Transcript A Mesoscale Convective Complex (MCC)

Mesoscale Convective Complexes
(MCCs)
Mesoscale
M. D. Eastin
MCCs
Definition
Climatology
Environmental Characteristics
Typical Evolution and Structure
Forecasting
Mesoscale
M. D. Eastin
Definition
A Mesoscale Convective Complex (MCC):
Examples of MCCs
• Defined by Maddox (1980) and based entirely
on IR satellite observations
IR temperature criteria:
• Continuous cold cloud with IR temps < -32ºC
over an area greater the 100,000 km2
• Inner cold cloud region with IR temps < -52ºC
over an area > 50,000 km2
Duration: The IR criteria must be met for > 6 hours
Shape: Minor axis / major axis ratio must be > 0.7
• Within the “MCS spectrum”, mesoscale convective
complexes are large, long lived, and quasi-circular.
Mesoscale
M. D. Eastin
MCC Climatology
Basic Characteristics:
• Examined by Bartels et al. (1984)
• Documented 160 MCCs, including
their lifecycle and severe weather
16-31 May
1-15 Jun
16-30 Jun
1-15 Jul
• Most often occur May to August
in the central U.S.
• Rarely observed along the East
Coast or west of the Rockies
• Often produce severe straight-line
winds and heavy amounts of rain
and localized flash flooding
• Can produce hail and tornadoes
• Almost 25% of MCCs result in
injuries or death
• Produce ~10% of total annual
rainfall in many areas
• Produce up to ~30% of total rainfall
during the growing season
Mesoscale
M. D. Eastin
MCC Environments
Common Characteristics:
• Often occur along quasi-stationary surface frontal zones east of a mid-level trough
• Large region of conditional instability (CAPE >1000 J/kg) south and east of the region
• Strong low-level jet advecting warm, moist air into the region (high-θe air at 850 mb)
Synoptic Environments for Four Severe-wind MCCs
[Solid contour = MCC cloud edge
Dashed contours = Equiv. Pot. Temp.
Wind barbs and Streamlines]
MCC
MCC
Jet
Warm & Moist Air
Mesoscale
M. D. Eastin
MCC Environments
Common Characteristics:
• Mid-level inflow advecting dry air into the region (low θe air at 700-500 mb)
• Strong anticyclonic divergence in upper-levels (at 200 mb)
• Moderate vertical shear through the depth
Synoptic Environments for Four Severe-wind MCCs
[Solid contour = MCC cloud edge
Dashed contours = Equiv. Pot. Temp.
Wind barbs and Streamlines]
MCC
MCC
Mesoscale
M. D. Eastin
MCC Environments
Common Characteristics:
Severe Wind Producers:
• Greater inflow of dry air at mid-levels (helps to initiate downdrafts)
• Stronger deep layer vertical shear
• Faster moving
Heavy Precipitation Producers:
• Deeper layer of warm moist inflow
• Less inflow of dry air
• Pronounced cyclonic circulation at mid-levels
(helps protect against mid-level dry air intrusions)
• Weaker deep layer vertical shear
• Slower moving
Mesoscale
M. D. Eastin
MCC Evolution and Structure
Basic Evolution:
• Often develop in the late afternoon from
the merger of storms originating in different
locations (e.g., over the Rockies and along
the dryline)
• Some develop from squall lines that
acquire MCC characteristics over time
• Severe weather most often occurs during
the initial late afternoon development
(when the largest CAPE exists)
• Reach a mature stage around local midnight
(when the nocturnal low-level jet is the
strongest and thus can maintain a large
continuous supply of warm moist air)
• Dissipate in the early morning hours in
response to a more stable environment
and a smaller supply of warm, moist air
(low-level jet is weakest in the morning)
Mesoscale
M. D. Eastin
MCC Evolution and Structure
Internal Precipitation Structure:
Developing Stage:
• Area covered by deep convective dominates the
area covered by stratiform precipitation
• Upper-level cold cloud shield much larger in area
than the total precipitation area
Mature Stage:
• Areal coverage of stratiform precipitation dominates
with embedded regions of deep convection
• Cold cloud shield and precipitation area roughly equal
Dissipating Stage:
• Primarily stratiform precipitation
• Minimal cold cloud shield that often no longer satisfies
the MCC definition
Mesoscale
M. D. Eastin
MCC Evolution and Structure
Structure at Mature Stage:
• Shallow anticyclone flow
at upper and lower levels
• Deep layer inflow generates
strong mesoscale (and
convective) updrafts that
produces the large anvil
• Large diabatic heating aloft
(in the updrafts) produces
a mid-level warm anomaly
• Evaporational cooling due to
widespread stratiform rain,
produces a near-surface cold
dome (or cold pool)
• A low- to mid-level mesoscale
convective vortex (MCV) develops
as a hydrostatic response to the
warm and cold anomalies
Mesoscale
M. D. Eastin
MCC Evolution and Structure
Structure at Mature Stage:
• Note the vertical structure of the
MCV that passed to the north
of Lathrop, MO
Vertical wind profiles from a NOAA
atmospheric sounder
Mesoscale
M. D. Eastin
MCC Evolution and Structure
Structure at Dissipating Stage:
• As the upper-level cold cloud
shield and stratiform precipitation
dissipate in the early morning, the
MCV becomes “visible” on satellite
• The MCV will often persist
throughout the following day
Why?
Redevelopment:
• If the “old” MCV maintains itself,
the MCC often re-develops in
the late afternoon if an ample
supply of CAPE is available
Mesoscale
M. D. Eastin
Forecasting MCCs
Guidelines:
• Look for development in moderate CAPE and vertical shear environments along
quasi-stationary boundaries when deep convergence of warm, moist is expected
• Move with the mean flow in the 700-500 mb layer
• Potential for severe weather greatest in late afternoon
• Potential for localized flash floods greatest overnight
Total Accumulated Rainfall from an MCC
Mesoscale
M. D. Eastin
MCCs
Summary:
Definition
• IR temperature criteria
• Duration
• Shape
Climatology
Environmental Characteristics
• Severe wind producers
• Heavy precipitation producers
Typical Evolution and Structure
• Basic evolution
• Mature Precipitation and Kinematic Structure
• Dissipation and Redevelopment
Forecasting Guidelines
Mesoscale
M. D. Eastin
References
Bartels, D. L., and R. A. Maddox, 1991: Midlevel Cyclonic Vortices Generated by Mesoscale Convective Systems.
Mon. Wea. Rev., 119, 104-118.
Bartels, D. L., J. M. Skradski, and R. D. Menard, 1984: Mesoscale convective systems: A satellite based climatology. NOAA
Tech Memo, ERL ESG-6, Environmental Research laboratories, NTIS No. PB85-187862, 58 pp.
Johnson, R. H., and D. L. Bartels, 1992: Circulations associated with a mature-to-decaying midlatitude mesoscale
convective system. Part II: Upper-level features. Mon. Wea. Rev., 120, 1301-1321.
Maddox, R.A., 1980: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61, 1374-1387.
Maddox, R.A., 1981: Satellite depiction of the life cycle of a mesoscale convective complex. Mon. Wea. Rev., 109,
1583-1586.
Maddox, R. A., 1983: Large-scale meteorological conditions associated with mid-latitude mesoscale convective complexes.
Mon. Wea. Rev., 111, 1475-1495.
Maddox, R. A., K. W. Howard, D. L. Bartels, and D. M. Rodgers, 1986: Mesoscale convective complexes in the middle
latitudes. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed., Amer. Meteor. Soc., 390–413.
Wetzel, P.J., W.R. Cotton, and R.L. McAnelly, 1983: A long-lived mesoscale convective complex, Part II: Evolution and
structure of the mature complex. Mon. Wea. Rev., 105, 1919-1937.
Mesoscale
M. D. Eastin