This work investigates direct and semi-direct aerosol radiative forcing and the associated climatic impacts over the West African region during the dry-season. The regional climate model version 3 (RegCM3) is used in combination with in-situ observations from the AMMA-SOP0/DABEX field campaigns and remote sensing observations from sunphotometry (AERONET/PHOTON) and satellite platforms (PARASOL, MODIS, OMI and MISR). RegCM3 is specifically configured to represent West African aerosols and is evaluated for the 2006 dry season. In this setup, RegCM3 is found to represent aerosol loadings accurately enough for climatic applications, with the model simulating consistent aerosol single scattering albedo variations. In December and January, when smoke aerosols dominate the background aerosol loading, the aerosol single scattering albedo over the Sahel ranges from 0.81to 0.83 (at 440 nm). During the months of March and April, when dust aerosol are mainly observed, the simulated aerosol single scattering albedo ranges between 0.90 and 0.92 (at 440 nm). The direct aerosol radiative forcing (visible + infrared) estimated at top of the atmosphere is essentially negative over the whole domain, with values ranging from -5 W/m² to -4.0 W/m². Over the Sahara, the direct aerosol radiative forcing at top of the atmosphere (TOA) is close to zero (-0.15 W/m²). The large difference between the TOA and surface direct radiative forcing indicates strong radiative absorption in the atmosphere (+11.47 and +24.40 W/m² over the Sahara and Sahel, respectively). Due to their relatively low single scattering albedo, smoke aerosols are the dominant contributors to atmospheric heating. At the regional scale, this results in a daily average atmospheric heating rates ranging between +0.2 and +0.6 K/day within the main smoke layers (approximately 2 and 5 km above the ground surface). Two longer simulations covering the 2001-2006 period are also conducted in order to investigate the effects of this radiative forcing on the regional climate during the dry season. A simulation including dust aerosols (DUSTexp) and a simulation including both dust and smoke aerosols (BBDUSTexp) are performed in order to take into account the dynamical feedbacks associated with direct and semi-direct aerosol radiative forcing. The strong aerosol radiative forcing at surface decreases available radiation, which leads to significant perturbations of the surface energy balance. Over the Sahara, sensible heat flux anomalies are similar in the two experiments (-5.52 W/m² and -6.65 W/m², in the DUSTexp and BBDUSTexp, respectively). Over the Sahel, the decrease is more significant in BBDUSTexp simulation (-16.59 W/m² compared to -5.37 W/m² in DUSTexp). Changes in latent heat fluxes are more complex and depend simultaneously on aerosols emission locations and the aerosol species present. As a result, the precipitation changes due to aerosol radiative effects are very different within the two experiments. In the DUSTexp, precipitation is decreased over most of the domain with a maximum decrease over the central part of the continent. For the BBDUSTexp, smoke aerosols tend to enhance precipitation over this sub-region. This increase seems to be related to a local increase of convective activity above 500 hPa, resulting from an elevated heat pump mechanism.
