Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test S... more Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50–80 m. At higher levels deviations from
The method traditionally used for measuring the wind speed and direction for wind energy purposes... more The method traditionally used for measuring the wind speed and direction for wind energy purposes is to record the output from cup or propeller anemometers and vanes at several heights on a mast. As turbines have grown in height, this method has become increasingly more expensive and difficult, and new methods have been sought. One alternative method for measurement of wind speed and direction over depths of around 200m is the SODAR (SOund Detection And Ranging). This instrument is installed on the ground and every few seconds transmits a short pulse of sound upward into the atmosphere. As the sound propagates upward, some acoustic energy is continuously reflected back to the ground by the variable atmospheric turbulence encountered. By analysing the Doppler frequency shift of echoes received from sound transmitted at a small angle to the vertical, wind speed components are estimated as a function of height. SODAR technology is well established as a tool for visualising and quantify...
The need for alternative means to measure the wind speed for wind energy purposes has increased w... more The need for alternative means to measure the wind speed for wind energy purposes has increased with the increase of the size of wind turbines. The cost and the technical difficulties for performing wind speed measurements has also increased with the size of the wind turbines, since it is demanded that the wind speed has to be measured at the rotor center of the turbine and the size of both the rotor and the hub height have grown following the increase in the size of the wind turbines. The SODAR (SOund Detection And Ranging) is an alternative to the use of cup anemometers and offers the possibility of measuring both the wind speed distribution with height and the wind direction. At the same time the SODAR presents a number of serious drawbacks such as the low number of measurements per time period, the dependence of the ability to measure on the atmospheric conditions and the difficulty of measuring at higher wind speeds due to either background noise or the neutral condition of the...
Concentration fluctuation data from surface-layer released smoke plumes have been investigated wi... more Concentration fluctuation data from surface-layer released smoke plumes have been investigated with the purpose of finding suitable scaling parameters for the corresponding two-particle, relative diffusion process. Dispersion properties have been measured at downwind ranges between 0.1 and 1 km from a continuous, neutrally buoyant ground level source. A combination of SF 6 and chemical smoke (aerosols) was used as tracer. Instantaneous crosswind concentration profiles of high temporal (up to 55 Hz) and spatial resolution (down to 0.375 m) were obtained from aerosolbackscatter Lidar detection in combination with simultaneous gas chromatograph (SF 6) reference measurements. The database includes detailed crosswind concentration fluctuation measurements. Each experiment, typically of 1 2-hour duration, contains plume mean and variance concentration profiles, intermittency profiles and exceedence and duration statistics. The diffusion experiments were accompanied by detailed in-situ micrometeorological mean and turbulence measurements. In this paper, a new distance-neighbour function for surface-released smoke plumes is proposed, accompanied by experimental evidence in its support. The new distance-neighbour function is found to scale with the surface-layer friction velocity, and not with the inertial subrange dissipation rate, over the range of distance-neighbour separations considered.
The structures of mean flow and turbulence in the atmospheric surface boundary layer have been ex... more The structures of mean flow and turbulence in the atmospheric surface boundary layer have been extensively studied on Earth, and to a far less extent on Mars, where only the Viking missions and the Pathfinder mission have delivered in-situ data. Largely the behaviour of surfacelayer turbulence and mean flow on Mars is found to obey the same scaling laws as on Earth. The largest micrometeorological differences between the two atmospheres are associated with the low air density of the Martian atmosphere. Together with the virtual absence of water vapour, it reduces the importance of the atmospheric heat flux in the surface energy budget. This increases the temperature variation of the surface forcing the near-surface temperature gradient and thereby the diabatic heat flux to higher values than are typical on the Earth, resulting in turn in a deeper daytime boundary layer. As wind speed is much like that of the Earth, this larger diabatic heat flux is carried mostly by larger maximal values of T * , the surface scale temperature. The higher kinematic viscosity yields a Kolmogorov scale of the order of ten times larger than on Earth, influencing the transition between rough and smooth flow for the same surface features. The scaling laws have been validated analysing the Martian surface-layer data for the relations between the power spectra of wind and temperature turbulence and the corresponding mean values of wind speed and temperature. Usual spectral formulations were used based on the scaling laws ruling the Earth atmospheric surface layer, whereby the Earth's atmosphere is used as a standard for the Martian atmosphere.
This study examines the dependence of the surface drag coefficient on stability, wind speed, meso... more This study examines the dependence of the surface drag coefficient on stability, wind speed, mesoscale modulation of the turbulent flux and method of calculation of the drag coefficient. Data sets over grassland, sparse grass, heather and two forest sites are analyzed. For significantly unstable conditions, the drag coefficient does not depend systematically on z/L but decreases with wind speed for fixed intervals of z/L, where L is the Obukhov length. Even though the drag coefficient for weak wind conditions is sensitive to the exact method of calculation and choice of averaging time, the decrease of the drag coefficient with wind speed occurs for all of the calculation methods. A classification of flux calculation methods is constructed, which unifies the most common previous approaches. The roughness length corresponding to the usual Monin-Obukhov stability functions decreases with increasing wind speed. This dependence on wind speed cannot be eliminated by adjusting the stability functions. If physical, the decrease of the roughness length with increasing wind speed might be due to the decreasing role of viscous effects and streamlining of the vegetation, although these effects cannot be isolated from existing atmospheric data. For weak winds, both the mean flow and the stress vector often meander significantly in response to mesoscale motions. The relationship between meandering of the stress and wind vectors is examined. For weak winds, the drag coefficient can be sensitive to the method of calculation, partly due to meandering of the stress vector.
The paper reviews the status of modelling offshore wind resources and wind conditions using the W... more The paper reviews the status of modelling offshore wind resources and wind conditions using the WAsP and WAsP Engineering models-experiences gained over the last decade, current engineering practices and challenges for the future. The paper aims at answering the basic question: how well can we estimate site wind conditions, described in the IEC 61400-1 and other standards, for offshore wind farm sites? Whether modelling the wind in the coastal zone or further offshore, the physical modelling should reliably describe the wind speed profiles over water, including the influence of the surface roughness length and atmospheric stability. The modelling outputs required and analysed here are: mean wind speed (V ave) and Weibull parameters (A, k), reference wind speed (V ref) and characteristic turbulence intensity (I 15). WAsP and WAsP Engineering modelling results are compared to measurements at offshore met. masts, as well as onshore data sets and other modelling results. The available meteorological input data may be observations from ships or lighthouses, measurements from offshore masts, wind atlas data sets, NCEP/NCAR reanalysis data, and data from meso-scale models (e.g. KAMM). The topographical input data discussed are: simple land/sea maps, WAsP roughness rose descriptions, Coastline Extractor coastline maps and hand-digitised coastline / roughness maps. The importance of roughness classification and map diameter is also discussed. Current engineering practices with respect to vertical datum, water roughness length, wake decay constant, WAsP parameters, wind indices etc. are summarised and discussed. The challenges offshore are mostly related to the wind field and wind conditions at heights between 50 and 200 m a.s.l.-partly outside the operational envelope of the WAsP programs. In addition, reliable and representative data sets are scarce at these heights. Another challenge is related to the modelling of wake effects in large offshore farms. Possible extensions of today's engineering procedures and practices are outlined.
Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test S... more Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50–80 m. At higher levels deviations from
The method traditionally used for measuring the wind speed and direction for wind energy purposes... more The method traditionally used for measuring the wind speed and direction for wind energy purposes is to record the output from cup or propeller anemometers and vanes at several heights on a mast. As turbines have grown in height, this method has become increasingly more expensive and difficult, and new methods have been sought. One alternative method for measurement of wind speed and direction over depths of around 200m is the SODAR (SOund Detection And Ranging). This instrument is installed on the ground and every few seconds transmits a short pulse of sound upward into the atmosphere. As the sound propagates upward, some acoustic energy is continuously reflected back to the ground by the variable atmospheric turbulence encountered. By analysing the Doppler frequency shift of echoes received from sound transmitted at a small angle to the vertical, wind speed components are estimated as a function of height. SODAR technology is well established as a tool for visualising and quantify...
The need for alternative means to measure the wind speed for wind energy purposes has increased w... more The need for alternative means to measure the wind speed for wind energy purposes has increased with the increase of the size of wind turbines. The cost and the technical difficulties for performing wind speed measurements has also increased with the size of the wind turbines, since it is demanded that the wind speed has to be measured at the rotor center of the turbine and the size of both the rotor and the hub height have grown following the increase in the size of the wind turbines. The SODAR (SOund Detection And Ranging) is an alternative to the use of cup anemometers and offers the possibility of measuring both the wind speed distribution with height and the wind direction. At the same time the SODAR presents a number of serious drawbacks such as the low number of measurements per time period, the dependence of the ability to measure on the atmospheric conditions and the difficulty of measuring at higher wind speeds due to either background noise or the neutral condition of the...
Concentration fluctuation data from surface-layer released smoke plumes have been investigated wi... more Concentration fluctuation data from surface-layer released smoke plumes have been investigated with the purpose of finding suitable scaling parameters for the corresponding two-particle, relative diffusion process. Dispersion properties have been measured at downwind ranges between 0.1 and 1 km from a continuous, neutrally buoyant ground level source. A combination of SF 6 and chemical smoke (aerosols) was used as tracer. Instantaneous crosswind concentration profiles of high temporal (up to 55 Hz) and spatial resolution (down to 0.375 m) were obtained from aerosolbackscatter Lidar detection in combination with simultaneous gas chromatograph (SF 6) reference measurements. The database includes detailed crosswind concentration fluctuation measurements. Each experiment, typically of 1 2-hour duration, contains plume mean and variance concentration profiles, intermittency profiles and exceedence and duration statistics. The diffusion experiments were accompanied by detailed in-situ micrometeorological mean and turbulence measurements. In this paper, a new distance-neighbour function for surface-released smoke plumes is proposed, accompanied by experimental evidence in its support. The new distance-neighbour function is found to scale with the surface-layer friction velocity, and not with the inertial subrange dissipation rate, over the range of distance-neighbour separations considered.
The structures of mean flow and turbulence in the atmospheric surface boundary layer have been ex... more The structures of mean flow and turbulence in the atmospheric surface boundary layer have been extensively studied on Earth, and to a far less extent on Mars, where only the Viking missions and the Pathfinder mission have delivered in-situ data. Largely the behaviour of surfacelayer turbulence and mean flow on Mars is found to obey the same scaling laws as on Earth. The largest micrometeorological differences between the two atmospheres are associated with the low air density of the Martian atmosphere. Together with the virtual absence of water vapour, it reduces the importance of the atmospheric heat flux in the surface energy budget. This increases the temperature variation of the surface forcing the near-surface temperature gradient and thereby the diabatic heat flux to higher values than are typical on the Earth, resulting in turn in a deeper daytime boundary layer. As wind speed is much like that of the Earth, this larger diabatic heat flux is carried mostly by larger maximal values of T * , the surface scale temperature. The higher kinematic viscosity yields a Kolmogorov scale of the order of ten times larger than on Earth, influencing the transition between rough and smooth flow for the same surface features. The scaling laws have been validated analysing the Martian surface-layer data for the relations between the power spectra of wind and temperature turbulence and the corresponding mean values of wind speed and temperature. Usual spectral formulations were used based on the scaling laws ruling the Earth atmospheric surface layer, whereby the Earth's atmosphere is used as a standard for the Martian atmosphere.
This study examines the dependence of the surface drag coefficient on stability, wind speed, meso... more This study examines the dependence of the surface drag coefficient on stability, wind speed, mesoscale modulation of the turbulent flux and method of calculation of the drag coefficient. Data sets over grassland, sparse grass, heather and two forest sites are analyzed. For significantly unstable conditions, the drag coefficient does not depend systematically on z/L but decreases with wind speed for fixed intervals of z/L, where L is the Obukhov length. Even though the drag coefficient for weak wind conditions is sensitive to the exact method of calculation and choice of averaging time, the decrease of the drag coefficient with wind speed occurs for all of the calculation methods. A classification of flux calculation methods is constructed, which unifies the most common previous approaches. The roughness length corresponding to the usual Monin-Obukhov stability functions decreases with increasing wind speed. This dependence on wind speed cannot be eliminated by adjusting the stability functions. If physical, the decrease of the roughness length with increasing wind speed might be due to the decreasing role of viscous effects and streamlining of the vegetation, although these effects cannot be isolated from existing atmospheric data. For weak winds, both the mean flow and the stress vector often meander significantly in response to mesoscale motions. The relationship between meandering of the stress and wind vectors is examined. For weak winds, the drag coefficient can be sensitive to the method of calculation, partly due to meandering of the stress vector.
The paper reviews the status of modelling offshore wind resources and wind conditions using the W... more The paper reviews the status of modelling offshore wind resources and wind conditions using the WAsP and WAsP Engineering models-experiences gained over the last decade, current engineering practices and challenges for the future. The paper aims at answering the basic question: how well can we estimate site wind conditions, described in the IEC 61400-1 and other standards, for offshore wind farm sites? Whether modelling the wind in the coastal zone or further offshore, the physical modelling should reliably describe the wind speed profiles over water, including the influence of the surface roughness length and atmospheric stability. The modelling outputs required and analysed here are: mean wind speed (V ave) and Weibull parameters (A, k), reference wind speed (V ref) and characteristic turbulence intensity (I 15). WAsP and WAsP Engineering modelling results are compared to measurements at offshore met. masts, as well as onshore data sets and other modelling results. The available meteorological input data may be observations from ships or lighthouses, measurements from offshore masts, wind atlas data sets, NCEP/NCAR reanalysis data, and data from meso-scale models (e.g. KAMM). The topographical input data discussed are: simple land/sea maps, WAsP roughness rose descriptions, Coastline Extractor coastline maps and hand-digitised coastline / roughness maps. The importance of roughness classification and map diameter is also discussed. Current engineering practices with respect to vertical datum, water roughness length, wake decay constant, WAsP parameters, wind indices etc. are summarised and discussed. The challenges offshore are mostly related to the wind field and wind conditions at heights between 50 and 200 m a.s.l.-partly outside the operational envelope of the WAsP programs. In addition, reliable and representative data sets are scarce at these heights. Another challenge is related to the modelling of wake effects in large offshore farms. Possible extensions of today's engineering procedures and practices are outlined.
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Papers by Hans Jørgensen