Variational retrievals using SSM/I and SSM/T-2 brightness temperatures in clear and cloudy situations

The direct assimilation of brightness temperatures (Tb's) from the Defense Meteorological Satellite Program Special Sensor Microwave Imager (SSM/I) an d Special Sensor Microwave Water Vapor Profiler (SSM/T-2) in a one-dimensio nal variational (1D-Var) assimilation system is studied over the oceans. Th e control variables of the 1D-Var are the natural logarithm of specific hum idity (ln q), near-surface wind speed (SWS), and cloud liquid water (CLW) p ath. Synthetic Tb's, with and without noise added, were simulated and used to es timate the strength of the 1D-Var assimilation (weight given to the observa tions) and the information content of the Tb. In clear skies, it is shown t hat except for very dry profiles (TPW, < 5-6 kg m(-2)) SSM/I Tb's are super ior to the SSM/T-2 Tb's for the determination of total precipitable water ( TPW). In the presence of clouds, the SSM/T-2 TPW retrievals are underestima ted and the underestimation increases with CLW. Cloudy profiles should be f iltered out. It is also shown that the SSM/I 1D-Var retrievals for low wind speeds are e rroneous (<2-3 m s(-1)) and so are retrievals of low CLW (<0.1 kg m(-2)). O therwise, SSM/I Tb retrievals of TPW, SWS, and CLW perform well. 1D-Var ana lyses using bias-corrected observed Tb's were also computed for the same ca se as that using synthetic Tb's and were compared with retrievals obtained using published regression equations. When both SSM/I and SSM/T-2 Tb's were assimilated simultaneously (in collocation) in the 1D-Var system, the retr ievals of TPW, SWS, and CLW were very similar to those when only SSM/I Tb's were assimilated. However, the SSM/T-2 Tb also provided specific humidity information in the upper troposphere.