Characterization of water-vapour retrievals from TOVS/HIRS and SSM/T-2 measurements

This paper studies the retrieval characteristics of water-vapour retrievals from infrared TOVS (TIROS Operational Vertical Sounder) radiances and micr owave SSM/T-2 (Special Sensor Microwave/Water-vapour Sounder) radiances. An optimal-estimation retrieval scheme is built around a radiative-transfer m odel that uses a spectral-band model based on the Malkmus line distribution for the infrared and the MPM92 transmission model for the microwave. Chara cteristics from synthetic retrievals show that the sensitivity to water vap our in the lower troposphere is low for TOVS and SSM/T-2 over land, but hig h for SSM/T-2 over ocean. The sensitivity to water vapour in the middle and upper troposphere is high for TOVS, but low for SSM/T-2. The vertical reso lution of the retrievals is restricted to a few kilometers for both instrum ents. The dependence on a priori information is therefore substantial, espe cially in the lower troposphere. A global retrieval with real data from the TOVS instrument and a priori data from the National Centers for Environmen tal Prediction re-analysis constrained by an extra total-column constraint from SSM/I(Imager) data shows that the retrieval scheme is able to produce monthly-mean fields similar to other data on monthly-mean precipitable-wate r fields and layered information about column water vapour. The approach pr esented allows for a useful error characterization that quantifies the depe ndence on the a priori information. In general, the best performance of the retrieval scheme is achieved in the tropical upper troposphere.