presented by wes junker at comet for the hydrometeorology faculty course 2000 monday, 12 june 2000...
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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.
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