A. Laing, 25 Oct 2006

Patterns of convection in Africa: Implications Patterns of convection in Africa: Implications for prediction of precipitationfor prediction of precipitation

Arlene Laing (NCAR/MMM)• Societal need for improved precipitation of prediction in Societal need for improved precipitation of prediction in

Africa (economy, health, water resource management, …).Africa (economy, health, water resource management, …). Prediction depends on improved understanding of initiation and evolution of organized convection

• Substantial fraction of organized convective events exhibits systematic propagation at regional to continental scales. Coherence behavior implies qualitative predictability beyond one or two days

• Long-term statistics of organized convection (from satellite, new radars, etc…) =>

Better understanding of water cycle at various scales, role of role of regenerating convection in tropical cyclogenesis => regenerating convection in tropical cyclogenesis => improve models, prediction

A. Laing, 25 Oct 2006

Seasonal & Diurnal CyclesSeasonal & Diurnal CyclesAverage Hourly Percentage of Cold Cloud, Tbb < 253K

• Convection triggered daily in lee of high terrain

• Local & Propagating Convection

• Preferred zones of initiation and propagation

Letters mark mountain peaks0.51.01.5Average Elevation (km)

0º – 20ºN 0.51.01.5Average Elevation (km)

0º – 20ºN

A. Laing, 25 Oct 2006

Mean Diurnal CyclesMean Diurnal Cycles

Average frequency of cold cloud as function of longitude & time

0

6

12

18

18

12

6

propagation

propagation

0.61.2

10E 20E 30E 40E

Average elevation (km)35S – 20S

35S-20S 5S-15N0 - 20N

0

6

12

18

18

12

6

20W 10W 0 10E 20E 30E 40E

Pro

paga

tion

Pro

paga

tion

Average Elevation (km)5S – 15N

+1 +2-1 0

Pro

pag

atio

n

Pro

pag

atio

n

0.51.01.5

20W 10W 0 10E 20E 30E 40E

Average Elevation (km)0º – 20ºN

0

6

12

18

18

12

6

UTC

• Convection triggered daily by heating of elevated terrain• Local diurnal heating maxima & propagating convection

16-31 Dec 200316-31 Oct 199916-30 June 1999

(Tbb< 233K)

A. Laing, 25 Oct 2006

Convection Episodes & PredictionConvection Episodes & Prediction

• Real-time propagation stats combined with climatology for short term forecasts (0-6hrs) of deep convection, precipitation

• Classify synoptic regime favoring frequent episodes of organized propagating convection. Aid in predicting high impact precipitation events

• Radar rainrates and lifecycle of convection => improve QPF

Mean

Median

Span (km)

1062

700

Duration (h)

25.4

18.0

Phase Speed (m/s)

12.0

11.2

30 ms-1

7 ms-1

Tropical N. Africa (60deg longitude)

~5 episodes per day across zonal domain

A. Laing, 25 Oct 2006

Patterns of convective precipitation over W. Africa resemble those in specific regions around the World

... specifically, environmental shear controls convective organization (c.f., US continent during summer - Moncrieff and Liu, 2006)

A. Laing, 25 Oct 2006

Data & MethodsData & Methods

• Meteosat-7 IR, 30min NCEP GFS Analyses May 1999 - March 2004

• 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

020W 20E

20S

20N

0

40E

0.61.2

0

Average Elevation (km) 35S - 20S

Nov - FebNov - Feb

Autumn,SpringAutumn,Spring

0.51.01.5

20W 10W 0 10E 20E 30E 40E

Average Elevation (km)0 – 20N

May - AugMay - Aug