Retrieval of water vapor profiles from a combination of SSM/T-2 and SSM/I observations

Abstract

A physical relaxation algorithm has been developed for the microwave radiometric retrieval of water vapor profiles from the radiances measured by the SSM/T-2. The algorithm includes statistical constraints to provide stability in the layers for which there is little information in the radiances. It also includes an explicit treatment of clouds. The SSM/I with its better resolution and lower frequencies provides the potential for improved retrievals. However, under most conditions this improvement is marginal at best and easily negated by the practical problems in merging two sensors with dissimilar scan patterns. The advantage of adding the SSM/I occurs in the presence of thick clouds. Using only the radiances from the SSM/T-2, it is difficult to estimate the liquid water content of the clouds and thereby the cloud impact on the radiances. As a result, the quality of the retrievals suffers. Over an ocean background, the inclusion of the SSM/I observations provides a more reasonable basis for quantitative estimates ofthe cloud liquid water content as well as a precipitable water constraint on the profile resulting in significant improvement in the retrievals. The resolution ofthe 91 GHz channel ofthe SSM/T-2 is 100 km, twice that ofthe 183 GHz channels, making it of limited value in the presence of gradients. The SSM/I can be used to synthesize a dual polarized pair of 85.5 GHz observations with 50 km resolution which can be substituted for the 91 GHz channel. The SSM/I can also be used to filter for rain events which would result in erroneous retrievals. >