The diurnal cycle and propagation of convection in Africa

1. The diurnal cycle and propagation of convection in Africa Arlene Laing National Center for Atmospheric Research

2. Introduction • African societies need better precipitation of prediction (economy, health, water resource management, …). Prediction depends on improved understanding of initiation and evolution of organized convection • Radar-based studies in US and satellite-based studies in East Asia, Australia, and Europe found that organized convective precipitation displays systematic propagation at regional to continental scales • Coherence behavior implies qualitative predictability beyond one or two days. • Focus on properties of organized convection in Africa =>Better understanding of water cycle at various scales, improve models & prediction Laing 2006

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4. Seasonal distribution of African MCCs Laing 2006

5. Data & Methods Average Elevation (km) 0 – 20N 20N May - Aug • Reduced Dimension (Hovmoller) techniques. Cold cloud as proxy for precipitating convection • Compute phase speeds, duration, span • Compare with easterly wave statistics • Examine large-scale influences 0 Autumn, Spring 1.5 1.0 0.5 20S 20W 10W 0 10E 20E 30E 40E Nov - Feb 40E 20W 0 20E Average Elevation (km) 35S - 20S 1.2 0.6 • Meteosat-7 IR, 30min • NCEP GFS Analyses • May 1999 - March 2004 Laing 2006

6. US Mainland, Warm Season Episodes of Precipitation and Cold Clouds exhibit coherence in phase Laing 2006

7. Tropical North Africa Episodes of Cold Clouds exhibit coherence in phase, Average frequency of cold cloud as a function of longitude and time occur daily Laing 2006

8. AFRICA: Transition Season (5S – 15N) Cold cloud episodes: coherent in phase Mean Diurnal Cycle 16 - 31 Oct 1999 (Tbb< 233K) Laing 2006

9. SH AFRICA Summer (35S–20S) 1100 UTC 9 Dec 1100 UTC 9 Dec 8 -11 Dec 2003 1500 UTC 9 Dec 1800 UTC 9 Dec 1800 UTC 9 Dec 2000 UTC 9 Dec 2000 UTC 9 Dec 2330 UTC 9 Dec 10E 20E 30E 40E 1.2 Average Elevation (km) (35S – 20S) 0.6 0430 UTC 10 Dec 0430 UTC 10 Dec Cold Cloud Episodes exhibit coherence in phase Laing 2006

10. AFRICA: SH Summer 16 – 31 Dec 2003 Tropical Cyclone Cela Mean Diurnal Cycle Eastward propagation from high terrain Laing 2006

11. AFRICA: SH early summer (35S -15S) 16 - 30 Nov 2003 Mean Diurnal Cycle Less propagation, more convection with diurnal heating maximum Laing 2006

12. Daily Mean U wind, 500hPa Cold Cloud Episodes 01 1-15 Dec 2003 03 06 09 12 15 0 m/s Mid-Latitude S. Africa • Propagation influenced by synoptic regime • Convection Episodes are less frequent than Sahel and Central Africa Cela Laing 2006

13. Span vs Duration for Four Continents Laing 2006

14. Mid-latitude Southern Africa Span vs Duration Weak correlation Phase Speed Laing 2006

15. Comparing Continental Domains Laing 2006

16. Summary: Mid-latitude Southern Africa • Coherent pattern of deep convection • Episodes of organized convection are less frequent than other regions. Needs additional forcing from synoptic scale waves • Phase speeds less than other continents • High terrain aids in convective initiation • Regime with propagation exhibit delayed-phase shift in diurnal max • Examining westerly wind shear to determine influence on frequency and propagation (propagating vs non-propagating regimes) Laing 2006

17. Dynamical similarity of mesoscale convective systems globally • Genesis of convective systems in high terrain • Systematic propagation on regional-continental scale (coherent behaviour) • Diurnal-maximum in precipitation shifted because of propagating cloud-systems • Scale-interaction between convective systems and synoptic waves • Propagating systems with vertical shear • Tropical north Africa (African Easterly Jet, West African Monsoon) • Southern Africa, East Asia, US, Europe, Southern Australia (Westerly shear with jet streams undulations) Laing 2006