Exploring the importance of horizontal transport terms in a katabatic flow over a glacier

Common turbulence parametrization in numerical weather prediction models and traditional boundary layer theory are predominantly designed for horizontally homogeneous flat terrain and only consider vertical transport processes. However, these assumptions fail in valleys, where the horizontal constrictions to the flow as well as prevalent surface heterogeneity mean that horizontal terms in the budget equations (e.g. advection, horizontal flux divergence) become important. Over a mountain glacier, in addition, the acceleration of the katabatic wind downslope, a decrease in wind speed from the centerline towards the margin due to lateral variation in the forcing (glacier ice vs. rocky sides), and horizontal temperature gradients necessitate consideration of horizontal terms in the budgets of mean and turbulent quantities.

Here we investigate the importance of horizontal term in the budgets of momentum, heat, TKE and sensible heat flux, for deep katabatic flows over the Hintereisferner glacier in Austria. The analysis is based on data collected during the three-week Hintereisferner Experiment (HEFEX) field campaign that took place in the summer of 2018, where four turbulence towers were installed in an along- and across-glacier transect, allowing the estimation of horizontal terms in the down-glacier and cross-glacier direction. Towers were equipped with two levels of turbulence sensors, and one level of mean wind and temperature sensors. The focus of the study is on deep flows where both turbulence observational heights were below the potential jet maximum height, so that all the estimated budget terms are located within the same layer.

The results indicate that, for certain selected periods with deep flow, horizontal terms have an important contribution to the budget equations. The largest contribution comes from the horizontal advection terms, and they are shown to enhance TKE destruction by buoyancy and TKE production by advection and shear over Hintereisferner. These results highlight the importance of considering horizontal processes to correctly capture the flow dynamics in complex terrain.