Seasonal and intra-seasonal patterns of African tropical aerosols and their influence on Kenyan rainfall.

Abstract

This paper presents an investigation of the seasonal and intra-seasonal variablity of absorbing and non-absorbing aerosols over the equatorial African region with respect to rainfall over Kenya. The data set used in this study was the daily Aerosol Index (A.I) over the region obtained from satellite based Total Ozone Mapping Spectrometer (TOMS) observations. For aerosol index, seven stations over the equatorial African region were used, these include stations situated in Cameroon, Central African Republic, Congo, Ethiopia, Kenya, Senegal and Seychelles.The period considered was from 1979 to 2003 inclusive, the choice of the study period was based on all available TOMS aerosols data. Data quality control is perfomed by intercomparison of the satellite based A.I. with ground based A.I. measurements for 3 months. Statistical analysis showed that for each station considered there is a unique distribution of aerosol particles over the year and the behaviour of occurrence is more less seasonal, the peak of non-absorbing aerosols mainly occurs during the wet months of the year. The variability of the aerosols coincides with the seasonal patterns of the prevailing wind. The inter-hemispheric winds play an important role in the distribution of aerosols. The type of aerosols found over a particular time depend on the airmass' time lapse from oceanic origin. This is confirmed by the uniques aerosols distribution over the island station of Mahe(Seychelles). The results obtained from the correlation between Aerosol Index (A.I) over Equatorial Africa and Kenyan rainfall show a strong relationship between the two. Over some parts of Kenya the non-absorbing aerosols dominate in the formation of rainfall whereas in other parts the absorbing aerosols dominate. The relationships depict that aerosol indices on average lead rainfall by about two months. This may be useful in enhancing predictability of rainfall over East Africa.