Characteristics of turbulent structures in the unstable atmospheric surface layer

An atmospheric surface-layer (ASL) experiment conducted at a meteorological site in the Oostelijk-Flevoland polder of the Netherlands is described. Turbulent fluctuations of wind velocity, air temperature and static pressure were measured, using three 10 m towers.Simultaneous turbulent signals at several heights on the towers were used to investigate the properties of the turbulent structures which contribute most significantly to the turbulent vertical transports in the unstable ASL. These turbulent structures produce between 30 and 50% of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50% of the mean turbulent vertical heat transport; in both cases this occurs during 15 to 20% of the total observation time.The translation speed of the turbulent structures equals the wind speed averaged over the depth of the ASL, which scales on the surface friction velocity. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is significantly smaller than that of the internal shear layer, which is associated with the temperature interface. The turbulent structures in the unstable ASL are determined by a large-scale temperature field: Convective motions, which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The curvature of the vertical profile of mean horizontal alongwind velocity forces the alignment of the convective cells in the flow direction (Kuettner, 1971), which have an average length of several hundreds of metres and an average width of a few tens of metres. This mechanism leads to the formation of turbulent structures, which extend throughout the depth of the ASL.     

On the correlation between turbulent velocity and temperature derivatives in the atmospheric surface layer

Velocity and temperature derivatives were obtained at a height of 4 m in the atmospheric surface layer above land. With the assumption of local isotropy, these measurements are used to obtain some statistics of the turbulent energy and scalar dissipation fields. These statistics include the variances of the logarithms of the scalar and velocity dissipation fields and the correlation between these logarithms. When used in conjunction with the hypotheses for fluctuations in turbulent dissipation rates of Obukhov and Kolmogorov, the statistics suggest that the dependence of the flatness factor of temperature derivative on the turbulent Rynolds number R is not as large as that which had been previously reported in the literature. The experimental data indicate a R ^0.5 dependence for the kurtosis of the temperature derivative and a R ^0.15 variation for the strain rate-scalar dissipation correlation.     

The detection and measurement of turbulent structures in the atmospheric surface layer

Turbulence data from the planetary boundary layer (PBL) indicate the presence of deterministic turbulent structures. These structures often show up as asymmetric ramp patterns in measurements of the turbulent fluctuations of a scalar quantity in the atmospheric surface layer (ASL). The sign of the slope of the sharp upstream edge of such a triangular pattern depends on the thermal stability conditions of the ASL.The turbulent structures in the ASL have been tracked by a detection method which searches for rapid and strong fluctuations in a signal — the VITA (variable interval time averaging) technique. This detection method has previously been employed in laboratory boundary layers. The VITA detection method performs well in the ASL and reveals the presence of vertically coherent turbulent structures, which look similar to those in laboratory shear flows. At the moment that a sharp temperature interface appears, the horizontal alongwind velocity shows a sharp increase, along with a sudden decrease of vertical velocity, independent of the thermal stability conditions of the ASL. The fluctuating static pressure reveals a maximum at that moment. The vertical turbulent transports show a twin-peak character around the time that the sharp jumps in the temperature and the velocity signals appear.     

On the turbulent Prandtl number in the stable atmospheric boundary layer

This study focuses on the behaviour of the turbulent Prandtl number, Pr _t , in the stable atmospheric boundary layer (SBL) based on measurements made during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA). It is found that Pr _t increases with increasing stability if Pr _t is plotted vs. gradient Richardson number, Ri; but at the same time, Pr _t decreases with increasing stability if Pr _t is plotted vs. flux Richardson number, Rf, or vs. ζ = z/L. This paradoxical behaviour of the turbulent Prandtl number in the SBL derives from the fact that plots of Pr _t vs. Ri (as well as vs. Rf and ζ) for individual 1-h observations and conventional bin-averaged values of the individual quantities have built-in correlation (or self-correlation) because of the shared variables. For independent estimates of how Pr _t behaves in very stable stratification, Pr _t is plotted against the bulk Richardson number; such plots have no built-in correlation. These plots based on the SHEBA data show that, on the average, Pr _t decreases with increasing stability and Pr _t < 1 in the very stable case. For specific heights and stabilities, though, the turbulent Prandtl number has more complicated behaviour in the SBL.     

湍流频散对边界层风廓线的影响

应用包括湍流粘性和频散的新的Reynolds平均动量方程，分析了边界层的垂直风速廓线，发现包含湍流频散的地面层的风速廓线对经典的风廓线指数规律有一个对数规律的修改；而且在不稳定层结下比在稳定层结下，湍流的频散效应更为显；在中性条件下，指数规律退化为对数规律并且Karman常数被另外一个常数所代替，而这个新常数也可以通过相似理论来获得。     

城郊大气近地面层湍流特征的初步研究

本文对北京城郊近地面层的大气湍流特征做了初步的研究.结果表明,以局地稳定度参数z/L代替通常的Monin-Obukhov稳定度参数,对湍流的二阶矩以及谱和协谱的惯性副区特征而言,均匀近地面层湍流的相似理论可以推广到城郊近地面层湍流的情形,但谱和协谱的低频(含能区)特征发生显著的变化:含能区谱密度往往由许多尖峰构成;在某些低频范围上,湍流运动的垂直分量可能比水平分量强得多.在城郊近地面层中,湍流动能的产生与耗散基本上是平衡的.     

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.     

Turbulence Reynolds number and the turbulent kinetic energy balance in the atmospheric surface layer

The relation between the turbulence Reynolds numberR and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale and the Kolmogorov length scale .     

Non-dimensional wind and temperature profiles in the atmospheric surface layer: A re-evaluation

Previous results of non-dimensional wind and temperature profiles as functions of ( = z/L) show systematic deviations between different experiments. These discrepancies are generally believed not to reflect real differences but rather instrumental shortcomings. In particular, it is clear that flow distortion has not been adequately treated in most previous experiments. In the present paper, results are presented from a surface-layer field experiment where great care was taken to remove any effects from this kind of error and also to minimize other measuring errors. Data from about 90 30-min runs with turbulence measurements at three levels (3, 6, and 14 m) and simultaneous profile data have been analysed to yield information on flux-gradient relationships for wind and temperature.The flux measurements themselves show that the fluxes of momentum and sensible heat are constant within ± 7% on average for the entire 14 m layer in daytime conditions and when the stratification is slightly stable. For more stable conditions, the flux starts to decrease systematically somewhere in the layer 6 to 14 m. From a large body of data for near-neutral conditions (¦¦ 0.1), values are derived for von Kármán's constant: 0.40 ± 0.01 and for _h at neutrally, 0.95 ± 0.04. The range of uncertainty indicated here is meant to include statistical uncertainty as well as the effect of possible systematic errors.Data for _m and _h for an extended stability range (1 > > – 3) are presented. Several formulas for _m and _h appearing in the literature have been used in a comparative study. But first all the formulas have been modified in accordance with the following assumptions: = 0.40 and ( _h ) _{= 0} = 0.95; deviations from this result in the various studies are due to incomplete correction for flow distortion. After new corrections are introduced, the various formulas were compared with the present measurements and with each other. It is found that after this modification, the most generally used formulas for _m and _h for unstable conditions, i.e., those of Businger et al. (1971) and Dyer (1974) agree with each other to within ± 10% and with the present data. For stable conditions, the various formulas still disagree to some extent. The conclusion in relation to the present data is not as clear as for the unstable runs, because of increased scatter. It is, however, found that the modified curve of Businger et al. (1971) for _h fits the data well, whereas for _m , Dyer's (1974) curve appears to give slightly better agreement.     

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.     

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 interaction of turbulent and radiative transfers in the surface layer

The turbulent heat flux is usually assumed constant with height in the atmospheric surface layer. The validity of this hypothesis is known to be questionable, due to a possible variation along the vertical of the infrared radiative heat flux.This problem is approached theoretically, using semi-empirical expressions for emissivities, and assuming logarithmic temperature and humidity profiles. A first approximation of the radiative heat flux divergence is thus obtained analytically, as a function of the surface layer parameters. Numerical application to the case of an underlying water surface reveals appreciable variations of the radiative and turbulent heat fluxes in the first ten meters of the atmosphere, especially when wind velocity is low and humidity is high.These preliminary results are presented here for discussion. If accepted, they could lead to a reinterpretation of some experimental data, and should permit an extension of turbulent transfer theories to the case of a variable heat flux.Chargé de Recherches au C.N.R.S.Assistant I.N.R.A., Station de Bioclimatologie de Montfavet.Contributed paper to IUGG-IAMAP-AMS Conference on Planetary Boundary Layers, Boulder (Colorado), March 18–21, 1970.     

The turbulent characteristics in the lowest part of the sea breeze front in the atmospheric surface layer

Tower measurements for the sea breeze front in the surface layer were carried out over the Kochi plain about 2 km inland from Tosa Bay in Shikoku, Japan during the period from August 1986 to October 1987. The study shows that the penetration time of the sea breeze has an annual variation, which is around 0830 JST in summer and 12 JST in winter, and that the width of the sea breeze front depends on the ratio of the sea breeze speed and the opposing flow speed. Moreover, the frontal width also shows a seasonal variation.The characteristics of the vertical winds (w) found just before and just after the passage of the sea breeze front lead to remarkable downdrafts and updrafts, respectively, with relatively large vertical velocities. Such behaviour ofw is shown to be consistent with the flow relative to the head of the front as reviewed by Simpson (1987), influencing the magnitude of the turbulence scale and the turbulent energy dissipation near the ground surface.     

On condensation and precipitation of atmospheric moisture in the surface layer

Presented are the computations of condensation and precipitation of atmospheric moisture at night depending on the diurnal variations of air temperature and air humidity. Used are the meteorological data for northern Dagestan. Determined is the contribution of the dew to the moistening of the soil. The examples of the exponential and lognormal distribution of droplets revealed that the use of the velocity of medium-size droplet underestimates the value of the moisture mass precipitated on the soil. A concept of the mean mass precipitation velocity is introduced, and the formulae are provided for determining the average mass of deposited condensate depending on humidity, the temperature drop, and size distribution of droplets. Posed is a new problem of diurnal condensate precipitation in the soil in summer and of technology of its increase.     

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.     

Third- and fourth-order mixed moments of turbulent velocity and temperature fluctuations in the atmospheric surface layer

Fourth-order mixed moments of velocity and temperature fluctuations, measured within the atmospheric surface layer, are compared with results obtained by assuming the quasi-Gaussian approximation. Standard deviations of the products uw, u and w(u and w are the longitudinal and vertical velocity fluctuations; is the temperature fluctuation) are in good agreement with those obtained using the quasi-Gaussian assumption. Good agreement is also obtained between measured and Gaussian estimates of fourth-order moments including all three fluctuations u, w, Schwarz inequalities, commonly used in the clipping approximation in turbulence modelling, are found to provide bounds for third-order moments of w, that are too conservative. More reasonable, tighter, bounds for these moments are given by inequalities obtained by Lumley.     

宝鸡热电厂近地层温度场、风场及大气稳定度特征分析

根据2005-03-12—28在宝鸡热电厂建设区取得的大气边界层污染气象资料,以及MM 5中尺度气候数值模式,模拟了2004-07-13—27地面至1 500 m范围大气边界层温度场、风场资料,分析评价区近地层风场特征和温度场层结特征,为电厂工程设计、建设布局及大气污染治理提供依据。根据MM 5模式模拟结果,能较好反映近地层大气温度层结和风场主要特征,说明MM 5模式在近地层大气温度和风模拟中,有一定的应用前景。     

Turbulent dispersion in the atmospheric surface layer

By non-dimensionalizing a trajectory-simulation (TS) model of turbulent dispersion, it is shown that the dimensionless concentration z ₀cu_*/kQ (cu _*/kQ) due to a continuous line (area) source of strength Q in the atmospheric surface layer depends only on z/z ₀, x/z ₀, z ₀/L and z _s/z₀, where z _s is the source height. The TS model is used to tabulate concentration profiles due to ground-level line and area sources. Concentration profiles generated by the TS model for elevated sources are shown to be inconsistent with the Reciprocal Theorems of Smith (1957) and it is suggested that this is because the flux-mean gradient closure scheme inherent in the Reciprocal Theorem is invalid for an elevated source.     

Two-point coherence in the atmospheric surface layer

Two-point, one-dimensional coherence in horizontally homogeneous atmospheric turbulence is studied, both by experiment and analysis. Measurements are carried out using horizontally spaced sensors with the separation perpendicular to the mean velocity. Two-dimensional spectral models and three-dimensional inertial-range spectral tensors are used in the coherence calculations. The one-dimensional coherence for both velocity and scalar fluctuations is found to roll off at a wavenumber much smaller than we would expect from the classical notion of eddy correlation. This is a consequence of the cancellation of Fourier components aliased from the direction of the sensor separation into the streamwise direction. However, the coherence for the three velocity components behaves somewhat differently, reflecting the relative orientations of the velocity component, sensor separation and the mean velocity. These features are well predicted by the calculation. The analysis is also extended to calculate the two-point scalar-vertical velocity cospectrum and the results are in good agreement with our experimental data. The ratio of two- to one-point cospectra decreases at slightly larger wavenumber than the two-point scalar coherence does.