Short-wave radiances: comparison between SEVIRI and the Unified Model



The radiation scheme in the Met Office Unified Model (UM) is extended to allow the calculation of short-wave radiances. Comparisons between short-wave reflectances simulated with the global forecasting configuration of the UM and observations from SEVIRI (Spin Enhanced Visible and Infrared Imager) on board Meteosat 8 are presented. One of the main purposes for such a comparison is model validation, since it allows us to directly compare model and satellite reflectances, without having to make use of retrieval systems. Eliminating the retrieval algorithm has the advantage that we can be certain that any discrepancies between the modelled and the measured radiances are due to inaccuracies in the model, rather than differences in the assumptions made both in the model and the retrieval algorithm. In principle, this should allow us to detect weaknesses in the model parametrizations.

In particular, we compare the four short-wave channels of SEVIRI to the corresponding radiance simulations obtained from the UM over oceanic regions. Generally good agreement between the simulated and observed reflectances was obtained in the visible channels, but greater disagreement was found in the NIR 1.6 and IR 3.9 channels, hinting at deficiencies in the model's representation of particle size. Sensitivity studies are carried out to explore these sensitivities.