Articles | Volume 9, issue 4
Research article
07 Aug 2015
Research article |  | 07 Aug 2015

Meteorological, elevation, and slope effects on surface hoar formation

S. Horton, M. Schirmer, and B. Jamieson

Abstract. Failure in layers of buried surface hoar crystals (frost) can cause hazardous snow slab avalanches. Surface hoar crystals form on the snow surface and are sensitive to micro-meteorological conditions. In this study, the role of meteorological and terrain factors was investigated for three layers of surface hoar in the Columbia Mountains of Canada. The distribution of crystals over different elevations and aspects was observed on 20 days of field observations during a period of high pressure. The same layers were modelled over simplified terrain on a 2.5 km horizontal grid by forcing the snow cover model SNOWPACK with forecast weather data from a numerical weather prediction model. Modelled surface hoar growth was associated with warm air temperatures, high humidity, cold surface temperatures, and low wind speeds. Surface hoar was most developed in regions and elevation bands where these conditions existed, although strong winds at high elevations caused some model discrepancies. SNOWPACK simulations on virtual slopes systematically predicted smaller surface hoar on south-facing slopes. In the field, a complex combination of surface hoar and sun crusts were observed, suggesting the simplified model did not adequately resolve the surface energy balance on slopes. Overall, a coupled weather–snow cover model could benefit avalanche forecasters by predicting surface hoar layers on a regional scale over different elevation bands.

Short summary
We investigate how various meteorological and terrain factors affect surface hoar formation in complex terrain. We modelled the distribution of three surface hoar layers with a coupled NWP - snow cover model, and verified the model with field studies. The layers developed in regions and elevation bands with warm moist air, light winds, and cold snow surfaces. Possible avalanche forecasting applications are discussed.