The Dependence of Sea-Surface Microwave Emission on Wind Speed, Frequency, Incidence Angle, and Polarzation over the Frequency Range from 1 to 40 GHz

Abstract

The dependence of sea-surface microwave emissive prop erties on wind speed, frequency, incidence angle, and polarization arn discussed in detail, in terms of wind-speed sensitivity of the brightnes temperature (defined as a change in brightness temperature with a un, change in wind speed) to the last three observational conditions, ovet the frequency range from 1 to 40 GHz, comparing our results at 6.7 and 18.6 GHz with many investigators' results at various levels from surface to satellite. This wind-speed sensitivity shows marked incidence angle and polarization dependencies. In vertical polarization, a wind-induced increase in the brightness temperature decreases with incidence angle. Then, the vertically polarized brightness temperature of the roughened sea surface is equal to that of the calm surface at an incidence angle between 50 ° and 600, and is higher below, and lower above, this angle; in other words, the above sensitivity is higher and takes a positive value at lower incidence angle and it is lower and takes negative value at higher incidence angle, with a wind-speed-insensitive angle existing between 500 and 600. A wind-induced change in the horizontally polarized brightness temperature shows no marked incidence angle dependence, but a uniform increase over the incidence angle. Both polarizations do not essentially change their characteristics in the above dependencies but gain wind-speed sensitivity with frequency; however, the horizontal polarization is much more sensitive to wind speed than the vertical one over the entire incidence and frequency ranges.