Terrain and drift influences on snow surface aerodynamics A. Clifton 1, K. C. Leonard 1, C. Manes 2,...

Terrain and drift influences on snow surface aerodynamics

A. Clifton1, K. C. Leonard1, C. Manes2, M. Lehning1.

1. SLF Davos, Switzerland2. Politecnico di Torino, Turin, Italy

AGU Fall Meeting 2010C11C-02

Surface aerodynamics

• Interaction of boundary layer and surface

• Log law framework– Friction velocity (m/s)– Roughness length (m)

0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.5

1

1.5

2

2.5

0.3 m/s, 1 mmIncreased friction velocity (0.5 m/s)Increased roughness (10 mm)Both increase (0.5 m/s, 10 mm)

Speed [m/s]

Z [m]

Relevant processes

Anything that alters momentum transfer• Drift• Crystal structure• Snow metamorphosis• Surface forms• Local terrain

Wind tunnel measurements

Wind tunnel measurements

Wind tunnel measurements

Clifton, A., Rüedi, J.-D., Lehning, M. (2006).Snow saltation threshold measurements in a drifting snow wind tunnel.J. Glaciol., 52(179), 585-596. DOI: 10.3189/172756506781828430

Alpine test site measurements

• Fluxes of momentum, heat and water vapour– Sonic anemometer and

fast hygrometer– Concurrent surface

observations– 3 months of

observations– 5m measurement height

Stössel, F., M. Guala, C. Fierz, C. Manes, and M. Lehning (2010)Micrometeorological and morphological observations of surface hoar dynamics on a mountain snow cover.Water Resour. Res., 46, W04511. DOI: 10.1029/2009WR008198.

Alpine test site measurements

• Fluxes of momentum, heat and water vapour– Sonic anemometer and

fast hygrometer– Concurrent surface

observations– 3 months of

observations– 5m measurement height

Davos 3 km

Stössel, F., M. Guala, C. Fierz, C. Manes, and M. Lehning (2010).Micrometeorological and morphological observations of surface hoar dynamics on a mountain snow cover.Water Resour. Res., 46, W04511. DOI: 10.1029/2009WR008198.

Alpine test site measurements

• Fluxes of momentum, heat and water vapour– Sonic anemometer and

fast hygrometer– Concurrent surface

observations– 3 months of

observations– 5m measurement height

Davos 3 km

Stössel, F., M. Guala, C. Fierz, C. Manes, and M. Lehning (2010).Micrometeorological and morphological observations of surface hoar dynamics on a mountain snow cover.Water Resour. Res., 46, W04511. DOI: 10.1029/2009WR008198.

Williams Field, Antarctica

Williams Field, Antarctica

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York U. particle counter(P. Taylor)

Williams Field, AntarcticaWillie Field AWSAntarctic Automatic Weather Station Program AMRC, SSEC, UW-Madison

Williams Field, Antarctica

Role of snow structure

Clifton, A., C. Manes, J.-D. Ruedi, M. Guala, and M. Lehning (2008)On shear-driven ventilation of snow. Boundary-Layer Meteorol., 126, 249-261.

DOI: 10.1007/s10546-007-9235-0.

Images courtesy M. Schneebeli, SLF

1 mm

New snow Polyester foam

Results

Wind tunnel, without drift

Results

Hydraulically smooth wall

Wind tunnel (no drift)

Results

Wind tunnel, sustained drift

Wind tunnel (no drift)

Smooth wall

Results

Drifting sand, soil, waves over open water (Owen, 1960)

Wind tunnel (no drift)

Smooth wall

Wind tunnel (drift)

Results

William Field, without drift

Wind tunnel (no drift)

Smooth wall

Wind tunnel (drift)

Drifting sandand soil

Results

Williams Field, with sustained drift(neutral conditions only)

Wind tunnel (no drift)

Smooth wall

Wind tunnel (drift)

Drifting sandand soil

Williams Field(no drift)

Results

Alpine test site, all data(neutral conditions & NW flows only)

Wind tunnel (no drift)

Smooth wall

Wind tunnel (drift)

Drifting sandand soil

Williams Field(no drift)

Williams Field (drift)

Results

Revised Davenport ClassificationDavenport (2000)

Wind tunnel (no drift)

Smooth wall

Wind tunnel (drift)

Drifting sandand soil

Williams Field(no drift)

Williams Field (drift)

Alpine Site(all data)

ResultsDavenport

Classification

Wind tunnel (no drift)

Smooth wall

Wind tunnel (drift)

Drifting sandand soil

Williams Field(no drift)

Williams Field (drift)

Alpine Site(all data)

Conclusions

• Log law is a useful analogy near the ground

Conclusions

• Log law is a useful analogy near the ground• Roughness length of ‘snow’ is a function of– Friction velocity– Drift rates (increase or decrease)– Surface features (increase)– Fetch (increase)

Conclusions

• Log law is a useful analogy near the ground• Roughness length of ‘snow’ is a function of– Friction velocity– Drift rates (increase or decrease)– Surface features (increase)– Fetch (increase)

• Next steps– Wind and drift profiles coupled with surface

characterization