EXTRATROPICAL INFLUENCE OF UPPER TROPOSPHERIC WATER VAPOR ON GREENHOUSE WARMING image

The figure shows that the increase in upper tropospheric humidity (a) is collocated with a decrease in outgoing longwave radiation (b) and an increase in greenhouse warming (c) along the strom track in the North Pacific and Atlantic; a clear demonstration of the role of upper tropospheric humidity in trapping radiation. The mechanism of enhenced upper tropospheric humidity over the storm track seems to be associated with the increased deep convective cloud (d) which transports water vapor upward to the tropopause. The generation of these deep convective clouds are associated with large-scale baroclinic waves along the storm tracks; The Eady growth rate is a measure of baroclinic instability (e). The distribution of integrated water vapor (f) closely follow sea surface temperature (g) and is more less zonally oriented. Therefore, the upper tropospheric water vapor is likely to be governed by the large-scale dynamics and not the local temperature, in contrary to some popular postulations.The pattern of upper tropospheric humidity provided by the GSFC Data Assimilation Office does not resemble the observations (h). It agrees more with the surface temperature distribution, and may imply the deficience in model simulation of upper atmosphere hydrologic balance.

The upper tropospheric humidity is measured by the Microwave Limb sounder on UARS. The deep convective cloud amount is provided the Interanational Satellite Cloud Climatology Project, the sea surface temperature is from AVHRR, the outgoing longwave radiation is from the Earth Radiation Budget Experiment, the integrated water vapor is from Special Sensor Microwave Imager, the Eady growth rate is computed using output from atmospheric general circulation model.

More details can be found in the article by Hu, H., and W.T. Liu, 1998: The impact of upper troposheric humidity from Microwave Limb Sounder on the midlatitude greenhouse effect. Geophys. Res. Lett., 25, 3151-3154