Mesoscale Convective Complexes in Southern Africa: A Case study

1. Mesoscale Convective Complexes in Southern Africa: A Case study Yemi Adebiyi

2. Jul.(above line) and Jan. (below line) 1985-1986 Jun. – Aug. (above line) and Dec.-Feb.(below line) 1974-78 -- Laing and Fritsch, 1997 MCC event: 07 July 1997 over Nebraska -- GOES-8 IR

3. MCC in Southern Africa Laing and Fritsch, 1993

4. Definition • 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. Example from Markowski and Richardson, 2011

5. MCC Evolution and Structure Developing Stage: • Often develop in the late afternoon from the merger of storms originating in different locations or squall lines with MCC characteristics • Usually in area of LLJ, surface front with small CIN and large CAPE • Strong vertical motion and heating in lower troposphere Cotton et al, 1989

6. MCC Evolution and Structure Cotton et al, 1989 Matured Stage: • Reach a mature stage around local midnight or early morning hours (when the nocturnal low-level jet is the strongest) • Level of maximum vertical velocity and heating shift upward producing warm anomaly • Deep layer inflow generates strong mesoscale (and convective) updrafts that produces the large anvil • A low- to mid-level mesoscale convective vortex (MCV)develops as a hydrostatic response to the warm and cold anomalies

7. MCC Evolution and Structure Cotton et al, 1989 Dissipating Stage: • Dissipate in the early morning hours or around noon, in response to a more stable environment and a smaller supply of warm, moist air (low-level jet is weakest in the morning) • As the upper-level cold cloud shield and stratiformprecipitation dissipate in the early morning, the MCV becomes “visible” on satellite

8. Southern Africa MCC: 29- 30th Dec 2008

9. 00Z 30 Dec 03Z 30 Dec • Maximum size: ~200,000km2 • Duration: >≅6hours • Eccentricity: >0.7 • Cloud-top temperature ~ -80degC 06Z 30 Dec 09Z 30 Dec 12Z 30 Dec h) 15Z 30 Dec

10. Synoptic pattern Wind speed > 5m/s Blue lines: geopotential height Orange shade: >1010hPa Light blue: <998hPa

11. Moisture Flux convergence Contour: Moisture flux values(interval=40gm/kgs) Vector: moisture flux vector(scale: 150 gm/kgs) Shaded: Moisture convergence

12. Vertical Structure

13. Vertical Structure

14. Precipitation 3-hourly precipitation Daily average

15. Conclusion • MCC event on 29-30th Dec 2008 in southern Africa satisfies the Maddox (1980) criteria. • It was initiated by strong low-level wind aided by surface front and moisture convergence • Eastward movement of surface high consistent with previous study. • Daily average precipitation >80mm/day

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