Wave-like wind fluctuations observed in the stable surface layer over a plant canopy(2)

Observations of wind velocity and air temperature fluctuations were made in the nocturnal surface inversion layer over a sorghum field. Wave-like fluctuations of temperature and wind velocity with a period of 15–20 min were observed for about 2 hours, 3 to 5 hours before sunrise. Wave-like fluctuations of temperature were observed in the air layer above and within a plant canopy and were most noticeable at the top of the plant canopy. Spectral analysis of temperature and wind velocity fluctuations reveals a separation of energy into wave-like and turbulent fluctuations. Cospectral analysis shows that for both momentum and heat, vertical transports are partitioned almost equally in the frequency ranges characteristic of wave-like and turbulent fluctuations. This suggests wave- turbulence interactions at low frequencies in the air layer near a plant canopy.     

Statistics of wind direction fluctuations in the surface layer over plant canopies

Boundary-Layer Meteorology - Height dependency of statistics of horizontal and vertical wind direction fluctuations over plant canopies have been studied, using observational data in the surface...     

On characteristics of wind direction fluctuations in the surface layer

It is shown that the ratio of standard deviation of lateral velocity to the friction velocity, /u _*, and therefore wind direction fluctuations, are sensitive to mesoscale terrain properties. Under neutral conditions, /u _* is almost 40% larger in rolling terrain than over a horizontal surface. In the lee of a low mountain, the fluctuations may be 2.5 times as strong as over horizontal terrain. In contrast, vertical velocity fluctuations are little influenced by mesoscale terrain features.Now with Air Weather Service, Offutt AFB, Omaha, Nebraska.     

Step-like temperature fluctuations associated with inverted ramps in a stable surface layer

Ensemble averages of temperature before and after step-like temperature fluctuations reveal the presence of inverted ramps in a stable surface layer. Normalized frequency of upward steps increases with increasing stability, whereas normalized magnitude of the temperature step decreases with stability and becomes constant at about R _i = 1. These results suggest that the significance of temperature steps increases as stability increases. In moderate stability, the temperature pattern shows a gradual decrease after an upward step, which can be called a time-inverted ramp. Descending air and large downward heat flux are observed in a time-inverted ramp, suggesting a contribution from an ordered motion in wind. On the other hand, the temperature steps are related to gravity waves in strong stability.     

Temperature oscillations observed in the stable boundary layer over four different underlying surfaces

夜间边界层内的温度时空特征决定了边界层的稳定性。由于稳定边界层的间歇性和复杂性,模式在夜间的参数化方案还不完善,研究夜间边界层的温度变化有助于改进模式在夜间的表现。本文利用四座在不同下垫面上的气象塔的数据,研究了夜间边界层中的温度波动。研究表明,无论是海边、草原还是沙漠下垫面,夜间形成的稳定边界层中,振幅大约2摄氏度,周期在数分钟的温度波动非常频繁地出现。而在北京,由于城市下垫面较大的热容量和人为热源使得稳定边界层不容易形成,温度波动也较弱。用布朗特维萨拉频率代表的静力稳定度是影响温度脉动强弱的重要因子。     

Some observations of water vapor fluctuations in a stable atmosphere over a lake surface

Summary The crain microwave refractometer and a bead thermistor have been used to measure 2 to 30-second period fluctuations in water vapor in a stable atmosphere over a lake surface. It appears that such fluctuations can be related to air flow over water waves and that they permit an estimate of a mixing length for water vapor.

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Zusammenfassung Das Crain-Mikrowellenrefraktometer und ein Perlenthermistor wurden verwendet, um in einer stabilen Atmosphäre über einer Seeoberfläche Wasserdampfschwankungen von 2 bis 30 Sekunden Dauer zu messen. Es scheint, daß solche Schwankungen zur Luftströmung über Wasserwellen in Beziehung gesetzt werden können, und daß sie eine Schätzung des Mischungsweges für Wasserdampf gestatten.

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Résumé Le réfractomètre de Crain et un thermistor ont été utilisés pour mesurer les fluctuations en teneur de vapeur d'eau d'une durée de deux à trente secondes dans un air stable au-dessus d'un lac. Il semble que de telles variations dépendent du mouvement de l'air au-dessus des vagues et qu'elles permettent d'apprécier le parcours de mélange de la vapeur d'eau.

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With 6 Figures     

利用行星边界层顶上方最邻近标准等压面的实测风推算地面风的方法及其比较

文中提出了一种在实际地形条件下,利用行星边界层顶上方最邻近标准等压面的实测风推算地面风的方法——修正的Taylor螺线法。试算表明,按这种方法和Taylor螺线法推算的地面风场的基本流型均与实测流场相似,但在风速模的均方根误差上,前者较之后者有显著改进.按文中方法推算得到的地面风场所求得的散度场的分布与天气区的配置也较合理,且散度值与实况比较接近.     

The horizontal distribution of space correlation coefficients of wind fluctuations in the atmospheric surface layer

The horizontal distribution of space correlation coefficients of wind fluctuations was investigated in the atmospheric surface layer. The observational network of wind sensors was arranged to form a two-dimensional extension in the horizontal plane. The shape of the distribution of the correlation coefficients was approximated by a group of concentric ellipses; streamwise and lateral integral scales were estimated as 75 m and 25 m, respectively. Taylor's frozen eddy hypothesis was tested using streamwise and time integral scales.     

An analysis of wind velocity fluctuations in the atmospheric surface layer using an orthonormal wavelet transform

A wavelet analysis can supply information of both the location (time) and the scale of fluctuations. This method is applied to the fluctuations of the natural wind and the turbulent transport of momentum in the atmospheric surface layer. The shapes of both the wavelet spectra and the Fourier spectra of the three components of the wind velocity fluctuations are similar to each other. The quadrant representation of momentum transport shows the scale difference of the transport. The large-scale fluctuations mainly contribute to the downward transport of momentum.     

Microscale temperature fluctuations in the atmospheric surface layer

Microscale temperature fluctuations were measured at 2 m above a grassy surface. The temperature-derivative spectrum was in general agreement with earlier results but the bump at nondimensional wavenumbers higher than 0.02 was not as pronounced as has been observed. The Obukhov-Corrsin constant for the one-dimensional temperature spectrum was evaluated to be 0.92 ± 0.05, consistent with recent results. The effects of instability and the vertical variation of temperature variance and kinetic energy dissipation are postulated to explain some of the difference with other spectra.     

Large-sample estimates of wind fluctuations over the ocean

The best quality wind data from the Norwegian sector of the North Sea, consisting of 3662 20-min time series measured at the top of the Statfjord A drilling derrick, are analyzed. Identification of Autoregressive wind models with Akaike's AIC and Achwarz's BIC measures appears to give rather arbitrary results. Spectral estimation with FFT- and AIC-identified AR-methods give almost identical results in the mean. At the higher frequencies (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzaaaa!36D7!\[f\] > 10^–2 s^–1) the spectrum is estimated to follow the usual inertial subrange law with little variability. The small-scale turbulent intensity is estimated to be very low, even in hurricane conditions. Comparatively, the low-frequency (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzaaaa!36D7!\[f\] ~ 10^–3 s^–1) fluctuations are more energetic than expected. None of the chosen low-frequency characteristica appear to be significantly linearly correlated to the available mean weather variables. However, some nonlinear relations appear to exist.     

A numerical study on the meso-scale pollutant dispersion over a sloped surface in the stable boundary layer

A numerical model is used to study the properties of pollutant dispersion over a large uniformly-sloped surface in the stable atmospheric boundary layer. By simulating the structure of boundary layer flow to improve the advective wind field and vertical eddy diffusivities included in the advection-diffusion equa-tion, this numerical model permits an estimation of the distribution of pollutant concentration for more real-istic atmospheric diffusion conditions.     

非均匀地面近地层风速廓线的实验研究

本文以Monin-Obukhov(1954)相似理论为基础,分析研究了北京西郊八角村地区80米高的铁塔上观测的风速和温度资料。 指出在均匀粗糙地面条件下,中性平均风速廓线在77米高度内很好地满足对数规律;稳定层结下,对数一线性规律在(0.03 ≤R_i≤0.69)一个很宽的稳定范围内都是适用的,但是必须十分注意其线性项常数β是随稳定度R_i而变的。本文给出了β-R_i之间的经验关系为β=0.35R(-0.9)对于不稳定层结描写风速切变函数φ_m,我们得到:弱不稳定范围使用|z/L| ~((-1)/4)规律     

On the extension of the wind profile over homogeneous terrain beyond the surface boundary layer

Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50–80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200–300 m and considering only wind speeds above 3 m s^?1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale (L _MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L _MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L _MBL , for applied use is suggested, based on measurements from the two sites.     

The velocity spectra in the stable surface layer

A simple model of the scaling parameter (f ₀) as a function of z/L for a stable surface layer is developed. This model is used to derive empirical forms for curves presented by Kaimal et al. (1972). Also obtained is the peak of the frequency and the maximum value of the logarithmic spectra as a function of z/L.Partially financed by CAPES.On leave from Faculdade de Engenharia de Joinville, SC.     

Estimates of velocity-pressure and velocity-pressure gradient interactions in the surface layer over plant canopies

The budget equations of turbulent kinetic energy and shear stress contain interaction terms of velocity-pressure and velocity-pressure gradient. These terms were estimated in the surface layer using the air pressure observed at the surface and wind velocity components over plant canopies. The magnitude of the pressure interaction terms was significantly large; it was not negligible compared with the production terms in each budget equation. The present results obtained over a rough surface also confirmed previous results that pressure terms play an important role in the turbulent kinetic energy budgets and the shear stress budget. The height dependency of nondimensional pressure terms versus (z - d)/z ₀ was not clear.     

Performance of various similarity functions for nondimensional wind and temperature profiles in the surface layer in stable conditions

The linear functions for non-dimensional wind and temperature profiles are commonly used to describe the surface layer fluxes in atmospheric models. However, their applicability is limited to smaller values of the stability parameter z/L (where z is the height above ground and L is the Obukhov length) i.e. z/L < 1.0. These linear functions have been modified (Webb 1970, Quart. J. Roy. Meteor. Soc. 96, 67–90; Clarke 1970, Quart. J. Roy. Meteor. Soc. 96, 91–114; Hicks 1976, Quart. J. Roy. Meteor. Soc. 102, 535–551; Beljaars and Holtslag 1991, J. Appl. Meteorol. 30, 327–341; Cheng and Brutsaert 2005, Boundary-Layer Meteorol. 114, 519–538) over the years for calculating fluxes when z/L > 1.0 under strongly stable conditions. In view of this, the objective of the present study is to analyze the performance of these similarity functions to compute surface fluxes in stable conditions.The meteorological observations from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) experiment are utilized for computing the surface fluxes in stable conditions. The computed fluxes are found to be reasonably close to those observed. The ratio of observed to computed fluxes reveals that the computed fluxes are close to the observations for all the similarity functions for z/L < 1.0 whereas the computed values show relatively a large scatter from observations for z/L > 1.0. The computed values of u and heat flux do not show significant differences from those observed at 99% confidence limit. The performance of all the similarity functions considered here is found to be comparable to each other in strongly stable conditions.     

On the downward transport of turbulent kinetic energy in the surface layer over plant canopies

Downward fluxes of turbulent kinetic energy have been frequently observed in the air layer just above plant canopies. In order to investigate the mechanism for such downward transport, analysis of observational data is attempted. Height-dependency of turbulent kinetic energy flux and turbulence statistics including higher order moments is represented as a function of a non-dimensional height z/H, where z is an observational height and H an average height of plant canopies. Downward fluxes and non-Gaussianity of wind velocity fluctuations are predominant just above plant canopies and decrease with increasing height. The downward flux is closely related to the high intensity of turbulence and the non-Gaussianity of wind velocity fluctuations, especially with a positive skewness in the longitudinal wind and a negative skewness in the vertical wind. The analysis method of conditional sampling and averaging is applied to the present observations. The results show that the predominance of the intermittent inrush phase over the intermittent ejection phase leads to the above-mentioned non-Gaussianity. Finally, a simple explanation is given in order to interpret the turbulent flow structure in the air layer near the plant canopies, which is associated with the downward energy transport process.     

Turbulence spectra of CO2, water vapor,temperature and wind velocity fluctuations over a crop surface

A prototype rapid-response CO₂ sensor was used in conjunction with a Lyman-alpha hygrometer, fine-wire thermocouples and a three-dimensional drag anemometer to measure CO₂, humidity, temperature and wind velocity fluctuations. Measurements were made over a soybean crop grown on relatively flat terrain near Mead, Nebraska.Temperature, humidity and CO₂ spectra measured under near neutral conditions were most similar in shape while longitudinal velocity (U) spectra appeared to be somewhat broader. Peaks occurred around f = 0.02 to 0.06 (where f is the non-dimensional frequency) in all spectra except for vertical velocity which had a peak near f = 0.5. As thermal stability changed from neutral to stable, spectra exhibited lower peaks, were narrower in shape and were shifted towards higher frequencies. Opposite behavior was observed with increasing instability.Cospectra for momentum, heat, water vapor and CO₂ measured during neutral conditions had similar peak frequencies (near f = 0.15). Among the cospectra studied, CO₂ and water vapor cospectra had the greatest similarity in shape.Published as Paper No. 7481, Journal Series, Nebraska Agricultural Experiment Station. The work reported here was conducted under Nebraska Agricultural Experiment Station Project 27-003 and Regional Research Project 11-33.Post-doctoral Research Associate and Professor, respectively, Center for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728.