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Wind statistics from a forested landscape
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
2015 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, no 1, 53-71 p.Article in journal (Refereed) Published
Abstract [en]

An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40∘ wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.

Place, publisher, year, edition, pages
2015. Vol. 156, no 1, 53-71 p.
Keyword [en]
Above canopy turbulence statistics Atmospheric boundary layers, Decreasing roughness, Forest Canopy Flows, Wind power
National Category
Meteorology and Atmospheric Sciences
URN: urn:nbn:se:uu:diva-237762DOI: 10.1007/s10546-015-0016-xISI: 000355153200004OAI: oai:DiVA.org:uu-237762DiVA: diva2:768813
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2015-06-29Bibliographically approved
In thesis
1. Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
Open this publication in new window or tab >>Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine.

This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile.

Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower.

The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space.

Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 47 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1212
Wind power, Forest, Turbulence, Waves, Spectra, Wind model, Atmospheric stability, Vindkraft, Skog, Turbulens, Vågor, Spektra, Modell, Atmosfärisk stabilitet
National Category
Meteorology and Atmospheric Sciences
urn:nbn:se:uu:diva-237764 (URN)978-91-554-9123-9 (ISBN)
Public defence
2015-02-06, Ekmansalen, EBC, Norbyvägen 14, Uppsala, 13:00 (English)
Vindforsk III, Wind power in forestsVindforsk IV, Forest wind
Swedish Energy Agency
Available from: 2015-01-16 Created: 2014-12-04 Last updated: 2015-03-09

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