SEMINAR SERIES and COLLOQUIA

Precipitation during the boreal winter dry season in tropical West Africa is rare but occasionally connected to high-impacts for the local population reaching from the rotting of harvests to improved grazing conditions and mango crop yields. The dynamics and predictability of this phenomenon have been studied very little.

In the first part of the presentation, an observationally based synoptic and dynamical analysis of an abundant rainfall event during the dry season 2003/04 that affected the countries of Nigeria, Benin, Togo, and Ghana will be presented. The results point to a forcing of the rainfalls from the extratropics in the following way: (I) Upper-level clouds and moisture to the east of a weak, quasi-stationary extratropical disturbance enhance the greenhouse effect over the Sahel and the adjacent Sahara, and thereby cause a net-column warm anomaly and falling surface pressure. (II) One day before the precipitation event negative pressure tendencies are further enhanced through warm advection and subsidence associated with the penetration of a more intense upper-trough into Algeria. (III) The resulting northward shift and intensification of the weak wintertime heat low allows low-level moist southerlies from the Gulf of Guinea to penetrate into the Soudanian zone. (IV) Finally, daytime heating of the land surface and convective dynamics initiate heavy rainfalls.

In the second part of the seminar, a statistical evaluation of the climatology, dynamics, and predictions of such events for the region 7.5–15°N, 10°W–10°E based upon GPCP merged satellite-gauge pentad rainfall estimates and five-day ERA-40 precipitation forecast for the 23 winter seasons (November–February) 1979/80–2001/02 will be presented. Wet events are defined as pentads with a more than +200% area averaged precipitation anomaly with respect to the mean seasonal cycle, resulting in 43 identified events during the study period. The events are associated with a trough over northwestern Africa, a tropical plume on its eastern side, unusual precipitation at the northern and western fringes of the Sahara, and reduced surface pressure over the Sahara, which allows an inflow of moist southerlies from the Gulf of Guinea to feed the unusual dry-season rainfalls. Interestingly, the results give evidence for a pre-conditioning by another disturbance about one week prior to the precipitation event. The forecast evaluation reveals a high temporal correlation with observations, a general wet bias, and a somewhat too low number of wet events resulting in a probability of detection of 53% and a false alarm rate of 32%. This indicates a moderate skill in contrast to many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborate the previously proposed hypothesis that a strong extratropical influence enhances the quality of predictions in this region. The results should encourage weather services in West Africa to take advantage of available dry-season precipitation forecasts in terms of the dissemination of early warnings more extensively.

Back to NWC Seminar Series and Colloquia