1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
An extreme wind analysis of wind speed calculated in the NCEP/NCAR reanalysis is done for grid points over and near Denmark. Winds at 10 m, 850 hPa, and geostrophic winds at 850 hPa, 1000 hPa, and at the sea level are analyzed. At 10 m height the expectedextreme wind with a return period of 50 years at the North Sea west of Denmark is 27 ms-1. It is approximately 11 % less than estimates from observations. However, values at grid points over land in Denmark cannot be compared with observations because theroughness length of these land surfaces is far to big in the model. A transformation to a common roughness length of 5 cm using the geostrophic drag law yields too high values. At points in northern Germany, where the surface roughness of the model isless, the transformed 50-years wind speed is 22-23 ms-1, which agrees well with estimates obtained from measurements. The analyses of the wind at 850 hPa and the geostrophic wind at 850 hPa or 1000 hPa yield very similar extreme winds of approximately 42ms-1. The geostrophic wind calculated from the surface pressure is approximately 45 ms-1 in central Denmark. The geostrophic winds at 1000 hPa are slightly stronger than at 850 hPa, which are somewhat greater than the actual wind at 850 hPa.Transformations to a wind at 10 m over a surface with roughness 5 cm with the help of the drag law yield extreme winds, which are approximately 10-12 % less than from surface measurements. The 850 hPa winds and the geostrophic wind calculated from thesurface pressure indicate a weak decrease from west to east, whereas the geostrophic wind data at constant pressure levels show almost constant extreme winds across Denmark. All upper-air and and geostrophic wind data show higher extreme winds in northernGermany than in Denmark. Further investigations are necessary to find out if the underestimation of the extreme wind by approximately 10-12 % is valid in most mid-latitudes.
Vindenergi og atmosfæriske processer; Risø-R-1238; Risø-r-1238(EN)