The lifecycle of snow in the Sierra Nevada USA: from snowfall to snowmelt and effects on endangered bighorn sheep

Abstract

Snow is a valuable resource in California. Snow from the Sierra Nevada sustains a diverse ecosystem and provides 3/4 of California’s Agricultural water supply. Because of its importance in water supply and global climate, snow accumulation, melt, and sublimation were ranked as the most important objectives in the 2017 Decadal Survey. This study employs a fully coupled meteorology‐chemistry‐snow model to investigate the impacts of both global warming and light‐absorbing particles (LAPs) on snow in the Sierra Nevada. Using self-organizing map (SOM) analysis with dust deposition and flux data from model and observations, we identify four typical dust transport patterns across the Sierra Nevada, associated with the mesoscale winds, Sierra barrier jet, North Pacific High, and long-range cross-Pacific westerlies, respectively. The satellite retrievals and model results show that LAPs in snow reduce snow albedo by 0.013 (0–0.045) in the Sierra Nevada during the ablation season (April-July), producing a midday mean radiative forcing of 4.5 W m−2 which increases to 15–22 W m−2 in July. LAPs in snow accelerate snow aging processes and reduce snow cover fraction, which doubles the albedo change and radiative forcing caused by LAPs. The impurity-induced snow darkening effects decrease snow water equivalent and snow depth by 20 and 70 mm in June in the Sierra Nevada bighorn sheep habitat. The earlier snowmelt reduces root-zone soil water content by 20%, deteriorating the forage productivity and playing a negative role in the survival of bighorn sheep. We also conduct the simulations using our coupled regional model to compare the impact of global warming vs. LAPs on snow melting by adopting the pseudo-global warming (PGW) approach to generate projections of future meteorological forcing. These results will be used to examine snow effects on endangered Sierra Nevada bighorn sheep and how a future climate might modify habitat and behavior.