Satellite studies of sub-Saharan Africa and the adjacent equatorial Atlantic Ocean are beginning to provide a differing perspective on the nature of mesoscale convective systems (MCSs) affecting these regions. Observations from infrared and passive and active microwave sensors show that MCSs over land are generally faster, shorter-lived, and stronger than their oceanic counterparts. MCSs over land also have larger ice particles, more lightning, and smaller proportions of stratiform rain. The next step is to try to explain the physical mechanisms behind these differences.

We suggest that stability and shear variations over land and ocean are major factors in the observed MCS differences. Over sub-Saharan Africa, strong heating during the day leads to intense, short-lived convection. This convection produces large ice particles, which are conducive to lightning production. However, the large ice particles fall out rapidly and do not assist in the growth of stratiform rain regions. In addition, strong wind shear associated with the African Easterly Jet (AEJ) and dry intrusions from the Saharan Desert may increase sublimation and evaporation, resulting in less stratiform rain. Over the east Atlantic, the sustainability of convection by a warm, moist boundary layer with a weak diurnal cycle allows the continual production of convective cells that produce only smaller ice particles aloft. While these smaller ice particles are not conducive to lightning production, they favor the formation of large stratiform rain regions. In addition, influences from the AEJ and Saharan air layer are less pronounced in the east Atlantic, and thus have less of an effect in reducing stratiform rain amounts.

This study focuses on the large-scale environmental factors that accompany variations in stratiform rain production over west Africa and the tropical east Atlantic. We analyze NCEP reanalysis fields of relative humidity, temperature, and wind in relation to observations from the Tropical Rainfall Measuring Mission (TRMM) satellite before and during the full cycle of the African monsoon to understand what drives the observed temporal and geographical variations in the percent of total rain that is stratiform rain. Results are consistent with the hypothesis that convective sustainability and large-scale wind shear play important roles in the observed nature of MCSs over sub-Saharan Africa and the tropical east Atlantic.