Direction interval retrieval with thresholded nudging: a method for improving the accuracy of QuikSCAT winds

The SeaWinds on QuikSCAT scatterometer (QSCAT) was developed by NASA JPL to measure the speed and direction of ocean surface winds. The accuracy of the majority of the swath, and the size of the swath are such that QSCAT meets its science requirements despite shortcomings at certain cross track positions. None the less, it was desirable to modify the baseline processing in order to improve the quality of the less accurate portions of the swath, in particular near the far swath and nadir. Two disparate problems have been identified for far swath and nadir. At far swath, ambiguity removal skill is degraded due to the absence of inner beam measurements, limited azimuth diversity, and boundary effects. Near nadir, due to nonoptimal measurement geometry, (measurement azimuths approximately 180/spl deg/ apart) there is a marked decrease in directional accuracy even when ambiguity removal works correctly. Two algorithms were developed, direction interval retrieval (DIR) to address the nadir performance issue, and thresholded nudging (TN) to improve ambiguity removal at far swath. The authors illustrate the impact of the two techniques by exhibiting post-launch statistical performance metrics with respect to European Center for Medium-range Weather Forecasting (ECMWF) wind fields.