Explaining the large drying ratios observed in flows past Patagonia

In orographic precipitation events over the Patagonia region of South America, recent observations suggest that airstreams on average lose around 50% of their total water content as they sweep eastward across the mountainous landscape. By contrast, analogous observations for the comparably sized Sierra Nevada range in California suggest a much smaller moisture loss (30-40%). Deciphering the physical mechanisms responsible for such differences in drying ratio (the ratio of the water removed by orographic precipitation to that impinging on the orography) is vital for explaining the larger-scale impacts of orographic precipitation and the response of mountainous terrain and regional hydrology to climate change.

To better understand the large drying-ratio estimates over Patagonia, we conduct a series of quasi-idealized WRF simulations of precipitation events over this region. Initialized with a family of upstream soundings from observed precipitation events, these simulations resolve the complex interplay between numerous influential physical processes acting across a range of physical and temporal scales. In interpreting these simulations, we draw from previous studies of orographic precipitation and develop new theories to describe how certain physical processes interact to control the drying ratio. These results form a basis for understanding why some mountain ranges are more effective than others in removing moisture from the air.