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This article presents observations and model simulations of the low-level nocturnal structure of the atmosphere over West Africa. The measurements are taken from the dry-season Special Observing Period (SOP-0) of the African Monsoon Multidisciplinary Analysis (AMMA), at Niamey, Niger, on 23 and 24 January 2006. During this time, mesoscale structures in the atmospheric aerosol loadings were observed. The available observations indicate that these mesoscale features at Niamey are consistent with the passage of gravity currents or bores in the northerly Harmattan winds. Model simulations at resolutions down to 1 km indicate that the mesoscale structures are caused by nocturnal frontogenesis in the baroclinic zone to the south of the Sahara in the winter months. This frontogenesis is a continental-scale phenomenon, which has significant implications for the uplift and transport of dust and biomass-burning aerosols in the region. An accompanying frontogenetic feature appears further south in the model simulations, associated with the winter intertropical front. The frontogenesis is possibly linked with the development of structures showing characteristics of canonical mesoscale phenomena, including internal bores and gravity currents. Representation of these features in the UK Met Office Unified Model at different resolutions (from 12 km to 1 km) is discussed. [ABSTRACT FROM AUTHOR]

A multi-column radiative transfer code is used to assess the direct radiative effect of biomass burning aerosols over the southern African region during September. The horizontal distribution of biomass smoke is estimated from two sources; i) General Circulation Model (GCM) simulations combined with measurements from the Aerosol Robotic Network (AERONET) of Sun photometers; ii) data from the Moderate resolution Imaging Spectrometer (MODIS) satellite. Aircraft and satellite measurements are used to constrain the cloud fields, aerosol optical properties, vertical structure, and land surface albedo included in the model. The net regional direct effect of the biomass smoke is -3.1 to -3.6Wm-2 at the top of atmosphere, and -14.4 to -17.0Wm-2 at the surface for the MODIS and GCM distributions of aerosol. The direct radiative effect is shown to be highly sensitive to the prescribed vertical profiles and aerosol optical properties. The diurnal cycle of clouds and the spectral dependency of surface albedo are also shown to play an important role. [ABSTRACT FROM AUTHOR]