Profiles of katabatic flow in summer and winter over Coats Land, Antarctica

Observations from a novel autonomous Doppler sodar wind profiling system are described and analysed. These include the first continuous wintertime soundings of katabatic winds over Antarctica—a continent with which they are synonymous. During 2002 and 2003 over 2600 wind profiles were taken during ‘case-studies’ of high-resolution sounding lasting hours to days. These case-studies have been subjectively classified as: synoptically driven, katabatically influenced (28 days); primarily katabatically driven flows (a subset of 16 days); or other flow types. The Doppler sodar observations were augmented by automatic weather station observations at the field site and further up the slope, as well as synoptic and upper-air observations at Halley Research Station, some 50 km distant on the Brunt Ice Shelf.

In primarily katabatic flows there is a systematic change in the shape and depth of the low-level katabatic jet with wind speed. Relatively strong katabatic flows (maximum winds of typically 8–10 m s−1) have a jet maximum between 20 and 60 m above the surface and are relatively deep (up to 200 m); while moderate katabatic flows (4–8 m s−1) typically have a jet maximum between 3 and 30 m and are shallower (∼100 m), although they can also be more diffuse in structure with a wind speed maximum at higher altitude. In all katabatic flows there is backing of wind direction with height, consistent with decreasing friction away from the surface. During summertime katabatic flows there is a clear diurnal signature at all heights, although this is less pronounced in the surface layer where there seems to be a persistent 2–4 m s−1 katabatic flow during all case-studies. Where the diurnal forcing results in an abrupt katabatic flow deceleration, i.e. what may be a katabatic ‘jump’, there is a concurrent vertical acceleration. Wind profiles from a recent numerical weather prediction study of idealized katabatic flows at this site compare favourably with selected mean profiles; the only significant difference is that the model's wind speed is too low over the lowest ∼10 m.