Rainfall estimation in the Sahel: the EPSAT-NIGER experiment

Abstract

EPSAT-NIGER (Estimation of Precipitation by SATellite—NIGER experiment) has been designed to improve the understanding of the precipitation systems of Sudano-Sahelian Africa and to develop operational rainfall estimation algorithms for this region. It is based on the combined use of a very dense raingauge network (93 gauges over a study area of 16 000 km₂) and a C-band weather radar system. The experiment is scheduled to last three years, 1990–1992. The network pattern, a regular grid with nodes spaced at 12.5 km and a 16 gauge target area where the distance between stations is decreased to 1 km, has allowed for some preliminary studies on the rainfall distribution at various space and time scales. Whereas the long term average rainfall gradient is uniform, rainfall increasing north to south, a single rainy season can be markedly different. The local variability may be extremely large. That variability is enhanced at smaller sampling time steps and the computation of reference areal rainfall for satellite imagery validation is extremely sensitive to the design of the ground-based validation system. The joint processing of gauge and radar data has led to the identification of a few typical features of the drop size distribution of the African squall lines, which could lead to deriving specific algorithms for radar calibration in this region. The data provided by EPSAT-NIGER will be used in various international projects for the assessment of water input from the atmosphere to the continent over the Sahel.