Visible bidirectional-reflectance measurements for rounded grain and surface hoar snow crystal morphologies

Abstract

An understanding of the optical properties of snow is vital to accurately quantifying the effect of snow cover on the Earth's radiative energy balance. Existing radiative transfer models often simplify complex crystal habits by utilizing spheres of equivalent specific surface area (SSA). While these models have had some success in accurately predicting snow albedo, more complex models strive to predict the directional reflectance properties of snow. These models require accurate bidirectional reflectance values for various snow crystal habits against which to compare their results. However, few studies in this area exist and none focus specifically on surface hoar--a well-known surface crystal type often responsible for avalanches once buried by subsequent snows. In this study, it is hypothesized that microstructural changes due to near surface metamorphism, traced by crystal size and type, will alter snow's solar bidirectional reflectance. Specifically, 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, thereby allowing the remote detection of surface hoar presence. To test this hypothesis, a methodology for reliably growing surface hoar in a lab setting was developed. Temporal changes in crystal mass and specific surface area were documented using computed tomography and visible microscopic imaging while a suite of meteorological instrumentation recorded environmental chamber conditions. A spectrometer was used to measure bidirectional-reflectance factors (BRF) both before growth (rounded grains) and after growth (surface hoar) from 42 different incident lighting and viewing geometries. These BRF values provide an accurate data set for comparison to modeling studies. Analysis of the result revealed three primary conclusions: 1) In the transition from rounded grains to surface hoar, mean BRF values (essentially albedo) decrease slightly (d2.9%) likely due to an increase in grain size; 2) Accompanying surface hoar growth is an increase in SSA and a departure from Lambertian scattering. That is, surface hoar has significantly brighter peak values and significantly darker minimum values than rounded grains; 3. Incident lighting and viewing geometries at which these maximum and minimum BRF values occur show no discernible pattern.