Tropical tropospheric water vapor budget, maintenance of the lapse rate, and distribution of the extratropical tropospheric temperature and wind

The first goal of this thesis research is to assess the nature of water vapor feedback in climate change. 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 stablizing role in climate change through a coupling with the tropical tropospheric lapse rate. Based on the observed vertical structure of tropical convection, we first formulated a model for the vertical distribution of tropical tropospheric temperature. In the model, heating from deep convection is parameterized as subsidence heating and the magnitude of the subsidence is determined through the surface moisture budget. The radiative-convective equilibrium profiles predicted by the model successfully depict the observed characteristics. The model lapse rate in the free troposphere is related to the speed at which air outside of deep clouds subsides and the radiative cooling rate it suffers in the course of subsiding. 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. Using the large-scale subsidence deduced from the energy budget and a simple parameterization for the moistening from the evaporation of hydrometeors, we are able to simulate the observed distribution of water vapor in the context of a simple conceptual model. 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. Under the assumption that the vertical structure of the averaged convective mass flux is not subject to change, we find that only a significant increase of relative humidity in the middle and upper troposphere (2040% percent greater than at present) can lead to the 20% increase of the lapse rate. This conclusion remains unchanged even when a substantial decrease in the surface and low level relative humidity is assumed to accompany the surface cooling. It is noted that the profiles observed in CLIMAP have much more convective available potential energy (CAPE) than present profiles, which can lead to enhanced production of hydrometeors in the middle and the upper troposphere whose reevaporation can moisturize the free troposphere. It is shown that the water vapor change leads to an extremely strong negative feedback which would greatly reduce the response of the tropical atmosphere to a doubling of CO 2. It is also found that the lapse rate feedback is also negative and can contribute as much as the water vapor feedback in reducing the tropical climate sensitivity to the increase of CO2. We stressed that results of the present analyses should be taken with caution since observations of the current climate and more sophisticated modeling do not support the idea that the tropical tropospheric temperature can deviate considerably from a moist adiabatic profile. The second goal of the thesis research is to obtain the zonal mean temperature and wind distribution of an extratropical troposphere with zero gradient of potential vorticity (PV) along isentropic surfaces and estimate the tropical impact on the extratropical troposphere in this extreme case of PV mixing. 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. Nevertheless, the overall structure is quite similar to the observed including the width of the jet. The presence of a planetary vorticity gradient limits the meridional extent of tropical impact on the extratropical temperature and wind distribution. However, to a greater or lesser degree, the temperature and wind distribution in the extratropical troposphere is found to be sensitive to the vertical distribution of PV and the position of the edge of the Hadley circulation. 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. Though the temperature and wind distribution cannot be accurately determined without the reference to the global momentum balance, the meridional distribution of wind has negligible effects on the temperature structure. We also showed how the thermal structure within the surface boundary layer may affect a large portion of the zonal mean temperature and wind distribution. The sensitivity of the temperature and wind distribution to 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. Thesis Supervisor: Richard S. Lindzen Title: Sloan Professor in Meteorology