Impact of a large-scale offshore wind farm on meteorology

Author:  Rooijmans, Pim

Abstract

In this thesis the meteorological effects of a large-scale (9000 km²) offshore wind farm in the North Sea were simulated using the MM5 mesoscale model. The wind farm was simulated by introducing a higher roughness length (0.5 m) in the area of the wind farm. The meteorological effects were examined by comparing model runs with and without wind farm. Turbulent kinetic energy, cloud formation, precipitation and wind speed reduction were studied. Two case studies with westerly flows were performed. The first case study begins at 00 UTC July 1st 2001 and ends at 18 UTC July 3rd 2001. The second is from 00 UTC October 2nd 1999 to 18 UTC October 4th 1999.

The model was evaluated by comparing observational measurements with the model output. This was done at two places near the Dutch coast, de Kooij and Schiphol. Observational values of wind speed, wind direction and temperature were compared using two boundary layer schemes (ETA and MRF). Then these standard runs were compared with two other runs that included the wind farm.

We compared the wind reduction results of the mesoscale model with a conceptual model. The conceptual model is based on a model by Emeis and Frandsen (1993), where the reduction of horizontal wind speed is computed from a balance between a loss of horizontal momentum and replenishment from above by turbulent fluxes. As regards wind reduction calculations the MM5 model yields comparable results as obtained by the Emeis and Frandsen model.

In case 1, where no clouds were present, the model simulated enhanced cloud formation above the wind farm. The second case, which was partly cloudy, showed no significantly cloudiness increase. The rainfall however did show dramatic changes. The Dutch coast received less but large parts of the North Sea more precipitation.

Master Thesis
Pim Rooijmans
Utrecht University
September 2003 – January 2004

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