Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

Abstract

Precipitation during the boreal winter dry season in tropical West Africa is rare but occasionally results in significant impacts on the local population. The dynamics and predictability of this phenomenon have been studied very little. Here, a statistical evaluation of the climatology, dynamics, and predictions of dry-season wet events is presented for the region 7.58-158N, 108W-108E. The analysis is based upon Global Precipitation Climatology Project (GPCP) merged satellite-gauge pentad rainfall estimates and 5-day 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) precipitation forecasts, and covers the 23 dry seasons (November-February) during 1979/80-2001/02. Wet events are defined as pentads with an area-averaged precipitation anomaly of more than 1200% with respect to the mean seasonal cycle. Composites of the 43 identified events indicate an association 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. The results give evidence for a preconditioning by another disturbance about 1 week prior to the precipitation event. The ERA-40 forecasts show a high temporal correlation with observations, a general wet bias, but a somewhat too low number of wet events. With 53% of all identified events correctly forecasted and only 32% of forecasted events not verified, the model shows moderate skill in contrast to the prediction of many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborates 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.

Ensemble-Based Forecast Uncertainty Analysis of Diverse Heavy Rainfall Events

“…This choice is also consistent with past research that has analyzed ''pentad'' rainfall (e.g., Knippertz and Fink 2009), and with the issuance of 5-and 6-10-day precipitation forecasts by the U.S. Hydrometeorological Prediction Center and Climate Prediction Center, respectively. All 5-day periods in which 100 mm of rain fell over more than 350 grid points (approximately 800 000 km 2 ) were identified; these will be referred to as widespread rain events.…”

“…Predictions of 5-day rainfalls in Africa were analyzed by Knippertz and Fink (2009). Their approach is similar to that taken here, where we examine multiple-day, widespread rain events that occur on scales that should be well resolved by global NWP models, pose flooding threats over large areas, and have implications for seasonal climate predictions.…”

Populations and Climatic Evolution in North Tropical Africa from the End of the Neolithic to the Dawn of the Modern Era

“…In fact, in the twentieth century, during the dry season that intervened from November to April in the Sudano-Guinean zone, exceptional rainfall that formed in the north from the front of the monsoon was observed. These rains were not monsoonal rains, but were dependent on the extra-tropical depressions of the cyclonic type and relate to the westerly wind circulation dominating in the middle latitudes of the Northern Hemisphere (Knippertz 2005;Knippertz and Fink 2009). We could therefore envisage a certain frequency of these extra-tropical depressions when the ITCZ remained blocked in the Sudano-Guinean zone, followed by the monsoon rains once the ITCZ moves to the north.…”

Driftsonde Observations to Evaluate Numerical Weather Prediction of the Late 2006 African Monsoon

“…Rainfall events during the dry season, although rare, can have a significant impact locally (Knippertz andFink 2008, 2009). The annual rainfall cycle over West Africa is linked to the variability of the West African monsoon (WAM) system associated with major changes in the atmospheric circulation over the region (Sultan and Janicot 2003).…”