The Benefit of HH and VV Polarizations in Retrieving Extreme Wind Speeds for an ASCAT-Type Scatterometer

The wind retrieval performance of a fixed fan-beam scatterometer operating at C-band in VV polarization [Advanced SCATterometer (ASCAT) type] is well established in the range of 0-25 m/s. This work evaluates prospective extensions with HH- and VH-polarized beams aimed at improving retrievals at extreme wind speeds, from 25 to 65 m/s. The geophysical model functions for C-band VV-, HH-, and VH-polarized backscatter used in the wind retrieval simulations are defined, along with an objective assessment of the quality scores ascribed to the optional beam configurations. Our study finds that the introduction of a VH capability in the midbeam improves the determination of wind speed over the entire scatterometer swath, with wind speed root-mean-square (rms) errors of about 0.5 m/s. The introduction of HH beams in the fore/aft antennas improves the determination of the ocean surface wind vector (wind speed and direction) but over a more limited portion of the outer swath, with wind vector rms errors as low as 6 m/s, but conditioned to a priori information to resolve the directional ambiguity. If the determination of wind speed primes over the determination of wind vectors at high wind speeds, then the introduction of a single VH capability in the midbeam antenna comes forth as the most simple and cost-effective manner to extend the wind retrieval capabilities of an ASCAT-type scatterometer into the domain of extreme winds.