THE EFFECTS OF AEOLIAN DUST ON WEST AFRICAN CLIMATE SYSTEM USING REGIONAL CLIMATE MODEL (RegCM4.4

Abstract

In this study, a Regional Climate Model (RegCM4.4) has been used to investigate the dynamic effect of aerosol loading radiative forcing and its impact on West African climate. The simulation was performed with the non-aerosol version of the model (control) and another simulation was made using the dust module for the year 2010. The spatial and temporal distribution of the AOD derived from the dust run was compared with observed aerosol data from Aerosol Robotic Network (AERONET) and other satellite products.The results of the simulations performed show that dust outbreak has a significant impact on both the wind and temperature profile at different levels, inducing observed changes in West-Africa Monsoon (WAM) system during JJAS seasons. The dust induced shortwave surface radiative forcing (SWSRF) was found to be negative at the surface with minimum value reaching upto -48W/m2 over the source during the monsoon season. However, the seasonal mean spatial distributions of dust aerosol Long-Wave Surface Radiative Forcing (LWSRF) showed positive throughout the year which indicates the warming effect of dust over West Africa. At the top of the atmosphere, The SW radiative forcing was negative with maximum values (varying between 1 and 9Wm-2) during MAM. The dust particles induced SW radiative forcing at the TOA exhibited heating effect in the study area during DJF. In contrast, the dust particle's LW radiative effects exhibit their atmospheric radiative cooling influences during DJF. However, the LW radiative forcing at the TOA is maximum during JJA with the core (4.3Wm-2) over the source. The highest degrees of correlation (r> 0.7) between modeled AOD and AERONET, MODIS, OMI, and MISR were observed over CapeVerde. Moderate (0.5 < r < 0.7) to poor correlations (r < 0.5) were also observed over Ilorin, Zinder, Dakar, Ouagadougou, Agoufou, and Banizoumbou. Generally, the Root Mean Bias (RMB) values over all the stations revealed that, RegCM underestimates AOD over Ilorin, Zinder, Cape Verde, Ouagadugou, Agoufou, and Banizoumbou when compared with AERONET and satellite observations except in Dakar where RegCM overestimates AOD with Moderate Resolution Imaging Spectroradiometer (MODIS) observations. All the RegCM experiments were found to perform well over Guinea and whole West Africa with low RMSE. However, the control case (CTRL), and dust with radiative feedbacks (DUST+RAD) had low performance in representing rainfall variability due to high RMSE when compared with the TRMM over Savannah and Sahel respectively. In conclusion the Regional Climate Models are capable in predicting the effects of dust outbreak over West Africa.