Analysis of the ASCAT inversion residual for quality control and forward modelling improvement

An important part of the scatterometer wind data processing is the quality control (QC). This report shows the implementation of a new scatterometer QC procedure, based on a comprehensive analysis of the wind inversion residual, which significantly improves the effectiveness of the wind data QC. The method is applied on the Advanced Scatterometer (ASCAT) onboard Metop-A, but is generic and can therefore be applied to any scatterometer system. The method is also used to analyse the C-band geophysical model function (CMOD5n). It turns out that for winds around 4 m/s, the GMF does not match the ASCAT measurements, therefore indicating a GMF misfit or error. In this study, the rain impact on the ASCAT QC and retrieved winds is thoroughly investigated using the European Centre for Medium-range Weather Forecasts (ECMWF) model winds, the Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI) rain data, and tropical buoy wind and precipitation data as reference. In contrast to Ku-band, it is shown that C-band is much less affected by direct rain effects, such as ocean splash, but effects of increased wind variability appear to dominate ASCAT wind retrieval. ECMWF winds do not well resolve the air flow under rainy conditions. ASCAT winds do, but also show artefacts in both the wind speed and the wind direction distributions for high rain rates. The operational QC proves to be effective in screening these artefacts, but at the expense of many valuable winds.