Characterizing energy budget variability at a Sahelian site: a test of NWP model behaviour

Anna Mackie, Paul I. Palmer, Helen Brindley
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> We use observations of surface and top-of-the-atmosphere (TOA) broadband radiation fluxes determined from the Atmospheric Radiation Measurement programme mobile facility, the Geostationary Earth Radiation Budget (GERB) and Spinning Enhanced Visible and Infrared Imager (SEVIRI) instruments and a range of meteorological variables at a site in the Sahel to test the ability of the ECMWF Integrated Forecasting System cycle 43r1 to describe energy budget variability. The
more » ... et variability. The model has daily average biases of −12 and 18<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup> for outgoing longwave and reflected shortwave TOA radiation fluxes, respectively. At the surface, the daily average bias is 12(13)<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup> for the longwave downwelling (upwelling) radiation flux and −21(−13)<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup> for the shortwave downwelling (upwelling) radiation flux. Using multivariate linear models of observation–model differences, we attribute radiation flux discrepancies to physical processes, and link surface and TOA fluxes. We find that model biases in surface radiation fluxes are mainly due to a low bias in ice water path (IWP), poor description of surface albedo and model–observation differences in surface temperature. We also attribute observed discrepancies in the radiation fluxes, particularly during the dry season, to the misrepresentation of aerosol fields in the model from use of a climatology instead of a dynamic approach. At the TOA, the low IWP impacts the amount of reflected shortwave radiation while biases in outgoing longwave radiation are additionally coupled to discrepancies in the surface upwelling longwave flux and atmospheric humidity.</p>
doi:10.5194/acp-17-15095-2017 fatcat:ux5fctyuabhpdfp2ytdsvykkli