A rapid-response 2-D drag anemometer for atmospheric turbulence measurements

A rapid response drag anemometer for measuring streamwise and lateral components of horizontal windspeed is described. Theory of operation, design and calibration are discussed with emphasis on the electronic preconditioning of signals and problems associated with using a mechanically resonant system as a sensor. Field comparisons showed half-hourly means and standard deviations of the streamwise component to be within 8% and 5% of respective values obtained from a 3-dimensional sonic anemometer. The lateral component from the drag anemometer was significantly more noisy than that from the 3-D sonic due to induced oscillations arising from vortex shedding. After mechanical and electronic filtering, half-hourly standard deviation comparisons agreed to within 6% for this component. Friction velocities obtained from the drag anemometer in combination with a 1-D sonic, agreed with measurements from the 3-D sonic anemometer to within 4% over a measured range of 0.05 to 1.2 m s^-1     

How Well Can We Measure the Vertical Wind Speed? Implications for Fluxes of Energy and Mass

Sonic anemometers are capable of measuring the wind speed in all three dimensions at high frequencies (10–50 Hz), and are relied upon to estimate eddy-covariance-based fluxes of mass and energy over a wide variety of surfaces and ecosystems. In this study, wind-velocity measurement errors from a three-dimensional sonic anemometer with a non-orthogonal transducer orientation were estimated for over 100 combinations of angle-of-attack and wind direction using a novel technique to measure the true angle-of-attack and wind speed within the turbulent atmospheric surface layer. Corrections to the vertical wind speed varied from −5 to 37% for all angles-of-attack and wind directions examined. When applied to eddy-covariance data from three NOAA flux sites, the wind-velocity corrections increased the magnitude of CO₂ fluxes, sensible heat fluxes, and latent heat fluxes by ≈11%, with the actual magnitude of flux corrections dependent upon sonic anemometer, surface type, and scalar. A sonic anemometer that uses vertically aligned transducers to measure the vertical wind speed was also tested at four angles-of-attack, and corrections to the vertical wind speed measured using this anemometer were within ±1% of zero. Sensible heat fluxes over a forest canopy measured using this anemometer were 15% greater than sensible heat fluxes measured using a sonic anemometer with a non-orthogonal transducer orientation. These results indicate that sensors with a non-orthogonal transducer orientation, which includes the majority of the research-grade three-dimensional sonic anemometers currently in use, should be redesigned to minimize sine errors by measuring the vertical wind speed using one pair of vertically aligned transducers.     

A comparison of turbulence measurements by different instruments; Tsimlyansk field experiment 1970

In the summer of 1970 an international intercomparison of turbulence measurement sensors was carried out at the Tsimlyansk field station of the Institute of Atmospheric Physics (IFA) Academy of Sciences of the U.S.S.R. Improved sonic anemometer sensors of the IFA were compared with the Kaijo sonic used by the Canadian group and with the fluxatron from CSIRO, Australia.Results from the experiment indicate that spectral and cospectral shapes are basically similar between the sonic anemometers. The spectra and cospectra of the fluxatron in the high-frequency region showed an attenuation which can be explained in terms of the response characteristics of the fluxatron. The maintenance of absolute calibration of the sensors in the field was found to be difficult, but when properly corrected for this variation, the measurements were found to agree within 13%, with a standard error of 0.2. A single point observation of turbulent fluxes at a few meters above the surface and averaged over a period of the order of one hour was shown to be representative of turbulent fluxes at the site.     

Sonic Anemometer as a Small Acoustic Tomography Array

The spatial resolution of a sonic anemometer is limited by the distance between its transducers, and for studies of small-scale turbulence and theories of turbulence, it is desirable to increase this spatial resolution. We here consider resolution improvements obtainable by treating the sonic anemometer as a small tomography array, with application of appropriate inverse algorithms for the reconstruction of temperature and velocity. A particular modification of the sonic anemometer is considered when the number of its transducers is doubled and the time-dependent stochastic inversion algorithm is used for reconstruction. Numerical simulations of the sonic anemometer and its suggested modification are implemented with the temperature and velocity fields modelled as discrete eddies moving through the sonic’s volume. The tomographic approach is shown to provide better reconstructions of the temperature and velocity fields, with spatial resolution increased by as much as a factor of ten. The spatial resolution depends on the inverse algorithm and also improves by increasing the number of transducers.     

Eddy flux measurements over Lake Ontario

Wind, temperature and humidity fluctuations have been recorded using a sonic anemometer-thermometer, a thrust anemometer, and a La humidiometer. The two anemometers agree in wind speed and stress. Exchange coefficients for momentum, heat and moisture are found to agree with values measured over other bodies of water.Contribution 391, Bedford Institute of Oceanography.Project 75 BL, International Field Year for the Great Lakes.     

New Equations For Sonic Temperature Variance And Buoyancy Heat Flux With An Omnidirectional Sonic Anemometer

Many new types of sonic anemometer obtain sonic temperature from an average value of temperature measured along three paths, unlike previous sonic anemometers that generally used one path. New equations are derived to calculate temperature variance from sonic temperature variance and sensible heat flux from buoyancy flux considering the influence of a crosswind. These equations can be applied to CSAT3, Solent R2, R3, R3A, HS, and USA-1 sonic anemometers with the corresponding correction factors given in this paper. The equations are verified by data measured by a CSAT3 sonic anemometer in the LITFASS-1998 field study.     

Thermal Image Velocimetry

A method for measuring the two-dimensional distribution of wind velocity vectors near a surface exposed to solar radiation, by tracking brightness temperature images instead of particle images, is proposed. It is based on time-sequential thermography with the algorithm used for particle image velocimetry. This thermal image velocimetry (TIV) was tested on a full-scale building wall covered by polystyrene boards attached side-by-side over a vertically elongated area measuring 22.2 m by 2.73 m. A thermal infrared camera was installed 8 m from the test wall to capture the wall-surface temperature at 30 Hz frequency. A sonic anemometer was also installed 35 mm from the surface used for validation of the TIV. The advection velocity estimated from thermal infrared imagery had a linear relationship with the wind velocity measured by the sonic anemometer, irrespective of the wind speed and direction. This linear slope was multiplied by the advection velocity of the thermal infrared image to rescale it to the wind velocity, and the term ‘TIV velocity’ was then used. A histogram and power spectra of the TIV velocity showed quantitatively good agreement with the velocity measured by the sonic anemometer, except for the high-frequency region of the spectra, where the TIV velocity was overestimated compared with that of the sonic anemometer. The method was also tested on ground covered by artificial turf to demonstrate its application to a horizontal plane with a wider area, extending for more than 80 m by 60 m.     

风杯风速仪过高效应的订正研究

该文基于对风杯风速仪动力方程的数值计算和风杯风速仪与超声风速仪平行对比的观测结果,发现专用于近地面层精细风廓线测量的轻型低阈值风杯风速仪测风的过高效应引起的u-error约为1%,因风速平均方法不同（标量平均和矢量平均）引起的DP-error是风杯风速仪测风过高的主要原因,在小风情况下更为明显。DP-error经修正后可获得合理的风速值。     

A Comment on “How Well Can We Measure the Vertical Wind Speed? Implications for Fluxes of Energy and Mass” by Kochendorfer et al.

Kochendorfer et al. (Boundary-Layer Meterol, 145:383–398, 2012) conducted an experiment to evaluate azimuth and angle-of-attack dependent errors of sonic anemometer measurements. Several questions are raised regarding the experimental design and the presented results. The finding that instruments with non-orthogonal sonic paths underestimate fluctuations of vertical wind speed and consequently also scalar fluxes by about 10 % is compared with the results of a hitherto unpublished side-by-side field comparison and other past intercomparison experiments. Scale considerations are presented that raise considerable doubts on the validity of the implicit assumption of Kochendorfer et al. (2012) that the turbulent wind vector is highly correlated across a distance of 1.2 m at a height of 2.5 m over flat grassland, which corresponds to the separation between the sonic anemometers tested in their experiment. Nevertheless, new developments in sonic anemometer design to minimize transducer-shadow effects are desirable.     

Infrared device for simultaneous measurement of fluctuations of atmospheric carbon dioxide and water vapor

An infrared device designed to measure simultaneous fluctuations of atmospheric CO₂ and water vapor concentrations is described. The measuring frequency is 30 Hz. The sensing path length is 20 cm. It is compatible with the path length of the standard type of a sonic anemometer. The noise level of the device is equivalent to fluctuations of about 0.8 ppm peak-to-peak for CO₂ and 0.02 g kg^-1 peak-to-peak for water vapor. Field tests have showed that the device is suitable for simultaneous measurement of turbulent fluxes of CO₂ and water vapor in conjunction with a sonic anemometer.     

Detection of Flow Distortions and Systematic Errors in Sonic Anemometry Using the Planar Fit Method

We describe the coordinate transformations that can be used to convert the velocity components measured by a set of sonic anemometers with time-dependent tilt fluctuations into a single, time-independent coordinate system. By applying the planar fit method (PFM) to each anemometer dataset, it is possible, for planar flows, to locate the flow plane at each measurement point and compare its orientation with the topography. Installation on a ship is also considered. An application of this method to intercomparison data has led to the detection of an instrument error due to a misalignment between the assembly of the sonic transducers and the anemometer pedestal. If this error occurs, pedestal levelling does not guarantee that measurements are unbiased. A correction method is proposed and the results of two experiments are shown. Flow planarity at different levels and flow distortion caused by the mast are highlighted. The influence of the error on the evaluation of the Reynolds stresses using PFM or the double rotation method and the triple rotation method is discussed and the tilt corrected stresses calculated using the three methods compared.     

A revaluation of the Kansas mast influence on measurements of stress and cup anemometer overspeeding

For the 1968 Kansas atmospheric surface-layer experiment, a supplementary analysis is made of the evaluation procedure. Available data on the ratio of wind speeds measured on separate booms show a variation with wind direction which is too large for an open mast. Actually the Kansas mast appears to have carried a bulky array of apparatus at the sonic anemometer levels. It is shown that the air flow interference caused by this obstacle can be satisfactorily estimated by way of potential flow calculations. From these it follows that the sonic anemometer measurements probably have undervalued the free-flow eddy stress by 20% to 30%, which implies that the simultaneous drag plate measurements of stress were generally correct. Also the overestimation of the mean wind speed by the Kansas cup anemometer is found to have been 6% rather than 10%. Some Kansas evaluation results are amended accordingly. The von Kármán constant is found to be 0.41 rather than 0.35, and the near-adiabatic eddy diffusivity ratio K _H /K _M becomes 1.0 rather than 1.3. The flux-gradient relations (Businger et al., 1971) after similar revision no longer differ significantly from those obtained elsewhere.     

Occurrence and features of ducted modes of internal gravity waves over western Europe and their influence on microwave propagation

Several years of intense investigation of internal gravity waves propagating in low-level ducts in the atmosphere over a rather flat region not influenced by land-sea circulation or mountains are reported. This study gives a rather complete picture of the occurrence, features, and of some of the excitation conditions of these waves. Simultaneous direct measurements made on a meteorological tower, 250 m high, using temperature, vector wind, and sonic anemometer sensors at different altitudes are reported along with indirect measurements using an array of microbarographs with separations over a range of more than 100 km. The study also included microwave propagation links crossing over the meteorological site. Individual wave crests some tens of kilometers long could be followed over a path of more than 100 km. Because of the variability and non-stationary conditions of the lower atmosphere, a direct correlation between a propagating internal wave group and the fading pattern of EM propagation could be found in only a few cases.It is interesting to note Goldie's references to Kelvin and Helmholtz in this regard.     

Observation of gravity waves in a boreal forest

In this paper we report the results of the analysis of two 60-min wave events that occurred in a boreal aspen forest during the 1994 BOREAS (Boreal Ecosystems-Atmosphere Study) field experiment. High frequency wind and temperature data were provided by three 3-D sonic anemometer/thermometers and fourteen fine-wire thermocouples positioned within and above the forest. Wave phase speeds, estimated from information revealed by spectral analysis and linear plane wave equations, are 2.2 and 1.3 m s^-1 for the two events. The wavelengths are 130 m and 65 m respectively and are much larger than the vertical wave displacements. There is strong evidence from the present analysis and from the literature supporting our postulate that these waves are generated by shear instability. We propose that wind shear near the top of the stand is often large enough to reduce the gradient Richardson number below the critical value of 0.25 and thus is able to trigger the instability. When external conditions are favorable, the instability will grow into waves.     

The Effects of Probe-Induced Flow Distortion on Velocity Covariances: Field Observations

We carried out measurements to test a simple theory of the effect of probe-induced flow distortion on turbulence measurements. We used two three-component sonic anemometers mounted 1.8m apart at a height of 6.7 m. Behind one was a horizontal circular cylinder of radius 0.15 m and length 1.2 m, chosen to model two-dimensional probe-induced flow distortion in the limit where the scale of the turbulence is very large compared to the scale of the probe. The second sonic anemometer measured the undistorted flow. The measured flow-distortion effects on the Reynolds shearing stress and the variances of streamwise and vertical velocity agree well with the theory.     

Temperature measurement with a sonic anemometer and its application to heat and moisture fluxes

Boundary-Layer Meteorology - The possibility of measuring heat and moisture fluxes using sonic anemometer data is investigated. Theoretical relations for the temperature variance and heat flux are...     

Surface renewal analysis for sensible and latent heat flux density

High frequency temperature measurements were recorded at five heights and surface renewal (SR) analysis was used to estimate sensible heat flux density (H) over 0.1 m tall grass. Traces of the temperature data showed ramp-like structures, and the mean amplitude and duration of these ramps were used to calculate H using structure functions. Data were compared with H values measured with a sonic anemometer. Latent heat flux density (E) was calculated using an energy balance and the results were compared with E computed from the sonic anemometer data. SR analysis provided good estimates of H for data recorded at all heights but the canopy top and at the highest measurement level, which was above the fully adjusted boundary layer.     

Corrections to Inertial-Range Power Spectra Measured by Csat3 and Solent Sonic Anemometers,1. Path-Averaging Errors

Sonic anemometer path-averaging errors are determined for measurements of inertial-range velocity and temperature power spectra as a function of k ₁ p, where k ₁ is the streamwise wave number and p is the sonic path length. The attenuation of vertical velocity spectra is found to be quite similar for the CSAT3 and Solent anemometers and to be insensitive to wind direction. The attenuation of the horizontal wind component spectra is noticeably greater for the Solent sonics than for the CSAT3, but those for the CSAT3 have a greater dependence on wind direction. The attenuation of sonic temperature spectra is also found to be quite similar for the CSAT3 and Solent R3 sonics and to be insensitive to wind direction, while that of the Solent R2 is less than that of the other two sonics and has a minor dependence on wind direction.     

Evaluation of the Mark 8 thrust anemometer-thermometer for measurement of boundary-layer turbulence

The most recent of a series of thrust anemometers, designed for measurement of wind turbulence and eddy fluxes and for long-term unattended operation, is evaluated. Calibration and data analysis procedures are outlined, and results of field trials are given, showing agreement with data from a sonic anemometer.     

New omnidirectional anemometer with no moving parts

An anemometer based upon measurement of the tangential windspeed around a sphere with hot-film probes is described. The anemometer determined the windspeed with a root-mean-square (rms) error of 5%, and the direction with an rms error of 5.6 °. A comparison between omnidirectional and sonic anemometers in the field gave practically identical results for the vertical sensible heat flux using eddy correlation procedures. Other turbulence statistics are also reported. The new instrument should be useful for measurements in canopies, where turbulence intensities are often large.