Mesoscale Variations of the Atmospheric Snowline over the Northern Sierra Nevada: Climatology, Case Study, and Mechanisms

Observations from several major mountain ranges reveal that the height of the transition from snowfall to rainfall, the snowline, can intersect the terrain at an elevation hundreds of meters below its elevation in the free air upwind. This mesoscale lowering of the snowline may have important implications for both the accumulation of mountain snowpack and the generation of storm runoff. Previous work attributes such modifications of the snowline to some combination of spatial variations in latent cooling from melting hydrometeors, in adiabatic cooling from lifting, and in hydrometeor melting distance.

A unique climatological view of this behavior comes from from profiling radars in the northern Sierra Nevada deployed as part of NOAA's Hydrometeorlogy Testbed. They show that the mesoscale lowering of the snowline is a robust feature common to nearly all major storms. Typically the snowline drops by at least 200 m as it approaches the terrain, but this lowering has significant storm-to-storm variations.

The mesoscale behavior of the snowline 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 the WRF model show that WRF is capable of reproducing the observed lowering of the snowline in a realistic manner. Diagnosis of model output reveals that pseudo-adiabatic transport related to orographic blocking, localized cooling due to melting of orographically enhanced snowfall, and spatial variations in hydrometeor melting distance all play important roles in lowering the snowline. Simulation of this behavior is surprisingly insensitive to model horizontal resolution, but has important sensitivities to microphysical parameterization.