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

2. Laing 2006 2 Introduction African societies need better precipitation of prediction (economy, health, water resource management, …).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 =>Focus on properties of organized convection in Africa => Better understanding of water cycle at various scales, improve models & prediction

3. Laing 2006 3

4. 4 Seasonal distribution of African MCCs

5. Laing 2006 5 Data & Methods Meteosat-7 IR, 30min NCEP GFS Analyses May 1999 - March 2004 0 20W 20E 20S 20N 0 40E 0.6 1.2 Average Elevation (km) 35S - 20S Nov - Feb Autumn, Spring Autumn, Spring 0.5 1.0 1.5 20W10W010E20E30E 40E Average Elevation (km) 0 – 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

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

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

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

9. Laing 2006 9 SH AFRICA Summer (35S–20S) 1100 UTC 9 Dec 1800 UTC 9 Dec 2000 UTC 9 Dec 0430 UTC 10 Dec 1100 UTC 9 Dec 1500 UTC 9 Dec 1800 UTC 9 Dec 2000 UTC 9 Dec 2330 UTC 9 Dec 0430 UTC 10 Dec 8 -11 Dec 2003 0.6 1.2 10E20E30E40E Average Elevation (km) (35S – 20S) Cold Cloud Episodes exhibit coherence in phase

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

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

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

13. Laing 2006 13 Span vs Duration for Four Continents

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

15. Laing 2006 15 Comparing Continental Domains Region (Longitude of Domain) Span (km) Duration (h)Phase Speed All episodes(ms - 1 ) Contiguous US (37deg) 838 (1 per day mean) 18.5 (1 per day mean) Median – 13.6 East Asia (50deg) 620( 1 per day mean) 11.6 (1 per day mean) Mean – 12.4 Europe (50 deg) Mean – 469.16Mean – 8.56Mean – 14.88 Median – 13.6 Tropical N. Africa (60deg) ~5 episodes per day Mean - 1062 Median – 700 Mean – 25.4 Median – 18.0 Mean – 12.0 Median – 11.2 Mid-latitude S. Africa (35 deg) PRELIMINARY (6months) Mean- 788 Median- 597 Mean – 23.5 Median - 18.5 Mean – 11.6 Median – 10.4

16. 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)

17. 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)