Characterizing Seasonal Snowfall Accumulation in the Sierra Nevada (USA): Inter-annual Variability of Snowstorms, Atmospheric Rivers, and Orographic Snowfall Gradients

A high-resolution picture of seasonal snowfall accumulation from the basin-scale to the range-scale remains a missing piece in our overall understanding of water storage and availability in snow-dominated mountainous regions. In particular, a spatially and temporally complete understanding of storm-driven accumulation rates, orographically-enhanced snowfall gradients, and atmospheric river contributions to the cumulative snowfall (CSNWFL) remain largely unknown across entire mountain ranges due to limited spatially-distributed snowfall information. We therefore aim to close this gap by utilizing a high-resolution (daily, 90-m), multi-decadal (1985-present), and spatially and temporally continuous snow water equivalent (SWE) reanalysis, which was derived by assimilating remotely-sensed Landsat fractional snow-covered data over Sierra Nevada, USA. Using the SWE reanalysis, we quantify CSNWFL accumulation rates, orographically-enhanced CSNWFL gradients, and snowstorm contributions over the range. Our analysis extends previous work with a more robust characterization of the climatology and inter-annual variability of snowstorm-driven snowfall accumulation than possible with traditional methods (e.g. in situ point-scale measurements, transects, etc.). Moreover, when distributed SWE data has previously been used, the SWE volume was typically quantified in terms of peak or 01 April SWE. We however, quantify the CSNWFL volume from November to the basin-average day-of-peak SWE, which on average is about 22.4 km³, and ranges from 4.4 to 41.3 km³. On average, about 83-93% of the CSNWFL is accumulated during large snowstorms, with about 27% derived from the largest snowstorm each season. Annually, the Sierra Nevada experiences about 11 snowstorms and 21 snow days, on average, which results in very rapid snowfall accumulation rates across the range and significant variability in orographic CSNWFL gradients. Several synoptic atmospheric drivers of orographically-driven snowfall accumulation are explored. Of the season’s largest snowstorms, atmospheric rivers are often among the mechanisms that transport large amounts of moisture to the Sierra Nevada and promote orographic enhancement during the cold season. Results suggest that prediction of future orographic CSNWFL gradients across high-elevation montane regions may be possible with parameterizations utilizing synoptic atmospheric fields from large-scale climate models.