Turbulence characteristics of the stable boundary layer over a mid-latitude glacier. Part II: pure katabatic forcing conditions.

Observations obtained over a glacier surface in a predominantly katabatic flow and with a distinct wind maximum below 13-m height are presented. The data were collected using a 13-m high profile mast and two sonic anemometers (at about 2.5-m and 10-m heights). The spectra at frequencies below that of the turbulence range appear to deviate considerably from the curves obtained by Kaimal and co-workers during the 1968 Kansas experiment. The characteristics of these deviations are compared to the observations of others in surface-layers disturbed by any kind of large-scale outer-layer (or inactive) turbulence. In our case the disturbances are likely to be induced by the high mountain ridges that surround the glacier. Moreover, the deviations observed in the cospectra seem to result from an, as yet, unspecified interaction between the inactive outer-layer turbulence and the local surface-layer turbulence. Near the distinct wind maximum turbulence production ceased while turbulence itself did not, probably the result of turbulence transport from other levels. Consequently, we studied the local similarity relations using σ(w) instead of u(*) as an alternative velocity scale. Well below the wind maximum, and for relatively low stability (0 < Ri(g) < 0.2), the flow behaves according to well established local-scaling similarity relationships in the stable boundary layer. For higher stability (Ri(g) > 0.2), and near or above the wind maximum, the boundary-layer structure conforms to that of z-less stratification suggesting that the eddy size is restricted by the local stability of the flow. In line with this we observed that the sensible heat fluxes relate remarkably well to the local flow parameters.