Abstract
Abstract. Two-way feedback occurs between offshore wind and waves. However, the influence of the waves on the wind profile remains understudied, in particular the momentum transfer between the sea surface and the atmosphere. Previous studies showed that for swell waves it is possible to have increasing wind speeds in case of aligned wind–wave directions. However, the opposite is valid for opposed wind–wave directions, where a decrease in wind velocity is observed. Up to now, this behavior has not been included in most numerical models due to the lack of an appropriate parameterization of the resulting effective roughness length. Using an extensive data set of offshore measurements in the North Sea and the Atlantic Ocean, we show that the wave roughness length affecting the wind is indeed dependent on the alignment between the wind and wave directions. Moreover, we propose a new roughness length parameterization, taking into account the dependence on alignment, consisting of an enhanced roughness length for increasing misalignment. Using this new roughness length parameterization in numerical models might facilitate a better representation of offshore wind, which is relevant to many applications including offshore wind energy and climate modeling.
Highlights
During the past years there has been increased interest in wind turbines
In this study we combined two large data sets to investigate the influence of thealignment between wind and wave directions on the momentum transfer between the sea surface and the atmosphere
We identified a clear difference in roughness length between aligned and opposed wind and wave directions
Summary
During the past years there has been increased interest in wind turbines. Wind energy has been proposed as an ideal alternative for nonclean energy sources, and a good candidate to meet the rising energy demands. By 2020, 20 % of the total energy should be renewable in order to meet the renewable energy directive (directive 2009/28/EC). As such the estimated installed wind energy capacity will be 40 GW (EWEA, 2011). Due to the high cost of offshore wind turbines, it is important to have accurate information about the vertical structure of the wind profile at offshore wind farm locations. In order to model offshore wind profiles accurately, the wind–wave interaction should be better understood. This physical understanding will result in more physical relationships that can be included in coupled atmosphere–wave models. It is important to have offshore measurement data available to improve our understanding of the wind–wave interaction
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