EFFECTS OF PRE-MONSOON BIOMASS BURNING AEROSOLS ON RAINFALL CHARACTERISTICS OVER WEST AFRICA

Abstract

This study investigate the effects of pre-monsoon biomass burning aerosols (BBA) on rainfall characteristics over West Africa. The specific objectives aimed to be achieved are; estimating the distribution of pre-monsoon biomass burning aerosols (BBA) over the study area; analyzing rainfall characteristics over the study area; evaluating the capability of the regional climate model (WRF-Chem) on capturing BBA effect on monsoon rainfall and determining the influence of BBA on rainfall characteristics and cloud formation. AERONET Aerosol Optical Depth (AOD) and Angstrom Exponent (AE) data were used to estimate the temporal distribution of AOD and AE and classification of aerosol types over the five selected AERONET sites namely, Agoufou, Banizoumbou, Dakar, IER_Cinzana and Ilorin. Rainfall data from ERA5 for the period of 1998-2021 was used to evaluate rainfall characteristics such as distribution, variability, normal, wet, and dry (drought) condition and rainfall trend over the five selected sites. Simulations were also run using WRF-Chem model to evaluate the capability of the model in capturing BBA and to investigate the effect of BBA on rainfall and cloud formation. The study found out that AOD peaks in March-June in all the sites except Ilorin where it peaks in January. The maximum values of AE were in December-January for all the sites except Ilorin where maximum AE value was in August. This shows the presence of fine mode aerosols. It has been found that desert dust aerosol was the dominant aerosol in all the sites throughout the study period. The normal and wet climatic conditions were dominant for both annual and seasonal rainfall at all the sites during the study period. High rainfall variability throughout the study period and all the seasons with no trend for annual and negative trend for MAM and JJA season. This means that it is easier to use mean to predict rainfall performance for the annual rainfall but difficult the seasonal rainfall performance in the study area. Lastly, the study found out that the WRF-Chem model overestimated the rainfall characteristics and the BBA radiative effects either increase or decrease rainfall amount depending on the period/season over West Africa. The WRF-Chem model also underestimates the values of Outgoing Longwave radiation (OLR) and the BBA radiative effects increased the rate of convective cloud formation over West Africa