In the terrestrial environment, ice exists near its phase change temperature. Consequently, snow on the ground is a thermodynamically active material with a granular structure that is continuously changing. This is important, since the snowpack microstructure influences virtually all of its thermo-mechanical and optical properties. In this research we the coupled environmental parameters governing near surface metamorphism connecting the consequent morphology to the snow strength and energy balance at the terrestrial/atmosphere interface. The research integrated field, laboratory and modeling. The modeling component has relevance to metamorphism deeper in the snowpack as well.

Intellectual Merit

Near surface snow structure is important from the standpoint of avalanche potential and terrestrial/atmosphere energy exchange. In this project we coupled the environmental parameters that lead to the development of near surface metamorphism. Microstructural changes that occur due to natural atmospheric boundary conditions were replicated in a laboratory environment and the resulting thermo-mechanical properties measured. The resultant anisotropic morphology of the snow was quantified and related to thermal conductivity and mechanical properties. This was accomplished by using detailed meteorological data and snow structure observations, which was simulated in a state of the art “cold climate simulation chamber”. The laboratory component, in addition to producing relevant snow morphologies, examined thermo-mechanical properties, including thermal conductivity and shear/normal strength of the layer. A thermal model accounting for topography and terrain thermal properties was implemented in field studies to assess spatial variability. A fabric tensor to describe thermo-mechanically relevant anisotropic directional morphology, which develops due to metamorphism, was derived as a means to quantify the microstructure. This tensor was used to analytically and empirically estimate this thermally induced evolution of the microstructure and its subsequent effect on snow’s effective material properties. An effective thermal conductivity, toward which a snow layer will tend for a given environmental energy input, was analyzed using a non-equilibrium thermodynamics approach. The approach demonstrated the tendency of temperature gradient driven metamorphism to geometrically reorganize, (i.e. metamorphose) to optimize the transport of heat. Thus the contributions of the individual heat transfer processes (conduction, diffusion, convection) tend toward the most efficient cumulative heat transport constant (effective thermal conductivity). A clear reduction in shear strength of radiation recrystallized near surface metamorphism was demonstrated, with clear implication to snowpack strength and stability. Additionally, laboratory produced surface hoar indicated that there is a solar directional dependence and departure from lambertian scattering at the snow surface.

Broader Impacts

Two PhD and two MS degrees have been awarded as a direct result of this project. Although not completed by the project end, another PhD and an MS will also be awarded this summer and spring. A visiting Swiss Post-Doctoral scientist contributed to the project. In addition, a number of undergraduate students benefited from laboratory research experience related to the study and each summer two recent high school graduates participated, through a separate program. Six of these high school students were from underrepresented groups with respect to STEM studies.

Dissemination of findings was accomplished through publication in technical literature and international scientific conferences. More broadly, substantial dissemination of the science was accomplished through mass media and public lectures. Our Subzero Science and Engineering research Facility r...

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