Incorporation of spectral and directional radiative transfer in a snow model
Present radiative transfer methods in physically based energy budget models of snow do not include adequate spectral or directional resolution to deal with the scattering of solar radiation. This paper reports on results from an advanced physically-based snow energy budget model, SNTHERM, linked with a discrete ordinate radiative transfer model, DISORT, at nine wavelengths assuming spherical snow grains. Scattering properties were averaged over a small range of grain sizes (as seen in real snow) to eliminate interference induced fluctuations. A method was derived to split a single measurement of spectrally integrated solar radiation into its direct and diffuse components at nine wavelengths. The split of radiation is required as input to the radiative transfer model, and is produced as a weighted average from days of total cloud cover and days of clear skies (from the 6S atmosphere radiative transfer model). The fully linked model was tested on a data set from a field campaign on the Uranus Glacier, Antarctica, December 1994–February 1995. An automated weather station provided meteorological inputs, with additional measurements made of solar radiation. A vertical array of thermistors made continuous measurements of snowpack temperature in the top metre of snow, though those thermistors near the surface were contaminated by solar heating and melting. Snow pit data were used to initialize the model. The combined model was tested against simpler radiative transfer parameterizations, with variations in grain size and solar zenith angle, or fixed albedo and extinction techniques. Albedo predictions from the discrete ordinate radiative transfer model show large variations in albedo with solar zenith angle and the diffuse and direct spectral radiative split. The spectral method provided superior results for snowpack temperatures over the test period.