Comparison of scatterometer and radiometer wind vector measurements

Coincident measurements with a 37-GHz polarimetric radiometer and a 10-GHz scatterometer during the Coastal Ocean Probing Experiment (COPE) in September and October of 1995 offered a unique opportunity to compare their relative sensitivity and performance in observing sea surface winds. The scatterometer cross section so and the radiometer's second and third Stokes parameters, Q and U, were measured. The dependence of the angular signature of the radiometer on friction velocity was investigated by combining the COPE data with data collected in the Labrador Sea in February and March of 1997 at higher wind speeds than were encountered during COPE. The results of these experiments showed that the first harmonics of the radiometer azimuthal response were relatively insensitive to the friction velocity but that their second harmonics increased rapidly in amplitude with increasing friction velocity, with approximately the same sensitivity as a scatterometer cross section. The sensitivity of the radar and radiometer to the wind direction were then compared by computing the ratio of the angular signal to the inherent variability of the measurements for both instruments, their variability ratio. These ratios were comparable for the radar and radiometer and both increased with wind speed. However, the variability ratio of the radiometer decreased with increasing incidence angle while that of the radar increased. The similar magnitudes of the variability ratios of the two instruments are interpreted by the authors to indicate that their azimuthal signatures are caused by the same geophysical process: the angular dependence of short waves on the ocean surface and their tilting by longer waves. Their different dependence on incidence angle ui is explained as a result of the fact that partial derivative sigma (o)/partial derivative theta (i) decreases with increasing incidence angle, while partial derivativeQ/partial derivative theta (i) and partial derivativeU/partial derivative theta (i) increase with incidence angle.