J2.2
Turbulence measurements with light weight cup anemometers and a modified inertial dissipation method
Anna Sjöblom, Bjerknes Centre for Climate Research, Bergen, Norway; and C. Nilsson and H. Bergström
By using light weight cup anemometers and a modified inertial dissipation method, an estimation of turbulence parameters, such as the friction velocity, can be made in the marine atmospheric surface layer. This is a way to increase the number of turbulence measurements over sea, since cup anemometers are much cheaper than ordinary turbulence measurements and therefore common as a meteorological instrument.
The method was developed using measurements from Östergarnsholm, situated east of Gotland, Sweden, in the middle of the Baltic Sea. This air-sea interaction station represents open sea conditions with turbulence instruments at three levels and slow response “profile” measurements of wind (light weight cup anemometers) and temperature at five levels. In addition, wave measurements are made.
The “classical” inertial dissipation method has been tested on the cup anemometers and compared to eddy-correlation measurements from the turbulence instruments. The conclusion was that corrections have to be made to account for problems with the cup anemometers, the method used, and also for a possible imbalance between local production and dissipation in the turbulent kinetic energy budget. Therefore, an “effective” Kolmogorovs constant as a function of stability and wind speed has been determined. Applying these new constants in the calculations improves the agreement between the two methods substantially.
The modified method was then applied to measurements from an off-shore wind farm called Bockstigen, south-west of the island Gotland, with an over water fetch of more than 70 km. In addition to five wind turbines, a 40 m meteorological tower is situated in approximately 6 m water depth. The measurements consist of profile measurements of wind and temperature at four heights and also wave measurements outside Bockstigen.
The modified inertial dissipation method works best at approximately 10 m measuring height and at near neutral conditions. Although the measurements are in relatively shallow water, with disturbed wave fields, a wave influence can be seen on the turbulence structure, which differs from its land equivalent. This has important implications for example off-shore wind power turbines, which are often placed in relatively shallow water.
Joint Session 2, Turbulent transport phenomena across the air-sea interface and surface layer gas fluxes. (Joint between the 15th Symposium on Boundary Layers and Turbulence and the 13th Symposium on the Interaction of the Sea and Atmosphere)
Wednesday, 11 August 2004, 8:30 AM-11:45 AM, New Hampshire-Vermont Room
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