Articles | Volume 9, issue 3
The Cryosphere, 9, 1277–1301, 2015

Special issue: Interactions between climate change and the Cryosphere: SVALI,...

The Cryosphere, 9, 1277–1301, 2015

Research article 23 Jun 2015

Research article | 23 Jun 2015

Parameterization of single-scattering properties of snow

P. Räisänen1, A. Kokhanovsky2,3, G. Guyot4, O. Jourdan4, and T. Nousiainen1 P. Räisänen et al.
  • 1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
  • 2EUMETSAT, Eumetsat Allee 1, 64295 Darmstadt, Germany
  • 3Institute of Remote Sensing, University of Bremen, 28334 Bremen, Germany
  • 4Laboratoire de Météorologie Physique (LaMP) Université Blaise Pascal/CNRS/OPGC, 24 avenue des Landais, 63177 Aubiére CEDEX, France

Abstract. Snow consists of non-spherical grains of various shapes and sizes. Still, in many radiative transfer applications, single-scattering properties of snow have been based on the assumption of spherical grains. More recently, second-generation Koch fractals have been employed. While they produce a relatively flat phase function typical of deformed non-spherical particles, this is still a rather ad hoc choice. Here, angular scattering measurements for blowing snow conducted during the CLimate IMpacts of Short-Lived pollutants In the Polar region (CLIMSLIP) campaign at Ny Ålesund, Svalbard, are used to construct a reference phase function for snow. Based on this phase function, an optimized habit combination (OHC) consisting of severely rough (SR) droxtals, aggregates of SR plates and strongly distorted Koch fractals is selected. The single-scattering properties of snow are then computed for the OHC as a function of wavelength λ and snow grain volume-to-projected area equivalent radius rvp. Parameterization equations are developed for λ = 0.199–2.7 μm and rvp = 10–2000 μm, which express the single-scattering co-albedo β, the asymmetry parameter g and the phase function P11 as functions of the size parameter and the real and imaginary parts of the refractive index. The parameterizations are analytic and simple to use in radiative transfer models. Compared to the reference values computed for the OHC, the accuracy of the parameterization is very high for β and g. This is also true for the phase function parameterization, except for strongly absorbing cases (β > 0.3). Finally, we consider snow albedo and reflected radiances for the suggested snow optics parameterization, making comparisons to spheres and distorted Koch fractals.

Short summary
While snow grains are distinctly non-spherical, spheres are often assumed in radiative transfer calculations. Here, angular scattering measurements for blowing snow are used to select an optically equivalent snow grain shape model. Parameterizations are then developed for the asymmetry parameter, single-scattering co-albedo and phase function of snow. The parameterizations will help to improve the treatment of snow in radiative transfer applications, including remote sensing and climate models.