A simple model for deriving ocean surface wind direction from the DMSP Special Sensor Microwave Imager (SSM/I)

Abstract

The Special Sensor Microwave Imagers (SSM/I) flying on the DMSP polar orbiting derive relatively high resolution surface wind speeds over the oceans. The SSM/I is limited to ocean surface wind speed only. Recently, techniques to infer wind direction from passive microwave measurements have been developed. Polarimetric measurements are currently of interest. Active devices produce very high quality vector wind measurements. It is the goal of this paper to discuss a simple technique that could be used in conjunction with polarimetry and scatterometry to augment the complete description of boundary layer winds. Th authors discuss a simple technique first developed by T.S. Yu (1987), that yields a closed form solution to the horizontal momentum equations for the boundary layer vector wind field using in situ satellite measurements as a source of wind speed data, and surface pressure gradient data from state-of-the-art numerical weather prediction models. The SEASAT scatterometer and altimeter data were originally used as a proof of concept for this technique. The authors describe an implementation using the SSM/I as the source of wind speed data, and the Air Force Global Weather Central (AFGWC) High Resolution Analysis System (HIRAS) as the source of boundary layer pressure analysis. They calculate a wind field at the resolution of the SSM/I, with time and space interpolation of the model surface pressure analysis to match the satellite measurements. It should be noted that the HIRAS resolution is much coarser than the SSM/I data, therefore they are relying on the general smoothness ofthe surface pressure during the interpolation process.