Diurnal Cycle of Warm Season Rainfall over West Africa: Observations and Regional High-Resolution Simulation

Abstract:

The diurnal cycle is a fundamental mode of tropical rainfall variability, and its accurate portrayal remains a challenging issue for numerical weather and climate prediction. The purpose of this study is to better understand the diurnal cycle of warm season (June-September) rainfall over West Africa and its underlying physical processes. We use rainfall data from the Tropical Rainfall Measuring Mission (TRMM) and atmospheric dynamics fields from reanalyses to explore the rainfall diurnal cycle in the climatology (1998-2013) and a case study of 2006. In addition, a simulation with the Weather Research and Forecasting (WRF) model at a convection-permitting resolution of 3 km resolution is analyzed to evaluate the model’s ability to capture the rainfall diurnal cycle in the warm season of 2006.

The rainfall in the warm season of 2006 shows a mixture of afternoon and nocturnal peaks over West Africa, which is similar to the climatology. Further examination of the 2006 case is focused on three domains: (1) the AF domain (3°W-0°W, 9°N-11.5°N) with late afternoon peaks (2) the SN domain (2°E-5°E, 9°N-11.5°N) with nocturnal peaks, and (3) the NN domain (2°E-5°E, 13.5°N-16.5°N) with nocturnal peaks. For each domain, the rainfall diurnal cycle is more related to the diurnal cycle of rainfall frequency than of the intensity, and it is largely shaped by the diurnal cycle of the frequency of extreme rainfall events. The afternoon rainfall peaks over the AF domain are associated with unstable atmospheric profiles below 850 hPa. The nocturnal rainfall peaks over the NN and SN domains are associated with westward-propagating rainfall systems initiated on the previous afternoon around the complex topography to the east (about 5-15°E).

The rainfall produced by the convection-permitting simulation shows similar westward-propagating features. The model realistically captures the nocturnal rainfall peaks over the SN domain, because it reproduces the westward-propagating rainfall systems originated from the Jos Plateau located just east of the domain. However, the model fails to simulate the rainfall peaks over the northern part of the NN domain due to a dry bias, and over the AF domain due to an overestimation of nocturnal peaks.