Observation of a Surface Wind around a Typhoon by Seasat-1 Scatterometer

Abstract

   Seasat-1 has a wind-measuring scatterometer (SASS) which is designed to measure globally the ocean surface wind vectors (accuracy ±2m/s and ±20°) independent of day or night, and in all weather conditions. The technique of SASS has its physical base mainly in the Bragg scattering of microwaves by capillary waves on the ocean (λ_ω∼1 cm). Normalized radar cross section (σ°) of the ocean is proportional to the capillary wave amplitude which is proportional to a wind speed 19.5 m height over the sea surface in neutral stability. We can derive the wind direction, too, from some different azimuthal σ° measurements, since σ° is anisotropic.
   Data of revolution numbers 693 and 700 in SASS IGDR contain the sea surface wind observations of Typhoon Carmen (T7811). We can see by GMS that the SASS swaths are almost rain-free. So it is assumed that σ° is almost free from attenuation by the atmosphere, and that a comparison between SASS data and ground truth gives the evaluation for the SASS algorithm. Here is a study of SASS data in comparison with the winds by a two-layer baroclinic model for the marine boundary layer. The comparison is not a direct verification of SASS data, because there are some assumptions and boundary conditions in this model. However we can indirectly evaluate the significance of the data in the ocean where we can not make any observations of a typhoon region.
   The most significant result obtained is that a SASS wind speed is somewhat stronger than a surface truth wind from 5 to 15 m/s wind speed region. It is suggested that the SASS algorithm is too sensitive to wind speed, especially in this wind speed region. And the result of this study gives support to that of the GOASEX workshop reports.