Impacts of absorbing aerosol deposition on snowpack and hydrologic cycle in the Rocky Mountain region based on variable-resolution CESM (VR-CESM) simulations

摘要

The deposition of light-absorbing aerosols (LAAs), such as black carbon (BC) anddust, onto snow cover has been suggested to reduce the snow albedo andmodulate the snowpack and consequent hydrologic cycle. In this study we usethe variable-resolution Community Earth System Model (VR-CESM) with aregionally refined high-resolution (0.125°) grid to quantifythe impacts of LAAs in snow in the Rocky Mountain region during the period1981–2005. We first evaluate the model simulation of LAA concentrations bothnear the surface and in snow and then investigate the snowpack and runoffchanges induced by LAAs in snow. The model simulates similar magnitudes ofnear-surface atmospheric dust concentrations as observations in the RockyMountain region. Although the model underestimates near-surface atmosphericBC concentrations, the model overestimates BC-in-snow concentrations by35 % on average. The regional mean surface radiative effect (SRE) due to LAAsin snow reaches up to 0.6–1.7 W m in spring, and dust contributes toabout 21–42 % of total SRE. Due to positive snow albedo feedbacks inducedby the LAA SRE, snow water equivalent is reduced by 2–50 mm and snow coverfraction by 5–20 % in the two regions around the mountains (eastern SnakeRiver Plain and southwestern Wyoming), corresponding to an increase insurface air temperature by 0.9–1.1 °C. During the snow meltingperiod, LAAs accelerate the hydrologic cycle with monthly runoff increases of0.15–1.00 mm day in April–May and reductions of 0.04–0.18 mm day in June–July in the mountainous regions. Of all the mountainousregions, the Southern Rockies experience the largest reduction of total runoff by15 % during the later stage of snowmelt (i.e., June and July). Compared toprevious studies based on field observations, our estimation of dust-inducedSRE is generally 1 order of magnitude smaller in the Southern Rockies,which is ascribed to the omission of larger dust particles (with the diameter 10 µm) in the model. This calls for the inclusion of largerdust particles in the model to reduce the discrepancies. Overall theseresults highlight the potentially important role of LAA interactions withsnowpack and the subsequent impacts on the hydrologic cycles across the RockyMountains.