Bidirectional-reflectance measurements for various snow crystal morphologies

•We present bidirectional reflectance factor data for rounded snow and surface hoar.•Surface hoar was found to be less Lambertian than rounded snow grains.•Surface hoar's departure from Lambertian scatter is linked to grain size and shape.•The geometry at which max/min BRF values are found is not predictable.•The surface hoar demonstrated lower spectral albedo than rounded snow grains.

An understanding of snow optical properties is vital to accurately quantifying the effect of snow cover on the Earth's radiative energy balance. Central to these models is the need for accurate bidirectional reflectance data for various snow surface types. However, few studies in this area exist and none focus specifically on surface hoar—a well-known surface crystal type often responsible for avalanches.In this study, it is postulated that the bidirectional reflectance distribution of the snow's surface before and after surface hoar growth will be predictably and quantifiably different when viewed in the visible wavelengths. To test this hypothesis, a methodology for reliably growing surface hoar in a lab setting was developed. Temporal changes in crystal habit were documented using computed tomography and visible microscopic imaging. A spectrometer was used to measure bidirectional-reflectance factors (BRF) both before and after surface hoar growth.Analysis of the results revealed three primary conclusions: 1) Surface hoar growth is accompanied by a departure from Lambertian scattering. The effect is more apparent the larger the surface hoar grains. 2) The incident lighting and viewing geometries at which maximum and minimum BRF values occur are difficult to discern. 3) In the transition from rounded grains to surface hoar, spectral albedo (as calculated by averaging the BRF over a hemispheric solid angle at 510 nm) decreases slightly.