Abstract
Abstract. The structural anisotropy of snow characterizes the spatially anisotropic distribution of the ice and air microstructure and is a key parameter for improving parameterizations of physical properties. To enable the use of the anisotropy in snowpack models as an internal variable, we propose a simple model based on a rate equation for the temporal evolution. The model is validated with a comprehensive set of anisotropy profiles and time series from X-ray microtomography (CT) and radar measurements. The model includes two effects, namely temperature gradient metamorphism and settling, and can be forced by any snowpack model that predicts temperature and density. First, we use CT time series from lab experiments to validate the proposed effect of temperature gradient metamorphism. Next, we use SNOWPACK simulations to calibrate the model with radar time series from the NoSREx campaigns in Sodankylä, Finland. Finally we compare the simulated anisotropy profiles against field-measured full-depth CT profiles. Our results confirm that the creation of vertical structures is mainly controlled by the vertical water vapor flux through the snow volume. Our results further indicate a yet undocumented effect of snow settling on the creation of horizontal structures. Overall the model is able to reproduce the characteristic anisotropy variations in radar time series of four different winter seasons with a very limited set of calibration parameters.
Highlights
Deposited snow is a porous material that continuously undergoes microstructural changes in response to the external, thermodynamic forcing imposed by the atmosphere and the underlying soil
To describe the evolution of the anisotropy, the model considers only two contributions: temperature gradient metamorphism (TGM), which was confirmed to create vertical structures, and snow settling for which we think that the strain leads to preferentially horizontally oriented ice grains in the snow microstructure
We minimized the difference between the depth average of the modeled anisotropy and the depth-averaged radar anisotropy by adjusting a single-fit parameter
Summary
Deposited snow is a porous material that continuously undergoes microstructural changes in response to the external, thermodynamic forcing imposed by the atmosphere and the underlying soil. The thermal conductivity can vary by an order of magnitude at a given density: this variability is discussed with respect to the theoretical limits defined by a microstructure of either vertical or horizontal series of ice plates (Sturm et al, 1997). The structural anisotropy is commonly characterized by different variants of geometrical or structural fabric tensors These can be computed from mean intercept lengths (Srivastava et al, 2016), contact orientations (Shertzer and Adams, 2011), surface normals (Riche et al, 2013), or other second-order orientation tensors that can be constructed from the two-point correlation function of a two-phase medium (Torquato and Lado, 1991; Torquato, 2002).
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