On the assimilation of Ku-band scatterometer winds for weather analysis and forecasting

Abstract

Following the successful assimilation of European remote sensing satellite (ERS) scatterometer winds for weather analysis and forecasting, the authors further develop this methodology for the assimilation of the NASA scatterometer (NSCAT) and QuikSCAT Ku-band scatterometer data. Besides retrieval problems in cases of a confused sea state, the quality control (QC) developed identifies cases with rain on a wind vector cell (WVC) by WVC basis. The elimination of such geophysical conditions is a prerequisite to arrive at a successful assimilation of Ku-band scatterometer data. Moreover, the authors propose to assimilate ambiguous winds rather than radar backscatter measurements, as is being done at most meteorological centers assimilating ERS scatterometer data. After their quality assessment, NSCAT winds still have more difficult ambiguity removal properties than ERS winds. A further testing of the data assimilation method proposed is being carried out at the European Center for Medium-range Weather Forecasts in NSCAT impact experiments. A normalized wind inversion residual is used for QC. In order to determine a threshold for the rejection of poor quality wind solutions, the inversion residual and the wind vector departure from the ECMWF model are correlated. They end up rejecting around 7.4% of wind vector solutions and 4.2% of the NSCAT WVCs. In order to perform a qualitative assessment of this rejection, comparisons to collocated SSM/I rain and ECMWF winds are used. Confused sea state and presence of rain seem to be the most likely causes for the rejection of WVCs.