Basic Science of Rain on Snow

TitleBasic Science of Rain on Snow
Publication TypeConference Proceedings
Year of Conference2021
AuthorsMcGurk, Bruce, Osterhuber Randall, and Kattelmann Rick
Conference Name88th Annual Western Snow Conference
Conference LocationBozeman, MT
Keywordsflow fingers, ice crusts, Rain on snow, snow layers, snowpack, water transmission

Rain on snow is a common event in the Sierra Nevada that occasionally generates floods, but has also generated coFnsiderable speculation and conjecture. This paper will summarize basic snowpack formation, structure, and response to melt and rain on its surface. Water transmission through snow is complex because the snowpack structure depends on the characteristics of the storms that created it, and no two years ever produce a snowpack with the same characteristics. The Sierran snowpack above 2100 m ranges from 2-5 m in depth at peak accumulation, and by spring, it is composed of 8-12 major layers that correspond to the prior major storm events. Melt and/or rain water transmission through snowpack is complex because the layers have varying temperature and densities, and the presence of crustal boundary layers that form during sunny, inter-storm periods. Rain on snow is transmitted both vertically and laterally along layers and interfaces, and steeper slope increases the horizontal speed and amount that may be delivered directly to stream channels. Vertical flow channels are often created that allow relatively rapid delivery of water to the soil surface. Rain can melt snow, particularly when lower or mid-elevations become snow-covered in cold storms and then are subsequently exposed to warm storms with high temperatures and winds. Contributions of that melted snowpack are typically small compared to the amount of rain that fell, some of which creates immediate runoff.

Rain-on-snow frequency and magnitude are increasing in the central Sierra Nevada, and the rain is reaching higher elevations more often (Osterhuber, 2019). Because runoff magnitude and timing are directly linked to the extent of the basin that receives rain as well as snowpack condition, this issue demands increased awareness and study of the snowpack as well as improved forecasting and verification of the rain-snow elevation of an incoming storm.