Abstract
AbstractEffective snow grain radius (re) is mapped at high resolution using near-infrared hyperspectral imaging (NIR-HSI). The NIR-HSI method can be used to quantify re spatial variability, change in re due to metamorphism, and visualize water percolation in the snowpack. Results are presented for three different laboratory-prepared snow samples (homogeneous, ice lens, fine grains over coarse grains), the sidewalls of which were imaged before and after melt induced by a solar lamp. The spectral reflectance in each ~3 mm pixel was inverted for re using the scaled band area of the ice absorption feature centered at 1030 nm, producing re maps consisting of 54 740 pixels. All snow samples exhibited grain coarsening post-melt as the result of wet snow metamorphism, which is quantified by the change in re distributions from pre- and post-melt images. The NIR-HSI method was compared to re retrievals from a field spectrometer and X-ray computed microtomography (micro-CT), resulting in the spectrometer having the same mean re and micro-CT having 23.9% higher mean re than the hyperspectral imager. As compact hyperspectral imagers become more widely available, this method may be a valuable tool for assessing re spatial variability and snow metamorphism in field and laboratory settings.
Highlights
Snow grain size is a physical property of snow that exerts controls on snow reflectance (Wiscombe and Warren, 1980), and is used to characterize mechanisms of snow metamorphism and stratigraphy (Colbeck, 1991)
We presented a new high spatial resolution near-infrared hyperspectral imaging (NIR-HSI) method to map re from spectral reflectance data measured with a Resonon Pika NIR-320 compact hyperspectral imager
To demonstrate the effectiveness of the method over a wide range of re, the method was applied to laboratory-prepared snow samples that had undergone wet snow metamorphism induced by a simulated solar lamp
Summary
Snow grain size is a physical property of snow that exerts controls on snow reflectance (Wiscombe and Warren, 1980), and is used to characterize mechanisms of snow metamorphism and stratigraphy (Colbeck, 1991). The vertical distribution of snow grain size, is critical for modeling microwave radiative transfer (Brucker and others, 2011) and understanding mechanical (Pielmeier and Schneebeli, 2003) and thermodynamic (Hammonds and others, 2015) properties of a snowpack. Snow grain size can be described as an effective grain radius (re), which is used to approximate the scattering and absorption properties of snow. Grenfell and Warren (1999) demonstrated that a collection of spherical ice particles with the same volume-to-surface area ratio as snow (i.e. characterized with stereology methods) could be used to model hemispherical reflectance. The effective grain radius is directly related to SSA by: cambridge.org/jog
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