Winter Snowpack Responses to Near-Freezing Storms in the Sierra Nevada, USA

Abstract:

Seasonal snow water equivalent (SWE) accumulation in California’s Sierra Nevada is known to be primarily governed by a relatively small number of orographically enhanced snowstorms. However, as air temperatures gradually rise resulting in a shift from snow to rain, the governing processes determining SWE accumulation versus ablation become ambiguous. We address two fundamental questions: first, what temperatures and storm conditions control SWE accumulation, and how do they vary across the range, and second, under what conditions and mechanism(s) do near-freezing snow or rain result in SWE gain or loss? We answer these questions by using an elevational and latitudinal gradient of quality controlled hourly measurements of SWE, temperature, precipitation, and snow depth at 28 sites along a north-south transect of the Sierra Nevada observed by the California Department of Water Resources’ snow pillow network. We supplement these primary observations with atmospheric reanalyses and radar bright band information. We identify distributions of critical temperatures and corresponding storm and snowpack characteristics that describe how SWE accumulation varies across the Sierras and in wet versus dry years. We demonstrate that temperatures restrict SWE accumulation with warm and cold boundaries representing snowfall and precipitable water requirements, respectively, and that the most productive regions balance these boundaries and lay in orographically favorable positions. We then investigate a series of rain-on-snow and other near-freezing storm event cases to diagnose the antecedent and persisting snowpack and atmospheric conditions that control SWE response.