Dynamics and Predictability of a Heavy Dry-Season Precipitation Event over West Africa-Sensitivity Experiments with a Global Model

In January 2002 the Cape Verde region in tropical West Africa was hit by an exceptionally heavy precipitation event. Rain rates of up to 116 mm (48 h)(-1) caused harmful impacts on the local population. The rainfall was triggered by a series of two upper-level disturbances penetrating from the extratropics to the West African coast. This study investigates the dynamics and predictability of this event on the basis of simulations with the global model Global Model Europe (GME) of the German Weather Service [i.e., Deutscher Wetterdienst (DWD)] initialized by the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data. Free forecasts satisfactorily reproduce the upper-level disturbances and the precipitation up to a lead time of 7 days. Several sensitivity experiments are conducted to unveil the reasons for this comparably high predictability and to identify dynamical precursors. The relevance of the upper-level wave structure in the extratropics is examined by modifications of the initial conditions using a quasigeostrophic potential vorticity (PV) inversion technique. While a reservoir of high PV over the North Atlantic and a PV ridge over Europe are found to be crucial for the upper-level wave amplification and the rainfall over West Africa, latent heating over the North Atlantic affects the event rather little in contrast to previous case studies. Surface properties like orography and sea surface temperature anomalies modify the precipitation quantity, but appear not to be essential for the occurrence of the extreme event on the simulated time scale.