Smith Lecture: The Fate of Water in Tropical Clouds and the Subtropical Humidity Balance
The humidity of the tropics and subtropics plays a disproportionate role in the radiative balance of the planet. Therefore understanding the processes that control the response of the tropical humidity to climate forcing is critical for determining climate sensitivity. A correct account of the complex set of processes involved requires consideration of large-scale transport, turbulence and cloud microphysical processes. While these are all included in climate models, many aspects associated with them are treated in a very simple parameterized form and are poorly observed. These limitations reduce the confidence one may place in the fidelity of simulated responses. Previous work has highlighted the importance of rain production, which is often modeled using a simplified treatment of autoconversion of cloud condensate. This quantity directly influences the fraction of total water lofted at the cloud based which is ultimately detrained by clouds to moisten the environment. Similarly, the degree to which falling condensate evaporates and moistens the lower troposphere is generally not well understood and not well observed over wide spatial scales. Using guidance provided by measurements of humidity and stable isotope ratios in tropics these processes can be constrained. We discuss both the importance of these constraints and some of the limitations associated with using stable isotope tracers in assessing the balance the microphysical controls against influences of large-scale moisture transport.