Understanding the summertime diurnal cycle of precipitation over sub-Saharan West Africa: regions with daytime rainfall peaks in the absence of significant topographic features

Abstract

Convection-permitting regional model output is analyzed to better understand the diurnal cycle of rainfall during the height of the West African summer monsoon. This investigation focuses on two regions, the western Bodele of Chad and eastern Burkina Faso, that have a propensity for daytime rainfall, but why this occurs does not conform to our conventional understanding of being directly associated with the presence of significant topographic feature(s). The August diurnal cycle of rainfall for the Bodele is characterized by an afternoon peak, with mesoscale convective systems (MCSs) originating within 500 km accounting for 80% of the afternoon precipitation. These MCSs are associated with a deepening of the monsoon trough over the western Sahara related to northern storm track African easterly wave (AEW) disturbance activity, combined with anomalous ridging over eastern Chad associated with cold pool outflow from convection that originates over the Marra Mountains the previous afternoon. These circulation features enhance the moist low-level southwesterly flow and increase instability over the Bodele. Over Burkina Faso rainfall has a primary afternoon peak, and a secondary morning peak. MCSs account for 95% of the total rainfall. Morning rainfall is primarily due to MCSs forming over the Damergou Gap of Niger, while the afternoon rainfall is associated with MCSs that originate over the Damergou Gap as well as locally. While both types of MCSs are associated with an approaching southern storm track AEW disturbance, it is differences in northern storm track activity that helps explain why some MCSs originate over the Damergou Gap.