Vertical Wind Velocity Research Articles

Turbulence structures and entrainment length scales in large offshore wind farms

Abstract. The flow inside and around large offshore wind farms can range from smaller structures associated with the mechanical turbulence generated by wind turbines to larger structures indicative of the mesoscale flow. In this study, we explore the variation in turbulence structures and dominant scales of vertical entrainment above large offshore wind farms located in the North Sea, using data obtained from a research aircraft. The aircraft was flown upstream, downstream, and above wind farm clusters. Under neutrally stratified conditions, there is high ambient turbulence in the atmosphere and an elevated energy dissipation rate compared to stable conditions. The intensity of small-scale turbulence structures is increased above and downstream of the wind farm, and it prevails over mesoscale fluctuations. But in stable stratification, mesoscale flow structures are not only dominant upstream of the wind farm but also downstream. We observed that the vertical flux of horizontal momentum is the main source of energy recovery in large offshore wind farms, and it strongly depends on the magnitude of the length scales of the vertical wind velocity component. The dominant length scales of entrainment range from 20 to ∼60 m above the wind farm in all stratification strengths, and in the wake flow these scales range from 10 to ∼100 m only under near-neutral stratification. For strongly stable conditions, negligible vertical entrainment of momentum was observed even just 2 km downstream of large wind farms. We also observed that there is a significant lateral momentum flux above the offshore wind farms, especially under strongly stable conditions, which suggests that these wind farms do not satisfy the conditions of an “infinite wind farm”.

True eddy accumulation – Part 1: Solutions to the problem of non-vanishing mean vertical wind velocity

Abstract. The true eddy accumulation (TEA) method provides direct measurements of ecosystem-level turbulent fluxes for a wide range of atmospheric constituents. TEA utilizes conditional sampling to overcome the requirement for a fast sensor response demanded by the state-of-the-art eddy covariance (EC) method. The TEA method is formulated under the assumption of ideal conditions with a zero mean vertical wind velocity during the averaging interval. However, this idealization is rarely met under field conditions. Additionally, unlike in EC, this assumption cannot be imposed in post-processing due to the real-time nature of sampling and the absence of high-frequency measurements of the scalar. Consequently, fluxes measured with the TEA method are biased with a non-turbulent advective term that scales with the scalar mean concentration. Here, we explore the magnitude of this biased advective term and potential ways to minimize or remove it. We propose a new formulation to calculate TEA fluxes that minimizes the bias term. The new formulation shows that the magnitude of the error is constrained to w‾/|w|‾ when the stationarity criterion is fulfilled. Here, w is the vertical wind velocity, and the overbar denotes time averaging. The error is shown to be dependent on the asymmetry of atmospheric transport, represented by the coefficient αc. Two methods of estimating the coefficient αc are proposed: a probabilistic treatment of turbulent transport and a method utilizing the assumption of scalar similarity. We show how other formulas for calculating the TEA flux are linked to the new formulation and explore the different corrections in a numerical simulation. The new formulation avoids the direct dependence of the bias term on the scalar background concentration. This result increases confidence in applying the TEA method to measuring fluxes of atmospheric constituents. This is particularly relevant to scalars with a large background concentration and a small flux. This paper is Part 1 of a two-part series on true eddy accumulation.

Wuhan MST Radar Observations of a Tropopause Descent Event during Heavy Rain on 1–2 June 2015

During heavy rain on 1–2 June 2015 in central China, the Wuhan mesosphere–stratosphere–troposphere (MST) radar was applied to record the atmospheric responses to the rain with a 30 min period. According to the vertical gradient of the echo power above 500 hPa, the tropopause height could be determined by MST radar detection. The tropopause descent was clearly observed by the Wuhan MST radar a few hours before the rain, and then the tropopause recovered to usual heights during the rain. The observation of the radiosonde in Wuhan was in line with that of the radar. Both the potential vorticity and the ozone mass mixing ratio variations at 100 hPa level implied the fall of the tropopause. During the tropopause decent, enhanced radar echoes appeared in the upper troposphere, the echo spectral widths became broader, and the large vertical wind velocities were recorded and indicated the occurrence of strong convective activities. The relative humidity was also found to increase at all tropospheric heights, including the region close to the tropopause. The convective flow may have transported water vapor to the tropopause heights, and a temperature decrease in this region was also recorded. It is very likely that water vapor cooling induced the tropopause descent.

Performance Modeling of a Diode-Laser-Based Direct-Detection Doppler Lidar for Vertical Wind Profiling

Abstract Micropulse differential absorption lidars (MPD) for water vapor, temperature, and aerosol profiling have been developed, demonstrated, and are addressing the needs of the atmospheric science community for low-cost ground-based networkable instruments capable of long-term monitoring of the lower troposphere. The MPD instruments use a diode-laser-based (DLB) architecture that can easily be adapted for a wide range of applications. In this study, a DLB direct-detection Doppler lidar based on the current MPD architecture is modeled to better understand the efficacy of the instrument for vertical wind velocity measurements, with the long-term goal of incorporating these measurements into the current network of MPD instruments. The direct-detection Doppler lidar is based on a double-edge receiver that utilizes two Fabry–Pérot interferometers and a vertical velocity retrieval that requires the ancillary measurement of the backscatter ratio, which is the ratio of the total backscatter coefficient to the molecular backscatter coefficient. The modeling in this paper accounts for the major sources of error. It indicates that the vertical velocity can be retrieved with an error of less than 0.56 m s−1 below 4 km with a 150-m range resolution and an averaging time of 5 min. Significance Statement Monitoring the temperature, relative humidity, and winds in the lower atmosphere is important for improving weather forecasting, particularly for severe weather such as thunderstorms. Cost-effective micropulse differential absorption lidar (MPD) instrumentation for continuous temperature and humidity monitoring has been developed and demonstrated, and its effects on weather forecasting are currently being evaluated. The modeling study described in this paper studies the feasibility of using a similar cost-effective MPD instrument architecture for monitoring vertical wind velocity in the lower atmosphere. Modeling indicates that wind velocities can be measured with less than 0.56 m s−1 accuracy and demonstrates the feasibility of adding vertical wind velocity measurements to the MPD instruments.

Towards vertical wind and turbulent flux estimation with multicopter uncrewed aircraft systems

Abstract. Vertical wind velocity and its fluctuations are essential parameters in the atmospheric boundary layer (ABL) to determine turbulent fluxes and scaling parameters for ABL processes. The typical instrument to measure fluxes of momentum and heat in the surface layer are sonic anemometers. Without the infrastructure of meteorological masts and above the typical heights of these masts, in situ point measurements of the three-dimensional wind vector are hardly available. We present a method to obtain the three-dimensional wind vector from avionic data of small multicopter uncrewed aircraft systems (UAS). To achieve a good accuracy in both average and fluctuating parts of the wind components, calibrated motor thrusts and measured accelerations by the UAS are used. In a validation campaign, in comparison to sonic anemometers on a 99 m mast, accuracies below 0.2 m s−1 are achieved for the mean wind components and below 0.2 m2 s−2 for their variances. The spectra of variances and covariances show good agreement with the sonic anemometer up to 1 Hz temporal resolution. A case study of continuous measurements in a morning transition of a convective boundary layer with five UAS illustrates the potential of such measurements for ABL research.

Scaling effects of fixed-wing ground-generation airborne wind energy systems

Abstract. While some airborne wind energy system (AWES) companies aim at small, temporary or remote off-grid markets, others aim at utility-scale, multi-megawatt integration into the electricity grid. This study investigates the scaling effects of single-wing, ground-generation AWESs from small- to utility-scale systems, subject to realistic 10 min, onshore and offshore wind conditions derived from a numerical mesoscale Weather Research And Forecasting (WRF) model. To reduce computational cost, vertical wind velocity profiles are grouped into 10 clusters using k-means clustering. Three representative profiles from each cluster are implemented into a nonlinear AWES optimal control model to determine power-optimal trajectories. We compare the effects of three different aircraft masses and two sets of nonlinear aerodynamic coefficients for aircraft with wing areas ranging from 10 to 150 m2 on operating parameters and flight trajectories. We predict size- and mass-dependent AWES power curves, annual energy production (AEP) and capacity factors (cf) and compare them to a quasi-steady-state reference model. Instantaneous force, tether-reeling speed and power fluctuations as well as power losses associated with tether drag and system mass are quantified.

Enhancement of thrust force of an atmospheric pressure positive corona discharge by DC superimposed AC high voltage

The ability of corona discharge as an electrohydrodynamic propulsion system has been considered by many physicists and aerospace researchers. The results show that the most important factor in increasing the thrust force and thrust effectiveness is increasing the momentum transmission frequency in other words reduction of ion mobility that leads to a reduction of the average velocity. By configuring the wire-cylinder in atmospheric conditions, in an experimental study, using a new strategy in generating corona discharge, and without changing the system configuration, the thrust force is increased by increased of exciting species and reducing the ion mobility. DC superimposed AC (AC-DC) voltage source was utilized to achieve higher thrust force efficiency. Results show that the thrust force generated by the AC-DC source is increased by 4–2 times, with the applied voltage range of 10–20 kV compared to the DC source, respectively; while the thrust effectiveness has also been increased. A theory is introduced to calculate the thrust force due to ionic wind generation in the corona discharge regime. Accordingly, a relation is obtained for calculating thrust force and ion mobility using the average vertical ionic wind velocity on the side of the grounded electrode to support experimental results.

건물 밀집도에 따른 도시협곡 풍환경 평가

In this study, based on conditions observed for different building densities, the conditions for changing the width between buildings around the reference building were assigned. Accordingly, a sensitivity test for changes in wind conditions in urban canyons was performed, and the pattern and changes in wind formed under different test conditions were compared and analyzed. In addition, the characteristics of the wind environment occurring in the canyon, the wind speed variability, and wind velocity components were considered. The results showed that the wind speed in urban canyons increased or decreased according to the density of the buildings. In particular, it was confirmed that some vortex flow occurred in the canyons (y/H = 1.5), and various surface flow types were observed depending on the experimental conditions regarding the vertical and horizontal wind velocity components. In the case of horizontal flow, it was confirmed that the wind speed strength of the wall increased as the x/H ratio increased, regardless of the y/H condition. However, the simulation of the characteristics differed depending on the canyon. Therefore, changes in wind field and wind velocity components that occur due to building density centered on the reference building were confirmed.

Forecasting pressure drop and maximum sustained wind speed associated with cyclonic systems over Bay of Bengal with neuro-computing

The current research anticipates developing a model based on adaptive neuro-computation to foresee the minimum pressure drop (PD) at the centre as well as the maximum sustained wind speed (MSWS) accompanying with cyclonic systems over Bay of Bengal (BOB). The cyclonic systems taken in this work contain systems of different ranges starting from deep depression to extreme severe cyclones. For predicting PD and MSWS, suitable predictors have been sorted using factor analysis and it is observed that low-level vorticity (LLV), mid-tropospheric relative humidity (MRH) and vertical wind velocity at 850, 500 and 200 hPa pressure levels are appropriate parameters to create input matrix of neural network (NN). The adaptive NN representations are skilled with the data from 1990 to 2015 to estimate the PD as well as MSWS over BOB for 47 cyclonic systems. The outcome divulges that the multi-layer perceptron (MLP) NN model delivers decent precision at 6- and 30-h lead time in foretelling the PD. But the lowest error has been found at 6-h lead time in forecasting the central PD during mature stage of cyclonic systems. The result also illustrates that the MLP model is the utmost capable in forecasting the MSWS during mature stage of cyclonic structures with the lowest prediction error at 60-h lead time. The model results were validated and compared with the operational forecast by IMD for the 10 cyclonic systems from 2016 to 2019.

Evaluation of the Wind Environment around Multiple Urban Canyons Using Numerical Modeling

This study aimed to evaluate the wind environment in step-up and step-down urban canyons through a computational numerical experiment using the computational fluid dynamics (CFD) model. Spatial structural conditions were considered according to the location of high-rise buildings, and the changing wind patterns inside canyons were compared and analyzed by varying the building heights. Under the step-up to step-down condition, wind velocity inside the canyon weakened, a vertical vortex formed, and vertical air flow separated; additionally, in shallow and deep canyons, wind velocity and detailed flow differed slightly according to each additional condition. For the step-down to step-up condition, the building located in the center appeared to be isolated, and a general wind environment phenomenon consistent with the step-up and step-down structures was observed. However, depending on the isolated area, an additional roof-top canyon was formed, and the wind field in the canyon was found to affect the wind velocity and detailed flow in other canyons. The wind velocity components of the inflow and outflow winds into the canyon differed based on the step-up to step-down or step-down to step-up conditions, and according to the conditions in the first and second canyons. Furthermore, the vertical wind velocity components were greatly affected by the step-up and step-down structures. Accordingly, the height and structural location of the building could affect various phenomena, such as the separation of vortices and air currents inside the canyon, and a variable wind environment was formed according to a series of conditions for the building.

A cold front induced co-occurrence of O3 and PM2.5 pollution in a Pearl River Delta city: Temporal variation, vertical structure, and mechanism

In this study, the spatiotemporal variabilities and characteristics of ozone (O3) and fine particulate matter (PM2.5) were reconstructed, and the interaction between meteorological conditions and the co-occurrence of O3 and PM2.5 in Zhuhai, a city in the Pearl River Delta (China), was analysed. The vertical distributions of lower tropospheric O3, aerosol extinction coefficient, and wind velocity were measured using a ground-based LiDAR system. The diurnal variations in air pollutant concentrations and meteorological conditions at ground level were examined from 28 November to December 8, 2020 considering the weather conditions in Zhuhai. Heavy pollution episodes with increased concentrations of O3 and PM2.5 were observed from 6 to 7 December after a period of cold air invasion. The maximum hourly average concentrations of O3 and PM2.5 at the ground level reached up to 190 μg/m3, 98 μg/m3, respectively. The horizontal wind speed rapidly decreased to less than 2 m/s during the heavy pollution episodes driven by O3 and PM2.5, whereas the vertical wind velocity was dominated by the downdraught. When the large-scale synoptic winds were weak, a strengthening sea breeze in the afternoon could promote the landward propagation of warm marine air masses, and a lower surface wind speed was driven by the convergence of cold air from the north and warm air from the south. In turn, this increased the residence time of air pollutants and promoted their conversion to secondary pollutants. Regarding the pollution sources, the results indicated that the Pearl River Estuary represented a ‘pool’ of O3 and PM2.5 pollution. In addition, the contribution of regional pollutant transport could not be ignored when considering the accumulative increase in air pollution. Overall, the relatively weak synoptic winds, low mixing height, and high generation of pollution around Zhuhai collectively resulted in high concentrations of O3 and PM2.5.

Investigating the Linkage between Extreme Rainstorms and Concurrent Synoptic Features: A Case Study in Henan, Central China

Extraordinary floods are linked with heavy rainstorm systems. Among various systems, their synoptic features can be quite different. The understanding of extreme rainstorms by their causative processes may assist in flood frequency analysis and support the evaluation of any changes in flood occurrence and magnitudes. This paper aims to identify the most dominant meteorological factors for extreme rainstorms, using the ERA5 hourly reanalysis dataset in Henan, central China as a case study. Past 72 h extreme precipitation events are investigated, and six potential factors are considered in this study, including precipitable water (PW), the average temperature (Tavg) of and the temperature difference (Tdiff) between the value at 850 hPa and 500 hPa, relative humidity (RH), convective available potential energy (CAPE), and vertical wind velocity (Wind). The drivers of each event and the dominant factor at a given location are identified using the proposed metrics based on the cumulative distribution function (CDF). In Henan, central China, Wind and PW are dominant factors in summer, while CAPE and Wind are highly related factors in winter. For Zhengzhou city particularly, Wind is the key driver for summer extreme rainstorms, while CAPE plays a key role in winter extreme precipitation events. It indicates that the strong transport of water vapor in summer and atmospheric instability in winter should receive more attention from the managers and planners of water resources. On the contrary, temperature-related factors have the least contribution to the occurrence of extreme events in the study area. The analysis of dominant factors can provide insights for further flood estimations and forecasts.

Characterizing warm atmospheric boundary layer over land by combining Raman and Doppler lidar measurements.

PBL plays a critical role in the atmosphere by transferring heat, moisture, and momentum. The warm PBL has a distinct diurnal cycle including daytime convective mixing layer (ML) and nighttime residual layer developments. Thus, for PBL characterization and process study, simultaneous determinations of PBL height (PBLH) and ML height (MLH) are necessary. Here, new approaches are developed to provide reliable PBLH and MLH to characterize warm PBL evolution. The approaches use Raman lidar (RL) water vapor mixing ratio (WVMR) and Doppler lidar (DL) vertical velocity measurements at the Southern Great Plains (SGP) atmospheric observatory, which was established by the Atmospheric Radiation Measurement (ARM) user facility. Compared with widely used lidar aerosol measurements for PBLH, WVMR is a better trace for PBL vertical mixing. For PBLH, the approach classifies PBL water vapor structures into a few general patterns, then uses a slope method and dynamic threshold method to determine PBLH. For MLH, wavelet analysis is used to re-construct 2-D variance from DL vertical wind velocity measurements according to the turbulence eddy size to minimize the impacts of gravity wave and eddy size on variance calculations; then, a dynamic threshold method is used to determine MLH. Remotely-sensed PBLHs and MLHs are compared with radiosonde measurements based on the Richardson number method. Good agreements between them confirm that the proposed new algorithms are reliable for PBLH and MLH characterization. The algorithms are applied to warm seasons' RL and ML measurements at the SGP site for five years to study warm season PBL structure and processes. The weekly composited diurnal evolutions of PBLHs and MLHs in warm climate were provided to illustrate diurnal and seasonal PBL evolutions. This reliable PBLH and MLH dataset will be valuable for PBL process study, model evolution, and PBL parameterization improvement.

Investigating hydrometeorology of the Western Himalayas: Insights from stable isotopes of water and meteorological parameters

The Himalayas govern the hydrometeorology of the entire Indian subcontinent and feed 19 major rivers on which a large population is dependent. Despite its vast socio-economic relevance, there exist knowledge gaps in the detailed understanding of Himalayan hydrometeorology. The present study attempts to understand subtle hydrometeorological processes concerning precipitation in Western Himalaya (WH), which is further complex due to several mountain ranges in addition to the Great Himalaya and varied land cover. In this backdrop, oxygen and hydrogen isotopic analyses of daily precipitation samples collected from Jammu was done. Major processes and features identified are: (1) Raindrop re-evaporation is dominant, indicated by lower δ18O – δD regression slope (6.6 ± 0.2). (2) Maximum raindrop re-evaporation occurs during Western Disturbances (WD), evident from the low d-excess (<10‰) and high δ18O (>0‰) with low rainfall. (3) Contrary to expectations, negligible moisture is derived from the Mediterranean region (MR) through WD. In contrast, maximum moisture (~97%) is derived from local terrestrial sources, estimated through backward wind trajectories and confirmed by vertical wind velocity (ω) and OLR. (4) The Arabian Sea (AS) moisture contributes more (~11%) compared to the Bay of Bengal (BOB) moisture (~8%) during the monsoon. (5) Isotopically, the most depleted precipitation during July-Aug is associated with continental recycling from the Indo-Gangetic plains and not with the BOB moisture. (6) Isotopic enrichment in precipitation towards end of monsoon is attributed to raindrop re-evaporation, lesser rainout fraction and moisture derived from enriched local sources. (7) Annually, 87% of the moisture for precipitation is derived through continental recycling, and only 13% of the moisture is derived through marine sources (11% from AS and 2% from BOB). Moisture from MR (<0.1%) is negligible.

Representation model of wind velocity fluctuations and saltation transport in aeolian sand flow

Since the stream-wise and vertical turbulent fluctuations have an important influence on the movement of sand and dust particles, the accurate quantitative estimation of natural wind fluctuations has an important role in improving the forecast level of wind erosion and dust storm. The high Reynolds number turbulence characteristics of aeolian sand flow and the complexity of sand-laden flows are the main difficulties affecting the accuracy of prediction. In this study, the characteristics of the stream-wise and vertical wind velocity fluctuations in sand-laden flows are studied by analyzing the real-time data of the wind velocity field in the atmospheric surface layer (ASL). Then, a simple empirical model representing the stream-wise and vertical wind velocity fluctuations is proposed. The model not only considers the effects of large-scale and small-scale fluctuations, but also has the advantage of predicting the stream-wise and vertical wind velocity fluctuations at any height. Verification shows the results predicted by the model are in good agreement with the observation results, and the predicted saltation sand flux is closer to the experimental results than that without considering the turbulence effect or the vertical wind velocity fluctuations. Moreover, it is a convenient model for application especially in predicting aeolian sand transport.

Atmospheric response to the annular solar eclipse of 26 December 2019 over Cochin, India

• Atmospheric response to annular solar eclipse of December 2019. • Drastic reduction in incoming solar radiation and variations in meteorological variables. • Mean PBL and Tropopause Heights were reduced by 50% and 5.5%. • Detected the presence of internal gravity waves with periodicities of 40–60 min. • Study useful for radiative transfer calculation and also for testing numerical models. The rapid response of the atmosphere to the reduction in solar radiation during the annular solar eclipse on 26th December 2019 with maximum obscurity of 91.16% over Cochin, located in the southwest peninsular India, has been investigated. A suite of instruments encompassing an advanced VHF wind profiling Radar operating at 205 MHz, GPS radiosonde, and automatic weather station was employed to record the temporal and vertical variations in various meteorological parameters. Compared to the corresponding values averaged for two control days neighboring the eclipse day, the temperature, relative humidity, net radiation, pressure and wind speed at the surface were altered by −4.0 °C, +39.6%, −383.0 Wm −2 , +0.62 mb, and −2.6 ms −1 , respectively. The corresponding percentage variations are −13.6%, +71.0%, −67.0%, +0.06%, and −99.0% during the maximum phase of the eclipse, and approached close to the control-day values within about 2 h. Multiple temperature inversions as large as 1 °C were observed up to a height of about 7.3 km. The heights of mean planetary boundary layer (PBL) and tropopause were decreased by about 355 m (−50%) and 900 m (−5.5%), respectively during the peak of obscuration. Meanwhile, the vertical profiles of both the horizontal and vertical wind velocities demonstrate substantial reduction. The vertical wind dispersion, which characterizes the fluctuations in vertical air motion, was reduced by 34% in the PBL compared to its corresponding values on the control days. Subsequently, wavelet spectral analysis of the vertical wind yields a quasi-periodic oscillation indicating the presence of internal gravity waves with periodicities of 40–60 min in the lower altitudes up to 3 km during the eclipse day. Besides reporting the characteristic variations in various atmospheric parameters, the present study reveals the height-time variations in the vertical winds, a vital parameter of significance in the studies of convection, turbulence, and atmospheric waves.

Analysis of Atmospheric Stability for the Prevention of Coastal Disasters and the Development of Efficient Coastal Renewable Energy

Kim, H.; Moon, C.-J.; Kim, Y.-G.; Chon, K.-H.; Joo, J.Y., and Ryu, G.H., 2021. Analysis of atmospheric stability for the prevention of coastal disasters and the development of efficient coastal renewable energy. In: Lee, J.L.; Suh, K.S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 241–245. Coconut Creek (Florida), ISSN 0749–0208. The ground friction force is lower in coastal and marine areas than it is on land, and there is a relatively high frequency of strong winds in such areas, which substantially affects the life and stability of wind turbines. In particular, the strong wind at the bottom of the atmospheric boundary layer is generated by a combination of thermal and mechanical factors, so there is always a risk due to vertical wind shear and strong turbulent gusts. Therefore, to prevent physical loss of wind turbines located in the atmospheric boundary layer, it is necessary to analyze the wind condition through a more accurate atmospheric stability analysis. In this study, observation data from the Boseong Standard Meteorological Tower are examined to select strong wind cases in the lower atmospheric boundary layer, and the vertical distribution characteristics of the wind resources are analyzed according to the Monin-Obukhov length (MOL). The results confirmed that the vertical wind velocity gradient caused by atmospheric stability mainly changes when the atmosphere is in the near-neutral state at night during the winter.

Toward inclusion of atmospheric effects in the aircraft community noise predictions.

This paper presents an atmospheric propagation model, based on ray acoustics, that accounts for realistic weather conditions in the evaluation of the noise footprint of an aircraft. Noise sources, obtained using the Ffowcs Williams and Hawkings acoustic analogy applied to scale-resolved flow simulation data, are stored on a hemisphere surrounding the vehicle. These noise sources are propagated using a propagation model that takes into account the vertical variability of air temperature and wind velocity. The electric vertical takeoff and landing aircraft, presented by Casalino, van der Velden, and Romani [(2019). in Proceedings of the AIAA Scitech 2019 Forum, January 7-11, San Diego, CA, pp. 1834-1851], is used as a case study; noise footprints, obtained considering various vertically varying temperature and wind velocity distributions, are compared. It is shown that weather conditions in the acoustic wave propagation can contribute to mismatch up to 4 dBA in the illuminated zone and a significant drop in the refractive shadow zone caused by the vertical air temperature and wind velocity gradients. This work constitutes the first accomplishment in including realistic atmospheric effects in aircraft community noise prediction based on scale-resolved flow simulations.

The high-frequency response correction of eddy covariance fluxes – Part 2: An experimental approach for analysing noisy measurements of small fluxes

Abstract. Fluxes measured with the eddy covariance (EC) technique are subject to flux losses at high frequencies (low-pass filtering). If not properly corrected for, these result in systematically biased ecosystem–atmosphere gas exchange estimates. This loss is corrected using the system's transfer function which can be estimated with either theoretical or experimental approaches. In the experimental approach, commonly used for closed-path EC systems, the low-pass filter transfer function (H) can be derived from the comparison of either (i) the measured power spectra of sonic temperature and the target gas mixing ratio or (ii) the cospectra of both entities with vertical wind speed. In this study, we compare the power spectral approach (PSA) and cospectral approach (CSA) in the calculation of H for a range of attenuation levels and signal-to-noise ratios (SNRs). For a systematic analysis, we artificially generate a representative dataset from sonic temperature (T) by attenuating it with a first order filter and contaminating it with white noise, resulting in various combinations of time constants and SNRs. For PSA, we use two methods to account for the noise in the spectra: the first is the one introduced by Ibrom et al. (2007a) (PSAI07), in which the noise and H are fitted in different frequency ranges, and the noise is removed before estimating H. The second is a novel approach that uses the full power spectrum to fit both H and noise simultaneously (PSAA21). For CSA, we use a method utilizing the square root of the H with shifted vertical wind velocity time series via cross-covariance maximization (CSAH,sync). PSAI07 tends to overestimate the time constant when low-pass filtering is low, whilst the new PSAA21 and CSAH,sync successfully estimate the expected time constant regardless of the degree of attenuation and SNR. We further examine the effect of the time constant obtained with the different implementations of PSA and CSA on cumulative fluxes using estimated time constants in frequency response correction. For our example time series, the fluxes corrected using time constants derived by PSAI07 show a bias between 0.1 % and 1.4 %. PSAA21 showed almost no bias, while CSAH,sync showed bias of ±0.4 %. The accuracies of both PSA and CSA methods were not significantly affected by SNR level, instilling confidence in EC flux measurements and data processing in set-ups with low SNR. Overall we show that, when using power spectra for the empirical estimation of parameters of H for closed-path EC systems the new PSAA21 outperforms PSAI07, while when using cospectra the CSAH,sync approach provides accurate results. These findings are independent of the SNR value and attenuation level.

Evaluation of Profiles of Standard Deviation of Vertical Wind in the Urban Area of Rome: Performances of Monin–Obukhov Similarity Theory Using Different Scaling Variables

The parametrizations of meteorological variables provided by the Monin–Obukhov similarity theory (MOST) is of major importance for pollutant dispersion assessment. However, the complex flow pattern that characterizes the urban areas limits the applicability of the MOST. In this work, the performance of different existing parametrizations of the standard deviation of vertical wind velocity were tested in the city of Rome. Results were compared with experimental data acquired by a sonic detection and ranging (SODAR) and a sonic anemometer. Different scaling variables estimated from the anemometer data by considering two coordinate systems—one aligned with the geodetic reference frame and the other following the flow streamlines—were used to evaluate the effects of flow distortion due to the presence of buildings. Results suggest that the MOST parametrizations perform better if the scaling variables obtained using the coordinate system following the flow streamlines are used. This estimation of the scaling variables would make it possible to overcome the difficulties in conducting measurements of turbulent fluxes, either at different altitudes or even in the constant flux layer.