Polarimetric millimeter-wave thermal emission from anisotropic water surfaces: Application to remote sensing of ocean surface wind direction

An investigation of the geophysical nature of passive polarimetric wind-direction signatures over the ocean including techniques for measurement of the third Stokes parameter {dollar}(2Relangle Esbnu Esbsp{lcub}h{rcub}{lcub}*{rcub}rangle){dollar} in passive Earth remote sensing is presented. Satellite observations of the oceanic wind-direction signature were found to be explained by an asymmetric-wave/geometrical-optics (AWGO) model of the upwelling thermal radiation over the ocean surface. The combined electromagnetic and statistical hydrodynamic model was based on a Monte-Carlo ocean wave generator which incorporated both amplitude and phase spectral information into each surface realization. A prescribed amount of ocean wave asymmetry in the surface realizations was obtained by using a locked-phase relationship between the constituent ocean-wave Fourier components. Polarimetric surface emission was found using a geometrical optics (GO) electromagnetic emission model incorporating multiple-scattering and surface shadowing effects.; Partially polarimetric laboratory measurements of thermal emission over a sinusoidally-striated water surface were used to corroborate the GO model. The measurements were performed using a precision-calibrated 92-GHz polarization-correlating radiometer trained on a rotatable water-wave tank. Azimuthal emission scans at several incident angles from near-nadir to near-grazing were made. The radiometer was calibrated using a novel polarized calibration load; the procedure proved the viability of a new calibration technique for fully-polarimetric radiometers. A technique to precisely rotate the antenna's polarization basis by a simple matrix transformation was also demonstrated. Using the 92-GHz cross-correlating radiometer along with two additional X- and K-band radiometers, airborne observations of ocean surface emission were performed under light-wind conditions over nearly a decade-wide band during TOGA/COARE. The data show broad-band azimuthal variations in brightness temperature that are related to wind direction with an upwind/downwind asymmetry that is consistent with previous findings using 19- and 37-GHz Special Sensor Microwave Imager (SSM/I) satellite observations.; The measured and simulated brightness temperature signatures were analyzed using Fourier harmonics. Calculations using the AWGO model were compared with the SSM/I observations and found to be in general agreement for a variety of surface conditions. The investigation suggested that passive wind direction signatures over the ocean are largely caused by ocean wave asymmetry and foam. Furthermore, azimuthal variations in the upwelling third Stokes parameter over striated water surfaces were shown to be in phase quadrature with variations of the first and second modified Stokes parameters. Therefore, in principle the third Stokes parameter can be used to facilitate remote sensing of the oceanic wind-vector by removing directional ambiguities. For observation angles at {dollar}sim{dollar}60{dollar}spcirc{dollar}-70{dollar}spcirc{dollar} from nadir, the first three Stokes parameters have particularly large azimuthal brightness variations. These large variations suggest significant potential for measuring ocean wave direction from spaceborne platforms.