Assimilating TOVS humidity into the GEOS-2 data assimilation system

The humidity data retrieved from the TIROS Operational Vertical Sounder (TO VS) measurements is assimilated into the Goddard Earth Observing System (GE OS) data assimilation system. The study focuses on the impact of the TOVS h umidity on assimilated humidity, precipitation, clouds, and radiation. The GEOS assimilation system utilizes the TOVS humidity effectively at levels b elow 300 mb, while the net impact on the 300-mb humidity is much less. It h as been demonstrated that the impact results from direct and indirect effec ts. The direct effect is the analysis increment introduced by the humidity data, which draws the assimilated humidity toward the data. The indirect ef fect is realized through the interactions of humidity with physical process es, mainly with moist convection. The indirect effect is often opposite to the direct effect in the current assimilation system. The direct effect is dominant in the lower and middle troposphere while in the upper troposphere the indirect effect is more important. The impact of the TOVS humidity on the GEOS precipitation, clouds, and radiation is also significant due to st rong interactions with convection and other physical processes. There is cl ear evidence indicating that tuning of physical parameterizations explicitl y in the data assimilation mode is necessary for optimal use of the TOVS da ta in the assimilation system.