Abstract
AbstractWe present the first direct comparison of turbulence conditions measured simultaneously over exposed ice and a 0.08 m thick supraglacial debris cover on Suldenferner, a small glacier in the Italian Alps. Surface roughness, sensible heat fluxes (~20–50 W m−2), latent heat fluxes (~2–10 W m−2), topology and scale of turbulence are similar over both glacier surface types during katabatic and synoptically disturbed conditions. Exceptions are sunny days when buoyant convection becomes significant over debris-covered ice (sensible heat flux ~ −100 W m−2; latent heat flux ~ −30 W m−2) and prevailing katabatic conditions are rapidly broken down even over this thin debris cover. The similarity in turbulent properties implies that both surface types can be treated the same in terms of boundary layer similarity theory. The differences in turbulence between the two surface types on this glacier are dominated by the radiative and thermal contrasts, thus during sunny days debris cover alters both the local surface turbulent energy fluxes and the glacier component of valley circulation. These variations under different flow conditions should be accounted for when distributing temperature fields for modeling applications over partially debris-covered glaciers.
Highlights
Mass loss at the surface of a glacier is governed by the surface energy balance between the atmosphere and the glacier (Cuffey and Paterson, 2010)
2011), though we do not find unequivocal evidence of this in our data. This dataset contributes to the small population of studies with direct measurements of turbulent fluxes over glaciers, and for the first time attempts a simultaneous comparison of fluxes over clean and debris-covered ice at a single glacier
Given the paucity of turbulence data collected over debris-covered glacier surfaces, even the short duration of the measurements analyzed here provide valuable insights for understanding processes of glacier–atmospheric energy exchange
Summary
Mass loss at the surface of a glacier is governed by the surface energy balance between the atmosphere and the glacier (Cuffey and Paterson, 2010). The inclusion of turbulent energy fluxes in glacier surface energy-balance models usually relies on bulk approaches that derive exchange coefficients for potential temperature and specific humidity in the boundary layer (e.g. Braithwaite and others, 1998; MacDougall and Flowers, 2011; Nicholson and others, 2013) The theory underpinning such approaches was developed for neutrally stratified, horizontally homogeneous flat terrain with constant fluxes with height (Lettau, 1934; Prandtl, 1934), while the cold, sloping surfaces of mountain glaciers within steep mountain topography do not conform to these conditions (Denby and Greuell, 2000; Radic and others, 2017). Effects of low frequency oscillations or coherent turbulent structures associated with katabatic winds or mesoscale flows respectively, are not captured in turbulent fluxes over glaciers calculated using bulk methods (e.g. Smeets and others, 1998; Litt and others, 2014) This poor performance of traditionally-used methods of calculating turbulent fluxes is of increasing concern given that the relative importance of turbulent fluxes to glacier ablation is expected to increase with projected climate warming of glaciated regions. We investigate the nature of the turbulence and turbulent fluxes under different wind regimes, in the context of the glacier katabatic wind system, with the overall goal of providing valuable information for improving representations of turbulent fluxes over complex glacier surfaces
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