A space-time rainfall disaggregation model adapted to Sahelian mesoscale convective complexes

Abstract

A model adapted to the disaggregation of rainfields associated to Sahelian mesoscale convective complexes (MCCs) at small time (≃5 min) and space (1-10 km) scales is presented. Spatial and temporal disaggregation have been decoupled in order to simplify the problem. The average rain depth over the study area for each storm is first disaggregated in space by using the turning band method, a geostatistical technique able to generate spatially correlated fields, with a known covariance function. The method has been adapted in order to simulate non-Gaussian rainfall fields including in particular a probability of zero rainfall. The movement of the rain system is described by mapping the starting time of the rain event as the convective front moves through the domain. Finally, at each location, the temporal disaggregation of the storm rain depth is obtained by using a standard hyetogram model reproducing the typical sequence of a convective front followed by a stratiform trail. In its present form the only input of the disaggregation model is the average rain depth over the domain which can be provided either by a general circulation model output or a satellite estimate. To perform the various steps of the spatial and temporal disaggregation, some knowledge on the climatology of the zone is needed. Over the Sahel, this knowledge is provided by the EPSAT-Niger data set, which contains 170 mesoscale convective systems (MCSs) observed over a 4-year period by a dense recording raingauge network and a weather radar in the Niamey area. Half of these MCSs were identified as MCCs. The analysis of the MCCs' data set allowed the derivation of a probability distribution and a covariance function of the point storm rain depth. It also allowed the calculation of statistics for the average speed and direction of storm displacement and the inference of parameters describing the standard hyetograms. A complete example of the use of the model is presented showing that it produces realistic rainfall fields. Perspectives for further development and validation of the model are given in the conclusion.