Surface-layer turbulence, energy balance and links to atmospheric circulations over a mountain glacier in the French Alps
Over Saint-Sorlin Glacier in the French Alps (45° N, 6.1° E; ∼ 3 km<sup>2</sup>) in summer, we study the atmospheric surface-layer dynamics, turbulent fluxes, their uncertainties and their impact on surface energy balance (SEB) melt estimates. Results are classified with regard to large-...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-04-01
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| Series: | The Cryosphere |
| Online Access: | http://www.the-cryosphere.net/11/971/2017/tc-11-971-2017.pdf |
| Summary: | Over Saint-Sorlin Glacier in the French Alps (45° N,
6.1° E; ∼ 3 km<sup>2</sup>) in summer, we study the atmospheric
surface-layer dynamics, turbulent fluxes, their uncertainties and their
impact on surface energy balance (SEB) melt estimates. Results are classified
with regard to large-scale forcing. We use high-frequency eddy-covariance
data and mean air-temperature and wind-speed vertical profiles, collected in
2006 and 2009 in the glacier's atmospheric surface layer. We evaluate the
turbulent fluxes with the eddy-covariance (sonic) and the profile method, and
random errors and parametric uncertainties are evaluated by including
different stability corrections and assuming different values for surface
roughness lengths. For weak synoptic forcing, local thermal effects dominate
the wind circulation. On the glacier, weak katabatic flows with a wind-speed
maximum at low height (2–3 m) are detected 71 % of the time and
are generally associated with small turbulent kinetic energy (TKE) and small
net turbulent fluxes. Radiative fluxes dominate the SEB. When the large-scale
forcing is strong, the wind in the valley aligns with the glacier flow,
intense downslope flows are observed, no wind-speed maximum is visible below
5 m, and TKE and net turbulent fluxes are often intense. The net
turbulent fluxes contribute significantly to the SEB. The surface-layer
turbulence production is probably not at equilibrium with dissipation because
of interactions of large-scale orographic disturbances with the flow when the
forcing is strong or low-frequency oscillations of the katabatic flow when
the forcing is weak. In weak forcing when TKE is low, all turbulent fluxes
calculation methods provide similar fluxes. In strong forcing when TKE is
large, the choice of roughness lengths impacts strongly the net turbulent
fluxes from the profile method fluxes and their uncertainties. However, the
uncertainty on the total SEB remains too high with regard to the net observed
melt to be able to recommend one turbulent flux calculation method over
another. |
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| ISSN: | 1994-0416 1994-0424 |