-stable air towards the storm area. At 03 and 06 GMT,

the apparent advection again is small but indicated movement of somewhat

more stable air into the storm area. This initiation in the apparent

advection of more stable air into the storm corresponds to the time of

maximum extent. The approximate average of the Showalter index within the

240°K isotherm went from about -0.5 at 00 GMT to about -2.0 at 03 GMT and

then to -i.0 at 06 GMT. Thus, the most unstable value again corresponded to

the time of maximum extent. Again, the effect of the nocturnal cooling in

the lower troposphere was not reflected in the Showalter field until 06 GMT.

The PII (Potential Instability Index) stability field is depicted in

Figures 4a,b,and c for 00, 03, and 06 GMT, respectively. The most unstable

air, according to this index, is located to the north of the storm with an

axis of less-stable air to the east of the storm. This axis shifts somewhat

westward with time while the storm moves northwestward until it becomes

located on the axis at 06 GMT. Also during this period, the entire field of

PII undergoes a modification in values indicating greater stability with
30

o -._

Fig. 3a. Showalter index field for 0000 GMT ii April 1981.
31

_B

oB

_%_ _oB _'°

,P

S"

Fig. 3b. Same as Fig. 3a except for 0300 GMT.
32

Fig. 3e. Same as Fig. 3a except fo_ 0600 GMT.
33

Fig. 4a. Potential Instability Index field for 0000 GMT

ii April 1981.
34

I .,.....0 . j/f° _ jjjfJ/ " o. .

I'--.. " .... "-0.®"- 7 ,," i /-" i

I" "- ".. ----'_ -" .." _'_ -'t

,... "...-r_-a_._ ,'_-_'_ ..._ .. ..;'"I ,.,"'1

_ !-......""-e._ ...... ..- ./.." ./_ J

Fig. 4b. Same as Fig. 4a except for 0300 Q_T
35

Fig. 4c. Same as Fig. 4a except for 0600 GMT.
36

time.

At 00 GMT, the average PII value within "the 240 K isotherm was

approximately -4.5. By 03 GMT, this average has decreased to about -5.5

meaning the area was slightly less-stable. Finally, at 06 GMT, the average

was about -4.0. These changes in internal stability correspond nicely with

the intensity changes in the storm. The relative flow at 00 GMT suggests a

weak flux of less-stable air into the storm. But at 03 and 06 GMT, the flow

suggests an apparent flux of stable air into the storm area. Again, this

corresponds well to the observed intensity variations of the MCC.
37

IV. TENTATIVE CONCLUSIONS

One would expect that regardless of the structure of the stability

field provided by any of the given indices that certain changes and

relationships should be observed to occur during the lifetime of the MCC.

These are:

i. The stabilizing influences of the nocturnal radiative cooling

should be reflected in the overall stability field becoming more

stable with time.

2. The stability within the storm boundary should be reflected in

the intensity changes in the storm. Thus, the time of maximum

extent of the storm should occur near the time of maximum

instability in the storm. Increasing stability of the air in

the storm should lead to diminished storm intensity.

3. The storm-relatlve flow should indicate a flux of less-stable

air into the storm as it approaches its most intense stage and

a flux of more-stable air into the storm as its termination

time is approached.

In fact, the four stability indices examined here differed from one

another in some of these aspects while agreeing in others. Only the

Potential Instability Index (PII) met expectations in every category listed

above. The reason for this is probably a consequence of the quantities

which go into the calculation of each of these indices and the extent to

which they become modified by the lower tropospheric stabilization provided

by the interpolation scheme. The solution to this problem requires

additional analysis of Case I and the comparison with results from the other
38

cases under study.
39

V. REFERENCES

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NY, 461 pp.

Dupuis, L. R. and J. R. Scoggins, 1979 Differences between measured

linearly interpolated synoptic variables over a 12-h period during

AVE IV. NASA CR-3150, NASA Marshall Space Flight Center, Huntsville,

AL, 126 pp.

Fritsch, J. M., R. A. Maddox and A. G. Barnstron, 1981: The character

of mesoscale convective complex precipitation and its contribution

to warm season rainfall'in the U. S. Preprints, 4th Conf.

on Hydrometeorology, Reno, NV, Amer. Meteor. Soc., 94-99.

House, D. C., 1960: Remarks on the optimum spacing of upper air

observations. Mon. Wea. Rev., 88, 97-100.

Kreitzburg, C. W. and H. A. Brown, 1970: Mesoscale weather systems

within an occlusion. J. Appl. Meteorol.,

9, 417-432.

Maddox, R. A. 1980: Mesoscale convective complexes. Bull. Amer.

Meteor. Soc., 61, 1374-1387.

1981: The structure and llfecycle of midlatitude mesoscale

convective complexes. Atmos. Sci. Pap. No. 336, Colorado State

University 311 pp.

, D M. Rodgers and K. W. Howard, 1982: Mesoscale convective

complexes over the United States during 1981. Mon. Wea.

Rev. ii0, 1501-1514.

Overall, J. W. and J. R. Scoggins, 1975: Relationships between

motion on isentropic surfaces from 3-h rawinsonde data and

radar echoes. NASA CR-2558, NASA Marshall Space Flight Center,

Huntsville, AL, iii pp.

Read, W. L and J. R. Scogglns, 1977: Vorticity imbalance and stability

in relation to convection. NASA CR-2819, NASA Marshall Space

Flight Center, Huntsville, AL, iii pp.

Rodgers, D. M., K. W. Howard and E. C. Johnston, 1983 Mesoscale

convective complexes over the United States during 1982. Mon.

Wea. Rev., iii, 2363-2369.

, M. J. Magnano and J. H. Arns, 1985: Mesoscale convective

complexes over the United States during 1983. Mon. Wea.

Rev., 113, 888-901.
40

Welshinger, M. J., 1985: Factors leading to the formation of arc cloud

complexes. Masters Thesis, TexasA&MUniv., College Station, TX,

ii0 pp.

Wilson, G. S. and J. R. Scoggins, 1976: Atmospheric structure and

variability in areas of convective storms determined from 3-h

rawinsonde data NASA CR-2678, NASA Marshall Space Flight

Center, Huntsville, AL, 118 pp.