PNNL

Abstract

Regional climate models (RCMs) generally predict modest changes in winter precipitation on the windward slopes of mid-latitude mountain ranges but relatively large increases in the lee, indicating a weakening of the orographic rain shadow under global warming. Such changes could have profound consequences for water resources and ecosystems in mountainous regions, but their underlying causes are not well understood. Here we show that the pattern of rain-shadow weakening predicted by RCMs is well approximated by a simple model that is based on two assumptions: first, that precipitation at any point can be partitioned into a large-scale component and an orographic component; and second, that the large-scale component scales with warming at a greater rate than the orographic component. Applying this model to the western United States, we show that it captures much of the pattern of precipitation change predicted by an ensemble of RCMs, suggesting that rain-shadow weakening is one manifestation of a more general weakening of orographic influence under warming. This result can be explained by differences in the vertical structure of orographic versus large-scale ascent, with orographic ascent generally occurring lower in the atmosphere where condensation is thermodynamically less sensitive to warming.