How well do we understand the influence of drifting and blowing snow in extreme environments?

Extreme cold environments such as the polar regions and high mountain areas are characterised by unknown precipitation quantities and a high variability of snow and ice masses. A key player in understanding the surface mass balance in these areas is drifting and blowing snow. Apart from the surface mass balance, drifting and blowing snow may perturb the surface structure of the boundary layer resulting in feedback loops that have not been sufficiently explored, especially at large scales. In this contribution we review recent advances in measuring and modelling drifting and blowing snow as well as their effects on the atmosphere and the surface mass balance. In particular, we suggest that current models may underestimate drifting snow sublimation because they assume saturation close to the surface and do not adequately represent the effects of transient (turbulent) wind fields. We highlight the importance of knowing not only the frequency of snow transport occurrence but also the height distribution of both mass flux and humidity for an accurate assessment. Therefore, we present first ground-based micro rain radar measurements of blowing snow clouds in Antarctica and show examples of blowing snow events which are not represented in current weather and climate models. Additionally, recent advances made is simulations of blowing snow clouds in Antarctica are present and compared with measurements. These simulations play the critical role of understanding the effect of drifting and blowing snow at continental scale. Results from such simulations, particularly on vapor contribution due to blowing snow sublimation will be presented as examples to highlight their utility.

Local surface mass balance processes in East Antarctica

Francesco Comola, Wolf Hendrik Huwald, Michael Lehning, Eric Sauvageat, Varun Sharma, Nander Wever

In Antarctica, direct observations of key processes shaping the surface mass balance such as precipitation and snow deposition, wind transport of snow, and sublimation, are sparse due to the extreme environment and related difficulties in measuring the variables of interest. The ongoing project ‘Local Surface Mass Balance in East Antarctica’ (LOSUMEA) is designed to address these processes at the local scale with the objective of gaining insight on the dominant surface mass and energy balance components and their link to near-surface atmospheric dynamics and surface and sub-surface snow properties. To this end, two measurement stations have been installed near the Princess Elisabeth Station, Antarctica, in December 2016. Besides standard meteorological data the stations are equipped with snow particle counters and 3-D sonic anemometers relating wind, turbulence and mass flux. Pre- and post-storm Terrestrial Laser Scans (TLS) record the resulting surface morphological changes. While preliminary data is being analyzed, the full data set of the year 2017 is retrieved in early 2018. Addition of moisture sensors in 2018 is expected to provide valuable information on latent heat fluxes related to sublimation. This contribution presents an overview of the project experiments, the data obtained to date as well as their analysis and some first results.

2018

Wind Tunnel Experiments and Modelling of Particle-Laden Boundary Layer Flows

Michael Lehning, Benjamin Andreas Walter

Particle-laden boundary flows occur in many geophysical and industrial environments yet are difficult to understand and quantitatively describe because the interactions of an often turbulent boundary layer flow with surface and particle dynamics are complex. The SLF wind tunnel allows the investigation of snow and sand particle laden boundary layer flows over their natural surfaces with and without the presence of a grass canopy.The experimental results are used to investigate the two possible approaches in describing the surface dynamics: (i) Models of particle transport, which assume a stationary flow situation and predict a mass flux for an hypothetical equilibrium; (ii) Models that take the temporal variability of the mass flux explicitly into account such as Lagrangian Stochastic particle tracking Models (LSM) on the basis of large eddy simulation (LES) or direct numerical simulation (DNS) of flow and turbulence. This presentation shows that wind tunnel data support the form of semi-empirical equilibrium models, which predict mass flux, q, as a function of the mean wind speed or the friction velocity, u, and a threshold velocity, uth: q=a(u-uth)x. For the exponent "x", a value of approximately 3, as based on theoretical considerations, is consistent with the data. This simple form of equilibrium models as well as more complicated equilibrium models are all based on the hypothesis that the surface shear stress induced by a fluid on the ground during sediment saltation is constant, i.e. independent of the magnitude of the particle mass flux (Owen's second hypothesis). Our surface shear stress measurements in a drifting-sand wind tunnel show a constant value of the fluid shear stress for saltation layers of various mass-flux magnitudes, directly validating Owen’s second hypothesis for the first time. The equilibrium models, however, only insufficiently describe the full dynamics of particle-laden flows. The second part of the presentation therefore discusses non-equilibrium features such as a high variability of the particle mass flux caused by flow turbulence and surface heterogeneity. Mass flux intermittency is primarily observed around the threshold value uth. Using a combination of LES and LSM models, we show how the simulation of individual feed-back processes leads to a more complete understanding of the mechanisms behind the flux variability.

2013

Simulating the influence of snow surface processes on soil moisture dynamics and streamflow generation in an alpine catchment

Mathias Thierry Pierre Bavay, Francesco Comola, Michael Lehning, Nander Wever

The assessment of flood risks in alpine, snow-covered catchments requires an understanding of the linkage between the snow cover, soil and discharge in the stream network. Here, we apply the comprehensive, distributed model Alpine3D to investigate the role of soil moisture in the predisposition of the Dischma catchment in Switzerland to high flows from rainfall and snowmelt. The recently updated soil module of the physics-based multilayer snow cover model SNOWPACK, which solves the surface energy and mass balance in Alpine3D, is verified against soil moisture measurements at seven sites and various depths inside and in close proximity to the Dischma catchment. Measurements and simulations in such terrain are difficult and consequently, soil moisture was simulated with varying degrees of success. Differences between simulated and measured soil moisture mainly arise from an overestimation of soil freezing and an absence of a groundwater description in the Alpine3D model. Both were found to have an influence in the soil moisture measurements. Using the Alpine3D simulation as the surface scheme for a spatially explicit hydrologic response model using a travel time distribution approach for inter-flow and baseflow, streamflow simulations were performed for the discharge from the catchment. The streamflow simulations provided a closer agreement with observed streamflow when driving the hydrologic response model with soil water fluxes at 30 cm depth in the Alpine3D model. Performance decreased when using the 2 cm soil water flux, thereby mostly ignoring soil processes. This illustrates that the role of soil moisture is important to take into account when understanding the relationship between both snowpack runoff and rainfall and catchment discharge in high alpine terrain. However, using the soil water flux at 60 cm depth to drive the hydrologic response model also decreased its performance, indicating that an optimal soil depth to include in surface simulations exists and that the runoff dynamics are controlled by only a shallow soil layer. Runoff coefficients (i.e. ratio of rainfall over discharge) based on measurements for high rainfall and snowmelt events were found to be dependent on the simulated initial soil moisture state at the onset of an event, further illustrating the important role of soil moisture for the hydrological processes in the catchment. The runoff coefficients using simulated discharge were found to reproduce this dependency, which shows that the Alpine3D model framework can be successfully applied to assess the predisposition of the catchment to flood risks from both snowmelt and rainfall events.