A SPACE-TIME RAINFALL DISAGGREGATION MODEL ADAPTED TO SAHELIAN MESOSCALE CONVECTIVE COMPLEXES

A model adapted to the disaggregation of rainfields associated to Sahe lian mesoscale convective complexes (MCCs) at small time (approximate to 5 min) and space (1-10 km) scales is presented. Spatial and tempora l disaggregation have been decoupled in order to simplify the problem. The average rain depth over the study area for each storm is first di saggregated in space by using the turning band method, a geostatistica l 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 mappin g the starting time of the rain event as the convective front moves th rough the domain. Finally, at each location, the temporal disaggregati on of the storm rain depth is obtained by using a standard hyetogram m odel reproducing the typical sequence of a convective front followed b y a stratiform trail. In its present form the only input of the disagg regation model is the average rain depth over the domain which can be provided either by a general circulation model output or a satellite e stimate. To perform the various steps of the spatial and temporal disa ggregation, some knowledge on the climatology of the zone is needed. O ver 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 r adar in the Niamey area. Half of these MCSs were identified as MCCs. T he analysis of the MCCs' data set allowed the derivation of a probabil ity distribution and a covariance function of the point storm rain dep th. It also allowed the calculation of statistics for the average spee d and direction of storm displacement and the inference of parameters describing the standard hyetograms. A complete example of the use of t he model is presented showing that it produces realistic rainfall fiel ds. Perspectives for further development and validation of the model a re given in the conclusion.