Tropical Tropospheric Water Vapor Budget, Maintenance of the Lapse Rate, and Distribution of the Extratropical Tropospheric Temperature and Wind.

Utilizing observational data and conceptual models, we examined the tropical tropospheric water vapor budget and the maintenance of the lapse rate. In light of the mountain-snow line record for the last glaciation, we investigated the nature of water vapor feedback in climate change. The chief finding is that water vapor in the tropical free troposphere may play a stabilizing role in climate change through a coupling with the tropical tropospheric lapse rate. We formulated a model for the vertical distribution of tropical tropospheric temperature. The radiative-convective equilibrium profiles predicted by the model successfully depict the observed characteristics. We then examined the tropical tropospheric water vapor budget. We found that the evaporation of the hydrometeors pumped into the upper troposphere by deep convective towers appears to be the major moisturizer for the large-scale subsiding flow. The role of water vapor and lapse rate in different climate regimes was examined in light of the mountain-snow line record for the last glaciation. Together with CLIMAP data for the surface temperature estimate, the mountain snowline record suggests that the mean lapse rate in the low tropical troposphere during the last glaciation was about 20% larger than at present. Using the model for the tropical tropospheric temperature, we studied the dependence of the lapse rate on the relative humidity in the middle and upper troposphere. We found that for a fixed meridional distribution of surface temperature and a tropical lapse rate which are the same as the observed, the extratropical troposphere with constant PV along isentropic surfaces has a considerably colder upper troposphere near the jet and a stronger jet than the observed. We examined the mutual dependence between temperature, wind and PV, and highlighted the role of the stratospheric PV and its meridional gradient in affecting the zonal wind. The sensitivity of the temperature and wind distribution of the PV gradient along isentropic surfaces is also examined. For a fixed meridional distribution of surface temperature, we found that the larger the gradient along isentropic surfaces, the warmer the upper troposphere and the weaker the jet. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.).