Measurements and modelling of snow particle size and shortwave infrared albedo over a melting Antarctic ice sheet

Abstract

Abstract. The albedo of a snowpack depends on the singlescattering properties of individual snow crystals, which have a variety of shapes and sizes, and are often bounded in clusters. From the point of view of optical modelling, it is essential to identify the geometric dimensions of the population of snow particles that synthesize the scattering properties of the snowpack surface. This involves challenges related to the complexity of modelling the radiative transfer in such an irregular medium, and to the difficulty of measuring microphysical snow properties. In this paper, we illustrate a method to measure the size distribution of a snow particle parameter, which roughly corresponds to the smallest snow particle dimension, from two-dimensional macro photos of snow particles taken in Antarctica at the surface layer of a melting ice sheet. We demonstrate that this snow particle metric corresponds well to the optically equivalent effective radius utilized in radiative transfer modelling, in particular when snow particles are modelled with the droxtal shape. The surface albedo modelled on the basis of the measured snow particle metric showed an excellent match with the observed albedo when there was fresh or drifted snow at the surface. In the other cases, a good match was present only for wavelengths longer than 1.4 µm. For shorter wavelengths, our modelled albedo generally overestimated the observations, in particular when surface hoar and faceted polycrystals were present at the surface and surface roughness was increased by millimetre-scale cavities generated during melting. Our results indicate that more than just one particle metric distribution is needed to characterize the snow scattering properties at all optical wavelengths, and suggest an impact of millimetre-scale surface roughness on the shortwave infrared albedo.

The spectral and chemical measurement of pollutants on snow near South Pole, Antarctica

“…Observable snow grain sizes were found to be approximately 0.5 mm in diameter on average when measured in the laboratory, several months after sample collection. Thus, when measured spectrally in the field, snow grain sizes were likely smaller, as snow grains nominally grow with age [e.g., LaChapelle , ; Pirazzini et al ., ]. We note that observable snow grain size does not necessarily equate to optical snow grain size, which is the radius of the sphere that returns the same hemispherical flux.…”

Development and calibration of an automatic spectral albedometer to estimate near-surface snow SSA time series

“…This simple model is called hereinafter "one-parameter model" because SSA is the only unknown. However, it is not the most suitable because albedo measurements are subject to various artifacts, such as variations of the illumination or poor cross-calibration between the downwelling and upwelling measurements, slopes of the surface, etc., sometimes even resulting in values larger than 1 (Pirazzini et al, 2015). To cope with artifacts that do not depend on wavelength, a free wavelength-independent scaling parameter A is introduced:…”

“…Estimating the maximal extent of the dominant crystals using handlens or binocular is the most widely used technique to measure grain size (Fierz et al, 2009;Aoki et al, 2000;Pirazzini et al, 2015). However, not only is it known to be imprecise and operator dependent, but also it cannot be automated for unmanned monitoring of grain size evolution.…”

“…It is also easier to assess the data quality or develop corrections by exploiting the spectral signature. Many spectral albedo measurements have been conducted in Antarctica (e.g., Grenfell et al, 1994;Hudson et al, 2006;Hudson and Warren, 2007;Marks et al, 2015;Pirazzini et al, 2015) but they were limited to short-term campaigns and the temporal evolution was not the priority. Long-term monitoring of spectral albedo requires fully automatic instruments with a good robustness to cope with the Antarctic conditions (Wuttke et al, 2006;Salzano et al, 2016).…”

Surface energy budget on Larsen and Wilkins ice shelves in the Antarctic Peninsula: results based on reanalyses in 1989–2010

“…As the surface on the ice shelves is either snow or ice, the albedo depends on parameterisation of snow and ice albedos. Parameterisations of those properties have been summarised by Pirazzini (2009). ERAI applies the ECMWF albedo parameterisation (ECMWF, 2007), where the snow albedo is a function of temperature.…”