Simultaneous X-band radar and Ka-band radiometer observations of the ocean

Simultaneous microwave radar and radiometer observations of the ocean were conducted from aircraft during the ShoWEx´99 experiment. Both radar and radiometer wind dependences show similar signal growth with wind speed increase (except for the radiometer at vertical polarization and a near-Brewster angle). Analysis of the low-wind data shows that wind speed is a poor characteristic of the sea surface state in calm conditions because the wind field itself is quite non-uniform in space and time. Under such conditions both radar and radiometer data show strong scatter versus mean wind speed, but they are still well correlated with each other. We conclude that both active and passive instruments respond to the local variations of the sea surface roughness, which are not related to the mean wind speed at low winds. Our comparison shows that the best correlation is between the radar looking at 40-60deg and the near-nadir looking radiometer, which agrees with the theory of scattering/emission from a rough surface.