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Dive into the research topics where Stefan Horender is active.

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Featured researches published by Stefan Horender.


Water Resources Research | 2013

Measurements of the pore-scale water flow through snow using Fluorescent Particle Tracking Velocimetry

Benjamin Walter; Stefan Horender; Cb Christof Gromke; Michael Lehning

[1] Fluorescent Particle Tracking Velocimetry (FPTV) measurements of the pore-scale water flow through the pore space of a wet-snow sample are presented to demonstrate the applicability of this measurement technique for snow. For the experiments, ice-cooled water seeded with micron sized fluorescent tracer particles is either sprinkled on top of a snow sample to investigate saturated and unsaturated gravity-driven flow or supplied from a reservoir below the snow sample to generate upward flow driven by capillary forces. The snow sample is illuminated with a laser light sheet and the fluorescent light of the particles transported with the water in the pore space is recorded with a high-speed camera equipped with an optical filter. Tracking algorithms are applied to the images to obtain flow paths and flow velocities. A flow loop found in a pore space for the case of saturated gravity flow together with the tortuosity of the particle trajectories indicate the three-dimensionality of the water flow in wet snow. The average vertical flow velocities in the pore spaces were 11.2 mm s � 1 for the downward saturated gravity flow and 9.6 mm s � 1 for the upward flow that is driven by capillary forces for the limited cases presented as examples of the measurement technique. In the case of unsaturated gravity-driven flow, the average and the maximum flow velocities were found to be 30 times smaller than for the saturated gravity flow. Velocity histograms show that the fraction of the total water flowing against the main flow direction was about 3–5%, and that the horizontal velocities average to zero for both the saturated gravity-driven and the capillary flow.


Geophysical Research Letters | 2014

Experimental assessment of Owen's second hypothesis on surface shear stress induced by a fluid during sediment saltation

Benjamin Walter; Stefan Horender; Christian Voegeli; Michael Lehning

A widely used, yet thus far unproven, fluid dynamical hypothesis originally presented by P. R. Owen 50years ago, states that the surface shear stress induced by a fluid on the ground during equilibrium sediment saltation is constant and independent of the magnitude of the fluid velocity and consequently the particle mass flux. This hypothesis is one of the key elements in almost all current model descriptions of sediment erosion. We measured the surface shear stress in a drifting-sand wind tunnel and found Owens hypothesis being merely an approximation of the real situation. A significant decrease of the fluid stress with increasing wind velocities was measured for low to intermediate particle mass fluxes. For high particle mass fluxes, Owens hypothesis essentially holds, although a slight increase of the fluid stress was measured.


Boundary-Layer Meteorology | 2016

Quantifying Particle Numbers and Mass Flux in Drifting Snow

P. Crivelli; Enrico Paterna; Stefan Horender; Michael Lehning

We compare two of the most common methods of quantifying mass flux, particle numbers and particle-size distribution for drifting snow events, the snow-particle counter (SPC), a laser-diode-based particle detector, and particle tracking velocimetry based on digital shadowgraphic imaging. The two methods were correlated for mass flux and particle number flux. For the SPC measurements, the device was calibrated by the manufacturer beforehand. The shadowgrapic imaging method measures particle size and velocity directly from consecutive images, and before each new test the image pixel length is newly calibrated. A calibration study with artificially scattered sand particles and glass beads provides suitable settings for the shadowgraphical imaging as well as obtaining a first correlation of the two methods in a controlled environment. In addition, using snow collected in trays during snowfall, several experiments were performed to observe drifting snow events in a cold wind tunnel. The results demonstrate a high correlation between the mass flux obtained for the calibration studies (


Journal of Glaciology | 2014

Snow particle characteristics in the saltation layer

Cb Christof Gromke; Stefan Horender; Benjamin Walter; Michael Lehning


Boundary-Layer Meteorology | 2014

Modelling Small-Scale Drifting Snow with a Lagrangian Stochastic Model Based on Large-Eddy Simulations

C. D. Groot Zwaaftink; Marc Diebold; Stefan Horender; Jan Overney; G. Lieberherr; Marc B. Parlange; Michael Lehning

r \geqslant 0.93


The Cryosphere | 2016

Wind tunnel experiments: cold-air pooling and atmospheric decoupling above a melting snow patch

Rebecca Mott; Enrico Paterna; Stefan Horender; Philipp Crivelli; Michael Lehning


Boundary-Layer Meteorology | 2017

Estimating Sediment Mass Fluxes on Surfaces Sheltered by Live Vegetation

Benjamin Walter; Christian Voegeli; Stefan Horender

r⩾0.93) and good correlation for the drifting snow experiments (


International Snow Science Workshop Grenoble – Chamonix Mont-Blanc - October 07-11, 2013 | 2013

Intermittent Drifting Snow – Combining Experimental and Model Studies

Stefan Horender; Christine D. Groot Zwaaftink; Beni Walter; Michael Lehning


Bulletin of the American Physical Society | 2015

Experimental investigation of drifting snow in a wind tunnel

P. Crivelli; Enrico Paterna; Stefan Horender; Michael Lehning

r \geqslant 0.81


Geophysical Research Letters | 2014

Experimental assessment of Owen's second hypothesis on surface shear stress induced by a fluid during sediment saltation: OWEN'S SHEAR STRESS APPROXIMATION

Benjamin Walter; Stefan Horender; Christian Voegeli; Michael Lehning

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Michael Lehning

École Polytechnique Fédérale de Lausanne

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Christian Voegeli

École Polytechnique Fédérale de Lausanne

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Cb Christof Gromke

Eindhoven University of Technology

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C. D. Groot Zwaaftink

École Polytechnique Fédérale de Lausanne

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G. Lieberherr

École Polytechnique Fédérale de Lausanne

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Jan Overney

École Polytechnique Fédérale de Lausanne

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Marc Diebold

École Polytechnique Fédérale de Lausanne

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Marc B. Parlange

University of British Columbia

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