Modeling snow drift in the turbulent boundary layer

The modeling of snow redistribution in turbulent conditions plays an important role for the prediction of snow accumulation and avalanche risk. For this aim two Lagrangian models, one for the suspension and one for the saltation of snow were coupled. The saltation layer model, where the snow particles are initially entrained, acts as the lower boundary condition for the suspension model. Snow particles can be lifted into suspension and then bed with the flow. They thus no longer follow the characteristic ballistic trajectories of the saltation process. The wind field data for both models was previously generated by a large eddy simulation of the atmospheric boundary layer. The saltation model used in this work is an adapted version of the one implemented in the Alpine3D model used at the Swiss Federal Institute for Snow and Avalanche Research (SLF Davos). The suspended transport is modeled by a 3D Lagrangian Stochastic Model. The large eddy simulation and the Alpine 3D saltation model have been validated against field data, however the verification of the suspension model for snow particles is still pending. Further work will scope on the full coupling of Lagrangian saltation and suspension as well as the large-eddy simulation into one complete snow drift model.

A physics-based model for wind turbine wake expansion in the atmospheric boundary layer

Fernando Porté Agel, Dara Vahidi

Analytical wind turbine wake models are widely used to predict the wake velocity deficit. In these models, the wake growth rate is a key parameter specified mainly with empirical formulations. In this study, a new physics-based model is proposed and validated to predict the wake expansion downstream of a turbine based on the incoming ambient turbulence and turbine operating conditions. The new model utilises Taylor diffusion theory, the Gaussian wake model, turbulent mixing layer theory and the analogy between wind turbine wake expansion and scalar diffusion. These components ensure that the model conserves mass and momentum in the far wake and accounts for the ambient turbulence and turbine-induced turbulence effects on the wake expansion. To account for the turbulence relevant scales that contribute to the wake expansion, the model uses the root-mean-square of the low-pass filtered radial velocity component. A simplified version that only requires the unfiltered velocity standard deviation and turbulence integral scale is also proposed. In addition, a new relation for the near-wake length is derived. The model performance is validated using large-eddy simulation data of a wind turbine wake under neutral atmospheric conditions with a wide range of incoming turbulence levels. The results show that the proposed model yields reasonable predictions of the wake width, maximum velocity deficit and near-wake length. In the case with a relatively low incoming streamwise turbulence intensity of 0.05, the ambient and turbine-induced terms in the model contribute almost equally to the wake width, rendering them both crucial for reasonable wake predictions.

2022

The inFuence of drifting and blowing snow on surface (turbulent) mass- and energy exchange assessed with different methods

Michael Lehning, Wolf Hendrik Huwald, Jérôme François Sylvain Dujardin, Franziska Gerber, Daniela Brito Melo, Armin Sigmund, Benjamin Andreas Walter

Drifting and blowing snow is a dominant process shaping snow-covered surfaces in particular in extreme environments such as high mountains or polar regions. Because the process is complicated to describe, as it covers scales from below 1 mm to several kilometers, it is typically not considered in models or greatly simpli^ed. This impedes our ability to correctly describe the mass- and energy exchange above snow covered surfaces and therefore also to understand the relationship with precipitation patterns. Our contribution addresses this problem by reviewing the current state of understanding. We show based on measurements and large-eddy simulations (LES) that typical bulk Monin–Obukhov formulations for turbulent auxes do not work in the presence of snow transport. More specifically, surface exchange is much more intense than predicted by current large-scale models during snow transport. We further present a new model coupling, CRYOWRF, which introduces the snow model SNOWPACK and a detailed representation of blowing snow to the Weather and Forecasting model (WRF) in order to allow a more accurate representation of surface exchange. We show how this coupled version is able to reproduce observed patterns of blowing snow and its inauence on surface exchange. This includes cases of blowing snow over mountain ridges and large blowing snow clouds from katabatic winds, an hydraulic jump and waves in Antarctica (Figure). Overall, we find that the influence of drifting and blowing snow on surface exchange may have been underestimated in previous (model) assessments and that the highly dynamic exchange associated with drifting and blowing snow has not only implications for the surface mass and energy balance, but potentially also for the isotopic composition of deposited snow.

2021

The influence of drifting and blowing snow on surface (turbulent) mass- and energy exchange assessed with different methods

Varun Sharma, Michael Lehning, Wolf Hendrik Huwald, Jérôme François Sylvain Dujardin, Franziska Gerber, Daniela Brito Melo, Armin Sigmund, Benjamin Andreas Walter

Drifting and blowing snow is a dominant process shaping snow-covered surfaces in particular in extreme environments such as high mountains or polar regions. Because the process is complicated to describe, as it covers scales from below 1 mm to several kilometers, it is typically not considered in models or greatly simplified. This impedes our ability to correctly describe the mass- and energy exchange above snow covered surfaces and therefore also to understand the relationship with precipitation patterns. Our contribution addresses this problem by reviewing the current state of understanding. We show based on measurements and large-eddy simulations (LES) that typical bulk Monin–Obukhov formulations for turbulent fluxes do not work in the presence of snow transport. More specifically, surface exchange is much more intense than predicted by current large-scale models during snow transport. We further present a new model coupling, CRYOWRF, which introduces the snow model SNOWPACK and a detailed representation of blowing snow to the Weather and Forecasting model (WRF) in order to allow a more accurate representation of surface exchange. We show how this coupled version is able to reproduce observed patterns of blowing snow and its influence on surface exchange. This includes cases of blowing snow over mountain ridges and large blowing snow clouds from katabatic winds, an hydraulic jump and waves in Antarctica (Figure). Overall, we find that the influence of drifting and blowing snow on surface exchange may have been underestimated in previous (model) assessments and that the highly dynamic exchange associated with drifting and blowing snow has not only implications for the surface mass and energy balance, but potentially also for the isotopic composition of deposited snow.

2021

The inFuence of drifting and blowing snow on surface (turbulent) mass- and energy exchange assessed with different methods

Michael Lehning, Wolf Hendrik Huwald, Jérôme François Sylvain Dujardin, Franziska Gerber, Daniela Brito Melo, Armin Sigmund, Benjamin Andreas Walter

2021

The influence of drifting and blowing snow on surface (turbulent) mass- and energy exchange assessed with different methods

Varun Sharma, Michael Lehning, Wolf Hendrik Huwald, Jérôme François Sylvain Dujardin, Franziska Gerber, Daniela Brito Melo, Armin Sigmund, Benjamin Andreas Walter

Drifting and blowing snow is a dominant process shaping snow-covered surfaces in particular in extreme environments such as high mountains or polar regions. Because the process is complicated to describe, as it covers scales from below 1 mm to several kilometers, it is typically not considered in models or greatly simplified. This impedes our ability to correctly describe the mass- and energy exchange above snow covered surfaces and therefore also to understand the relationship with precipitation patterns. Our contribution addresses this problem by reviewing the current state of understanding. We show based on measurements and large-eddy simulations (LES) that typical bulk Monin–Obukhov formulations for turbulent fluxes do not work in the presence of snow transport. More specifically, surface exchange is much more intense than predicted by current large-scale models during snow transport. We further present a new model coupling, CRYOWRF, which introduces the snow model SNOWPACK and a detailed representation of blowing snow to the Weather and Forecasting model (WRF) in order to allow a more accurate representation of surface exchange. We show how this coupled version is able to reproduce observed patterns of blowing snow and its influence on surface exchange. This includes cases of blowing snow over mountain ridges and large blowing snow clouds from katabatic winds, an hydraulic jump and waves in Antarctica (Figure). Overall, we find that the influence of drifting and blowing snow on surface exchange may have been underestimated in previous (model) assessments and that the highly dynamic exchange associated with drifting and blowing snow has not only implications for the surface mass and energy balance, but potentially also for the isotopic composition of deposited snow.

2021