MODELLING SURFACE HOAR FORMATION AND EVOLUTION ON MOUNTAIN SLOPES

ABSTRACT: Predicting the spatial distribution and persistence of surface hoar layers is a challenge to avalanche forecasters and backcountry recreationists. This study evaluates surface hoar size predictions made with empirical weather based models and discusses how buried layers change over time. Surface hoar layers were observed during the 2011-2012 winter at flat, north and south facing slopes in the Columbia Mountains. Two models were developed to predict crystal size, one using a constant growth rate and the other estimating vapour deposit from the surface energy balance. Weather station data and forecasted data from the GEM15 numerical weather prediction model were used to simulate the growth of layers over seven winters. Reasonable size predictions were made with growth rates of 2.1 and 2.6 mm per day (r2 = 0.44), although specific rates appeared to change with season. The surface energy balance model made good predictions of crystal size with real station data (r2 = 0.74) and reasonable predictions with the output of GEM15 (r2 = 0.49). Time series of shear frame and propagation saw test results were made from observations on a buried surface hoar layer that formed in February 2012. The large crystals in this layer showed a steady increase in shear strength while the potential for fracture propagation remained high. The results of this study show promise for modelling surface hoar formation and evolution in snow cover models