Mesoscale convective systems
Mesoscale convective systems
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.
FRONTAL AND MESOHIGH (850
MB)
Why does the orientation of the low-level jet favor heavy rainfall?
(From Maddox et al. 1980)
Mesohigh or Frontal
Type
Outflow boundary or front provides focus for lifting. The area at
highest risk for heavy rainfall is in red.
Slide 6
About 60% of mesohigh and
frontal type heavy rainfall events occur near the ridge axis.
NEAR RIDGE AXIS YOU HAVE
EITHER 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
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
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.
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
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
MCSs can develop a number of
ways. Mature systems have a convective and “stratiform” precipitation shield
Predictions of MCS symmetry
and movement play a significant role in determining precipitation amounts
How does stratiform
precipitation form
MCSs over Southeast
Individual cells move
approximately with the 850-300 mean wind during early stages of an MCS
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
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
An example of a
quasi-stationary convective system
Factors favorable to
quasi-stationary convection
MOISTURE CONVERGENCE
STRENGTHENS OVER EASTERN NE AS PRESSURES FALL IN RESPONSE TO THE APPROACH OF A
WEAK SURFACE WAVE
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
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.
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 maximum observed
rainfall at a point versus the size of the 2” area
When the moisture
convergence is aligned with the 850-300 mb mean flow, a sizeable area of 3” precipitation is more likely.
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
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)
700 mb temperatures above 12oC
appear to limit the size of any convective system that forms.
COMPOSITE OF 12 LARGEST
EVENTS, THE HEAVIEST RAIN OCCURS AT THE NOSE OF THE LOW-LEVEL JET IN/OR NEAR
THE STRONGEST WARM ADVECTION
The moisture transport (flux
or qV) and moisture convergence are dependent on the low-level jet.
THE HEAVIEST RAIN USUALLY
OCCURS TO THE NORTHEAST OF THE THETA-E RIDGE, NEAR BUT JUST SOUTH OF THE
MAXIMUM IN THETA-E ADVECTION
IN SUMMARY
In summary (continued)
MCCs IN WEST, CLIMATOLOGY
LOCATIONS OF LARGEST 45
MCS/MCC SYSTEMS USED TO PREPARE 4 TYPES OF ATMOSPHERIC COMPOSITES
SOME CLIMATOLOGY
FREQUENCY
OF FLASH FLOODING OR 2”/24HR RAINFALL FOR 137 EVENTS IN WEST
NOTE
THE HIGH FREQUENCY IN LATE JULY AND AUGUST
Slide 43
The vast majority of front
range events occur during the late July and early August,
HEAVY RAIN EVENTS ALONG THE
FRONT RANGE
BIG THOMPSON, FORT COLLINS, CHEYENNE, MADISON COUNTY (VA)
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
ETA 500MB FORECASTS
NOTE
THE TILT OF THE UPPER RIDGE AXIS.
OBSERVED MAPS VALID 00Z
MODEL 12-36 HR QPF
VERIFICATION
In conclusion