Characteristics of Orographic Snowfall in a High Altitude Environment

The vertical structures of 22 snowstorms are analyzed to illustrate the variability of reflectivity, moisture, and stability during the Inhibition of Snowfall by Pollution Aerosols (ISPA 2010) field study conducted between 16 January and 28 February 2010 in Steamboat Springs, CO. Two vertically pointing radars, one at a valley site and one at a mountain peak site, provide a detailed dataset of the vertical structure of snowstorms. Information on the environment was obtained by a microwave radiometer in the valley, and surface meteorology sensors at the valley and peak. Orographic enhancement is observed in all 22 cases, diagnosed by a widening of the reflectivity distribution to include higher values near the surface, and upward motion over the peak. This study provides insights on the general vertical structure of 22 snowstorms separated into along-barrier and cross-barrier flow-regimes, while four cases are examined in detail. Hydrometeor growth occurs through the depth of the storm in some cases (~3 km AGL), and within a shallow layer (1.5 km AGL) above the mountain peak in others. Cooling due to sublimating snow particles is an important control on precipitation accumulation in the valley and the stability of the airmass between the valley and mountain peak. Additionally, dry air intrusions aloft are observed during six of the 22 cases. Before, or in the absence of intrusions, stable to neutral conditions and stratiform precipitation are observed. Unstable conditions and convective precipitation are observed post-intrusion. No two observed storms have identical atmospheric stability and synoptic forcing. Future studies need to address the multi-scale interaction between dynamical and thermodynamical processes controlling precipitation accumulation at the surface in order to improve numerical weather forecasts.