High-resolution passive polarimetric microwave mapping of ocean surface wind vector fields

The retrieval of ocean surface wind fields in both one and two dimensions i s demonstrated using passive polarimetric microwave imagery obtained from a conical-scanning airborne polarimeter. The retrieval method is based on an empirical geophysical model function (GMF) for ocean surface thermal emiss ion and an adaptive maximum likelihood (ML) wind vector estimator, Data for the GMF were obtained using the polarimetric scanning radiometer/digital ( PSR/D) on the NASA P-3 aircraft during the Labrador Sea Deep Convection Exp eriment in 1997, To develop the GMF, a number of buoy overflights and GPS d ropsondes were used, out of which a GMF of 10.7, 18.7, and 37.0 GHz azimuth al harmonics for the first three Stokes parameters was constructed for the SSM/I incident angle of 53.1 degrees. The data show repeatable azimuthal ha rmonic coefficient amplitudes of similar to2-3 K peak-to-peak, with a 100% increase in harmonic amplitudes as the frequency is increased from 10.7 to 37 GHz. The GMF is consistent with and extends the results of two independe nt studies of SSM/I data and also provides a model for the third Stokes par ameter over wind speeds up to 20 m/s, The aircraft data show that the polar imetric channels are much less susceptible to geophysical noise associated with maritime convection than the first two Stokes parameters. The polarime tric measurement technique used in the PSR/D also demonstrates the viabilit y of digital correlation radiometry for aircraft or satellite measurements of the full Stokes vector. The ML retrieval algorithm incorporates the additional information on wind direction available from multiple looks and polarimetric channels in a stra ightforward manner and accommodates the reduced SNRs of the first two Stoke s parameters in the presence of convection by weighting these channels by t heir inverse SNR. The Cramer-Rao bound for the retrieval operator is comput ed using the SNR estimates. Here, the measurement noise is a root-sum-squar ed combination of both instrument (radiometric) noise and geophysical (clou d and convection) noise. The retrieved wind directions are within 9 degrees of GPS-dropsonde wind-direction measurements, and the standard deviations are similar to2 times the Cramer-Rao bound. The retrieval method illustrate s the capability of mapping ocean surface wind fields from either airborne or spaceborne platforms using a two-look technique.