Observations and modelling of the solar and terrestrial radiative effects of Saharan dust: a radiative closure case-study over oceans during the GERBILS campaign

Mineral dust events exert a significant perturbation to the Earth's radiation balance via scattering and absorption in both solar and thermal infrared wavelengths. This study documents aircraft-based measurements of the solar and terrestrial radiative effects of a mineral dust outbreak off the west coast of Africa while the FAAM BAe-146 was in transit to the GERBILS observational measurement campaign. By comparing model and measurements of upwelling irradiance, an instantaneous top-of-atmosphere broadband solar direct radiative effect (DRE) of -33 +/- 6 W m(-2) is determined for an aerosol optical depth at 0.55 mu m (tau(0.55)) of around 0.26 +/- 0.04 with the variability in tau(0.55) being estimated from the uncertainty in the aerosol models used in the radiative transfer calculations. Measurements of the spectral dependence of the solar radiative effect indicate that this is well modelled both above and below the dust layer whether using spherical, spheroid or irregular particle models. The terrestrial radiative impact at the top of the atmosphere is estimated to be +9 +/- 3 W m(-2) or around 25-30% of the solar radiative effect in this particular case, although the relative magnitude will of course be dependent on the underlying surface albedo, surface skin temperature, and details of the vertical profile of dust, temperature and humidity. The DRE retrieved from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) is in reasonable agreement with the aircraft measurements. Mineral dust aerosol optical depths derived from the Met Office global numerical weather prediction model show a reasonable spatial distribution but a general underestimation when compared against SEVIRI. Additionally, as expected from a relatively low-resolution global-scale model, the high tau(0.55) values associated with mesoscale convective events are not well reproduced. Copyright (C) c 2011 Royal Meteorological Society and British Crown Copyright, the Met Office