Measurement of the mountain drag on South Georgia

The island of South Georgia is situated at 54 deg S, 37 deg W. It is entirely mountainous; the range is about 120 km from west to east and about 25 km from north to south. Typical mountain peaks are about 3000 m in altitude. South Georgia is well-known for spectacular mountain wave clouds (Ac,len). Conditions strongly favour the generation of gravity waves. Strong zonal winds are prevalent at these latitudes and in addition, although South Georgia is to the south of the Antarctic Convergence (and therefore the SST is low), air often arrives from north of the Convergence, leading to an extremely strong temperature inversion.

During 1998, four pairs of automatic weather stations with microbarographs were deployed along a transect across one of the highest parts of the mountain range. These enabled crude estimates of the mountain drag to be made. Several extraordinary downslope windstorm events were observed. During the most extreme, during a two hour period, each 10 minute average 5 m wind speed in the lee of the mountains was above 50 m/s whilst on the upwind side the wind speed was less than 10 m/s. This event was associated with a dynamically induced pressure difference of 10 hPa over a distance of 20 km across the range.

Estimates of the drag will be given and possible explanations for the frequency of severe downslope winds on South Georgia will be discussed.