Polarimetric brightness temperatures of sea surfaces measured with aircraft microwave radiometers

Aircraft dual-frequency (19 and 37 GHz) radiometer measurements of polarimetric sea surface brightness temperatures are reported in this paper. All measured Stokes parameters showed a few Kelvin azimuth modulations with respect to the wind direction. The wind directional 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 the range of 19 to 37 GHz. The harmonic coefficients of the wind direction signals were derived from experimental data versus incidence angle. It was found that the first harmonic coefficients, which are caused by the up and downwind asymmetric surface features, had a small increasing trend with the incidence angle. In contrast, the second harmonic coefficients, caused by the up and crosswind asymmetry, showed significant variations in T_υ and U data, with a sign change when the incidence angle increased from 45° to 65°. Besides the first three Stokes parameters, the fourth Stokes parameter, V, which had never, been measured for sea surfaces before, was measured using our 19-GHz channel. The Stokes parameter V has an odd symmetry just like that of the third Stokes parameter U, but with a smaller wind direction signal than that of U. Theoretical interpretation based on two-scale scattering models was performed to interpret the experimental data. In summary, the sea surface features created by the near surface winds are anisotropic in azimuth direction and modulate all Stokes parameters of sea surface microwave brightness temperatures by as large as a few Kelvin in the range of incidence angles from 45° to 65° applicable to spaceborne observations