Evaluation of the microwave ocean surface emissivity model FASTEM-5 in the IFS

TitleEvaluation of the microwave ocean surface emissivity model FASTEM-5 in the IFS
Publication TypeTechnical memorandum
Date Published02/2012
Secondary TitleECMWF Technical Memoranda
AuthorsBormann, N, Geer, AJ, English, SJ

The latest version of FASTEM, the fast ocean surface emissivity model for microwave frequencies, is evaluated in the ECMWF system. The performance of FASTEM-5 is assessed through a comparison between observed and simulated brightness temperatures for a range of sensors, and the results are compared to similar characteristics for earlier versions of FASTEM. FASTEM-5 shows significantly different bias characteristics compared to earlier versions. For the microwave imagers, the change in the foam cover parameterisation leads to an altered dependence of the biases on the 10m wind speed compared to FASTEM-4. At high 10m wind speeds, the behaviour is now closer to FASTEM-2, as expected from reverting the change to the foam cover model. For low wind speeds, the behaviour is similar to FASTEM-4 for vertically polarised channels, and similar to FASTEM-2 for horizontally polarised channels. At high wind-speeds, standard deviations of First-Guess departures suggest a small benefit from using FASTEM-4 rather than FASTEM-5, but overall the results do not clearly favour one parameterisation over the other. For the microwave sounders, FASTEM-5 leads to an overall reduction of biases compared to FASTEM-3 or -4, with a smaller dependence on the 10m wind speed. The impact on departure statistics is primarily confined to the window channels which are not assimilated in the ECMWF system, but used for quality control of the sounding channels. For these channels, a positive bias in terms of brightness temperatures remains. The variational bias correction successfully removes the differences in the bias characteristics, leaving bias-corrected departure characteristics that are not significantly different with FASTEM-5 or FASTEM-4. The forecast impact of the move from FASTEM-4 to FASTEM-5 is neutral overall.

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