Surface-layer turbulence, energy-balance and links to atmospheric circulations over a mountain glacier in the French Alps
Maxime Litt1, Jean-Emmanuel Sicart1,2,3, Delphine Six4,5, Patrick Wagnon1,2,3,6, and Warren D. Helgason71Univ. Grenoble Alpes, LTHE, F-38000 Grenoble, France 2CNRS, LTHE, F-38000 Grenoble, France 3IRD, LTHE, F-38000 Grenoble, France 4Univ. Grenoble Alpes, LGGE, F-38000 Grenoble, France 5CNRS, LGGE, F-38000 Grenoble, France 6Icimod, GPO Box 3226, Kathmandu, Nepal 7Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada
Received: 19 Apr 2016 – Accepted for review: 08 May 2016 – Discussion started: 13 May 2016
Abstract. Over mountain glaciers, large errors may affect turbulent surface heat fluxes estimated with the bulk-aerodynamic (BA) method. That might lead to uncertainties in estimating melt from surface energy balance (SEB). During the summers of 2006 and 2009, in the atmospheric surface layer of Saint-Sorlin Glacier (French Alps, 45° N, 6.1° E, ~3 km2), mean air-temperature and wind-speed vertical profiles and high frequency Eddy-Covariance (EC) data were collected to characterize the turbulence and the turbulent fluxes. We studied the influence of the BA method errors on the melt estimations, calculating the SEB alternatively with turbulent fluxes obtained from the BA and the EC methods. We classified our results in terms of large-scale forcing. In weak synoptic forcing, local thermal effects dominated the wind circulation. On the glacier, weak katabatic flows with a wind-speed maximum at low height (2–3 m) were detected 71 % of the time and were generally associated with weak turbulent kinetic energy (TKE) and turbulent fluxes. When the large-scale forcing was strong, the wind in the valley aligned with the glacier flow, intense downslope flows were observed, no wind-speed maximum was visible below 5 m, TKE and turbulent fluxes were often intense. For both regimes, the surface layer turbulence production was probably not at equilibrium with dissipation because of the interaction of large-scale orographic disturbances with the flow when the forcing was strong, or low-frequency oscillations of the katabatic flow when the forcing was weak. When TKE was low, all turbulent fluxes calculation methods provided similar fluxes. When TKE was large, the EC method provided larger fluxes than the BA method. This underestimation was compensated by increasing the BA flux estimates using melt-calibrated effective roughness lengths. Though strong forcing was more frequently associated with large TKE events than weak forcing conditions, differences between the different SEB estimates remained in both cases within the error range of observed melt.
Litt, M., Sicart, J.-E., Six, D., Wagnon, P., and Helgason, W. D.: Surface-layer turbulence, energy-balance and links to atmospheric circulations over a mountain glacier in the French Alps, The Cryosphere Discuss., doi:10.5194/tc-2016-93, in review, 2016.