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

Abstract

Abstract. We use observations of surface and top-of-theatmosphere (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 model has daily average biases of −12 and 18 W m −2 for outgoing longwave and reflected shortwave TOA radiation fluxes, respectively. At the surface, the daily average bias is 12(13) W m −2 for the longwave downwelling (upwelling) radiation flux and −21(−13) W m −2 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.

Observed and CMIP5‐Simulated Radiative Flux Variability Over West Africa

“…The Sahel is a semiarid region: typically dry and dusty with a high surface albedo in the dry season, contrasting with higher humidity and precipitation in the wet season leading to a large increase in vegetation and decrease in surface albedo (Milton et al, , ). For a more comprehensive overview of radiative processes in the Sahel, we refer the reader to our previous study (Mackie et al, ). The Sahel is also particularly vulnerable to changes in climate, having already suffered from extensive droughts in the past decades (L'Hôte et al, ), and been identified as one of a number of climate change hot spots (Diffenbaugh & Giorgi, ).…”

The GERB Obs4MIPs Radiative Flux Dataset: A new tool for climate model evaluation

“…Futyan et al 2005, Slingo et al 2006, Pearson et al 2010, Ansell et al 2014, Banks et al 2014) and to evaluate the representation of these processes in selected numerical weather prediction and climate models (e.g. Allan et al 2007, Greuell et al 2011, Haywood et al 2011, Mackie et al 2017).…”