UC Davis

In this dissertation I tell two stories about tropical climate. In the first story we investigate a negative climate feedback due to the lightness of water vapor. The molecular weight of water vapor is less than that of dry air, making humid air less dense than dry air at a given temperature and pressure. Using idealized cloud-resolving model simulations, we demonstrate that this vapor buoyancy in the humid regions of Earth’s tropics must be balanced by warmer temperatures in the dry regions. This horizontal temperature difference gives rise to a negative lapse-rate-type climate feedback of about -0.15 W/m2/K, which we call the vapor buoyancy feedback. Then, using a general circulation model, we show that this feedback is robust to the presence of planetary rotation and does not result in a countervailing water vapor feedback. Finally, using a one-dimensional radiative-convective climate model, we quantify the vapor-buoyancy feedback in relation to other clear-sky longwave feedbacks active in the tropical atmosphere.

In the second story we investigate the temperature of high clouds in the tropics. Tropical anvil clouds are generally observed to rise as the climate warms, causing their temperature to change little. However, the precise magnitude and mechanism of this temperature change remains the subject of some controversy. In this study, I conduct over 100 idealized experiments in a convection-permitting model. We note that the radiative tropopause – the location in the upper troposphere where radiative heating equals zero – warms at approximately the same rate as the cloud anvils so long as the simulations are performed using Earth-like parameters. From there, we show how radiative heating due to sunlight, carbon dioxide, and ozone all modify the temperature and warming trend of the anvil clouds.