QuikSCAT quality control: rain flag

A good assessment of the information content of scatterometer winds is particularly important in order to assimilate them in weather analysis. Besides retrieval problems in cases of a confused sea state, a particularly acute problem of Ku-band scatterometry is the sensitivity to rain. Elimination of poor quality data is therefore a prerequisite for the successful use of NSCAT or QuikSCAT winds. Following the Quality Control for the ERS and NSCAT scatterometers performed at KNMI, we further develop this methodology for QuikSCAT and define a quality indicator, called the normalized residual (Rn). In order to characterize and validate the normalized residual, we use collocated SSM/I rain and ECMWF wind data. The results show indeed correlation between Rn and data quality. A wind speed dependent Rn threshold is shown to be adequate in terms of rejecting poor quality data (particularly rain) and keeping fair quality data. This opens the way to a quantitative use of SeaWinds measurements in weather analysis.