Introduction

The importance of the multifrequency passive microwave imagers is that, irrespective of cloud cover, they can retrieve a large variety of surface and atmospheric variables. Figure 9.1 shows the atmospheric and surface variables that affect the transmissivities and emissivities, and are retrievable by passive microwave. In the atmosphere, these include columnar water vapor, the cloud liquid water and the surface rain rate. At the surface, they include sea ice extent and type, sea surface temperature (TS) and salinity (SS) and the scalar and vector wind speeds.

As this chapter shows, because the emissivity or transmissivity associated with each atmosphere or oceanic constituent has a different frequency dependence, the variables are retrieved from a set of multifrequency, multivariable simultaneous equations. The disadvantage of this formulation is that in most cases the solutions are not separable, so that if, for example, the only variable of interest is TS, then by necessity many of the other variables must also be retrieved. An advantage of the microwave is that except in regions with heavy rain the retrievals are independent of cloud cover. In the following, Section 9.2 discusses the frequency dependence of the atmospheric absorption and transmission, and shows that, for frequencies less than 10 GHz, the effects of clouds and water vapor are negligible. Section 9.3 discusses the microwave form of the radiative transfer equation, the problem of sun glint, radio-frequency interference and Faraday rotation. Section 9.4 describes the effect on the surface emissivity of ocean waves, surface roughness and foam and shows how the azimuthal distribution of capillary and gravity waves relative to the wind direction permits retrieval of the vector wind speed. Section 9.5 describes the effect of sea surface temperature and salinity on the emissivity; Section 9.6 describes the multichannel algorithms for retrieval of the different oceanic and atmospheric variables; Section 9.7 describes the WindSat retrieval of vector winds; Section 9.8 discusses the sea ice algorithms.