Abstract : E.5
Explaining the large drying ratios observed in flow over Patagonia

Daniel J. Kirshbaum, Ronald Smith
Yale University

Recent observations suggest that, in orographic precipitation events over the Patagonia region of South America, airstreams may on average lose around 50% of their water content as they sweep eastward across the mountainous landscape. By contrast, analogous observations over the comparably sized Sierra Nevada range in California suggest a substantially smaller moisture loss (~30%). Deciphering the physical mechanisms responsible for such differences in drying ratio (the ratio of the vertically integrated flux of water vapor downstream of the ridge to that upstream) is vital for explaining the larger-scale impacts of orographic precipitation and the response of mountainous terrain and regional hydrology to climate change.

Useful insights into this issue may be drawn from previous studies examining the effects of certain physical processes (e.g., flow blocking, mountain waves, latent heating, embedded convection, etc.) on orographic precipitation amounts and distributions. However, in realistic orographic flows where many such processes simultaneously interact, a full understanding is only provided when all of these processes are considered in conjunction. To this end, we perform a series of high-resolution simulations of moist flows impinging on an idealized mountain ridge.

Initialized with a family of upstream soundings from observed orographic precipitation events over the Patagonia region, these simulations resolve the influences of numerous processes across a range of physical and temporal scales. In doing so, they allow us to identify the physical parameters that most strongly control the drying ratio in realistic orographic flows past midlatitude mountain ridges. Insights from the simulations help us to explain why some mountain ranges are more effective than others at removing moisture from the incident flow and increasing the drying ratio.