Katabatic wind profiles over the Greenland ice sheet: observation and modelling

Profiles of wind and temperature have been observedabove the Greenland ice sheet, 90 km from its westernmargin, in July 1991. The terrain slopes downward tothe west. Measurements were performed with instrumentson a 30 m mast, combined with a Doppler SODAR and aRASS. Whereas the surface is usually at the meltingpoint, the temperatures in the free atmosphere areabove freezing. The depth of the boundary layer, in whichthe wind turns to the free atmosphere direction, is notmuch more than 100 m. The surface wind is always aboutfrom the southeast (hence with a downslope component),whereas winds from the southwest (with an upslopecomponent) often occur at the 100 m level.Mixing length profiles for momentum were estimatedby comparison of calculated and observed windprofiles. A good accordance between calculated andobserved wind speed was obtained. The neutralmixing length had a maximum of only a few metres, whichwas approached already at low height. The limiting valueis proportional to the 0.7-th power of the Froudenumber times a length scale obtained from thetemperature profile.