Complementary observations aid identification of the mountain rain-snow transition elevation

Abstract

The elevation of the mountain rain-snow transition is critical for short-term hazard forecasting and longer-term water supply considerations. Despite the transition's importance, direct in-situ observations are rare. Here we present two new methods that utilize "anomalous" snow observations to detect rainfall during rain-on-snow: (1) a mass fluctuation at snow pillow sites, and (2) inflated remotely sensed snow grain sizes. Using auxiliary data, we show snow pillows respond to rain-on-snow with distinct perturbations that appear as pulses, collapses and declines within the snow water equivalent. We use these responses to identify mountain-scale rain-snow transitions across California's Sierra Nevada. We also show how a threshold approach (>200 mm) for remotely sensed snow grain size can identify rain-on-snow as snow grain sizes artificially inflate due to a liquid water film. While the methods are not predictive, if paired retroactively with hydrometeorological models, these new methods have the potential to improve predictive streamflow capabilities.

Watershed memory amplified the Oroville rain-on-snow flood of February 2017

“…However, these daily values only correspond to roughly 1200Z, as “daily” values represent a single measurement between 0300 and 0400 local time rather than a 24-hour aggregation of measurements [ https://www.cnrfc.noaa.gov/awipsProducts/RNOFSTSWE.php ; California Department of Water Resources (DWR), personal communication]. In contrast, hourly SWE data paint a distinctly different picture, exhibiting SWE “pulses” ( 66 ) (Figs. 4B and S3 to S5B).…”

“…Given the steep SWE oscillations, rather than assuming this might be accumulating and ablating ice, it could be hypothesized that this is in fact a mass shift due to the exchange of liquid water (i.e., rainfall saturating and draining from the snowpack). Pulses also occur across the snow pillow network ( 66 ) earlier in the WY. A similar SWE pulse occurred during the 7J event (Figs.…”