On the characteristics of the rainfall events in the Sahel with a view to the analysis of climatic variabiilty

Abstract

The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and thoss of the core of the rainy seasons. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy seasons. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year to year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainall remains close to 12mm during the margins of the rainy season and close to 15mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660mm and for any given year the region between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events.