ANALYSIS OF CONVECTIVE ACTIVITY AND ITS RELATIONSHIP TO THE RAINFALL OVER THE RIFT-VALLEY LAKES OF EAST-AFRICA DURING 1983-90 USING THE METEOSAT INFRARED CHANNEL

The convective activity over the Rift Valley lakes of East Africa, as deduced from cloud tops colder than a predefined threshold, is examine d. Relationships between satellite-derived convective indices and rain fall measurements are also examined. The diurnal cycle of convective a ctivity over Lake Victoria and over the land is analyzed. The maximum convection is found to occur during the morning between 0500 and 0800 LST over Lake Victoria, and a second maximum occurs in the afternoon. In contrast, over surrounding land, the maximum occurs generally in la te afternoon and during the evening. It is also found that a linear re lationship exists between satellite-derived convective indices and rai nfall measurements: the correlation between the two is strong enough t hat the indices can be used to estimate annual and areally averaged mo nthly rainfall. The cold cloud indices explain more than 50% of observ ed variances of rainfall for the months of May through October. Howeve r, the performance is inadequate in several instances during February and March. The results show that the satellite algorithm is robust eno ugh to estimate spatial averages of monthly rainfall with satellite es timates accounting for between 75% and 95% of observed variances of ra infall. The results further show that there is an exceedingly high cor relation between convective rainfall over Lake Victoria and in the sur rounding catchment. This permitted the derivation of a relationship be tween rainfall over the lake and its catchment.