Interannual, monthly, and regional variability in the wet season diurnal cycle of precipitation in sub-Saharan Africa

Convective systems in sub-Saharan Africa were defined from measurements by the Tropical Rainfall Measuring Mission satellite Microwave Imager at 85 GHz for four wet seasons, May-September 1998-2001. By applying a convective-stratiform discrimination algorithm to each convective system, the pixels within were designated as convective or stratiform cloud, life cycle ages assigned, and rainfall rates calculated. The years 1998 and 1999 were wetter than the long-term (1898-2000) mean, while 2000 and 2001 were drier. The wetter years had about 10% more convective systems than the drier years, but the size and intensity distributions for the wetter and drier years were virtually identical. The wet season diurnal cycle of precipitation in the study area varied regionally, intraseasonally, and inter-annually. Analysis of precipitation versus time revealed different diurnal cycles for each of the three 10degrees zones south of the Sahara Desert. The diurnal cycle was bimodal north of 10degreesN and unimodal south of 10degreesN. The bimodal diurnal cycle was more pronounced north of 15degreesN. Diurnal cycles in each zone exhibited regional and seasonal variability of about 10% per four-hour time block. In wetter years the regional mean diurnal cycle was unimodal, but in drier years it was bimodal. The variability of the diurnal cycle appeared to be primarily influenced by variability in the frequency and life cycle of organized convective systems and thus the physical and dynamical factors responsible for their development.