Presented by Wes Junker

At COMET

For the Hydrometeorology Faculty Course 2000

Monday, 12 June 2000

Mesoscale convective systems

Mesoscale convective systems

Come in a variety of sizes and shapes MCSs account for 30-70 percent of precipitation during the warm

season (Apr-Sept.) (Fritsch et al, 1986* , Kane et al., (1987*) Found precipitation characteristics of MCS are

similar to those of MCCs except for size. Larger MCCs tend to produce more cumulative rainfall and to a lesser

extent point rainfall. (McAnelly and Cotton, 1989 MWR). Smaller MCSs typically are of shorter duration (Geerts, WAF 1998) Large MCCs in Plains generally reach their peak size and intensity at

about midnight (Houze MWR 1990).– However, between 35N and 35S, MCSs are twice as likely at sunset than

sunrise (Mohr and Zipser, BAM 1996).

*J. Climate Appl. Meteor., 1986)

Maddox et al. MCC papers revolutionized summer forecasting of precipitation. The paper noted the ingredients a

forecaster should look for when anticipating an MCC that might produce a flash flood.

MOST ARE ASSOCIATED WITH MCCs OR MCSs AND OCCUR AT NIGHT

ABNORMALLY MOIST, PWS USUALLY ARE 1.40” OR HIGHER AND AVERAGE ABOUT 1.62”.

VERTICAL SHEAR IS WEAK TO MODERATE ALLOWING SLOW MOVEMENT

MANY OCCUR NEAR THE 500 MB RIDGE POSITION OCCUR AT THE NOSE OF THE LOW LEVEL WIND

MAXIMUM

FROM MADDOX ET AL., 1979

Td=14 oC

FRONTAL AND MESOHIGH (850 MB)Why does the orientation of the low-level jet favor

heavy rainfall? (From Maddox et al. 1980)

FRONTAL Td=16 oC

Axi

s of

Max

Win

ds

MESOHIGH120 nm

Td=10 oC

T d=1

4 o C

Td=12 oC

Max

Win

ds

Axi

s of

120 nm

SCALE IS USUALLY SMALLER FOR MESOHIGH EVENTS (Kane et al., 1987, J. Climate Appl. Meteor., 26, 1345-1357)

Mesohigh or Frontal TypeOutflow boundary or front provides focus for lifting. The area at highest risk for heavy rainfall is in red.

L

Td=70oF

H

HTd=70oFTd=60oF

Td=60oF

H

BUBBLE HIGH

OUTFLOW BOUNDARY

L

COOL AND MOIST

WARM AND MOIST

Td=70oFTd=70oF Td=60oF

Td=60oF

SURFACE MESOHIGH

120 nm

WARM AND MOIST

COOL AND MOIST

120 nm

SURFACE FRONTAL

0

5

10

15

20

NU

MB

ER

OF

EV

EN

TS

J F M A M J J A S O N DMONTH

MESHOHIGH

0

2

4

6

8

10

12

14

NU

MB

ER

OF

EV

EN

TS

J F M A M J J A S O N DMONTH

FRONTAL

200 MB

500

850

700

300

SFC

-56

-36

6-10

17

3

4

7

1013

6570

PW=1.60”

(158%)

K=38

SI=-4

15 FRONTAL

200 MB

300

500

700

850

SFC1014

-36

10

-10

-57

674

183

7166

PW=1.64”

(162%)

K=39

SI=-5

MESOHIGH

About 60% of mesohigh and frontal type heavy rainfall events occur near the ridge axis.

500 mb

120 nmMESOHIGH HFRONTAL

500 mb

MOIST

MOIST

Maddox et al., 1980

NEAR RIDGE AXIS YOU HAVE EITHER WEAK INERTIAL STABILITY OR INERTIAL

INSTABILITY.

AREAS WITH STRONG ANTICYCLONIC SHEAR HAVE WEAK INERTIAL STABILITY OR INERTIAL INSTABILITY

Two Conceptual diagrams of the structure of an warm core MCS, from a circulation perspective on left (Scofield and Junker 1988),

and from an PV anomaly perspective on right (Fritsch et

al., JAS, 1994)

Mesoscale convective

vortices

Convection redevelops on afternoon and then strengthens during night. During day convection tends to develop on periphery near outflow boundary, at night often

redevelops near center of vortex. (Bartels and Maddox, MWR 1991)

Figure from Fritsch et al, (JAS, 1994)

0330 UTC0300 UTC

0500 UTC 0900 UTC

Latitudinal and monthly distribution of MCC centroids at maximum extent. Contours represent average distribution for period 1978-1985.

Dots make up individual yearly distribution, a) for 1986, b) 1987. Shaded area indicates null period

From Augustine and Howard (MWR,1991)

850 analysis of heights, temps, winds (full barb 5 ms-1. Dark and light shaded areas depict the 12 and 10 g kg-1 mean mixing ratio. On left null

period, on right active period.

From Augustine and Howard (MWR,1991)

MCCs need mositure and instability to form.

MCS area at maximum extent versus maximum 850 mb frontogenesis in the vicinity of the location of the

maximum extent at 00 UTC. L=large MCC, Small MCS

From Augustine and Caracena , 1994, Wea. Forecasting

Normalized composite precipitation (mm) pattern for 74 MCCs. Dashed and dotted lines are approximate centroid tracks of -32 and -54oC cloud-shields, respectively. The

horizontal axis is the axis of propagation and indicates the storm heading

From Kane et al., 1987*

Cluster around propagation axis the probability of 1 mm of rain is 100% but

for 75 mm drops to 10% nearly coincident with the 35 mm shown in figure above.

MCSs can develop a number of ways. Mature systems have a

convective and “stratiform” precipitation shield

Loerer and Johnson, Wea. Forecasting 1995

Stratiform rainfall can produce up to 50% of rain in some MCSs

Predictions of MCS symmetry and movement play a significant role in determining precipitation amounts

Possible cause of asymmetry

1) The Coriolis force acting upon the ascending front-to-rear flow turns the flow to the north, leading to an accumulation of hydrometeors and buoyancy

2) The Coriolis force acting on the surface cold pool helps drive cold air south leading to new cell generation

3). Position of low-level jet and strongest instability.

Loerer and Johnson, Wea. Forecasting 1995

How does stratiform precipitation form

From front to rear ascending air creating anvil melting of ice or snow crystals which induce

cool pool just below freezing level this tightens thermal gradient and is

frontogenetic. Thermally direct circulation enhances lifting

above freezing level and subsidence below it.

From Szeto et al, 1988 JAS

MCSs over Southeast

Are about twice as common is summer as winter

– occur mostly in afternoon but the amplitude of occurrence in the afternoon is not as great as for general thunderstorms

– despite being more common in summer, the probability of any point being affected by an MCS is about the same for winter as summer

in summer-usually are small and short lived in winter-are larger and longer lived in winter do not evolve into typical leading line trailing

stratiform precipitation very often.

Mean 850-300 mb wind speed

Ob

serv

ed c

ell s

pee

d

du

rin

g M

CC

gen

esis

25

35

30

35302520

20

15

15

10

10

5

5

0

0

r=.71180

180

210

210

240

240

270

270 300

300

330

330

360

360

Mean 850-300 mb wind direction

r=.76

Ob

serv

ed c

ell d

irec

tion

d

uri

ng

MC

C g

enes

is

From Corfidi et al, 1996

Individual cells move approximately with the 850-300 mean wind during early stages of an

MCS

Dir

ecti

on o

f lo

w-l

evel

jet

Systems with propagation vectors between 0-120 degrees have been plotted between 360 and 480 degrees

The direction of propagation is in the opposite direction of the low-level jet. This may be why MCCs tend to track to the right of the mean wind.

60

360 420 480

120

180

300180 240

240

300

Direction of MBE propagation

r=.65

From Corfidi

The direction of the MBE (the most active part of the MCS) is dependent on the direction of the low-level jet (Corfidi et al., 1997) and on the position of the most moist and unstable air relative to the MCS.

Movement of convective systems

Individual cells movement can be approximated using the mean wind

Movement of an MCS is dependent on, 1) the mean flow between 850-300 mb (Corfidi, 1994), and 2) the rate new cells are growing (propagation)

The propagation rate is strongly dependent on the low-level jet but is also dependent on the strength of the cold pool

The stronger the low-level jet (compared to the mean wind), the more the MCS will deviate from the mean wind.

UNSTABLE AIR

UNSTABLE AIR

DIRECTION OF PROPAGATION

MCS

AXIS OF LOW-LEVEL JET

1000-500 THICKNESS

1. FORWARD

2. BACKWARD

N

W E

SADOPTED FROM JIANG AND SCOFIELD, 1987

THE PROPAGATION OF A CONVECTIVE SYSTEM IS DEPENDENT ON THE LOCATION OF: 1) THE MOST UNSTABLE

AIR, 2) THE AXIS AND ORIENTATION OF THE LOW-LEVEL JET, AND 3) THE LOCATION OF THE STRONGEST LOW-LEVEL MOISTURE CONVERGENCE

The most unstable air is usually found upstream of the initial convection during backbuilding or quasi-stationary convective events

JUNKER AND SCNEIDER, 1997, NAT. WEA. DIGEST, ,21, 5-17

An almost e-w frontal band with PWS 1.80” or higher (shaded)

Area with most unstable Lifted Indices shaded. 35 TO 40 kt winds are feeding across KS into NE

An example of a quasi-stationary convective system

850-300 mb mean winds, 982 mb equivalent potential temperature (dashed) and msl pressure (solid)

1000-850 mb layer mean moisture flux (vectors)moisture flux magnitude (dashed) and moisture flux divergence (-4 x10-7s-1 are shaded), the red dot represents the location where convection started

1) mean winds almost parallel to the front but directed slightly away from it

00Z 00Z

2) a low-level e ridge to west, and

3) the location of the strongest moisture convergence west of the initial convection

JUNKER AND SCNEIDER, 1997, NAT. WEA. DIGEST, ,21, 5-17

Factors favorable to quasi-stationary convection

MSL PRESSURE (THICK SOLID), MOISTURE CONVERGENCE (HIGHEST VALUES SHADED), RED DOT IS WHERE INITIAL CELL FORMED

21Z 00Z

03Z 06Z

THE WIND AND MOISTURE CONVERGENCE FIELDS CAN CHANGE RAPIDLY AS A RESULT OF PRESSURE RISES OR FALLS.

MOISTURE CONVERGENCE STRENGTHENS OVER EASTERN NE AS PRESSURES FALL IN RESPONSE TO THE APPROACH OF A WEAK

SURFACE WAVE

21Z 00Z

02Z 06Z

1st cell

New cells form upstream Merger

Accumulated precipitation from the storm

DURING THE 1993 DSM FLASH FLOOD, THE CONVECTIVE SYSTEM REMAINED

STATIONARY FOR ABOUT 9 HOURS, WHY?

Investigation of the MCS during the Great Flood of 1993

MCSs were investigated for June-Sept. all 2, 3, 4 and 5 inch areas were measured for

each MCS identified systems were categorized based on the size

of the 3” coverage The largest scale, heaviest events were

compared with smaller scale events that produced less rain.

Average size of various precipitation thresholds for each category (km2) during June-Sept. 1993, (Junker et al 1999 WAF)

Cases where lower relative humidity and/or a stronger cap are more likely to have the convection

form north of the front.

THICKNESS VALUES FOR 70% SATURATION

P.W. P.W.THICKNESSTHICKNESS

.70

540

534

528

522

561

558

552

546

570

567

564

561

576

573

582

579

.55

.27

.22

.43

.35

.90

.80

1.90

1.15

1.05

.90

1.70

1.40

1.25

1.55

558552

552558564

564

570

570

PW=0.80”

PW=1.15”

L

L

=CONVECTIVE AREA

INFLOW

INFLOW

outflow boundary

From Funk (WAF, 1991)

THE LARGER SCALE HEAVY RAINS FELL WITH HIGHER RH VALUES. THERE WERE CATEGORIES BASED ON THE AREAL EXTENT OF THE 4 INCH. CAT 1 HAS NO 3 INCH AREA, WHILE

CAT 4 HAD 3600 SQ. NAUTICAL MI. OR MORE

The fact that few larger scale heavy rainfall events occurred to the of the line may be the reason preferred thickness appears to work

Junker et al 1999 WAF

The maximum observed rainfall at a point versus the size of the 2” area

Junker et al 1999 WAF

When the moisture convergence is aligned with the 850-300 mb mean flow, a sizeable area of 3”

precipitation is more likely.

3600 sq. nm area of 3” more likely

(inches)

a 3” area is less likely

THE Y-AXIS REPRESENTS THE LENGTH OF THE -2X10-7 S-1 OR GREATER MOISTURE FLUX CONVERGENCE MEASURED UPSTREAM ALONG A LINE DEFINED BY THE MEAN FLOW.

Junker et al 1999 WAF

AVERAGE SIZE OF THE 3” FOR THE VARIOUS CATEGORIES, NOTE THE SMALL SCALE OF THE MOST INTENSE RAINFALL. THE BOTTOM RIGHT FIGURE IS THE LARGEST 3” DURING THE

STUDY

CAT 1 CAT 3CAT 2

CAT 4 LARGEST 3”

BECAUSE OF THE SMALL SCALE, IT IS VERY HARD TO CORRECTLY FORECAST THE CORE OF HEAVIEST RAINS. SOME KIND OF PROBABILISTIC APPROACH TO FORECASTING MAY BE BETTER THAN A DETERMINISTIC ONE

ALL THE CATEGORY EVENTS OCCURRED WITH PWS AT OR ABOVE 1.40”. IN GENERAL THE SHEAR WAS WEAK TO

MODERATE (Mean winds are in knots)

0

10

20

30

40

50 850-3

00 M

EA

N W

IND

SP

EE

D

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 PRECIPITABLE WATER (INCHES)

0

0

2

1

0 2

1

10

0

0

11

1

1

1

30

00

0

1

1

0

0

0 0

100

0

0

1

0

00 1

21

1

0

0

1

1

3

32

4

3

2

2

21

342

4

4

4

21

4

4

224

4

1

34

4

1

2

1

4

1

4

3

23

4

3

44

3 INCH CATEGORIESIN SQUARE DEG. LAT.

CAT 0=NONE OBS.CAT 1=.01-.25CAT 2=.26-.50CAT 3=.51-1.0CAT 4>1.0

Junker et al 1999 WAF

700 mb temperatures above 12oC appear to limit the size of any convective system that forms.

0

2

4

6

8

10

12

14

16

700 M

B T

(C

ELS

IUS

)

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 PRECIPITABLE WATER (INCHES)

0 02

1

02 1

1 0

0

01

1

1

1

1 30

0

00

1

1

0

0

0

0

100

0 01

0

0

01

2

1

1

0

0

1

1

3

324

3 2

2 2

13

4

2

4

4

421

4

4

2

24

4

1

34

4

1

2

1

4

14 3

2

3

4

34

4

CATEGORIES OF 3 INCHBASED ON SQUARE DEG. LAT

CAT 0 = NON OBS.CAT 1 = .01-.25CAT 2 = .26-.50CAT 3 = .51-1.0CAT 4 > 1.0

The HPC rule of thumb that 700 mb temperatures above 12oC will provide an effective cap is a decent first guess BUT you should also look at the negative area on the sounding

COMPOSITE OF 12 LARGEST EVENTS, THE HEAVIEST RAIN OCCURS AT THE NOSE OF THE LOW-LEVEL

JET IN/OR NEAR THE STRONGEST WARM ADVECTION

850 MB WIND DIRECTION (ARROWS) AND ISOTACHS ON LEFT, 850 MB TEMPERATURE ADVECTION ON RIGHT, BLACK DOT IS CENTER OF HEAVIEST RAIN, 2 BY 2 DEG. LATITUDE GRID

1

The moisture transport (flux or qV) and moisture

convergence are dependent on the low-level jet.

850 mb moisture flux (left) and moisture flux divergence (right). Note that the heaviest rain occurred southeast of the strongest 850 mb moisture convergence. The red dot is the center of heaviest rainfall.

1918

0 2-2

2

4

4

6

6

8

8

0

-2

-4

-6

-4-8

-6-8

10

9

8

7

6

17

THE HEAVIEST RAIN USUALLY OCCURS TO THE NORTHEAST OF THE THETA-E RIDGE, NEAR BUT JUST SOUTH OF THE

MAXIMUM IN THETA-E ADVECTION

THETA-E (e) Ve

IN SUMMARY

The scale of precipitation associated with an MCS during the study appears to be related to – the relative humidity– the orientation of the moisture convergence

band with respect to the mean flow– the width of the axis of stronger moisture

convergence

In summary (continued)

Most of the MCSs formed to the north or northeast of the strongest 850 mb winds and moisture flux.

Most occurred in an area of 850 mb warm and theta-e advection

most occurred on the southern edge of the 250 mb divergence

the size of the heavy rainfall seems to be modulated at least in part by the RH and by the orientation of the moisture convergence to the mean flow

MCCs IN WEST, CLIMATOLOGYLOCATIONS OF LARGEST 45 MCS/MCC SYSTEMS USED TO

PREPARE 4 TYPES OF ATMOSPHERIC COMPOSITES

11

1

111

144

4444

4

22

2 22

2 2

2

2

2

2

22

2

2

221

1

1

1

1

1

11

33

33

LOCATION OF COMPOSITE CASES

3

LEGEND SYNOPTIC TYPE1

4

3

2

BLOCKING ANTICYCLONE

DEFORMATION ZONE

7

6

1715

SHORTWAVE TROUGH IN NORTHWEST FLOW

SHORTWAVE TROUGH IN ZONAL FLOW

#EVENTS (45)

FROM CHAPPELL COMET NOTES

Scale is usually smaller than East, so small, that predicting where the MCS will occur is almost impossible.

PWs OF 1.00” ARE HIGH

700 MB (DEWPOINTS IN WEST ARE TYPICALLY IN THE 6-8oC RANGE WHEN SIGNFICANT FLASH FLOODS OCCUR)

SURFACE DEWPOINTS ARE IN THE 50S

SOME CLIMATOLOGYFREQUENCY OF FLASH FLOODING OR 2”/24HR RAINFALL FOR 137

EVENTS IN WEST

NOTE THE HIGH FREQUENCY IN LATE JULY AND AUGUST

0

5

10

15

20

25

30

35

FR

EQ

UE

NC

Y O

F O

CC

UR

RE

NC

E

J F M A M J J A S O N D

SEMI-MONTHLY TIME OF YEAR

0

10

20

30

40

50

60

PE

RC

EN

T O

F C

AS

ES

N-4PM 4-8PM 8PM-M M-4AM 4-8AM 8-AM-N

LOCAL TIME OF OCCURRENCE

FROM CHAPPELL COMET NOTES

0

5

10

15

20

25

NU

MB

ER

OF

CA

SE

S

J F M A M J J A S O N DSEMI-MONTH TIME OF YEAR

INTERMOUNTAIN PLATEAU (83 EVENTS)

EVENTS IN INTERMOUNTAIN REGION ALSO HAVE A DISTINCT MAXIMUM DURING THE 6-HR PERIOD BETWEEN 2 PM AND 8 PM LOCAL DAYLIGHT TIME.

OCCUR MOSTLY IN AUGUST AND SEPTEMBER INTO EARLY OCTOBER.

The vast majority of front range events occur during the late July and early August,

0

5

10

15

20

NU

MB

ER

OF

CA

SE

S

J F M A M J J A S O N DSEMI-MONTHLY TIME OF YEAR DISTRIBUTION

FRONT RANGE EVENTS (49)DIVIDING INTO HALF-MONTH PERIODS

0

20

40

60

80

PE

RC

EN

T O

F C

AS

ES

8AM-2PM 2PM-8PM 8PM-2AM 2AM-8AM

LOCAL TIME OF DAY

and occur during the late afternoon and early evening hours (2-8 PM)

FROM CHAPPELL COMET NOTES

HEAVY RAIN EVENTS ALONG THE FRONT RANGEBIG THOMPSON, FORT COLLINS, CHEYENNE, MADISON COUNTY (VA)

FROM MADDOX ET AL., 1977

•A SLOW MOVING FRONT IS LOCATED UP JUST SOUTH OF THE AREA

•WINDS ALOFT ARE LIGHT AND SOUTHEASTERLY

•A LARGE AMPLITUDE NEGATIVE-TILTED UPPER RIDGE AXIS LIES NORTH AND EAST OF THE AREA

•A WEAK SHORTWAVE ROTATES NORTHWARD TOWARDS THE AREA RESULTING IN WEAK PVA

H

L

LLIDEALIZED SURFACE PATTERN

Cells develop east of highest terrain

* Cells then move slowly north and northwest* Redevelopment occurs on SE or S flank* Heaviest rain falls over a very small area* This pattern also occurs in east (ie. Madison County flash flood. Scale of rain is heaviest rain is small

THERMAL AXIS

AND MOISTURE TONGUE

ADOPTED FROM MADDOX ET AL., 1977

500 MBTROF

LOW LEVEL

LOW LEVEL JET

T-Td6oC

THREAT AREA

Td65oF

ETA 500MB FORECASTSNOTE THE TILT OF THE UPPER RIDGE AXIS.

L

NEGATIVE TILTING RIDGE AXIS

12 HR V.T. 00Z 24 HR V.T. 12Z

OBSERVED MAPS VALID 00Z

193

171

185

179

193

173

207

178

190

72

71

7254

72 7082

63

51

7158

6863

63

7364 13

7760

205

54

50 0 13

5

173

.. 1695

7666

6255 65 13

8267

193

7164

74

6848

6178

8270

8365

8270

.. 7169

13114882

63

6446

6556

8566

20

16

SURFACE 850 MB 500 MB

BOUNDARY?

Td15oCT-Td6oC

FROM THESE MAPS WHAT CAN BE INFERRED ABOUT THE PRECIPITATION EFFICIENCY OF ANY CELLS THAT FORM?

MODEL 12-36 HR QPF

.01”

.50”

1.0”

.2.0”

.3.0”

ETANGMAVN

WHICH MODEL DO YOU THINK HAS THE BEST FORECAST OF THE SCALE OF THE 2.00” OR GREATER AMOUNTS? WHAT ABOUT THE LOCATION OF THE MAXIMUM RAINFALL?

REMEMBER YOU NEED TO KNOW MODEL BAISES

VERIFICATION

.50”

1.0”

2.0”

3.0”.

VERIFYING ANALYSIS VALID 12Z JULY 29

8-10” OF RAIN ON FORT COLLINS & NEARBY FOOTHILLS

4 MILES AWAY ONLY .83” OBSERVED

295 HOUSES OR MOBILE HOMES DESTROYED, 4 KILLED

A LARGER MCS MOVED SOUTHEASTWARD AWAY FROM A SMALLER SCALE QUASI-STATIONARY CONVECTIVE STORM.

Note the barren dry expanse known as southeast Wyoming

In conclusion

MCSs remain a major forecast problem– However they produce a large proportion of the

rainfall in summer.– Probably produce the majority of flash floods– propagation effects need to be better forecast.

The current generation of operational models do not predict propagation very well. They have big problems handling outflow.