Ice exists near its phase change temperature in the terrestrial environment. Consequently, snow on the ground is a thermodynamically active material with a granular structure that is continuously changing. The snowpack microstructure influences virtually all of its thermo-mechanical and optical properties. We will better determine the coupled environmental parameters governing near surface metamorphism and tie the consequent morphology to snow strength (important to avalanche potential) and energy balance at the terrestrial/atmosphere interface. We will integrate field, laboratory and numerical modeling.
The three research hypotheses are: microstructural changes that occur due to natural atmospheric boundary conditions can be replicated in a laboratory environment and the resulting thermo-mechanical properties measured; anisotropic morphology of snow can be quantified and related to thermal conductivity and mechanical properties; process driven microstructure can be deduced based on thermal input.
Field studies will be carried out at two existing alpine research sites. Field meteorological data will dictate imposed laboratory conditions to accurately replicate the natural environment and consequent metamorphic processes. Important microstructure will be developed in the state-of-the-art Cold Climate Simulation Chamber through simulation of observed natural conditions. We will develop near surface metamorphism laboratory protocols for radiation recrystallization, surface hoar growth and diurnal recrystallization.
Theoretical aspects include developing a microstructure fabric tensor, non-equilibrium thermodynamics analyzing metamorphism and terrain modeling. A fabric tensor to describe thermo-mechanically relevant anisotropic directional morphology, which develops due to metamorphism, will be derived. Entropy production extremum concepts will be used to evaluate heat transport based on microstructure resulting from imposed temperature gradients. The contributions of the individual heat transfer processes (conduction, diffusion, convection) tend toward the most efficient cumulative heat transport (effective thermal conductivity). Taken together, these techniques will be used to analytically and empirically quantify this thermally-induced evolution in fabric and its subsequent effect on snow's effective material properties. We will measure thermo-mechanical properties, including; thermal conductivity, penetration resistance, shear/normal strength and bulk properties. An existing thermal model accounting for topography and terrain thermal properties will be implemented in field studies to assess spatial variability.
We will work with the USFS National Avalanche Center to assist its mission to provide information, new developments and technology to snow safety practitioners. Additionally we will interface with the local USFS avalanche center to investigate how best to exploit thermal modeling of the snowcover for practical application. Interaction with a local ski area snow safety team provides an opportunity for this group to be involved in a scientific study in a field in which they have an intense interest. They will then go on to share their findings with colleagues in the field, expanding the impact.
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 regularly accommodates tours of K-12 students and other interested groups, with this research a focus of the visit. Annual presentations have been made to local ski area snow safety personnel and there was direct collaboration with some of these practitioners who were directly involved in the field component. These specialists, who have an intense interest in snow mechanics as related to avalanches, go on to share their findings with colleagues in the field, expanding the impact.
