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

The snow albedo feedback (SAF) is an important climate feature of mountain regions with transient snow cover. In a warmed climate, the SAF enhances regional warming by increasing the amount of absorbed solar radiation where snow cover is reduced. In most mountain regions, patterns of snow cover are largely determined by the underlying terrain and, as a result, patterns of SAF-enhanced warming are highly variable in space and time. This variability may interact with the development of diurnally driven mountain-breeze circulations by altering the thermal contrast between high and low elevations that drives them, or by increasing downward momentum mixing via a more vigorous convective boundary layer. In this study, high-resolution regional climate model (RCM) experiments are used to investigate and characterize these interactions in the Rocky Mountains of Colorado and Utah during the late spring when SAF strength is at a seasonal maximum. We examine output from two separate 8-year climate change experiments with differing model configurations. A pseudo global warming approach is used to perturb the climate in order to focus on the regional climate response to a large scale thermodynamic forcing. In the experiment with a strong SAF, there is a clear increase in the strength of daytime mountain breeze circulations caused by an increased thermal contrast between local mountain slopes and the surrounding lowlands. The resulting enhancement of upslope flow increases convergence and cloudiness near the snow margin in the warmed climate. In the simulation with a weaker SAF results are largely similar but less pronounced. In both experiments we find that changes in the thermal contrast between high and low elevations, rather than enhanced boundary layer mixing, is the dominant mechanism linking the SAF and mountain-breezes.