A simplified interpretation of the radiances from the SSM/T-2

Summary

A microwave radiometer with channels near the 18 GHz water vapor line and in nearby windows, the Special Sensor Microwave/Temperature-2 (SSM/T-2) was launched on a Defense Meteorological Satellite Program (DMSP) satellite in November of 1991. The instrument is intended to provide data for the retrieval of atmospheric water vapor profiles. Because the relationship between the radiances observed by the instrument and the water vapor profile are both non-linear and non-local and because of the influence of clouds, the interpretation of the radiances is inherently complex. Here we develop a simplified, albeit approximate, algorithm for the profile retrievals and test it with simulation studies. Specifically it is shown that for each channel of the instrument near the 183 GHz line there is a nearly constant overburden of water vapor above the height at which the atmospheric temperature equals the observed brightness temperature. This relationship, in turn, provides the basis for a simple analytic algorithm for the relative humidity immediately above that height.

The simplified algorithm is useful as a first guess for iterative solutions to the non-linear equations and for a variety of analyses such as estimating the impact of uncertainty in the radiances or the temperature profile on the retrieved water vapor profile. It is also useful as a conceptual tool to aid in the understanding of the more complex algorithms.