Wind height reduction at fixed sea stations

Fixed sea stations fitted with cup anemometers, wind vanes or anemographs, should report the mean reading over a ten-minute period or, if the wind changes markedly in the ten-minute period, an average over the period after the change, at a level of 10 metres above MSL. However, the synoptic stations on the North Sea, for instance at oil platforms, do measure wind velocity and -direction at much higher levels: at AUK 103.3 metres above MSL, at K13: 73.8 metres above MSL, and so on. Especially for the values of wind velocity a procedure or an algorithm is therefore required to reduce these values to 10 metre level, before insertion into the synop. Present reduction factors for the wind velocity of the North Sea stations were determined in 1977 by the North Sea Meteorological Panel. In this panel all the involved countries were represented: Norway, Sweden, Denmark, Germany, Netherlands, Belgium, France, United Kingdom, Ireland. The factors are well correlated with the Power Law formula, recommended by the WMO (ref.3):

Model analyses, synchronic observations at ships and theoretical arguments from Wieringa (ref.2) suggested that the used factors caused an over reduction of the wind velocity. A study has therefore been carried out to derive more realistic reduction factors for the North Sea stations, the results of which are presented in a technical report (ref.20) which can be obtained at the KNMI . Finally the reduction factor for the average wind was concluded as :

If a fixed sea station has indicated (see: Maintenance -> Station data -> fixed sea station) that the anemometer reading does NOT indicate wind speed at 10 metres above MSL, the above stated formula will be applied by the program.