Dynamic and Thermodynamic Controls on the Amount and Distribution of Orographic Precipitation

Abstract

This thesis examines various mechanisms controlling the amount and distribution of precipitation in mountainous terrain. In Chapter 2, linear theory and numerical simulations are used to investigate how the tropopause affects the vertical structure of mountain waves, and in turn orographic precipitation. In idealized numerical simulations, variations in the height of the tropopause are found to strongly modulate the depth and magnitude of windward ascent, resulting in a factor-of-two difference in total precipitation across a ridge. The implications of this result are then extended to realistic terrain using a modified version of the Smith-Barstad orographic precipitation model. Chapter 3 addresses the causes of rain-shadow variability in the Washington Cascades. Fluctuations in the large-scale circulation over the North Pacific is found to explain around 70\% of interannual variability in the wintertime rain shadow. Across individual storms, the strongest rain shadows are found to be associated with warm-sector events, while the weakest rain shadows occur during warm-frontal passages. Inter-storm variability in the Cascade rain shadow is further explored in Chapter 4, via numerical simulations involving both real and idealized terrain. Storms with weak rain shadows are shown to exhibit much less evaporation east of the crest than storms with strong rain shadows. The suppression of east-slope evaporation during weak-rain-shadow storms is found to be caused by the presence of stagnant, stable air at low levels, which in turn is shown to be a consequence of warm-frontal passage. In Chapter 5, a simple numerical model is used to evaluate the response of orographic precipitation to surface warming under idealized conditions representative of some of the strongest orographic storms. An upward shift is found in the pattern of condensation with warming, caused by larger fractional changes in condensation at low temperature and amplified warming aloft. As a result, the distribution of precipitation shifts downwind, causing larger fractional changes in precipitation on the lee slope than on the windward slope. In addition, total precipitation is found to increase by a smaller fraction than near-surface water vapor, in contrast to expected changes in other types of extreme precipitation.