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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'The model chain: First steps towards tomorrow's technology' taking place on Thursday, 13 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Andrea N. Hahmann Technical University of Denmark, Denmark
Co-authors:
Claire Louise Vincent (1) F P Andrea N Hahmann (1) Jake Badger (1)
(1) Technical University of Denmark, Roskilde, Denmark

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Abstract

Mapping mesoscale wind variability over the North Sea region

Introduction

As the number of offshore wind farms in the North Sea region increases, the potential impact of persistent wind speed fluctuations on their optimal operation becomes more critical. In fact, the frequency and intensity of wind fluctuations could become a new siting criterion, together with existing criteria such as wind resource and proximity to grid connection points. We describe the use of mesoscale modelling and observations to create maps of mesoscale wind variability over the North Sea region. Data from the mesoscale part of the model-chain can thus be used to add new information to existing wind resource maps.

Approach

Mesoscale variability on time scales of 20 minutes to several hours is studied using mesoscale modelling with a horizontal grid spacing of 2 km. Wind speeds from the mesoscale model are stored every 10 minutes, allowing for analysis of temporal spectra at every grid point.

Maps of temporal variability for different time scales are created by integrating spectra over the time scale of interest. We present maps indicating the distribution and magnitude of temporal variability over a range of time scales. Results are verified, where possible, using equivalent 10 minute observations from tall meteorological masts.

Main body of abstract

Maps of mesoscale variability for different time scales indicate that the degree of variability is by no means spatially homogeneous, even over a large area of water such as the North Sea. For example, parts of the North Sea that are most vulnerable to the development of convection cells arising from undisturbed flow from the North Atlantic ocean have high average levels of hour-scale wind fluctuations. Conversely, parts of the North Sea that are sheltered by the surrounding land have low levels of mesoscale variability. Results also indicate that mesoscale variability is higher over the sea than the land, due largely to the existence of particular mesoscale phenomena over the sea.

Developing spatial maps of temporal variability requires data at both high temporal and spatial resolutions. Mesoscale modelling is an ideal tool to address this need. However, hour-scale variability is influenced by processes such as convection and boundary layer rolls that may not be fully resolved in a mesoscale model, so thorough verification of the results using observations from tall meterological masts in the North Sea region is essential.

Although the variability in the mesoscale model is generally suppressed compared with that in the observations, the spatial patterns in mesoscale variability appear to be consistent. While satellite data is not generally available at high temporal resolution, it can give a useful guide to spatial occurrence of the phenomena leading to hour-scale variability. Where possible, satellite data is therefore also used to validate the results from the mesocale modelling.

Conclusion

The results presented here show that there is large variation in the amount of mesoscale variability across the North Sea, and between the sea and the neighbouring land. In regions where the magnitude of hour-scale power fluctuations is a relevant design criteria for large offshore wind farms, this information can be an important consideration together with the resource assessment for wind farm siting.


Learning objectives
The importance of considering hour-scale wind variability for off-shore wind farm siting is discussed, and the use of mesoscale modelling for resolving variance on particular time scales is explained. The degree of spatial variation in hour-scale variability across the North Sea region is demonstrated and discussed.