Calculation of Wind Speed Variation with Height over the Sea

Abstract

Two one-dimensional wind models have been tested for their ability to describe the wind profile at sea up to a height of 125 m. The first model is based on Monin-Obukhov similarity theory with due consideration of effects of surface waves. The second model is a combination of Monin-Obukhov similarity for a surface layer and an Ekman layer above, with requirements for continuity of the wind components and their vertical derivatives at the top of the surface layer. Model predictions are compared with data from an instrumented tower up to 29 m and from a limited set of pilot balloon data up to greater heights. The first of these two models gives good predictions of the wind up to 29 m but not acceptable results for 75 m and 125 m. The second model gives quite good results for 29 m, 75 m and 125 m. Required input data for this latter model are measured wind at one height (typically 10 m) and measured temperature difference between two levels (e.g. 2 and 10 m). The test has been restricted to cases with long (>150 km) undisturbed over-water fetch. For wind energy assessment at other sites, the combined MO and Ekman layer model is recommended for sectors with such characteristics, but dedicated models must be used to deal with limited-fetch effects for other sectors.