Aerosol direct radiative forcing over Djougou (northern Benin) during the African Monsoon Multidisciplinary Analysis dry season experiment (Special Observation Period-0)

The purpose of this work is to investigate the direct radiative forcing of aerosols over the supersite of Djougou (northern Benin) during the African Monsoon Multidisciplinary Analyses dry season experiment. We focus our simulations on the top of atmosphere, bottom of atmosphere, and atmosphere radiative forcings. During the dry season period, Sun photometer measurements indicate a rather turbid atmosphere with a mean aerosol optical depth for the overall period of 0.78 +/- 0.24 (at 440 nm). The aerosol absorption coefficient estimated at the surface ranged between 2.3 and 37.3 Mm(-1) (mean value 15.2 +/- 10.6 Mm(-1) at 520 nm) and the scattering coefficient between 44.5 and 232.3 Mm(-1) (mean 145 +/- 59 Mm(-1) at 520 nm). This leads to a single scattering albedo of between 0.81 and 0.98 (at 520 nm) with a mean (and standard deviation) value of 0.91 +/- 0.05, indicating moderately absorbing aerosols. In parallel, micropulse lidar measurements indicate the presence of two distinct aerosol layers, with a first one located between the surface and 1 km and a second one located above 1.5-4.0 km. On the basis of surface and aircraft observations, sunphotometer measurements, lidar profiles, and Moderate Resolution Imagaing Spectroradiometer sensor an estimation of the daily clear sky direct radiative forcing has been estimated for the 17 - 24 January 2006 period. Simulations indicate that aerosols reduce significantly the solar energy reaching the surface (mean Delta F-BOA = -61.5 W/m(2)) by reflection to space (mean Delta F-TOA = -18.4 W/m(2)) but predominantly by absorption of the solar radiation into the atmosphere (mean Delta F-ATM = +43.1 W/m(2)). The mean heating rate at the surface and within the elevated biomass burning layer is considerably enhanced by 1.50 and 1.90 K day(-1), respectively.