Instruments and Methods Contact spectroscopy for determination of stratigraphy of snow optical grain size

We present a technique for in situ measurement of the vertical and spatial stratigraphic distribution of snow optical grain size with a coupled contact illumination probe and field spectroradiometer. Accurate measurements of optical-equivalent grain size are critical for modeling radiative properties of snow such as spectral albedo and microwave emission. Measurements of the spectral reflectance of the snow-pit surface are made at 2 cm intervals in the vertical plane under constant illumination and view geometries. We invert the integral of the continuum normalization of the ice absorption feature with maximum at 1.03 mm wavelength for optical-equivalent grain size using the validated model of Nolin and Dozier (2000) that has accuracy of 10–50mm across the grain-size range 50–900 mm. Results are presented for an alpine site in southwest Colorado, USA, across the ablation season and for a Greenland ice-sheet site at the onset of snowmelt. These results suggest that traditional measurements of grain size using a hand lens are nearly accurate only for rounded grains (R 1⁄4 0.41, rmse 1⁄4 160mm); for polycrystals and faceted grains the hand-lens approach is very inaccurate (R 1⁄4 0.03 and 0.24, rmse 1⁄4 1206 and 1010 mm, respectively). We demonstrate an order-ofmagnitude improvement in modeling of shortwave spectral albedo and net shortwave flux with contact spectroscopy measurements of grain-size stratigraphy over those from a hand lens.