Large scale Environment of Extreme Precipitation Events over West Africa

In the Sahel region, between 12° - 20 °N, the rainfall distribution is characterized by a climatological north to south gradient of about 1 mm km- (Lebel at al. 1992). Previous research by D'Amato and Lebel (1998) found that the main source of rainfall variability in this region is linked to the variability in the number of the events rather than in the magnitude of these events. However Panthou et al. (2014) showed that a different regime of precipitation is occurring since 2001, with an observed persisting deficit of the occurrence of convective events associated with a persistent amount of rainfall, meaning that the convective systems that occur are more intense than they used to be in the past. Panthou et al. (2014) gave a good description of the rainfall regimes and their temporal evolution based on station (rainfall) data but the atmospheric conditions leading to the rainfall events were not documented. The main objective of this study is to investigate the large scale atmospheric signature that characterized the extreme precipitation event in the Sahel over the rainy season (JJAS).

The series of extreme events has been identified by Panthou et al. (2014) using station data. It is used here to build a composite of extreme events over the 1979-2015 period which overlaps TRMM availability. A first statistical description of the occurrence of extreme given (diurnal cycle, season) and an extreme event composite over the period is build. A description is first given over the Ouagadougou (Burkina Faso). A large range of scales is found to be involved in the occurrence of the extremes. An African Easterly Wave associated with a large-scale moist anomaly favours the intensification of the meso-scale convective systems (MCS). Also a well developed vortex helps the MCS to turn into a high precipitating event. The results are extended to the rest of the Sahel depending on the north – south location of the extreme event.