The Diurnal Cycle of Warm Season Rainfall over West Africa

This study provides an improved understanding of the diurnal cycle of warm season (June-September) rainfall over West Africa, including its underlying physical processes. Rainfall from the Tropical Rainfall Measuring Mission and atmospheric dynamics fields from reanalyses are used to evaluate the 1998-2013 climatology and a case study for 2006. Convection-permitting simulations at 3-km resolution using a regional climate model are also analyzed for the warm season of 2006.

In both the climatology and the 2006 case study, most regions of West Africa are shown to have a single diurnal peak of rainfall in either the afternoon or at night. Averaging over a large West African domain produces a diurnal cycle with two peaks, but this type of diurnal cycle is quite atypical on smaller (e.g., 1 degree) space scales. Rainfall systems are usually generated in the afternoon and propagate westward, lasting into the night. Afternoon rainfall peaks are associated with an unstable lower troposphere. They occur either over topography or in regions undisturbed by nocturnal systems, allowing locally-generated instability to dominate. Nocturnal rainfall peaks are associated with the westward propagation of rainfall systems, and not generally with local instability.  Nocturnal rainfall peaks occur most frequently about 3°-10° of longitude downstream of regions with afternoon rainfall peaks. The diurnal cycle of rainfall is closely associated with the timing of extreme rainfall events. 

The regional model produces an accurate representation of the observed seasonal mean rainfall and lower-troposphere circulation, and captures the observed westward propagation of rainfall systems. Most of West Africa has a single diurnal peak of rainfall in the simulations, either in the afternoon or at night, in agreement with observations. However, the number of simulated rainfall systems is greater than observed in association with an overestimation of the initiation of afternoon rainfall over topography. The longevity of the simulated propagating systems is about 30% shorter than is observed, and their propagation speed is nearly 20% faster.

The model captures the observed afternoon rainfall peaks associated with elevated topography, e.g., the Jos Plateau. Nocturnal rainfall peaks downstream of topographic afternoon rainfall are also well-simulated. However, these nocturnal rainfall peaks are too widespread, and the model fails to reproduce the observed afternoon rainfall peaks over regions removed from topographic influence. This deficiency is related to a planetary boundary layer that is deeper than observed, elevating unstable profiles and inhibiting afternoon convection. This study concludes that increasing model resolution to convection-permitting space scales significantly improves the diurnal cycle of rainfall compared with the models that parameterize convection, but this is not sufficient to fully resolve the issue, perhaps because other parameterizations remain.