The Simulated Response of Mountain Breeze Circulations to Regionally Enhanced Warming Caused by the Snow Albedo Feedback

Snow over the Rocky Mountains can influence local and regional mountain-breeze circulations by modulating the surface energy budget. In the future as the climate warms, snow cover in this region is expected to decrease, especially near the low-elevation margin of the seasonal snowpack. This will lead to spatially heterogeneous warming through the snow albedo feedback (SAF). This heterogeneous warming may modify mountain breeze circulations by altering the thermal contrast that drives them or by increasing convective mixing within the boundary layer. High-resolution regional climate model experiments are used to examine the springtime response of mountain breeze circulations to SAF. We examine output from two separate 7-year high-resolution control and pseudo global warming experiments run at 4 and 12-km horizontal grid spacing. We evaluate the control simulations against a mesoscale network of surface observations within the region finding that diurnal wind systems are generally well represented in the models. At 4-km horizontal grid-spacing, there is a clear strengthening of daytime upslope flows and weakening of overnight downslope flows as snow cover is depleted under climate warming. These changes are due primarily to an enhanced thermal contrast between high and low elevation ranges caused by the SAF. Changes in mountain breezes may affect convective initiation. For instance, we find that stronger daytime mountain-breeze strength leads to increased convergence and cloudiness near the snow margin in the warmed climate. Increased boundary layer mixing during the daytime caused by the SAF also modulates mountain wind systems, though this mechanism appears to be of secondary importance. Over broad mountain regions, 12-km grid-spacing is sufficient to resolve changes in mountain breeze circulations.