Articles | Volume 10, issue 4
The Cryosphere, 10, 1771–1797, 2016
The Cryosphere, 10, 1771–1797, 2016

Research article 17 Aug 2016

Research article | 17 Aug 2016

Anisotropy of seasonal snow measured by polarimetric phase differences in radar time series

Silvan Leinss1, Henning Löwe2, Martin Proksch2, Juha Lemmetyinen3, Andreas Wiesmann4, and Irena Hajnsek1,5 Silvan Leinss et al.
  • 1Institute of Environmental Engineering, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
  • 2Institute for Snow and Avalanche Research SLF, Davos, Switzerland
  • 3Finnish Meteorological Institute FMI, Arctic Research, Sodankylä, Finland
  • 4GAMMA Remote Sensing AG, Gümlingen, Switzerland
  • 5Microwaves and Radar Institute, German Aerospace Center (DLR), Wessling, Germany

Abstract. The snow microstructure, i.e., the spatial distribution of ice and pores, generally shows an anisotropy which is driven by gravity and temperature gradients and commonly determined from stereology or computer tomography. This structural anisotropy induces anisotropic mechanical, thermal, and dielectric properties. We present a method based on radio-wave birefringence to determine the depth-averaged, dielectric anisotropy of seasonal snow with radar instruments from space, air, or ground. For known snow depth and density, the birefringence allows determination of the dielectric anisotropy by measuring the copolar phase difference (CPD) between linearly polarized microwaves propagating obliquely through the snowpack. The dielectric and structural anisotropy are linked by Maxwell–Garnett-type mixing formulas. The anisotropy evolution of a natural snowpack in Northern Finland was observed over four winters (2009–2013) with the ground-based radar instrument "SnowScat". The radar measurements indicate horizontal structures for fresh snow and vertical structures in old snow which is confirmed by computer tomographic in situ measurements. The temporal evolution of the CPD agreed in ground-based data compared to space-borne measurements from the satellite TerraSAR-X. The presented dataset provides a valuable basis for the development of new snow metamorphism models which include the anisotropy of the snow microstructure.

Short summary
Four years of anisotropy measurements of seasonal snow are presented in the paper. The anisotropy was measured every 4 h with a ground-based polarimetric radar. An electromagnetic model has been developed to measured the anisotropy with radar instruments from ground and from space. The anisotropic permittivity was derived with Maxwell–Garnett-type mixing formulas which are shown to be equivalent to series expansions of the permittivity tensor based on spatial correlation function of snow.