Glacier winds and parameterisation of the related surface heat fluxes

The katabatic flow over glaciers is studied with data from automatic weather stations (AWS). Weanalyse data from the Morteratschgletscher (Switzerland), Vatnajökull (Iceland) and West Greenland, and conclude that katabatic flow is very common over melting glacier surfaces and rarely disrupted by the large-scale flow. Over small and medium-size glaciers the height of the wind maximum is generally low (typically 10 m), and vertical temperature differences near the surface are very large(up to 15 K over 4 m). In glacier mass-balance models there is a great need for parameterisations of the surface heat flux. We develop a simple method to estimate the sensible heat flux F_h associated with the glacier wind. It is based on the classical Prandtl model for slope flows. We set the turbulent exchange coefficient proportional to the maximum wind speed (velocity scale) and the height of the wind maximum (length scale). The resulting theory shows that F_h increases quadratically with the temperature difference between the surface and the ambient atmosphere; F_h decreases with the square rootof the potential temperature gradient of the ambient atmosphere; and F_h is independent of the surfaceslope.