Turbulence Measurements with Dual-Doppler Scanning Lidars

Velocity-component variances can be directly computed from lidar measurements using information of the second-order statistics within the lidar probe volume. Specifically, by using the Doppler radial velocity spectrum, one can estimate the unfiltered radial velocity variance. This information is not always available in current lidar campaigns. The velocity-component variances can also be indirectly computed from the reconstructed velocities but they are biased compared to those computed from, e.g., sonic anemometers. Here we show, for the first time, how to estimate such biases for a multi-lidar system and we demonstrate, also for the first time, their dependence on the turbulence characteristics and the lidar beam scanning geometry relative to the wind direction. For a dual-Doppler lidar system, we also show that the indirect method has an advantage compared to the direct one for commonly-used scanning configurations due to the singularity of the system. We demonstrate that our estimates of the radial velocity and velocity-component biases are accurate by analysis of measurements performed over a flat site using a dual-Doppler lidar system, where both lidars stared over a volume close to a sonic anemometer at a height of 100 m. We also show that mapping these biases over a spatial domain helps to plan meteorological campaigns, where multi-lidar systems can potentially be used. Particularly, such maps help the multi-point mapping of wind resources and conditions, which improve the tools needed for wind turbine siting.

Download Full-text

Turbulence characterization from a forward-looking nacelle lidar

Wind Energy Science ◽

10.5194/wes-2-133-2017 ◽

2017 ◽

Vol 2 (1) ◽

pp. 133-152 ◽

Cited By ~ 11

Author(s):

Alfredo Peña ◽

Jakob Mann ◽

Nikolay Dimitrov

Keyword(s):

Wind Speed ◽

Radial Velocity ◽

Velocity Component ◽

Sonic Anemometer ◽

Velocity Spectrum ◽

Wave System ◽

Velocity Tensor ◽

Velocity Spectra ◽

Cup Anemometer ◽

Forward Looking

Abstract. We present two methods to characterize turbulence in the turbine inflow using radial velocity measurements from nacelle-mounted lidars. The first uses a model of the three-dimensional spectral velocity tensor combined with a model of the spatial radial velocity averaging of the lidars, and the second uses the ensemble-averaged Doppler radial velocity spectrum. With the former, filtered turbulence estimates can be predicted, whereas the latter model-free method allows us to estimate unfiltered turbulence measures. Two types of forward-looking nacelle lidars are investigated: a pulsed system that uses a five-beam configuration and a continuous-wave system that scans conically. For both types of lidars, we show how the radial velocity spectra of the lidar beams are influenced by turbulence characteristics, and how to extract the velocity-tensor parameters that are useful to predict the loads on a turbine. We also show how the velocity-component variances and co-variances can be estimated from the radial-velocity unfiltered variances of the lidar beams. We demonstrate the methods using measurements from an experiment conducted at the Nørrekær Enge wind farm in northern Denmark, where both types of lidars were installed on the nacelle of a wind turbine. Comparison of the lidar-based along-wind unfiltered variances with those from a cup anemometer installed on a meteorological mast close to the turbine shows a bias of just 2 %. The ratios of the unfiltered and filtered radial velocity variances of the lidar beams to the cup-anemometer variances are well predicted by the spectral model. However, other lidar-derived estimates of velocity-component variances and co-variances do not agree with those from a sonic anemometer on the mast, which we mostly attribute to the small cone angle of the lidar. The velocity-tensor parameters derived from sonic-anemometer velocity spectra and those derived from lidar radial velocity spectra agree well under both near-neutral atmospheric stability and high wind-speed conditions, with differences increasing with decreasing wind speed and increasing stability. We also partly attribute these differences to the lidar beam configuration.

Download Full-text

Turbulence characterization from a forward-looking nacelle lidar

10.5194/wes-2016-47 ◽

2016 ◽

Cited By ~ 1

Author(s):

Alfredo Peña ◽

Jakob Mann ◽

Nikolay Dimitrov

Keyword(s):

Wind Speed ◽

Radial Velocity ◽

Velocity Component ◽

Sonic Anemometer ◽

Velocity Spectrum ◽

Wave System ◽

Velocity Tensor ◽

Velocity Spectra ◽

Cup Anemometer ◽

Forward Looking

Abstract. We present two methods to characterize turbulence in the turbine inflow using radial velocity measurements from nacelle-mounted lidars. The first uses a model of the three-dimensional spectral velocity tensor combined with a model of the spatial radial velocity averaging of the lidars and the second uses the ensemble-averaged Doppler radial velocity spectrum. With the former, filtered turbulence estimates can be predicted, whereas the latter model-free method allows us to estimate unfiltered turbulence measures. Two types of forward-looking nacelle lidars are investigated: a pulsed system that uses a 5-beam configuration and a continuous-wave system that scans conically. For both types of lidars, we show how the radial velocity spectra of the lidar beams are influenced by turbulence characteristics and how to extract the velocity-tensor parameters that are useful to predict the loads on a turbine. We also show how the velocity-component variances and co-variances can be estimated from the radial-velocity unfiltered variances of the lidar beams. We demonstrate the methods using measurements from an experiment conducted at the Nørrekær Enge wind farm in northern Denmark, where both types of lidars were installed on the nacelle of a wind turbine. Comparison of the lidar-based along-wind unfiltered variances with those from a cup anemometer installed on a meteorological mast close to the turbine shows a bias of just 2 %. The ratios of the unfiltered and filtered radial velocity variances of the lidar beams to the cup-anemometer variances are well predicted by the spectral model. However, other lidar-derived estimates of velocity-component variances and co-variances do not agree with those from a sonic anemometer on the mast, which we mostly attribute to the small cone angle of the lidar. The velocity-tensor parameters derived from sonic-anemometer velocity spectra and those derived from lidar radial velocity spectra agree well under both near-neutral atmospheric stability and high wind-speed conditions with differences increasing with decreasing wind-speed and increasing stability. We also partly attribute these differences to the lidar beam configuration.

Download Full-text

An analysis of flow distortion in a multipath sonic anemometer

10.5194/egusphere-egu21-12396 ◽

2021 ◽

Author(s):

Ebba Dellwik ◽

Poul Hummelshøj ◽

Gerhard Peters

Keyword(s):

Vertical Velocity ◽

Velocity Component ◽

Sonic Anemometer ◽

Three Dimensional ◽

Turbulence Theory ◽

Flow Distortion ◽

Vertical Velocity Component ◽

Ongoing Research ◽

Sonic Anemometers

Sonic anemometers provide point observations of the three-dimensional velocity field at high sampling rates and are crucial instruments for understanding and quantifying the fluxes of momentum, energy and scalars between the atmosphere and Earth&#8217;s surface. Since the beginning of sonic anemometry 50 years ago, the characterization of flow distortion, i.e. how the instrument structure alters the flow, has been an ongoing research topic. Multi-path sonic anemometry provides a new opportunity to research and understand flow distortion on the vertical velocity component, since several positions in the small measurement volume can be measured simultaneously. In this work, we use data from a flat terrain measurement campaign in 2020, in which several sonic anemometers were mounted on 4m towers placed 4m apart. The analysis is focused on the Multipath Class-A sonic anemometer (Metek GmbH, Germany), which provides vertical velocity observations from three vertical paths 120 degrees and 0.1m apart. Vertical velocities are also calculated from several combinations of the tilted paths. We investigate how the vertical velocity component is altered depending on wind direction relative to different parts of the instrument structure. We demonstrate that by an optimal combination of the different paths, the vertical velocity variance and fluxes can be significantly enhanced. We also show spectra, and especially look at the high frequency end of the spectrum, where the relative behaviour of the velocity components is known from fundamental turbulence theory. Further, the relative importance of transducer shadowing and pressure-induced blockage effects is discussed.

Download Full-text

Validating precision estimates in horizontal wind measurements from a Doppler lidar

Atmospheric Measurement Techniques ◽

10.5194/amt-10-1229-2017 ◽

2017 ◽

Vol 10 (3) ◽

pp. 1229-1240 ◽

Cited By ~ 22

Author(s):

Rob K. Newsom ◽

W. Alan Brewer ◽

James M. Wilczak ◽

Daniel E. Wolfe ◽

Steven P. Oncley ◽

...

Keyword(s):

Wind Speed ◽

Radial Velocity ◽

Sonic Anemometer ◽

Direct Calculation ◽

Measurement Precision ◽

Horizontal Wind ◽

Retrieval Algorithm ◽

Doppler Lidar ◽

Wind Fields ◽

Wind Retrieval

Abstract. Results from a recent field campaign are used to assess the accuracy of wind speed and direction precision estimates produced by a Doppler lidar wind retrieval algorithm. The algorithm, which is based on the traditional velocity-azimuth-display (VAD) technique, estimates the wind speed and direction measurement precision using standard error propagation techniques, assuming the input data (i.e., radial velocities) to be contaminated by random, zero-mean, errors. For this study, the lidar was configured to execute an 8-beam plan-position-indicator (PPI) scan once every 12 min during the 6-week deployment period. Several wind retrieval trials were conducted using different schemes for estimating the precision in the radial velocity measurements. The resulting wind speed and direction precision estimates were compared to differences in wind speed and direction between the VAD algorithm and sonic anemometer measurements taken on a nearby 300 m tower.All trials produced qualitatively similar wind fields with negligible bias but substantially different wind speed and direction precision fields. The most accurate wind speed and direction precisions were obtained when the radial velocity precision was determined by direct calculation of radial velocity standard deviation along each pointing direction and range gate of the PPI scan. By contrast, when the instrumental measurement precision is assumed to be the only contribution to the radial velocity precision, the retrievals resulted in wind speed and direction precisions that were biased far too low and were poor indicators of data quality.

Download Full-text

Validating Precision Estimates in Horizontal Wind Measurements from a Doppler Lidar

10.5194/amt-2016-312 ◽

2016 ◽

Author(s):

Rob K. Newsom ◽

W. Alan Brewer ◽

James M. Wilczak ◽

Daniel E. Wolfe ◽

Steven P. Oncley ◽

...

Keyword(s):

Wind Speed ◽

Radial Velocity ◽

Sonic Anemometer ◽

Direct Calculation ◽

Horizontal Wind ◽

Retrieval Algorithm ◽

Doppler Lidar ◽

Wind Fields ◽

Wind Retrieval ◽

Turbulence Effects

Abstract. Results from a recent field campaign are used to assess the accuracy of wind speed and direction precision estimates produced by a Doppler lidar wind retrieval algorithm. The algorithm, which is based on the traditional velocity-azimuth-display (VAD) technique, estimates the wind speed and direction measurement precision using standard error propagation techniques. For this study, the lidar was configured to execute an 8-beam plan-position-indicator (PPI) scan once every 12 minutes during the 6 week deployment period. Several wind retrieval trials were conducted using different schemes for estimating the uncertainty in the radial velocity measurements. The resulting wind speed and direction precision estimates were compared to differences in wind speed and direction between the VAD algorithm and sonic anemometer measurements taken on a nearby 300-m tower. All trials produced qualitatively similar wind fields with negligible bias, but substantially different wind speed and direction precision fields. The most accurate wind speed and direction precisions were obtained when the radial velocity uncertainty was determined by direct calculation of radial velocity standard deviation along each pointing direction and range gate of the PPI scan. By contrast, setting the radial velocity uncertainty to the radial velocity precision (thereby ignoring turbulence effects) resulted in wind speed and direction precisions that were biased far too low and poor indicators of data quality.

Download Full-text

Comparison of turbulence measurements by a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar

Atmospheric Measurement Techniques ◽

10.5194/amt-13-969-2020 ◽

2020 ◽

Vol 13 (2) ◽

pp. 969-983 ◽

Cited By ~ 1

Author(s):

Matthias Mauder ◽

Michael Eggert ◽

Christian Gutsmuths ◽

Stefan Oertel ◽

Paul Wilhelm ◽

...

Keyword(s):

High Resolution ◽

Wind Speed ◽

Friction Velocity ◽

Sonic Anemometer ◽

Vertical Wind ◽

Doppler Lidar ◽

Flow Distortion ◽

Sonic Anemometers ◽

Vertical Wind Speed ◽

Standard Deviations

Abstract. Accurate measurements of turbulence statistics in the atmosphere are important for eddy-covariance measurements, wind energy research, and the validation of atmospheric numerical models. Sonic anemometers are widely used for these applications. However, these instruments are prone to probe-induced flow distortion effects, and the magnitude of the resulting errors has been debated due to the lack of an absolute reference instrument under field conditions. Here, we present the results of an intercomparison experiment between a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar, which is inherently free of any flow distortion. This novel remote sensing instrument has otherwise very similar spatial and temporal sampling characteristics to the sonic anemometer and hence served as a reference for this comparison. The presented measurements were carried out over flat homogeneous terrain at a measurement height of 30 m. We provide a comparative statistical analysis of the resulting mean wind velocities, the standard deviations of the vertical wind speed and the friction velocity and investigate the reasons for the observed deviations based on the turbulence spectra and co-spectra. Our results show an agreement of the mean wind velocity measurements and the standard deviations of the vertical wind speed with a comparability of 0.082 and 0.020 m s−1, respectively. Biases for these two quantities were 0.003 and 0.012 m s−1, respectively. Slightly larger differences were observed for friction velocity. Analysis of the corresponding co-spectra showed that the CSAT3B underestimates this quantity systematically by about 3 % on average as a result of co-spectral losses in the frequency range between 0.1 and 5 s−1. We also found that an angle-of-attack-dependent transducer-shadowing correction does not improve the agreement between the CSAT3B and the Physikalisch-Technische Bundesanstalt (PTB) lidar effectively.

Download Full-text

Improvement of vertical velocity statistics measured by a Doppler lidar through comparison with sonic anemometer observations

Atmospheric Measurement Techniques ◽

10.5194/amt-9-5833-2016 ◽

2016 ◽

Vol 9 (12) ◽

pp. 5833-5852 ◽

Cited By ~ 4

Author(s):

Timothy A. Bonin ◽

Jennifer F. Newman ◽

Petra M. Klein ◽

Phillip B. Chilson ◽

Sonia Wharton

Keyword(s):

Vertical Velocity ◽

Close Agreement ◽

Sonic Anemometer ◽

Lag Time ◽

Volume Averaging ◽

Doppler Lidar ◽

Established Method ◽

Velocity Statistics ◽

Sonic Anemometers ◽

Velocity Variance

Abstract. Since turbulence measurements from Doppler lidars are being increasingly used within wind energy and boundary-layer meteorology, it is important to assess and improve the accuracy of these observations. While turbulent quantities are measured by Doppler lidars in several different ways, the simplest and most frequently used statistic is vertical velocity variance (w′2) from zenith stares. However, the competing effects of signal noise and resolution volume limitations, which respectively increase and decrease w′2, reduce the accuracy of these measurements. Herein, an established method that utilises the autocovariance of the signal to remove noise is evaluated and its skill in correcting for volume-averaging effects in the calculation of w′2 is also assessed. Additionally, this autocovariance technique is further refined by defining the amount of lag time to use for the most accurate estimates of w′2. Through comparison of observations from two Doppler lidars and sonic anemometers on a 300 m tower, the autocovariance technique is shown to generally improve estimates of w′2. After the autocovariance technique is applied, values of w′2 from the Doppler lidars are generally in close agreement (R2 ≈ 0.95 − 0.98) with those calculated from sonic anemometer measurements.

Download Full-text

Improvement of Vertical Velocity Statistics Measured by a Doppler Lidar through Comparison with Sonic Anemometer Observations

10.5194/amt-2016-134 ◽

2016 ◽

Cited By ~ 1

Author(s):

Timothy A. Bonin ◽

Jennifer F. Newman ◽

Petra M. Klein ◽

Phillip B. Chilson ◽

Sonia Wharton

Keyword(s):

Vertical Velocity ◽

Sonic Anemometer ◽

Lag Time ◽

Volume Averaging ◽

Doppler Lidar ◽

Atmospheric Conditions ◽

Established Method ◽

Velocity Statistics ◽

Sonic Anemometers ◽

Velocity Variance

Abstract. Since turbulence measurements from Doppler lidars are being increasingly used within wind energy and boundary-layer meteorology, it is important to assess and improve the accuracy of these observations. While turbulent quantities are measured by Doppler lidars in several different ways, the simplest and most frequently used statistic is vertical velocity variance (σ2ω) from zenith stares. However, the competing effects of signal noise and resolution volume limitations, which respectively increase and decrease σ2ω, reduce the accuracy of these measurements. Herein, an established method that utilizes the autocovariance of the signal to remove noise is evaluated and its skill in also correcting for volume-averaging effects in the calculation of σ2ω is assessed. Additionally, this autocovariance technique is further refined by defining the amount of lag time to use for the most accurate estimates of σ2ω. Through comparison of observations from two Doppler lidars and sonic anemometers on a 300-m tower, the autocovariance technique is shown to improve estimates of σ2ω over a variety of atmospheric conditions. After the autocoviance technique is applied, values of σ2ω from the Doppler lidars are generally in close agreement (R2 ≈ 0.95–0.98) with those calculated from sonic anemometer measurements.

Download Full-text

Comparison of turbulence measurements by a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar

10.5194/amt-2019-133 ◽

2019 ◽

Cited By ~ 2

Author(s):

Matthias Mauder ◽

Michael Eggert ◽

Christian Gutsmuths ◽

Stefan Oertel ◽

Paul Wilhelm ◽

...

Keyword(s):

High Resolution ◽

Wind Speed ◽

Friction Velocity ◽

Sonic Anemometer ◽

Vertical Wind ◽

Doppler Lidar ◽

Flow Distortion ◽

Sonic Anemometers ◽

Vertical Wind Speed ◽

Standard Deviations

Abstract. Accurate measurements of turbulence statistics in the atmosphere are important for eddy-covariance measurements, wind energy research, and the validation of atmospheric numerical models. Sonic anemometers are widely used for these applications. However, these instruments are prone to probe-induced flow distortion effects, and the magnitude of the resulting errors has been debated due to the lack of an absolute reference instrument under field conditions. Here, we present the results of an intercomparison experiment between a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar, which is inherently free of any flow-distortion. This novel remote sensing instrument has otherwise very similar spatial and temporal sampling characteristics as the sonic anemometer and hence served as a reference for this comparison. The presented measurements were carried out over flat homogeneous terrain, at a measurement height of 30 m. We provide a comparative statistical analysis of the resulting mean wind velocities, the standard deviations of the vertical wind speed and the friction velocity and investigate the reasons for the observed deviations based on the turbulence spectra and cospectra. Our results show a very good agreement of the mean wind velocity measurements and the standard deviations of the vertical wind speed, with comparabilities of 0.082 and 0.017 m s−1, respectively. Biases for these two quantities were very low, being smaller than 0.01 m s−1, which corresponds to about 1 % in relative terms. Slightly larger differences were observed for friction velocity. Analysis of the corresponding cospectra showed that the CSAT3B underestimates this quantity systematically by about 3 % on average as a result of too steep a drop-off in the inertial sub-range. We also found that an angle-of-attack dependent transducer-shadowing correction does not improve this agreement effectively because it leads to an artificial correlation between the three wind components and therefore severely distorts the shape of the cospectra.

Download Full-text

Can Lidars compete with sonic anemometers? - Comparison of wind measurements from different Doppler Lidar scan strategies to sonic anemometer data

10.5194/ems2021-272 ◽

2021 ◽

Author(s):

Kevin Wolz ◽

Frank Beyrich ◽

Julian Steinheuer ◽

Carola Detring ◽

Ronny Leinweber ◽

...

Keyword(s):

Boundary Layer ◽

Spatial Data ◽

Technological Development ◽

Weather Prediction ◽

Sonic Anemometer ◽

Horizontal Wind ◽

Doppler Lidar ◽

Sonic Anemometers ◽

Wind Measurements ◽

Sonic Anemometer Data

The technological development of ground-based active remote sensing instruments has reached a point where they have the possibility to drastically increase the temporal and spatial data density compared to conventional instruments, which would allow for a better process understanding and is expected to enhance the forecasting skills of numerical weather prediction systems and reduce its uncertainties. To test the measurement uncertainty and feasibility of Doppler Lidar systems we participated in the FESST@MOL 2020 field campaign, organized by the German Meteorological Service (DWD) in Lindenberg, Germany. During this campaign, eight Doppler Lidars were operated at the boundary layer field site (GM) Falkenberg. We evaluated different scanning strategies for the determination of the wind profile in the Atmospheric Boundary Layer (ABL) using multiple different triple Lidar virtual tower (VT) scan patterns including range height indicator (RHI) and step/stare scan modes. We compared these Lidar-based wind measurements with the data from a sonic anemometer on a 99 m tall instrumented tower also located in Falkenberg over a period of four months. The lidar and the sonic anemometer data were processed to 10- and 30- minute averages and compared to each other. The VT measurements underestimated the mean horizontal wind compared to the sonic anemometer by around 0.2 m s&#8209;1. Besides that, we compared the VT data with those from a single fourth nearby Doppler Lidar which was running in a velocity-azimuth display (VAD) mode. The calculated mean horizontal wind values between the two different modes showed a good comparability but differed stronger with increasing height.

Download Full-text