Mesoscale Convective Complexes (MCCs)

1. Mesoscale Convective Complexes (MCCs) M. D. Eastin

2. MCCs Definition Climatology Environmental Characteristics Typical Evolution and Structure Forecasting M. D. Eastin

3. Definition Examples of MCCs • A Mesoscale Convective Complex (MCC): • 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. M. D. Eastin

4. MCC Climatology • Basic Characteristics: • Examined by Bartels et al. (1984) • Documented 160 MCCs, including • their lifecycle and severe weather • 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 16-31 May 1-15 Jun 16-30 Jun 1-15 Jul M. D. Eastin

5. 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 M. D. Eastin

6. 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 M. D. Eastin

7. 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 M. D. Eastin

8. 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) M. D. Eastin

9. 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 M. D. Eastin

10. 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 M. D. Eastin

11. 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 M. D. Eastin

12. 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 M. D. Eastin

13. 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 M. D. Eastin

14. 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 M. D. Eastin

15. 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. M. D. Eastin