Application of snow specific surface area measurement using an optical method based on near-infrared reflectance around 900-nm wavelength to wet snow zones in Japan

The specific surface area (SSA) of snow can be used as an objective measurement to define the optical sphere diameter of snow; it is therefore a helpful parameter to describe the physical properties of snow, such as albedo. Recently, measurement of snow SSA in the field has become easier with the use of optical methods based on near-infrared reflectance values (Ref). However, existing optical methods have only been validated for dry snow conditions in the field. In this study, we tested the possibility of applying the optical method using light with wavelength of around 900 nm (NIR photometry) to wet snow zones in Japan by comparing the findings with snow pit observation data. Our results indicated that NIR photometry can be applied to wet snow zones before the main melt season when the liquid water content is small, but problems arose during melt season due to the appearance of darker layers with more high liquid water content. To resolve these problems, we propose three improvements to NIR photometry: using three calibration targets ranging from high Ref to low Ref in coverage; establishing an estimation formula for SSA from measured Ref, including lower Ref values; and considering how water in the snow influences Ref.