Assimilating TOVS humidity into the GEOS-2 data assimilation system

The humidity data retrieved from the TIROS Operational Vertical Sounder (TOVS) measurements is assimilated into the Goddard Earth Observing System (GEOS) data assimilation system. The study focuses on the impact of the TOVS humidity on assimilated humidity, precipitation, clouds, and radiation. The GEOS assimilation system utilizes the TOVS humidity effectively at levels below 300 mb, while the net impact on the 300-mb humidity is much less. It has been demonstrated that the impact results from direct and indirect effects. The direct effect is the analysis increment introduced by the humidity data, which draws the assimilated humidity toward the data. The indirect effect is realized through the interactions of humidity with physical processes, 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 strong interactions with convection and other physical processes. There is clear evidence indicating that tuning of physical parameterizations explicitly in the data assimilation mode is necessary for optimal use of the TOVS data in the assimilation system.