The Role of Mesoscale Convective Systems in the Diurnal Cycle of Rainfall over Sub-Saharan Northern Africa

The purpose of this paper is to improve our understanding of the role of mesoscale convective systems (MCSs) in determining the diurnal cycle of rainfall in sub-Saharan northern Africa, and to evaluate the extent to which this role changes with location and the annual cycle. 

MCSs are identified using the 3-hourly TRMM precipitation product 3B42V7. We evaluate the number of MCSs using a series of thresholds for rain rate, similar to other studies. We focus on two thresholds, 25 mm/day and 100 mm/day, and evaluate the sensitivity of the results to this choice.  Since other studies suggest that MCSs have minimum contiguous areas of about 2000 km², we require that there be at least 3 adjacent grid cells that meet the rainfall threshold in order to be defined as a MCS. The University of Utah Precipitation Measuring Mission database also defines MCSs based on TRMM satellite, and we compare with those results. Related physical processes are evaluated by examining multiple atmospheric reanalyses.

The percentage of rainfall delivered by MCSs varies seasonally and regionally. For the 25 mm/day threshold, the percentages range from 65-85% in spring and fall and from 80-93% in summer. The percentage contribution in winter varies meridionally, from 0-35% over 12°-16°N and from 20-70% over 8°-12°N. For the 100 mm/day threshold, spatial and seasonal distributions are similar to those for the 25 mm/day threshold but with smaller percentages. We also investigate how the diurnal cycles of MCS characteristics vary seasonally and regionally. The diurnal cycles of the number, size and intensity of MCSs all coincide with the diurnal cycle of rainfall.

For all four seasons, over 95% of the sub-Saharan northern Africa region has a single diurnal peak of rainfall either in the afternoon or at night when we examine 1°×1° averaged grids in the 1998-2014 TRMM climatology. We classify diurnal cycle types into three categories.  There are two types with afternoon peaks. The first has an afternoon peak with rainfall persisting until midnight. The second has an independent afternoon peak that dissipates within 6 hours of the time of peak rainfall, with nearly no rainfall by midnight. The third type of diurnal cycle has a single nocturnal peak. In each season, we choose regions with homogenous diurnal cycles over which we examine the profiles of temperature, moisture and moist static energy. The results show that temperature profiles are similar, but moisture profiles change dramatically for different types of diurnal cycles and determine the timing of diurnal peaks and their persistence. A rainfall minimum throughout the region occurs at 12Z despite morning temperature increases. This suggests that the midday rainfall minimum is also closely related with moisture.