Retrieval of water vapor profiles over ocean using SSM/I and SSM/T-2

Abstract

The SSM/T-2 (Special Sensor Microwave Temperature-2) and SSM/I (Special Sensor Microwave Imager) are two microwave radiometers aboard the DMSP (Defense Meteorological Space Program) series of satellites. The SSM/T-2 was designed for retrieval of water vapor profiles, while the SSM/I can be used to retrieve, among other things, the total integrated water vapor (TIWV) in a column of air. It is theoretically possible to use SSM/I data to supplement the SSM/T-2 data, producing more accurate water vapor profiles. A physical relaxation algorithm is used to retrieve the relative humidity at each of several levels in the atmosphere from a set of SSM/T-2 brightness temperatures and the SSM/1-derived T@. In this retrieval process, TAW is treated formally as a separate channel. Retrievals are performed using the SSM/T-2 in conjunction with individual SSM/I channels and the TRW pseudochannel. The yield (the percentage of retrieved humidity profiles for which the computed and observed brightness temperatures agree to within the noise level of the radiometer) is found to be higher and the normalized brightness temperature error lower when the SSM/1-derived TAW is added to the inputs. When using the TAW, the 150 GHz channel can be dropped with minimal impact on results. It is shown that by using the SSM/T-2. Examples of retrieved profiles are shown to agree with the general features of profiles from radiosondes and the European Centre for Medium-Range Weather Forecasting (ECMWF) analyses. The stability of the solution to the iterative nonlinear retrieval algorithm with respect to the first guess profile is also investigated. The first guess is shown to have a small but significant effect on the retrieved profiles.