POLARIMETRIC MEASUREMENTS OF SEA-SURFACE BRIGHTNESS TEMPERATURES USING AN AIRCRAFT K-BAND RADIOMETER

This paper presents the first experimental evidence that the polarimet ric brightness temperatures of sea surfaces are sensitive to ocean win d direction in the incidence angle range of 30 to 50 degrees, Our expe rimental data were collected by a K-band (19.35 GHz) polarimetric wind radiometer (WINDRAD) mounted on the NASA DC-8 aircraft. A set of airc raft radiometer Eights was successfully completed in November 1993, We performed circle flights over National Data Buoy Center (NDBC) moored buoys deployed off the northern California coast, which provided ocea n wind measurements, The first WINDRAD flight was made on November 4, 1993, There was clear weather with a wind speed of 12 m/s at 330 degre es around the Pt, Arena buoy, We circled the buoy at three incidence a ngles, and all data when plotted as functions of azimuth angles show c lear modulations of several Kelvin, At 40 degrees incidence angle, the re is a 5 Kelvin peak-to-peak signal in the second Stokes parameter Q and the third Stokes parameter Ii, The Q data maximum is in the upwind direction and U has a 45 degrees phase shift in azimuth-as predicted by theory, There is also an up/downwind asymmetry of 2 Kelvin in the Q data, and 1 Kelvin in the U data, At 50 degrees incidence angle, the collected data show very similar wind direction signatures to the SSM/ I model function, Additional Eights were made on other days under clou dy conditions, Data taken at a wind speed of 8 m/s show that at 40 deg rees incidence Q and U have a smaller azimuthal modulation of 3 Kelvin , probably due to the lower wind speed, Additionally, the simultaneous ly recorded video images of sea surfaces suggested that Q and U data w ere less sensitive to unpolarized geophysical variations, such as clou ds and whitecaps, while the T-v and T-h increased by a few Kelvin when the radiometer beam crossed over clouds, or there was a sudden increa se of whitecaps in the radiometer footprint. The results of our aircra ft flights indicate that passive polarimetric radiometry has a strong potential for global ocean wind speed and direction measurements from space,