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.     

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...     

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.     

Direct measurement of turbulent fluxes on a cruising ship

The result of an attempt at the direct measurement of turbulent fluxes on the top of the mast of a cruising ship is presented. The three-dimensional components of wind relative to the ship measured by a sonic anemometer are corrected for ship motion; from these the fluxes of momentum, sensible heat and water vapor are computed using the outputs of a fine-wire thermocouple psychrometer. The observations were made by this method on the Northwestern Pacific. The results indicate that this technique is usable for determining the distribution of fluxes over the ocean.     

Stability Dependence of the Eddy-Accumulation Coefficients for Momentum and Scalars

From a set of turbulence data collected with a three-axis sonic anemometer/thermometer and described in a companion paper, we simulate the eddy-accumulation process for sensible heat and momentum fluxes. The resulting eddy-accumulation coefficient for momentum clearly depends on surface-layer stability; at neutral stability, its value is 0.63. On supplementing the scalar eddy-accumulation coefficients that we derive from our sensible heat flux data with values of sensible and latent heat flux coefficients reported by Businger and Oncley, we also find that scalar eddy-accumulation coefficients depend on stability, though more weakly than does the momentum coefficient. The coefficients for sensible and latent heat show no significant difference; we, thus, fit them with one function of stability whose value is 0.52 for neutral stratification.     

Cross‐shore variations of near‐surface wind velocity and atmospheric turbulence at the land‐sea boundary during CASP

Abstract

Airborne measurements of mean wind velocity and turbulence in the atmospheric boundary layer under wintertime conditions of cold offshore advection suggest that at a height of 50 m the mean wind speed increases with offshore distance by roughly 20% over a horizontal scale of order 10 km. Similarly, the vertical gust velocity and turbulent kinetic energy decay on scales of order 3.5 km by factors of 1.5 and 3.2, respectively. The scale of cross‐shore variations in the vertical fluxes of heat and downwind momentum is also 10 km, and the momentum flux is found to be roughly constant to 300 m, whereas the heat flux decreases with height. The stability parameter, z/L (where z = 50 m and L is the local Monin‐Obukhov length), is generally small over land but may reach order one over the warm ocean. The magnitude and horizontal length scales associated with the offshore variations in wind speed and turbulence are reasonably consistent with model results for a simple roughness change, but a more sophisticated model is required to interpret the combined effects of surface roughness and heat flux contrasts between land and sea.

Comparisons between aircraft and profile‐adjusted surface measurements of wind speed indicate that Doppler biases of 1–2 m s^?1 in the aircraft data caused by surface motions must be accounted for. In addition, the wind direction measurements of the Minimet anemometer buoy deployed in CASP are found to be in error by 25 ± 5°, possibly due to a misalignment of the anemometer vane. The vertical fluxes of heat and momentum show reasonably good agreement with surface estimates based on the Minimet data.     

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     

Evaluation of Noah-LSM for soil hydrology parameters in the Indian summer monsoon conditions

The micrometeorological observations, collected over a station in Ranchi (23°45′N, 85°30′E) which is under the monsoon trough region of India, were used in the Noah-LSM (NCEP, OSU, Air Force and Office of Hydrology Land Surface Model) to investigate the model performance in wet (2009 and 2011) and dry (2010) conditions during the south-west summer monsoon season. With this analysis, it is seen that the Noah-LSM has simulated the diurnal cycle of heat fluxes (sensible and ground) reasonably. The simulated heat fluxes were compared with its direct measurements by sonic anemometer and soil heat flux plate. The net radiation and sensible heat flux are simulated well by the model, but the simulation of ground heat flux was found to be poor in both dry as well as wet conditions. The soil temperature simulations were also found to be poor in 0–5- and 5–10-cm layers compared to other deeper layers. The observations were also correlated with the Modern Era Retrospective-analysis for Research and Applications (MERRA) data. The correlation between the observations and ground heat flux was better in MERRA dataset than that of the Noah-LSM simulation.     

An improved pressure-sphere anemometer

A pressure sphere anemometer for measuring the three wind components is described. It can be used within a meter of the surface. This pressure-sphere anemometer has a wider acceptance angle than the IMFL anemoclinometer, and is more easily constructed.     

Application of an infrared carbon dioxide and humidity instrument to studies of turbulent transport

A calibration equation and some results of the field performance of an infrared instrument, which is designed to measure simultaneous fluctuations of atmospheric carbon dioxide and water vapor, are described. Field observations show that the instrument is suitable for simultaneous measurement of turbulent fluxes of carbon dioxide and water vapor in conjunction with a sonic anemometer. Measured values of carbon dioxide and water vapor fluxes show diurnal variations characterized by crop activity with respect to assimilation, respiration and evapotranspiration. Carbon dioxide is transferred downward during the daytime and upward at night, while latent heat and sensible heat are transferred in the opposite sense. The non-dimensional gradient of carbon dioxide is expressed in the following form under weak unstable conditions: _c = (1 – 16 _v )^-1/2. Here, _v is the Monin-Obukhov stability parameter including the humidity effect. This relation was originally proposed for temperature and humidity. Thus, the results indicate that the turbulent mechanisms of carbon dioxide fluctuations are similar to those of other scalar entities. This is strongly supported by the high correlation coefficient found between fluctuations of carbon dioxide and temperature or humidity in the air layer over crop fields.     

Temperature and humidity flux-variance relations determined by one-dimensional eddy correlation

It may be possible to estimate surface fluxes of scalar quantities from measurement of their variance and mean wind speed. The flux-variance relation for temperature and humidity was investigated over prairie and desert-shrub plant communities. Fluxes were measured by one-dimensional eddy correlation, humidity by fast-response wet-bulb psychrometers and Krypton open-path hygrometers, temperature by fine-wire thermocouples, and mean windspeed by a cup anemometer. The quality of the flux-variance relation proved to be good enough for application to flux measurement. Regressions of flux estimated by the variance technique versus measured flux usually had r ² values greater than 0.97 for sensible heat flux and greater than 0.88 for water vapor flux. More uniform surfaces tended to yield the same flux-variance relations except when fluxes were small. This exception supported the hypothesis that sparse sources of flux may increase variance downwind. Nonuniform surfaces yielded flux-variance relations that were less predictable, although reasonably accurate once determined. The flux-variance relation for humidity was quite variable over dry surfaces with senescent vegetation.     

Surface and Mixed-Layer Variance Methods to Estimate Regional Sensible Heat Flux at the Surface

Turbulence measurements in the lower half of the convective boundary layer (CBL), which includes both mixed layer and surface layer, were carried out with five sonic anemometers mounted on a 213-m tower over a complex flat suburban area with patches of forest, agricultural land, houses and buildings. Also made were radiosoundings of temperature, humidity and wind speed, to determine the CBL height. The sonic anemometer data of wind speed and temperature were processed to derive the second-moment turbulent statistics and were analyzed to investigate the applicability of variance methods to estimate regional surface fluxes of sensible heat. It was found that the temperature variances in the lower mixed layer, coupled with universal functions, produced sensible heat fluxes H over the area with an rms error of the order of 40 Wm^-2 when compared with H values derived from the eddy correlation method. The variance of the vertical wind speed did not produce as good a result. In contrast, the surface-layer temperature variances yielded H values with rms error of the order of 20 Wm^-2, even though the underlying surface was non-uniform and highly non-isothermal, above which enhanced temperature variances could be suspected.     

黑河实验区非均匀地表能量通量的数值模拟

利用三维非静力RAMS模拟研究了黑河实验区非均匀地表能量通量，模拟结果表明：绿洲地表净辐射通量较沙漠戈壁大；绿洲及沙漠戈壁下垫面上的Bowen比分别为0．4和4．0；夜间绿洲上整晚维持蒸发，并有负感热通量。模拟结果与测站实测结果与卫星反演值的对比研究指出，RAMS对绿洲下垫面潜热通量的模拟和沙漠戈壁下垫面感热通量的模拟与实测值基本一致。卫星遥感反演及数值模拟方法对净辐射的估算与实测较吻合，绿洲地表感热通量的卫星反演值较数值模拟结果更接近于实际，但潜热通量的模拟值则较卫星估算值更接近于实际；沙漠戈壁地表则是感热通量的模拟值较卫星反演值更接近于实际。上述分析为今后结合卫星遥感改进RAMS陆面过程参数化，使使用于模拟研究干旱区非均匀下垫面地气相互作用提供了可靠依据。     

An Optimal Inverse Method Using Doppler Lidar Measurements to Estimate the Surface Sensible Heat Flux

Inverse methods are widely used in various fields of atmospheric science. However, such methods are not commonly used within the boundary-layer community, where robust observations of surface fluxes are a particular concern. We present a new technique for deriving surface sensible heat fluxes from boundary-layer turbulence observations using an inverse method. Doppler lidar observations of vertical velocity variance are combined with two well-known mixed-layer scaling forward models for a convective boundary layer (CBL). The inverse method is validated using large-eddy simulations of a CBL with increasing wind speed. The majority of the estimated heat fluxes agree within error with the proscribed heat flux, across all wind speeds tested. The method is then applied to Doppler lidar data from the Chilbolton Observatory, UK. Heat fluxes are compared with those from a mast-mounted sonic anemometer. Errors in estimated heat fluxes are on average 18 %, an improvement on previous techniques. However, a significant negative bias is observed (on average $-63\,\%$ ) that is more pronounced in the morning. Results are improved for the fully-developed CBL later in the day, which suggests that the bias is largely related to the choice of forward model, which is kept deliberately simple for this study. Overall, the inverse method provided reasonable flux estimates for the simple case of a CBL. Results shown here demonstrate that this method has promise in utilizing ground-based remote sensing to derive surface fluxes. Extension of the method is relatively straight-forward, and could include more complex forward models, or other measurements.     

Applying a simple three-dimensional eddy correlation system for latent and sensible heat flux to contrasting forest canopies

Summary A simple eddy correlation system is presented that allows on-line calculation of latent and sensible heat fluxes. The system is composed of a three dimensional propeller anemometer, a thermocouple and a capacitance relative humidity sensor. Results from two contrasting sites demonstrate the capability of the system to measure turbulent fluxes under varying conditions. A dry mixed (dominantly coniferous) forest in hilly terrain in Austria is compared to a well irrigated, heavily transpiring, deciduous pecan orchard in the Southwest of the US. The US site shows insufficient closure of the energy balance that is attributed to non-turbulent fluxes under advective conditions in a stable boundary layer (Blanford et al., 1991) while the Austrian site exhibits almost perfect closure with the use of the very same instruments when the boundary layer is convective and advection is negligible.With 4 Figures     

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.     

Laser-Doppler anemometry and its application to flow investigations related to the environment of vegetation

Accurate measurements in highly turbulent flows, as they occur in nature, require reliable velocity measuring techniques that permit instantaneous velocity components to be locally recorded. Hot-element techniques and optical methods are available for local measurements of instantaneous velocity and the present paper summarizes the advantages and disadvantages of different techniques when applied to flow fields in the environment of vegetation. The paper points out the advantages of laser-Doppler anemometry for velocity measurements in highly turbulent flows under laboratory conditions and stresses the reliability of the technique for measurements in polluted air and water flows. The basic principles of the method are explained and developments are described that have yielded optical anemometer systems for measurements of the magnitudes and signs of the instantaneous velocity components. Both quantities have to be known if accurate measurements of the mean flow properties and turbulence characteristics in flow fields with unknown flow directions are required. Electronic data-processing systems for laser-Doppler anemometer measurements are surveyed, embracing frequency analysers, automatic filter banks, frequency trackers, photon correlators and frequency counters. Photon-correlation and counting techniques are introduced as the methods most likely to be employed for the laser-Doppler anemometer. Laser-Doppler anemometer investigations are described in boundary-layer flows along bean leaves and a metal model of plant leaves. These measurements formed the basis of heat and mass transfer predictions near leaves for a specific leaf position relative to the free stream and with different turbulence properties imposed onto the oncoming flow. These data are presented and discussed in some detail. Measurements are also presented that were carried out to investigate the velocity fields in different flow regimes around a square obstacle in a water flow. These flow properties were needed to understand the different growth rates of sea-weed observed in differing flow regimes.     

卫星遥感确定沙特阿拉伯吉达地区非均匀地表区域地表参数和能量通量

对临海沙漠地区非均匀地表区域地表能量通量和蒸发(蒸散)的研究,是一个十分重要但又是一个难点问题。本文提出了1个基于卫星遥感和地面观测的参数化方案,并把其应用于沙特阿拉伯吉达地区,利用1个景的陆地资源卫星Landsat-7E^TM^ 资料进行了分析研究,得到了一些有关临海沙漠地区非均匀地表区域地表特征参数、植被参数和地表能量通量的新概念。最后讨论了所提出的参数化方案的适用范围和需改进之处。     

Limitations of an eddy-correlation technique for the determination of the carbon dioxide and sensible heat fluxes

A modified infrared CO₂ gas analyzer, a small thermocouple assembly, a heated-thermocouple anemometer for horizontal wind, and a propeller-type vertical wind sensor were used to measure the eddy fluxes of heat and CO₂ above a corn crop. Experimental results of these fluxes are discussed. The main sources of errors of the eddy fluxes using these instruments were estimated:

1. Sensors with a time constant of 0.5 s appear to be fast enough to detect most of the vertical CO₂ transfer as long as the sensors are located at least one meter above the crop surface.
2. The deviation from steady-state conditions for 10-min periods was found to have a significant effect on the eddy flux estimates.
3. Temperature fluctuations of the air sample passing through the CO₂ infrared gas analyzer were found to be non-negligible but could be easily corrected.
4. A 1° misalignment of the vertical anemometer affected these eddy fluxes by less than 10% under all circumstances studied.

     

Sensible heat flux measurements with a yaw sphere and thermometer

A yaw sphere has been designed that measures a quantity that is proportional to the vertical heat flux. The method is described, and some comparisons are given with measurements made with a three-dimensional pressure-sphere anemometer, in combination with a fast-response thermometer.