The dependence of the microwave emissivity of the ocean on hurricane force wind speed

There has been considerable controversy within the NASA and NOAA satellite remote sensing communities in the past several years about the relative strengths and weaknesses of active and passive microwave sensors for the measurement of ocean surface wind speed and direction. In particular, the ability of the two approaches to retrieve wind vector at hurricane force speeds and through the extremely high precipitation rates common in hurricanes has been much debated. The ability of the WindSat passive microwave satellite imager to satisfactorily retrieve wind vector at wind speeds below 20 m/s and through clear air and light precipitation has been well documented in the refereed literature. WindSat's sensitivity to wind direction in much higher (hurricane force) winds has also recently been demonstrated. Results are presented here of a study which examines WindSat's sensitivity to wind speed in hurricanes. The study involves an intercomparison between WindSat overpass measurements, made during 2005 of Hurricanes Dennis, Rita and Katrina, and surface wind fields for those overpasses, generated using NOAA's H*Wind system.

The raw WindSat observations are of fully polarimetric top-of-atmosphere radiances at 10.7, 18.7 and 37.0 GHz and dual linear polarization radiances at 6.8 and 23.8 GHz. The higher frequency vertically polarized channels are relatively insensitive to surface wind effects and so are used to estimate the absorbing and emitting atmospheric water vapor and liquid constituents between the satellite and the surface. Once the atmosphere has been characterized, its effects on the horizontally polarized radiance can be removed from those observations and the underlying ocean surface emissivity can be derived. The emissivity is then matched up against corresponding H*Wind surface wind speeds.

Our results clearly show that the ocean surface emissivity responds strongly to changes in near surface winds at wind speeds of up to 60 m/s (Category 4 hurricane). In addition, the emissivity response is clearly distinguishable in the presence of the extremely high precipitation rates found in the spiral rain bands of a hurricane. These results demonstrate the potential of polarimetric radiometers to retrieve wind fields in hurricane conditions.