Water Vapor Feedback in the Tropical Upper Troposphere: Model Results and Observations

The sensitivity of water vapor in the tropical upper troposphere to changes in surface temperature is examined using a single-column, radiative-convective model that includes couplings between the moistening effects of convective detrainment, the drying effects from clear-air subsidence, and radiative heating and cooling from water vapor. Equilibrium states of this model show that as the surface warms, changes in the vertical distribution and temperature of detraining air from tropical convection lead to higher water vapor mixing ratios in the upper troposphere. However, the increase in mixing ratio is not as large as the increase in saturation mixing ratio due to warmer environmental temperatures, so that relative humidity decreases. These changes in upper-tropospheric humidity with respect to surface temperature are consistent with observed interannual variations in relative humidity and water vapor mixing ratio near 215 mb as measured by the Microwave Limb Sounder and the Halogen Occultation Experiment. The analysis suggests that models that maintain a fixed relative humidity above 250 mb are likely overestimating the contribution made by these levels to the water vapor feedback.