EFFECT OF SAHARA DUST ON THE ONSET OF THE SUDANO-SAHELIAN RAINFALL IN NIGERIA

Abstract

The objective of this study is to analyse the effect of Sahara dust mobilization on the onset of rainfall over Sahel zone of Nigeria using the total ozone mapping spectrometer (TOMS), daily rainfall, relative humidity, temperature, pressure data from synoptic stations in the zone and outgoing long wave radiation (OLR) obtained from National Oceanic and Atmospheric Administration (NOAA). The analysis of the study covers a period of thirty years (1984-2013). The dust mobilization effect was evaluated by estimating the dust loads concentration 5-day before and after the onset date as determined using the anomalies of equivalent potential temperature (θe) which occurs as a result of the seasonal, monthly, and daily variations of moisture in the summer monsoon flow over West Africa. The onset of rainfall was defined to be the beginning of first three rainfalls of at least 10mm with not more a 7-day dry spell between any two such rainfalls. The 10mm threshold was chosen because it is the most prominent threshold to wash-out the dust in the atmosphere. Using this definition, the actual onset of rainfall was obtained from NIMET in situ data. The results revealed that there was high concentration of dust load during the periods of observed delayed onset and low concentration or no dust concentration during the period of observed early onset which showed atmospheric dust concentration and onset of rainfall are dependent of each other. It also indicated that the amount of dust loads in the atmosphere significantly influence the onset of rainfall. Furthermore, OLR at the top of the atmosphere was examined during both the delayed and early onset to show the cloudiness distinction in the atmosphere and the result revealed that during the early onset an OLR value of less or equal to 240W/M2 was observed which indicated the formation of cloud with high probability of rainfall and also during the delay onset OLR valued of around 279-286W/M2 was observed which indicated the suppressed cloudiness as a result of dusty atmosphere.