A SIMPLIFIED INTERPRETATION OF THE RADIANCES FROM THE SSM T-2/

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 Meterological Satellite Program (DMSP) satellite in November of 1991. The instrument is intended to provide data for the retrieval of atmospheric water vapor profiles. Because th e relationship between the radiances observed by the instrument and th e water vapor profile are both nonlinear and non-local and because of the influence of clouds, the interpretation of the radiances is inhere ntly complex. Here we develop a simiplified, albeit approximate, algor ithm for the profile retrievals and test it with simulation studies. S pecifically it is shown that for each channel of the instrument near t he 183 GHz line there is a nearly constant overburden of water vapor a bove the height at which the atmospheric temperature equals the observ ed brightness temperature. This relationship, in turn, provides the ba sis for a simple analytic algorithm for the relative humidity immediat ely above that height. The simplified algorithm is useful as a first g uess for iterative solutions to the non-linear equations and for a var iety of analyses such as estimating the impact of uncertainty in the r adiances or the temperature profile on the retrieved water vapor profi le. It is also useful as a conceptual tool to aid in the understanding of the more complex algorithms.