Spectral measurements of surface hoar crystals

Abstract

ABSTRACT. Surface hoar crystals are common on the surface of mountain snow covers. Once buried, layers of large plate-shaped surface hoar crystals are prone to releasing dangerous snow-slab avalanches. Since snow microstructure influences the optical properties of snow, remote sensors could potentially detect the formation of surface hoar and other snow types associated with avalanche release. The spectral reflectance of 377 snow samples was measured with a field spectrometer between 750 and 2500 nm, including 161 samples of surface hoar. Morphological snow shapes associated with critical avalanche layers (surface hoar, near-surface faceted particles and depth hoar) had lower average reflectance factors than non-critical snow shapes at infrared wavelengths. Needle-shaped surface hoar was more reflective than plate-shaped surface hoar, but there were no significant differences between different sizes of surface hoar. Normalized difference indices calculated with reflectance from two wavelength bands is presented as a potential method to classify critical snow surfaces remotely, although melt-freeze crusts near the surface complicated the classification. Accordingly, further studying on the effect of melt-freeze crusts and quantification of the bidirectional reflective properties of critical snow types is needed.

Seasonal Snowpack Classification Based on Physical Properties Using Near-Infrared Proximal Hyperspectral Data

“…However, it is possible to detect a small difference between certain spectral bands, such as in the 1000-1100 nm and 1200-1300 nm bands, in which different groups of snow with close or homogeneous physical characteristics can be distinguished. These spectral reflectance values show that the reflectance changes throughout the wavelength regions as snow ages [6,33]. Indeed, snow samples that were newly deposited on the snowpack (precipitation particles, decomposing, and fragmented precipitation particles) and that had a low to medium density (50-200 kg m −3 ) had higher reflectance values.…”

“…The snow cover, also called snowpack, is the set of snow layers that pile up on the ground throughout winter [ 5 ]. This snow cover is highly variable due to the complex distribution of precipitation, wind, and radiation after snow deposition [ 6 ]. Each layer is separated from the previous one by a superposition plane.…”

“…In economic and social terms, permission for many winter recreational activities depends on knowledge of the vertical structure of the snowpack, which makes this task paramount and valuable [ 18 ]. Also, meteorological conditions and solid precipitation data remain insufficient to predict avalanche risks [ 6 ]. To do so, it is necessary to document the internal evolution of the snowpack, hence the need for frequent and regular observations and measurements [ 6 ].…”

“…Also, meteorological conditions and solid precipitation data remain insufficient to predict avalanche risks [ 6 ]. To do so, it is necessary to document the internal evolution of the snowpack, hence the need for frequent and regular observations and measurements [ 6 ]. These measurement techniques are often based on direct observations using a magnifying glass to classify granulometry stratigraphy and snow grain shape, and a snow sampler to measure density.…”

Experimental and model-based investigation of the links between snow bidirectional reflectance and snow microstructure

“…They concluded that, as surface hoar grows, the snow surface becomes less and less Lambertian but that the geometry (illumination and viewing), at which the reflectance is maximum or minimum is difficult to predict. Horton and Jamieson (2017) used reflectance measurements and investigated the potentiality of normalised difference indices calculated from conical reflectance measurements at two wavelengths (860 and 1310 nm) to classify different crystal morphologies. They concluded that the bidirectional reflectance properties for different snow types must be investigated further.…”