Abstract
The partitioning of precipitation into frozen and liquid components influences snow-derived water resources and flood hazards in mountain environments. We used a 915-MHz Doppler radar wind profiler upstream of the northern Sierra Nevada to estimate the hourly elevation where snow melts to rain, or the snow level, during winter (December–February) precipitation events spanning water years (WY) 2008–2017. During this ten-year period, a Mann-Kendall test indicated a significant (p < 0.001) positive trend in snow level with a Thiel-Sen slope of 72 m year−1. We estimated total precipitation falling as snow (snow fraction) between WY1951 and 2017 using nine daily mid-elevation (1200–2000 m) climate stations and two hourly stations spanning WY2008–2017. The climate-station-based snow fraction estimates agreed well with snow-level radar values (R2 = 0.95, p < 0.01), indicating that snow fractions represent a reasonable method to estimate changes in frozen precipitation. Snow fraction significantly (p < 0.001) declined during WY2008–2017 at a rate of 0.035 (3.5%) year−1. Single-point correlations between detrended snow fraction and sea-surface temperatures (SST) suggested that positive SST anomalies along the California coast favor liquid phase precipitation during winter. Reanalysis-derived integrated moisture transported upstream of the northern Sierra Nevada was negatively correlated with snow fraction (R2 = 0.90, p < 0.01), with atmospheric rivers representing the likely circulation mechanism producing low-snow-fraction storms.
Highlights
As the climate warms, the partitioning of snowfall to rainfall in snow-dominated mountain watersheds is likely to change [1,2,3,4,5]
This study focuses on the northern Sierra Nevada, a 150 km wide north‐south trending mountain annual precipitation occurs during the winter months (December–February; hereafter winter) [16]
We identified a statistically significant positive trend in winter snow levels in the northern Sierra Nevada during the winters between WY2008 and 2017
Summary
The partitioning of snowfall to rainfall in snow-dominated mountain watersheds is likely to change [1,2,3,4,5]. Changes in precipitation phase alter seasonal snowpack dynamics, ecological processes, peak streamflow timing, and winter flood hazards [5,6,7]. These changes present different challenges for water supply forecasting and reservoir operations [8], in states that depend upon snow-derived water resources and are at risk for winter flooding [9,10]. Continued climate change, increased drought risk [11], and more intense extreme precipitation events [12] will necessitate adaptive water management strategies. Precipitation reductions during a multiyear temperature enhancing drought severity [14]
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