Analysis of the ASCAT wind inversion residual: towards an improved C-band geophysical model function

Abstract

The Advanced Scatterometer (ASCAT) instrument, a real aperture vertically polarised C-band radar with high radiometric stability, was launched onboard EUMETSAT’s MetOp in October 2006. Within the EUMETSAT Satellite Application Facilities (SAF), the Royal Netherlands Meteorological Institute (KNMI) has the responsibility of the ASCAT level 2 wind production. During the successful calibration and validation phase, in which the instrument has been precisely calibrated and outperforming in terms of retrieved wind accuracy, several aspects of the level 2 processing were reviewed, notably the forward modeling, the Quality Control (QC) and the wind retrieval. Several authors have shown that a proper characterization of the scatterometer inversion residual or Maximum Likelihood Estimator (MLE) is very useful for improving level 2 processing. Following previous experience with the European ERS scatterometer, we normalize the ASCAT MLE and look into its properties as a QC indicator and wind retrieval parameter. Furthermore, we show that by looking at the normalized MLE magnitude and sign, one can identify certain deficiencies in the Geophysical Model Function (GMF). This analysis sets the grounds for an improved C-band GMF, i.e., CMOD6