UC Berkeley

Mountains have profound local and remote impacts on Earth's rainfall distribution. While the mechanisms by which they concentrate rainfall on their windward side and deplete it in their lee are well-studied in midlatitudes, they remain poorly understood in the tropics. Yet, these unknown physics govern the spatial and temporal distribution of precipitation in dozens of tropical regions, including some of the rainiest places on Earth. This thesis aims to understand the main physical processes behind this phenomenon, termed mechanically forced tropical orographic precipitation.

We first build a conceptual picture for the time-mean distribution of tropical orographic rainfall given a fixed background wind. This distribution is set by the statistical response of entraining convective clouds to lower-free-tropospheric perturbations of temperature and moisture forced by orography. Upwind of orography, a cool and moist anomaly develops as a result of forced ascent, which increases cloud buoyancies, resulting in higher mean rainfall; the converse mechanism occurs downstream. A quantitative theory based on this picture accurately captures mean rainfall in idealized simulations of tropical orographic precipitation. The theory also explains the seasonal-mean rainfall distribution in several regions of Earth's tropics, only failing where large-scale ascent from non-orographic processes is important. It is also shown to explain part of the temporal variability in observed rainfall, along with variations in large-scale horizontal moisture advection.

We subsequently explore how tropical orographic rainfall may evolve under climate change. The theory predicts a large sensitivity of orographic rainfall to background wind changes, a result that is confirmed in idealized simulations (despite slight variations in the mechanisms involved). Observations of past interannual variability in five tropical regions confirm that orographic rainfall has varied at a rate of 20 -- 30 % per m/s change in cross-slope wind, consistent with theory and simulations. A suite of simulations suggests a surprising decrease in peak orographic precipitation with warming when winds are held fixed. Understanding this result requires a modification to the convective closure used in our theory, which may arise from a widening in the statistical distribution of cloud buoyancy anomalies. Combined together, these results suggest a potential for large decreases in orographic rainfall in regions of the tropics where background winds will weaken under warming.