The predictability of precipitation episodes during the West African dry season

Precipitation episodes in tropical West Africa (7-15 degrees N, 10 degrees W-10 degrees E) during the dry season from November to March are rare, but can have significant impacts on human activities reaching from greening of pastures to spoiling harvests and health implications. Previous work has shown a link between these unseasonal rainfalls and extratropical disturbances via a decrease of surface pressure over the Sahara/Sahel and a subsequent inflow of moist air from the Gulf of Guinea. This paper supports the previously stated hypothesis that the extratropical influence leads to a high rainfall predictability through a careful analysis of operational 5 day forecasts from the European Centre for Medium-Range Weather Forecasts' (ECMWF) ensemble prediction system (EPS), which are evaluated using Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) precipitation estimates for the 11 dry seasons 1998/99-2008/09. The long-term regional average of ensemble-mean precipitation lies between the two observational datasets, with GPCP being considerably wetter. Temporal correlations between the ensemble mean and observations are 0.8. Standard probabilistic evaluation methods such as reliability and relative operating characteristic (ROC) diagrams indicate remarkably good reliability, sharpness and skill across a range of precipitation thresholds. However, a categorical verification focusing on the most extreme ensemble mean values indicates too many false alarms. Despite the considerable observational uncertainty the results show that the ECMWF EPS is capable of predicting winter rainfall events in tropical West Africa with good accuracy, at least on regional spatial and synoptic time-scales, which should encourage West African weather services to capitalize more on the valuable information provided by ensemble prediction systems during the dry season. Copyright (c) 2012 Royal Meteorological Society