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Convection of water vapour in snowpacks

Mahdi Jafari, Michael Lehning, Varun Sharma

This paper studies numerically the convection of water vapour in snowpacks using an Eulerian-Eulerian two-phase approach. The convective water vapour transport in snow and its effects on snow density are often invoked to explain observed density profiles, e.g. of thin Arctic snow covers, but this process has never been numerically simulated and analysed in a systematic manner. Here, the impact of convection on the thermal and phase change regimes as a function of different snowpack depths, thermal boundary conditions and Rayleigh numbers is analysed. We find considerable impact of natural convection on the snow density distribution with a layer of significantly lower density at the bottom of the snowpack and a layer of higher density located higher in the snowpack or at the surface. We find that emergent heterogeneity in the snow porosity results in a feedback effect on the spatial organization of convection cells causing their horizontal displacement. Regions where the snowpack is most impacted by phase changes are found to be horizontally extended and vertically thin, `pancake'-like layers at the top and bottom of the snowpack. We further demonstrate that among the parameters important for natural convection, the snowpack depth has the strongest influence on the heat and mass transfer. Despite our simplifying assumptions, our study represents a significant improvement over the state of the art and a first step to accurately simulate snowpack dynamics in diverse regions of the cryosphere.

CAMBRIDGE UNIV PRESS2022

Modeling the small-scale deposition of snow onto structured Arctic sea ice during a MOSAiC storm using snowBedFoam 1.0.

Océane Hames, Mahdi Jafari, Michael Lehning, Marc Schneebeli Zeugin, David Nicholas Wagner

The remoteness and extreme conditions of the Arctic make it a very difficult environment to investigate. In these polar regions covered by sea ice, the wind is relatively strong due to the absence of obstructions and redistributes a large part of the deposited snow mass, which complicates estimates for precipitation hardly distinguishable from blowing or drifting snow. Moreover, the snow mass balance in the sea ice system is still poorly understood, notably due to the complex structure of its surface. Quantitatively assessing the snow distribution on sea ice and its connection to the sea ice surface features is an important step to remove the snow mass balance uncertainties (i.e., snow transport contribution) in the Arctic environment. In this work we introduce snowBedFoam 1.0., a physics-based snow transport model implemented in the open-source fluid dynamics software OpenFOAM. We combine the numerical simulations with terrestrial laser scan observations of surface dynamics to simulate snow deposition in a MOSAiC (Multidisciplinary Drifting Observatory for the Study of Arctic Climate) sea ice domain with a complicated structure typical for pressure ridges. The results demonstrate that a large fraction of snow accumulates in their vicinity, which compares favorably against scanner measurements. However, the approximations imposed by the numerical framework, together with potential measurement errors (precipitation), give rise to quantitative inaccuracies, which should be addressed in future work. The modeling of snow distribution on sea ice should help to better constrain precipitation estimates and more generally assess and predict snow and ice dynamics in the Arctic.

COPERNICUS GESELLSCHAFT MBH2022

Water vapor transport in snowpacks

Mahdi Jafari

This thesis is an in-depth treatment of water vapor transport in snowpacks and its impacts on snow structure. The aim is to better understand this transport process and to lay the basis for a model representation in physics-based multi-layer snow models. Overall the process should improve the representation of the impact of snow in diverse environments from mountains, ice sheets to sea ice.The vertical diffusion of water vapor in the snow cover is investigated first by solving a one-dimensional transient diffusion equation in the SNOWPACK model. The implications on the snow layering of this vertical diffusive flux are analyzed for four different regions: Alpine, Subarctic, Arctic, and Antarctic sea ice. The largest impact of diffusion is observed in snow on sea ice in the Weddell Sea and the shallow Arctic snowpack. The simulations show significant density reductions at the base of the snow cover upon inclusion of diffusive water vapor transport.Diffusion may in some snow covers have a small effect and it is the convection often invoked to explain observed density profiles e.g. of thin tundra snowpacks. The effects of convection on snowpack structure have never been studied systematically by looking at the thermal and phase change regimes for different snowpack conditions. Therefore, this thesis presents for the first time a numerical investigation for convection of water vapor in idealized snowpacks using an EulerianâEulerian two-phase approach. We find considerable impact of the natural convection on the snow density distribution with a layer of significantly lower density at the bottom of the snowpack and a layer of higher density located near the surface. We observe that emergent heterogeneity in the snow porosity results in a feedback effect on the spatial organization of convection cells causing their horizontal displacement. We additionally find that the thermal equilibrium assumption is not valid in our idealized system.Natural convection is rather not likely to have a large impact in typical Alpine or sub-arctic snow covers if conditions are horizontally homogeneous. Therefore, this thesis also studies numerically the potential impact of heterogeneity induced by shrubs or rocks on convection of water vapor in snowpacks. We find that the convection cells are formed even with sub-critical Rayleigh number as low as 5 due to heterogeneity induced in snow density. We show that contributions of diffusive and convective flux divergence on snow density change show strong variations. We further find that the strongest effect of convection is not for very thin or thick snow covers but for snow covers in between, which allow the formation of convection cells with a scale of 0.5 m.Among different snow covers, significant convection events are numerically observed by SNOWPACK-OpenFOAM coupling for a herb tundra permafrost at Bylot Island. We find that significant sublimation in downward flow generates a low-density base in the snow cover. We further observe a strong footprint of convection on snow density and temperature with significant lateral variations up to 80 kg m

^{-3}

and 6.5 K respectively. This effect of convection on the snow density profile, namely a low density foot and high density top, corresponds qualitatively to observations. Further work is required to adapt physical parameterizations in conventional snow models to represent this effect and its interaction with snow settling and metamorphism.

EPFL2022

Water vapor transport in snowpacks

Mahdi Jafari

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EPFL2022