POLARIMETRIC BRIGHTNESS TEMPERATURES OF SEA SURFACES MEASURED WITH AIRCRAFT K-BAND AND KA-BAND RADIOMETERS

Dual-frequency (19 and 37 GHz), multi-incidence measurements of the St okes parameters of sea surface microwave emission are reported in this paper, A series of aircraft polarimetric radiometer flights were carr ied out over the National Data Buoy Center (NDBC) moored buoys deploye d off the northern California coast in July and August 1994, Measured radiometric temperatures showed a few Kelvin azimuth modulations in al l Stokes parameters with respect to the wind direction, Wind direction al signals observed in the 37-GHz channel were similar to those in the 19-GHz channel, This indicates that the wind direction signals in sea surface brightness temperatures have a weak frequency dependence in t he range of 19-37 GHz. Harmonic coefficients of the wind direction sig nals were derived from experimental data versus incidence angle, It wa s found that the first harmonic coefficients, which are caused by the up and downwind asymmetric surface features, had a small increasing tr end with the incidence angle, In contrast, the second harmonic coeffic ients, caused by the up and crosswind asymmetry, showed significant va riations in T-v and U data, with a sign change when the incidence angl e increased from 45 degrees to 65 degrees. Besides the first three Sto kes parameters, the fourth Stokes parameter, V, which had never been m easured before for sea surfaces, was measured using our 19-GHz channel , The Stokes parameter V an odd symmetry just like that of the third S tokes parameter U, and increases with increasing incidence angles, In summary, sea surface features created by surface winds are anisotropic in azimuth direction and modulate all Stokes parameters of sea surfac e microwave brightness temperatures by as large as a few Kelvin in the range of incidence angles from 45 degrees to 65 degrees applicable to spaceborne observations.