Mesoscale Variations of the Atmospheric Snow Line over the Northern Sierra Nevada: Multiyear Statistics, Case Study, and Mechanisms

Observations from several mountain ranges reveal that the height of the transition from snowfall to rainfall, the snow line, can intersect the terrain at an elevation hundreds of meters below its elevation in the free air upwind. This mesoscale lowering of the snow line affects both the accumulation of mountain snowpack and the generation of storm runoff. A unique multiyear view of this behavior based on data from profiling radars in the northern Sierra Nevada deployed as part of NOAA’s Hydrometeorology Testbed is presented. Data from 3 yr of storms show that the mesoscale lowering of the snow line is a feature common to nearly all major storms, with an average snow line drop of 170 m. The mesoscale behavior of the snow line is investigated in detail for a major storm over the northern Sierra Nevada. Comparisons of observations from sondes and profiling radars with high-resolution simulations using theWeather Research and Forecasting model (WRF) show thatWRF is capable of reproducing the observed lowering of the snow line in a realistic manner. Modeling results suggest that radar profiler networks may substantially underestimate the lowering by failing to resolve horizontal snow line variations in close proximity to the mountainside. Diagnosis of model output indicates that pseudoadiabatic processes related to orographic blocking, localized cooling due to melting of orographically enhanced snowfall, and spatial variations in hydrometeor melting distance all play important roles. Simulations are surprisingly insensitive to model horizontal resolution but have important sensitivities to microphysical parameterization.