DISTRIBUTION OF TROPICAL TROPOSPHERIC WATER-VAPOR

Utilizing a conceptual model for tropical convection and observational data for water vapor, the maintenance of the vertical distribution of the tropical tropospheric water vapor is discussed. While deep convec tion induces large-scale subsidence that constrains the turbulent down gradient mixing to within the convective boundary layer and effectivel y dries the troposphere through downward advection, it also pumps hydr ometeors into the upper troposphere, whose subsequent evaporation appe ars to be the major source of moisture for the large-scale subsiding m otion. The development of upper-level clouds and precipitation from th ese clouds may also act to dry the outflow, thus explaining the low re lative humidity near the tropopause. A one-dimensional model is develo ped to simulate the mean vertical structure of water vapor in the trop ical troposphere. It is also shown that the horizontal variation of wa ter vapor in the tropical troposphere above the trade-wind boundary la yer can be explained by the variation of a moisture source that is pro portional to the amount of upper-level clouds. Implications for the na ture of water vapor feedback in global war-ming are discussed.