Anisotropy scaling of a sloping glacier boundary layer

Glacier boundary layers present an ideal atmospheric laboratory for studying persistently stable boundary-layer dynamics over inclined surfaces. On glaciers in summer, turbulence is strongly controlled by the katabatic flow dynamics that is intimately coupled with very stable stratification at the glacier surface and the slope angle. In these kind of conditions, the basic assumptions of Monin–Obukhov Similarity Theory (MOST) are rarely met, due to the significant flux divergence, and the imposition of an alternative limiting scale. Still, bulk approaches based on MOST have shown good agreement under very stringent conditions, while alternative scaling approaches that add the slope angle into the scaling parameter, or use jet maximum height have shown promise in providing scaling frameworks for such flows.

Here we use a dense network of atmospheric turbulence observations during the HEFEX II campaign, that took place on the Hintereisferner Glacier, Austria in 2023. The campaign features ten turbulence towers with multi-level observations, distributed across the entire glacier surface (from the accumulation area to downstream of the glacier tongue) and therefore experiencing different flow conditions (katabatic flow depth) or slope angles. We focus on the mathematical invariant representing turbulence anisotropy that has recently been used to extend MOST to more realistic terrain conditions. Focusing on the flux-variance relations we show that katabatic flows over glaciated terrain display distinct turbulence characteristics at varying degrees of anisotropy that differ considerably to the previous studies over non-glaciated terrain. These peculiarities are further examined to isolate the difference between katabatic and canonical flows in terms of their flow anisotropy. We also test alternative scaling approaches, including those based on the katabatic jet height, local terrain slope, and formulations designed to avoid the self-correlation that is shown to be an issue in very stable stratification.