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

Profiles of wind and temperature have been observed above the Greenland ice sheet, 90 km from its western margin, in July 1991. The terrain slopes downward to the west. Measurements were performed with instruments on a 30 m mast, combined with a Doppler SODAR and a RASS. Whereas the surface is usually at the melting point, the temperatures in the free atmosphere are above freezing. The depth of the boundary layer, in which the wind turns to the free atmosphere direction, is not much more than 100 m. The surface wind is always about from the southeast (hence with a downslope component), whereas winds from the southwest (with an upslope component) often occur at the 100 m level. Mixing length profiles for momentum were estimated by comparison of calculated and observed wind profiles. A good accordance between calculated and observed wind speed was obtained. The neutral mixing length had a maximum of only a few metres, which was approached already at low height. The limiting value is proportional to the 0.7-th power of the Froude number times a length scale obtained from the temperature profile.