Quantification of the snow albedo feedback in regional climate model simulations over the Rocky Mountains

The Snow/Ice Albedo Feedback (SAF) is one of the primary positive feedbacks that amplify anthropogenic climate change. While globally this feedback is well quantified, on regional scales it is less well understood. Over complex terrain the SAF may be particularly important for regional climate, but also poorly resolved by global models.

The SAF was studied over the region encompassing the Colorado Rockies by analysis of WRF regional climate model (RCM) simulations. The years 2002-2008 were simulated both using a control simulation with boundary forcing from reanalysis data, and an experimental simulation, which used a pseudo global warming (PGW) approach. The PGW experiment is designed to study the regional-scale response to large-scale thermodynamic and radiative forcings associated with future anthropogenic climate change in isolation from large-scale changes in circulation. The PGW experiment uses the same reanalysis boundary conditions, but with idealized thermodynamic changes applied that are consistent with GCM projections.

The framework of linear feedback analysis was adapted for analysis of the RCM results. This framework allows us to quantify the regional SAF using standard model output, as well as diagnose the different mechanisms causing variability in the SAF. Applying this method, we find that the magnitude of the SAF over the Colorado headwaters region is much stronger than estimates of the global SAF. There is a strong seasonal cycle in the SAF that peaks during the spring snowmelt season, but varies greatly year-to-year. Simulations with differing horizontal resolutions (4km, 12km, and 36km) produce qualitatively similar results, but the SAF persists longer throughout the late spring months as model resolution was increased and is substantially stronger in early spring at coarse resolution.

Circulations that transport heat through the boundaries of our regional domain may balance some of the radiative forcing associated with the SAF. Accordingly, these circulations may act as a negative feedback on warming over the regions of snow loss, producing SAF-enhanced warming remote from the high terrain. Feedback analysis allows us to compare the radiative perturbations from the SAF to energy flux divergence due to circulations and analyze how these feedbacks combine to determine regional climate change.