A comparison of satellite observations and model simulations of column-integrated moisture and upper-tropospheric humidity

Water vapor distributions obtained from the fourth generation ECHAM general circulation model are compared with satellite observations of total precipitable water (TPW) from the Special Sensor Microwave/Imager (SSM/I) and upper-tropospheric relative humidity (UTH) from TlROS-N Operational Vertical Sounder (TOVS). In general, the model simulations agree well with satellite observations of the climatological mean, seasonal variation, and interannual variation of moisture. There are, however, biases in the details. Underestimates in TPW and UTH are found off the west coast of continents, especially in the boreal summer over the eastern subtropical Pacific. These biases are related to both enhanced dry advection due to an excessively strong subtropical high and greater large-scale subsidence. A more intense tropical circulation in ECHAM4 is evidenced by the broadening of the high TPW and UTH zone that coincides with the equatorial convective regions. Additionally, interannual anomalies in equatorial UTH and TPW simulated by the model are found to be more sensitive to tropical SST anomalies than are the satellite data. The impact of changes in physical parameterizations upon the moisture distribution is also examined by comparing the simulations from the previous ECHAM3 and the current ECHAM4 models. The dry bias at the equator in ECHAM3 is related to the closure assumption used for deep convection, while the dry bias in UTH over the high-latitude winter hemisphere in ECHAM3 is a result of negative specific humidities produced by the spectral vapor advection scheme. With the new semi-Lagrangian advection scheme in ECHAM4, the simulated UTH over the same region becomes moister than TOVS observations suggest. The impact of discrepancies in the simulated water vapor distributions upon the radiation budget and cloud distribution in the model are also described.