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

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 TIROS-N Operational Vertical Sounder (TOVS). The model simulations generally agree with the satellite observed climatological mean and seasonal variation of moisture abundance. The interannual variabilities of vapor amount shown in the model are also comparable to the observed data. There are, however, biases in the details. The underestimates in TPW and UTH are found off the west coast of continents, especially in the boreal summer over the eastern subtropical Pacific. The biases are related to both an enhanced dray advection associated with a too strong subtropical high and more vigorous large scale subsidence in the model. A slightly different geographical location and more intense tropical circulation in ECHAM4 is manifest at the broadening of the high TPW and UTH zone that coincides with the active convective region. The anomalous equatorial UTH and TPW simulated in the model are found to be more sensitive to the tropical SST anomalies than the satellite data. The impact of changes in physical parameterizations on the moisture distribution is 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. The dry bias in UTH over the higher latitude of the winter hemisphere in ECHAM3 is a result of the 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 more moist than TOVS observations suggest. The discrepancies in simulating water vapor distributions affect the simulation of the radiation budget and cloud distribution in the model. (orig.)56 refs.Available from TIB Hannover: RR 1347(155) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman