Inter-comparison study of wind measurement between the three-lidar-based virtual tower and four lidars using VAD techniques

Abstract. In this study, we compare the wind measurements of a virtual tower triple Doppler lidar setup to those of a sonic anemometer located at a height of 90 m above ground on an instrumented tower and with those of two single Doppler lidars to evaluate the effect of the horizontal homogeneity assumption used for single Doppler lidar applications on the measurement accuracy. The triple lidar setup was operated in a 90 m stare and a step–stare mode at six heights between 90 and 500 m above ground, while the single lidars were operated in a continuous scan velocity–azimuth display (VAD) mode where one of them had a zenith angle of 54.7° and the other one of 28.0°. The instruments were set up at the boundary-layer field site of the German Meteorological Service (DWD) in July and August of 2020 during the FESST@MOL (Field Experiment on sub-mesoscale spatiotemporal variability at the Meteorological Observatory Lindenberg) 2020 campaign. Overall, we found good agreement of the lidar methods for the whole study period for different averaging times and scan modes compared to the sonic anemometer. For the step–stare mode wind speed measurements, the comparability between the triple lidar and the sonic anemometer was 0.47 m s−1 at an averaging time of 30 min with a bias value of −0.34 m s−1. For wind speed measured by one single lidar setup for the same period with an averaging time of 30 min, we found a comparability of 0.32 m s−1 at an averaging time of 30 min and a bias value of −0.07 m s−1 as well as values of 0.47 and −0.34 m s−1 for the other one, respectively. We also compared the wind velocity measurements of the single and triple lidars at different heights and found decreasing agreement between them with increasing measurement height up to 495 m above ground for the single lidar systems. We found that the single Doppler lidar with the increased zenith angle produced poorer agreement with the triple Doppler lidar setup than the one with the lower zenith angle, especially at higher altitudes. At a height of 495 m above ground and with an averaging time of 30 min the comparability and bias for the larger zenith angle were 0.71 and −0.50 m s−1, respectively, compared to values of 0.57 and −0.28 m s−1 for the smaller zenith angle. Our results confirm that a single Doppler lidar provides reliable wind speed and direction data over heterogeneous but basically flat terrain in different scan configurations. For the virtual tower scanning strategies, we developed a new filtering approach based on a median absolute deviation (MAD) filter combined with a relatively relaxed filtering criterion for the signal-to-noise ratio output by the instrument.

Halo-Photonics Streamline XR doppler lidar measurements performed using several scan configurations (Velocity Azimuth Display – VAD and Doppler Beam Swing – DBS) and elevation angles of 60° and 80°, are compared to wind observations conducted by various in situ instruments (tethered balloon, meteorological mast, and radiosondes). Good agreement is obtained, with an R2 over 0.90 for wind speed and a standard error <=18.90° for wind direction for the VAD scan method. Best performance was attained with VAD and lower elevation scans (60°). These results are consistent with the higher spatial lateral homogeneity exhibited by lower angle scans. The boundary layer structure along a diurnal cycle is analyzed by utilizing retrieved backscatter data and wind measurements in conjunction with WRF simulations. Presence of multiple inversions which allow the coexistence of different layers within the studied profile is also verified using data acquired by several radiosondes. Synergic use of Lidar data with WRF simulations for low SNR regions is demonstrated.

Weather radar is a critical tool in meteorology, providing invaluable insights into atmospheric conditions and in detecting precipitation, storm intensity, and movement.Several studies have been conducted using DWR observations to retrieve the local wind field.These winds are vital in transporting mass, momentum, and energy throughout the atmosphere, influencing various meteorological processes.Traditional methods like velocity-azimuth display (VAD) and volume-velocity processing (VVP) assume linearity in wind patterns, introducing potential inaccuracies in the case of non-uniform flow [1], [2].Furthermore, studies suggest that in traditional VVP, where multivariate regression is used to estimate the wind field coefficients, this leads to an ill-conditioned matrix and poses practical limitations in estimating the wind fields for small analysis volume.Studies indicate that the application of machine learning techniques for estimating wind fields from single Doppler radar data eliminates the constraint of linearity, resulting in more accurate predictions of the wind field [3].The present study aims to leverage machine learning (ML) techniques that can potentially overcome the challenges posed by ill-conditioned matrices and the linearity assumption in wind field retrievals.These models can adapt to complex wind patterns and learn from radar radial velocity data, leading to more realistic and accurate wind field representations.This study explores a support vector regression (SVR) based framework for retrieving wind fields, incorporating the k-means clustering algorithm to consider the analysis volume.The proposed methodology is shown in Fig. 1.In a simulated experiment, the SVR method effectively captured a uniform horizontal wind field, with a notably strong correlation between simulated and predicted winds.The comparison of retrieved winds at different altitudes with ERA5 winds reveals that the RMSE increases with altitude.The RMSE at lower altitudes is found to be about 5-6 m/s up to 6 km and further increases with altitude.Ongoing efforts involve model fine-tuning, comprehensive retrievals and validation for DWR data, contributing valuable insights to improve our understanding of extreme meteorological events and enhance forecasting accuracy.Fig 1: Flow chart representation of the proposed algorithm.

Wind turbine impacts on the atmospheric flow are investigated using data from the Crop Wind Energy Experiment (CWEX-11) and large-eddy simulations (LESs) utilizing a generalized actuator disk (GAD) wind turbine model. CWEX-11 employed velocity-azimuth display (VAD) data from two Doppler lidar systems to sample vertical profiles of flow parameters across the rotor depth both upstream and in the wake of an operating 1.5 MW wind turbine. Lidar and surface observations obtained during four days of July 2011 are analyzed to characterize the turbine impacts on wind speed and flow variability, and to examine the sensitivity of these changes to atmospheric stability. Significant velocity deficits (VD) are observed at the downstream location during both convective and stable portions of four diurnal cycles, with large, sustained deficits occurring during stable conditions. Variances of the streamwise velocity component, σu, likewise show large increases downstream during both stable and unstable conditions, with stable conditions supporting sustained small increases of σu, while convective conditions featured both larger magnitudes and increased variability, due to the large coherent structures in the background flow. Two representative case studies, one stable and one convective, are simulated using LES with a GAD model at 6 m resolution to evaluate the compatibility of the simulation framework with validation using vertically profiling lidar data in the near wake region. Virtual lidars were employed to sample the simulated flow field in a manner consistent with the VAD technique. Simulations reasonably reproduced aggregated wake VD characteristics, albeit with smaller magnitudes than observed, while σu values in the wake are more significantly underestimated. The results illuminate the limitations of using a GAD in combination with coarse model resolution in the simulation of near wake physics, and validation thereof using VAD data.

An all-fiber airborne pulsed coherent Doppler lidar (CDL) prototype at 1.54μm is developed to measure wind profiles in the lower troposphere layer. The all-fiber single frequency pulsed laser is operated with pulse energy of 300μJ, pulse width of 400ns and pulse repetition rate of 10kHz. To the best of our knowledge, it is the highest pulse energy of all-fiber eye-safe single frequency laser that is used in airborne coherent wind lidar. The telescope optical diameter of monostatic lidar is 100 mm. Velocity-Azimuth-Display (VAD) scanning is implemented with 20 degrees elevation angle in 8 different azimuths. Real-time signal processing board is developed to acquire and process the heterodyne mixing signal with 10000 pulses spectra accumulated every second. Wind profiles are obtained every 20 seconds. Several experiments are implemented to evaluate the performance of the lidar. We have carried out airborne wind lidar experiments successfully, and the wind profiles are compared with aerological theodolite and ground based wind lidar. Wind speed standard error of less than 0.4m/s is shown between airborne wind lidar and balloon aerological theodolite.

Combined effects of unsteady wake and free-stream turbulence on turbine blade film cooling with laid-back fan-shaped holes using PSP technique

Pollutants dispersion emitted from road traffic is a dominant source of urban air pollution. Therefore, it is necessary to quantify emission levels in urban street canyon to evaluate their impacts on the public health and the environment. In this article, we study the effect of different street geometrical layouts on dispersion emissions of traffic exhausts for winds perpendicular to the roadway. A wind tunnel experiment modeling three different street geometrical layouts was executed to examine the flow fields and the concentration distributions in urban canyon. The flow and pollutants concentration were measured using a hot-wire anemometer with a split-fiber probe and a fast flame ionization detector under neutral atmospheric conditions. Flow and Dispersion characteristics were studied that included mean velocity, turbulence intensity, Reynolds stress, turbulent kinetic energy (TKE), mean concentrations, and fluctuation intensity of concentration in the street canyon. The results show that the vertical velocity and turbulent energy increases as the aspect ratio increases. The pollutant concentration increases as the aspect ratio decreases. The concentration fluctuation intensities of the aspect ratio W/H = 1 was found lower than that of the aspect ratios W/H = ½ and ¾. The pollutant concentration at the leeward side of the aspect ratios W/H = ¾ and 1 was higher than that in the aspect ratios W/H = ½. The pollutant concentration distributions measured in the experiment indicates that the variability of the street geometrical layouts in urban environment are important parameters for estimating air quality in the urban canyon.

Experimental study of the wind flow in a coastal region of Japan

A recently developed mesoscale numerical model has been applied to the Athens Basin in order to provide a detailed description of the mean flow and pollution levels under neutral atmospheric conditions. The model is based on the numerical solution of the time‐averaged Navier‐Stokes equations written in their contravariant strong conservation form in a generalized non‐orthogonal co‐ordinate system. Before its application to the Athens Basin case, the reliability of the model was tested by predicting the flow and concentration fields over a three‐dimensional hill. The predicted results agree favourably with available experimental data, demonstrating the validity, flexibility and economy of the model for flows in three‐dimensional complex terrain.

Ammonia has been identified as a promising fuel to diminish greenhouse gas emission. However, ammonia combustion presents certain challenges including low reactivity and high NO emission. In the present study, three-dimensional direct numerical simulations (DNS) of ammonia/air premixed slot jet flames with varying Karlovitz numbers (Ka) and cracking ratios were performed. Three cases were considered, including two pure ammonia/air flames with different turbulence intensities and one partially cracked ammonia/air flame with high turbulence intensity. The effects of turbulence intensity and partial ammonia cracking on turbulence–flame interactions and NO emission characteristics of the flames were investigated. It was shown that the turbulent flame speed is higher for the flames with high turbulence intensity. In general, the flame displacement speed is negatively correlated with curvature in negative curvature regions, while the correlation is weak in the positive curvature regions for highly turbulent flames. Most flame area is consumed in negatively curved regions and produced in positively curved regions. It was found that the NO mass fraction is higher in the flame with partial ammonia cracking compared to the pure ammonia/air flames. The NO pathway analysis shows that the NH → NO pathway is enhanced, while the NO consumption pathway is suppressed in the partially cracked ammonia/air flame. The NO mass fraction is higher in regions of negative curvature than positive curvature. Interestingly, the NO mass fraction is found to be negatively correlated with the local equivalence ratio, which is consistent in both the DNS and the corresponding laminar premixed flames.

&amp;lt;p&amp;gt;We present exciting Doppler lidar and cloud radar measurements from a high-vantage mountain observatory in the hyper-arid United Arab Emirates (UAE) - initiated as part of the UAE Research Program for Rain Enhancement Science (UAEREP). The observatory was designed to study the clear-air pre-convective environment and subsequent convective events in the arid Al Hajar Mountains, with the overarching goal of improving understanding and nowcasting of seedable orographic clouds. During summer in the Al Hajar Mountains (June to September), weather processes are often complex, with summer convection being initiated by several phenomena acting in concert, e.g., interaction between sea breeze and horizontal convective rolls. These interactions can combine to initiate sporadic convective storms and these can be intense enough to cause flash floods and erosion. Such events here are influenced by mesoscale phenomena like the low-level jet and local sea breeze, and are constrained by larger-scale synoptic conditions.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The Doppler lidar and cloud radar were employed for approximately two years at a high vantage-point to capture valley wind flows and observe convective cells. The instruments were configured to run synchronized polar (PPI) scans at 0&amp;amp;#176;, 5&amp;amp;#176;, and 45&amp;amp;#176; elevation angles and vertical cross-section (RHI) scans at 0&amp;amp;#176;, 30&amp;amp;#176;, 60, 90&amp;amp;#176;, 120&amp;amp;#176;, and 150&amp;amp;#176; azimuth angles. Using this imagery, along with local C-band radar and satellite data, we were able to identify and analyze several convective cases. To illustrate our results, we have selected two cases under unstable conditions - the 5 and 6 September 2018. In both cases, we observed areas of low-level convergence/divergence, particularly associated with wind flow around a peak 2 km to the south-west of the observatory. The extension of these deformations are visible in the atmosphere to a height of 3 km above sea level. Subsequently, we observed convective cells developing at those approximate locations &amp;amp;#8211; apparently initiated because of these phenomena. The cloud radar images provided detailed observations of cloud structure, evolution, and precipitation. In both convective cases, pre-convective signatures were apparent before CI, in the form of convergence, wind shear structures, and updrafts.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;These results have demonstrated the value of synergetic observations for understanding orographic convection initiation, improvement of forecast models, and cloud seeding guidance. The manuscript based on these results is now the subject of a peer review (Branch et al., 2021).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Branch, O., Behrendt, Andreas Alnayef, O., Sp&amp;amp;#228;th, F., Schwitalla, Thomas, Temimi, M., Weston, M., Farrah, S., Al Yazeedi, O., Tampi, S., Waal, K. de and Wulfmeyer, V.: The new Mountain Observatory of the Project &amp;amp;#8220;Optimizing Cloud Seeding by Advanced Remote Sensing and Land Cover Modification (OCAL)&amp;amp;#8221; in the United Arab Emirates: First results on Convection Initiation, J. Geophys. Res.&amp;amp;#160; Atmos., 2021. In review (submitted 23.11.2020).&amp;lt;/p&amp;gt;

Abstract. A dual-lidar system comprising two WindCube v2.1 Doppler lidars – one oriented horizontally at a 45° angle relative to the other – was deployed to estimate atmospheric turbulence using the variance method. This approach derives second-order velocity statistics directly from line-of-sight (LOS) velocity variances, enabling the reconstruction of the three-dimensional velocity variance components without retrieving instantaneous wind vectors. Its performance is evaluated against the traditional method commonly used in the wind energy sector, which reconstructs instantaneous velocity components from single-lidar LOS measurements prior to computing turbulence statistics. Both methods are assessed at a single measurement height using a 30 d collocated dataset, with sonic anemometer measurements as reference and classification by atmospheric stability. The analysis focuses on turbulence intensity (TI), and performance is quantified using the mean relative bias error (MRBE) and relative root mean square error (RRMSE), following Det Norske Veritas (DNV) load-validation acceptance criteria (MRBE within ±5 % and RRMSE ≤ 15 %). Results show that the variance method demonstrates improved agreement with the reference measurements across nearly all wind-speed bins. For along-wind TI, MRBE values range between 0 % and −5 %, with RRMSE remaining below 15 % over the full wind-speed range, fully satisfying the DNV criteria. For cross-wind TI, the MRBE criterion is met, while the RRMSE threshold is exceeded. In contrast, the traditional method meets both DNV criteria for along-wind TI only at wind speeds above approximately 7 m s−1 and fails to satisfy either criterion for cross-wind TI across the investigated range. Overall, the results demonstrate that the variance method provides more robust and DNV-compliant TI estimates than the traditional reconstruction approach, particularly for along-wind turbulence relevant to wind turbine load validation.

The influence of upstream turbulence on the flow produced by a plane jet is investigated experimentally with hot-wire anemometry and smoke flow visualisation. An innovative active grid, where each wing can be independently controlled, is used to change the upstream turbulence conditions. Three cases are investigated: a canonical reference case, a case with the same integral scale as the reference case but an order of magnitude increase in turbulence intensity, and a case with both an order of magnitude increase in turbulence intensity and an order of magnitude increase in integral scale compared to the reference case. It is demonstrated that the wake width increases with turbulence intensity, but decreases with integral scale for constant turbulence intensity. In addition, the positional variability of the wake width is highest with high turbulence intensity and a short integral scale. Along the jet centreline, the potential core region is shorter with elevated upstream turbulence intensity; this is reflected in both the mean velocity and the variance. The decay of the centreline mean velocity is also retarded by incoming turbulence. In all, increased incoming turbulence results in increased jet spreading, and a shorter integral scale further increases the spreading.Graphic abstract

Experimental investigation of the evaporation of suspended mono-sized heptane droplets in turbulence intensities approaching unity

The turbulent energy dissipation rate (EDR) is a quantitative measure of turbulence intensity, and it is widely used across various fields. Accurate estimation of EDR using Doppler lidar depends on the choice of estimation technique and scanning strategy. Therefore, a comparison of the techniques is still required to achieve an accurate estimation. However, the effect of the choice on estimation accuracy remains uncertain. This study systematically evaluates the accuracy of EDR estimation techniques by utilizing two distinct scanning strategies: a vertically pointing scan (EDRVP) and a Plan Position Indicator scan (EDRVAD). We assess four different EDRVP estimation techniques and assess the accuracy of EDRVAD for each elevation angle by comparing it with the measurements from sonic anemometers on a 300 m tall meteorological tower. EDRVAD shows a positive correlation coefficient exceeding 0.5 with the sonic anemometers. EDRVAD demonstrates dependency on the elevation angle, with lower angles resulting in higher EDR values. Conversely, all of the EDRVP techniques exhibit high agreement, with correlation coefficients above 0.9. This study provides a comprehensive assessment of the accuracy of each technique, highlighting their respective characteristics and practical considerations.