On the development of orographic rain shadows during uplift of mountain ranges

Orographic rain shadows are a primary feature of Earth's surface climatology and potentially influence landscape evolution in mountainous regions. Disentangling the effects of tectonics and climate during mountain building is a primary goal of geological research, but progress has been hampered by a lack of theories regarding their relative infuences in geological processes. Fluid dynamical theories for stratified flow over topography provide a potentially powerful framework for understanding some aspects of how climate, topography, and precipitation co-evolve during mountain building. In this study, an idealized numerical weather prediction model is used to explore the parameters that control air flow and cloud processes over intramontane valleys during surface uplift. The controlling parameter is found to be Nh/U, where N is a measure of the atmospheric stability, h is the terrain relief, and U is the horizontal wind speed. When Nh/U is much smaller than unity, simple linear models are valid as air flows directly over the topography. In contrast, when Nh/U is much greater than unity, the air is deflected around the terrain, and orographic precipitation vanishes within the idealized intramontane valley. This nonlinear switching between 'flow-over' and 'flow-around' conditions is controlled by both climate and topography and is fundamental to many mountain ranges, including the Sierra Nevada, the Alps, and the Andes. These results may provide an improved framework for interpreting geological records of orogenesis and suggest several avenues for joint research between the atmospheric and geological sciences.