Excerpt
Seasonal precipitation forecasts are important sources of information for early drought and famine warnings in West Africa. This study presents an assessment of the monthly precipitation forecast of the Climate Forecast System Version 2 (CFSv2) for three agro-ecological zones (Sudan-Sahel, Sudan and Guinean Zone) of the Volta basin. The CFSv2 performance is evaluated for the Sahel drought 1983 and for all August months of the reforecast period (1982–2009) with lead times up to 8 months using a quantile-quantile transformation for bias correction. In addition, an operational experiment is performed for the rainy season 2013 to analyse the performance of a dynamical downscaling approach for this region. 22 CFSv2 ensemble members initialized in February 2013 are transferred to a resolution of 10 km x 10 km using the Weather and Research Forecasting model (WRF). Since the uncorrected CFSv2 precipitation forecasts are characterised by a high uncertainty (up to 175% of the observed variability), the quantile-quantile transformation can clearly reduce this overestimation with the potential to provide skillful and valuable early warnings of precipitation deficits and excess up to six months in ahead, particularly for the Sudan-Sahel zone. The operational experiment illustrates that CFSv2-WRF can reduce the CFSv2 uncertainty (up to 69%) for monthly precipitation and the onset of the rainy season but has still strong deficits regarding the northward progression of the rain belt. Further studies are necessary for a more robust assessment of the techniques applied in this study to confirm these promising outcomes.