The strategic location of the AERONET site in Ilorin, Nigeria, makes it possible to obtain information on several aerosol types and their radiative effects. The strong reversal of wind direction occasioned by the movement of the ITCZ during the West Africa Monsoon (WAM) plays a major role in the variability of aerosol nature at this site, which is confirmed by aerosol optical depth (AOD) (675 nm) and angstrom ngstrom exponent (AE) (440-870 nm) values with 1st and 99th percentile values of 0.08 and 2.16, and 0.11 and 1.47, respectively. The direct radiative forcing (DRF) and radiative forcing efficiency (RFE) of aerosol, as retrieved from the AERONET sun-photometer measurements, are estimated using radiative transfer calculations for the periods of 2005-2009 and 2011-2015. The DRF and RFE of the dominant aerosol classes-desert dust (DD), biomass burning (BB), urban (UB) and gas flaring (GF)-have been estimated. The median (+/- standard deviation) values of the DRF at the top of the atmosphere (TOA) for the DD, BB, UB and GF aerosol classes are -27.5 +/- 13.2 Wm(-2), -27.1 +/- 8.3 Wm(-2), -11.5 +/- 13.2 Wm(-2) and -9.6 +/- 8.0 Wm(-2), respectively, while those of the RFE are -26.2 +/- 4.1 Wm(-2) delta(-1), -35.2 +/- 4.6 Wm(-2) delta(-1), -31.0 8.4 Wm(-2) delta(-1) and -37.0 +/- 10.3 Wm(-2) delta(-1), respectively. Arguably due to its high SSA and assymetric values, the DD aerosol class shows the largest DRF but the smallest RFE. Its smallest AOD notwithstanding, the GF class can cause greater perturbation of the earth-atmosphere system in the sub-region both directly and indirectly, possibly due to the presence of black carbon and other co-emitted aerosol and the ageing of the GF aerosols. This study presents the first estimate of DRF for aerosols of gas flaring origin and shows that its radiative potential can be similar in magnitude to that of biomass burning and urban aerosol in West Africa.

This study assesses the direct and indirect effects of natural and anthropogenic aerosols (e.g., black carbon and sulfate) over West and Central Africa during the West African monsoon (WAM) period (June-July-August). We investigate the impacts of aerosols on the amount of cloudiness, the influences on the precipitation efficiency of clouds, and the associated radiative forcing (direct and indirect). Our study includes the implementation of three new formulations of auto-conversion parameterization [namely, the Beheng (BH), Tripoli and Cotton (TC) and Liu and Daum (R6) schemes] in RegCM4.4.1, besides the default model's auto-conversion scheme (Kessler). Among the new schemes, BH reduces the precipitation wet bias by more than 50% over West Africa and achieves a bias reduction of around 25% over Central Africa. Results from detailed sensitivity experiments suggest a significant path forward in terms of addressing the long-standing issue of the characteristic wet bias in RegCM. In terms of aerosol-induced radiative forcing, the impact of the various schemes is found to vary considerably (ranging from -5 to -25 W m(-2)).