Papers by Michael Lehning
Annals of Glaciology, 2004
ABSTRACT. SNOWPACK has been in operational use for five consecutive winters on approximately 100 ... more ABSTRACT. SNOWPACK has been in operational use for five consecutive winters on approximately 100 automatic weather stations in the Swiss Alps. It calculates snow pre-cipitation, snowdrift and the layered structure of the snow cover. An analysis routine has been implemented ...
The inhomogeneous snow distribution found in Alpine terrain is the result of wind and precipitati... more The inhomogeneous snow distribution found in Alpine terrain is the result of wind and precipitation interacting with the (snow) surface over terrain. We introduce and explain preferential deposition of precipitation as the deposition process without erosion of previously deposited snow and thus in the absence of saltation. A numerical model is developed, describing on a physical basis the relevant processes
Air & waste: journal of the Air & Waste Management Association
A series of measurements were made of the velocity boundary layer over snow and over open-celled ... more A series of measurements were made of the velocity boundary layer over snow and over open-celled polyester foams. Measurements were made in a wind tunnel, with zero streamwise pressure gradient. Logarithmic boundary layers formed, and the roughness length varied by several orders of magnitude between different foam and snow surfaces. Discrete, reynolds-number dependent, smooth and rough hydraulic regimes were not
A correct representation of new snow and its properties is crucial in order to initialize the sim... more A correct representation of new snow and its properties is crucial in order to initialize the simulation of snow-cover evolution. On the one hand, adding a new snow layer to an ongoing numerical snow-cover simulation is straightforward and adjusting the density of new snow according to the meteorological conditions is common practice in snow-cover modeling, e.g. using a statistical model. On the other hand, however, assigning proper initial microstructural and textural parameters such as grain size, bond size, dendricity or sphericity to this new snow layer is more involved. For the sake of simplicity, these parameters are usually set to arbitrarily flxed values for new snow.
The Cryosphere Discussions
ABSTRACT
Wind Energy
Most large-eddy simulation studies related to wind energy have been carried out either by using a... more Most large-eddy simulation studies related to wind energy have been carried out either by using a fixed pressure gradient to ensure that mean wind direction is perpendicular to the wind turbine rotor disk or by forcing the flow with a geostrophic wind and timely readjusting the turbines' orientation. This has not allowed for the study of wind farm characteristics with a time-varying wind vector. In this paper, a new time-adaptive wind turbine model for the large-eddy simulation framework is introduced. The new algorithm enables the wind turbines to dynamically realign with the incoming wind vector and self-adjust the yaw orientation with the incoming wind vector similar to real wind turbines. The performance of the new model is tested first with a neutrally stratified atmospheric flow forced with a time-varying geostrophic wind vector. A posteriori, the new model is used to further explore the interaction between a synthetic time-changing thermal atmospheric boundary layer and an embedded wind farm. Results show that there is significant potential power to be harvested during the unstable time periods at the cost of designing wind turbines capable of adapting to the enhanced variance of these periods. Stable periods provide less power but are more constant over time with an enhanced lateral shear induced by an increased change in wind direction with height.
Icing of structures is a general problem and especially critical for power lines and wind energy ... more Icing of structures is a general problem and especially critical for power lines and wind energy production. Icing of wind blades is a major problem because of safety concerns (falling ice) and economic aspects like production downtimes. The rising number of wind energy developments calls for a better assessment of regional icing probability. Therefore, we are investigating the regional probability of icing and present an icing map of Switzerland. In this study icing situations are calculated using the Makkonen algorithm, which estimates icing on a cylindrical metal surface, using temperature, wind speed, cloud and rain water content of the atmosphere as input. The input was calculated from an analysis run of the Swiss operational weather forecast model COSMO-2 with 2.2 km grid size. The meteorological data were modelled for different levels above ground in hourly resolution for the last two years. A verification against measured meteorological data proved the model output to be an ...
Hydrology and Earth System Sciences Discussions, 2015
ABSTRACT
A series of 17 wind-tunnel experiments was made to establish the saltation threshold conditions o... more A series of 17 wind-tunnel experiments was made to establish the saltation threshold conditions of natural snow. This information is of significant interest for numerical models of snow pack development. We characterised snow visually on site, using a penetrometer and through image processing of digital images. The profile of the velocity boundary layer over the snow was measured and results were fitted using a log-law. Friction velocities and roughness lengths up to and including the drift threshold will be presented. No relationship was seen between the boundary layer roughness length or threshold friction velocity and grain type, size or snow density alone. Aerodynamic roughness length was found to be independent of friction velocity until the drift threshold was reached. The relationship between the roughness length and friction velocity at the threshold was similar to that predicted by Owen in 1964 for flow over saltating particles. The boundary layer roughness length at the th...
The run-out distance depends, among other things, on the release mass and the snow conditions in ... more The run-out distance depends, among other things, on the release mass and the snow conditions in the avalanche path. So far, our knowledge on the effect of snow conditions on avalanche run-out is limited and qualitative at best. In the Interreg project STRADA we investigate the effects of snow cover conditions on avalanche dynamics, and thus run-out distance. We selected five avalanches from the Valí ee de la Sionne test site (Western Swiss Alps), with similar initial mass and flow paths but different flow dynamics. For each of these avalanches, the snow conditions were reconstructed using either field data from local snow pits or the numerical snow cover model SNOWPACK. Our data suggest that even for short vertical distances distinct flow regimes develop depending on the amount of entrained snow and its properties. In contrast, the lack of potentially erodible snow along the track or snow with relatively warm temperature did not only reduce the run-out distance but also strongly af...
Avalanche Warning has been a key task of the Swiss Federal Institute for Snow and Avalanche Resea... more Avalanche Warning has been a key task of the Swiss Federal Institute for Snow and Avalanche Research in Davos (SLF) since it was established over half a century ago. In the past, the predominant methods used for avalanche forecasting at SLF have been conventional, Le. snow stability and avalanche hazards were predicted without analytical techniques such as formal numerical and symbolic algorithms. A paradigm shift is currently taking place: Information systems and computer programs are becoming more and more important, assisting the forecaster in collecting and analysing large amounts of field data. Furthermore, computer models are available which simulate processes in the snow cover and calculate local and regional avalanche hazards, thereby supporting forecasters and decision makers. However, the forecaster with his intuition, experience and local knowledge still plays a decisive role in the forecasting process. While the computer helps to assimilate information, to assess the haz...
As the European wind market matures, energy prospectors are increasingly looking to more challeng... more As the European wind market matures, energy prospectors are increasingly looking to more challenging terrain and conditions, for example those found in the mountains and passes of the Alps. These locations present very different technical challenges to those found in the flatter plains of Northern Europe, the US midwest or offshore. There is little public data available on wind regimes in these areas, and what information there is is not often examined in conjunction with other data for the same area. Consequently it is difficult to estimate the effect of terrain or surface cover on the wind resource. We present selected data collected in a mountain pass during the winter of 2009 / 2010. Data were collected on site at 36, 54 and 77m above ground using sonic anemometers, and at the surface using small portable weather stations. Preliminary analysis of data from the sonic anemometers shows that flow in the pass is often low shear compared to an unconstrained boundary layer, although t...
The effects of permafrost degradation due to climate change on slope stability, and thus debris f... more The effects of permafrost degradation due to climate change on slope stability, and thus debris flows or rock falls generated in high alpine regions, seriously affects the safety of many mountain communities. Therefore, its spatial distribution and temporal evolution are of great interest for scientists and practitioners. Simulations as will be presented in this work can aid to identify, simulate and forecast impacts of climate change scenarios on permafrost areas. We present the three-dimensional atmospheric and surface process model Alpine3D in combination with the one-dimensional model of vegetation, snow, soil SNOWPACK and discuss its potential to calculate 4D, i.e. in space and time, potential permafrost distributions. The model setup was applied to the Veneto region (Italy) with a spatial resolution of 150 m. Information on land use and soil characteristics served as input data and for the initialization of the model, which was then run for 12 years with the meteorological dat...
Uploads
Papers by Michael Lehning