Matching ASCAT and QuikSCAT winds

Surface winds from two scatterometers, the Advanced scatterometer (ASCAT), available since 2007, and QuikSCAT, which was available through November 2009, show persistent differences during their period of overlap. This study examines a set of collocated observations during a 13-month period November 2008 through November 2009, to evaluate the causes of these differences. A difference in the operating frequency of the scatterometers leads to differences that this study argues depend on rain rate, wind velocity, and SST. The impact of rainfall on the higher frequency QuikSCAT introduces biases of up to 1 m s(-1) in the tropical convergence zones and along the western boundary currents even after rain flagging is applied. This difference from ASCAT is reduced by some 30% to 40% when data for which the multidimensional rain probabilities > 0.05 is also removed. An additional component of the difference in wind speed seems to be the result of biases in the geophysical transfer functions used in processing the two data sets and is parameterized here as a function of ASCAT wind speed and direction relative to the mid-beam azimuth. After applying the above two corrections, QuikSCAT wind speed remains systematically lower (by 0.5 m s(-1)) than ASCAT over regions of cold SST < 5 degrees C. This difference appears to be the result of temperature-dependence in the viscous damping of surface waves which has a greater impact on shorter waves and thus preferentially impacts QuikSCAT. The difference in wind retrievals also increases in the storm track corridors as well as in the coastal regions where the diurnal cycle of winds is aliased by the time lag between satellites.