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

Laing 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

Laing

4 Seasonal distribution of African MCCs

Laing Data & Methods Meteosat-7 IR, 30min NCEP GFS Analyses May March W 20E 20S 20N 0 40E Average Elevation (km) 35S - 20S Nov - Feb Autumn, Spring Autumn, Spring W10W010E20E30E 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

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

Laing 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

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

Laing 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 Dec E20E30E40E Average Elevation (km) (35S – 20S) Cold Cloud Episodes exhibit coherence in phase

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

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

Laing 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 m/s 1-15 Dec 2003 Cela

Laing Span vs Duration for Four Continents

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

Laing 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 – Mean – 8.56Mean – Median – 13.6 Tropical N. Africa (60deg) ~5 episodes per day Mean 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 Mean – 11.6 Median – 10.4

Laing 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 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)