Abstract
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.
Highlights
After deposition on the ground, snow crystals form a porous, sintered material which continuously undergoes metamorphism to adapt to the thermodynamic forcing imposed by the atmosphere and the soil
The density and dielectric measurements of the studies of Fujita indicate that a copolar phase difference (CPD) of φCPD = −80◦ per meter would have been measured for the radar parameters of the satellite TSX as used in the following study on seasonal snow: in Leinss et al (2014b) a CPD of 60–150◦ m−1 was measured for fresh snow (ρ = 0.2) in Finland at θ0 = 32.7◦ and 9.65 GHz, corresponding to elongated horizontal structures with an anisotropy between A = +0.2 and +0.5 (A −1 = 1.2 and 1.7)
We demonstrated a contact-less technique for monitoring the temporal evolution of the depth-averaged anisotropy of a seasonal snowpack
Summary
After deposition on the ground, snow crystals form a porous, sintered material which continuously undergoes metamorphism to adapt to the thermodynamic forcing imposed by the atmosphere and the soil. The porous microstructure, defined by the 3-D distribution of the ice matrix and the pores space, determines the thermal, mechanical, and dielectric properties of the snowpack. A spatially anisotropic distribution of the snow microstructure leads to macroscopically anisotropic snow properties. Macroscopic (point) methods commonly applied in the field can be used to determine snow properties averaged over sample volumes of several centimeters. Methods based on remote sensing complement these point methods in providing large spatial coverage of repetitive measurements with a sampling resolution between meters and kilometers for inaccessible locations as well. Radar remote sensing methods facilitate measurements of snow properties averaged over the microwave penetration depth. This makes it possible to estimate the depth-averaged dielectric anisotropy of seasonal snow with radar instruments. Leinss et al.: Anisotropy of seasonal snow measured by polarimetric phase differences in radar time series
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