Near-surface turbulence in the atmospheric boundary layer

Measurements of the near-surface turbulence in the atmospheric boundary layer have been made using hot-wire probes above the salt flats of northwestern Utah, where the momentum thickness Reynolds number, R_θ, is O(10⁶), and the surface is smooth and nearly devoid of flow obstructions. The measurements were made with arrays of up to 24 parallel straight sensors and with a modular 12-sensor probe capable of measuring all of the components of the instantaneous velocity vector and velocity gradient tensor. Measurements were also made in a laboratory wind tunnel at R_θ=1730 using 22 straight sensors. The data analysis focuses on the effects of the Reynolds number on turbulence properties and on the physics of the dissipation rate of turbulent kinetic energy.Some properties are found to be dependent on the Reynolds number when normalized with inner variables, while others are not. Among those that show the significant Reynolds number dependence are the rms and the skewness factor of the streamwise velocity fluctuations.Significant differences in flow structure, particularly those related to high rates of dissipation, are implied by the data. The joint PDF and covariance integrand of streamwise and wall normal vorticity fluctuations show less preferred orientation of the vorticity vector in the buffer layer at R_θ of O(10⁶) than at R_θ=1070. The largest contribution to the dissipation rate, at O(10⁶) is by the ∂w/∂z velocity gradient, while this term makes a quite small contribution to the dissipation rate at low R_θ. Here w and z are the spanwise velocity fluctuations and direction, respectively. Conditional analysis in the streamwise-wall normal (x−y) plane based on high instantaneous dissipation rate shows that the typical high dissipation rate events are generally similar at high and low Reynolds numbers, but display some significant differences.     

Determination of integral microphysical parameters of multicomponent aerosols from data of atmospheric sensing by lidars based on an Nd:YAG laser

Optics and Spectroscopy - A statistical model of an atmospheric aerosol is used to analyze the informativeness of lidar measurements of the optical characteristics of an aerosol at wavelengths of...     

Identification of lagrangian coherent structures in the turbulent boundary layer

Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vortices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is implemented to get the ensemble-averaged inclination angle θ _R of typical LCS. θ _R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y ⁺=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y ⁺=100. The ensemble- averaged convection velocity U _c of typical LCS is finally calculated from temporal-spatial correlation analysis of FTLE field. It is found that the wall-normal profile of the convection velocity U _c(y) accords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the downstream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond. Supported by the National Natural Science Foundation of China (Grant Nos. 10425207 and 10832001)     

Aspect sensitivity of sound backscattering in the atmospheric boundary layer

The dependence of backscatter intensity on the inclination of a SODAR beam was measured in the nocturnal boundary layer. The aspect sensitivity was revealed, but the angular dependence was not so strong as that of radiowave backscattering in the upper atmosphere. The mechanism of the phenomenon is under discussion as well as its influence on the accuracy of SODAR measurements of C _T ² values which are very useful for air-pollution meteorology.     

大气边界层各向异性的室内模拟研究

 边界层的各向异性研究对于了解实际大气的湍流特性和经典理论的适用范围均具有较大的意义。利用室内水槽实验模拟了实际大气边界层,使用小波变换方法分析了对数光强起伏谱的特征,将获得的标度指数与理论值进行比较,结果表明混合层在水平方向较为接近各向同性,而垂直方向则呈现一定的各向异性。     

Evolution of the atmospheric convective boundary layer monitored by the metric RASS

Summary The automatic metric RASS (Radio Acoustic Sounding System), operating at an acoustic wavelength ∼1 m, proved to be able to measure continuously the thermal vertical profiles as far as about 1 km in 50% of soundings with an accuracy generally within a few tenths ofK degrees and with an altitude resolution of (30÷40) m. A set of triennal data of the diurnal evolution of the convective boundary layer (CBL) during Summer, as monitored by a metric RASS operating in the Po Valley (Italy) is presented and discussed. Although atmospheric turbulence, pertinent to the considered unstable conditions, is one of the main factors limiting the radio-acoustic sounding range, by means of the metric RASS it is possible to follow the growth of the mixing layer and to estimate the multilayered structure of the CBL. In addition to the RASS data, the measurements of wind speed, temperature, radiation, humidity and so on, performed by conventional equipments near ground level, are considered.

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Riassunto Il RASS (Radio Acoustic Sounding System) operante in modo automatico alla lunghezza d'onda acustica ∼1 m, ha dimostrato di poter misurare con continuità il profilo termico verticale dell'atmosfera fino a ∼1 km nel 50%, dei sondaggi, con una precisione entro alcuni decimi di gradoK ed una risoluzione verticale di (30÷40) m. Vengono qui presentati ed analizzati i dati triennali (1984–86) sull'evoluzione dello strato limite convettivo durante l'estate, misurati da un RASS metrico operante in Valle Padana. Sebbene la turbolenza atmosferica, associata alle condizioni instabili considerate, sia uno dei principali fattori che limitano la quota di sondaggio del RASS, con il sistema metrico è possibile seguire la crescita dello strato rimescolato ed analizzare la struttura termica inomogenea dello strato limite convettivo. Oltre alle misure RASS sono stati considerati i dati di sistemi tradizionali per la misura della velocità del vento, della radiazione solare, dell'umidità, ecc. in prossimità del suolo.

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Резюме В исследованиях атмосферного граничиого слоя используется разнообразная аппаратура и опорные платформы. Однако имеется мало информации об относительно недоступных областях выше поверхностного слоя. Для измерений в более высоких областях используются летательные аппараты и болышие аэростаты, но они не могут обеспечить непрерывные наблюдения. Автоматическая метрическая радио-акустическая зондирующая система, действующая на длине волны ∼1 м, обеспечивает непрерывные измерения температурных вертикальных профилей до высоты 1 км с точностью до нескольких десятых кельвина и разрешением по высоте (30÷40) м. Приводятся и обсуждаются данные за трехлетний период для суточной эволюции конвективного граничного слоя втечение лета, которая контролировалавсь с помощью метрической радио-акустической зондирующей системы в долине По (Италия). Эта область характерется преобладающими слабыми ветрами. Хотя атмосферная турбулентность является основным фактором, ограничивающим применение радио-акустической зондирующей системы, с помощью метрической радио-акустической зондирующей системы можно проследить перемешивание слоев и оценить многослойную структуру конвективного граничного слоя. Кроме данных радио-акустической зондирующей системы, рассматриваются измерения скорости ветра, температуры, излучения, влажности и т.д., выполненные с помощью обычной аппаратуры.

     

Parameterization of meandering phenomenon in a stable atmospheric boundary layer

Accounting for the current knowledge of the stable atmospheric boundary layer (ABL) turbulence structure and characteristics, a new formulation for the meandering parameters to be used in a Lagrangian stochastic particle turbulent diffusion model has been derived. That is, expressions for the parameters controlling the meandering oscillation frequency in low wind speed stable conditions are proposed. The classical expression for the meandering autocorrelation function, the turbulent statistical diffusion theory and ABL similarity theory are employed to estimate these parameters. In addition, this new parameterization was introduced into a particular Lagrangian stochastic particle model, which is called Iterative Langevin solution for low wind, validated with the data of Idaho National Laboratory experiments, and compared with others diffusion models. The results of this new approach are shown to agree with the measurements of Idaho experiments and also with those of the other atmospheric diffusion models. The major advance shown in this study is the formulation of the meandering parameters expressed in terms of the characteristic scales (velocity and length scales) describing the physical structure of a turbulent stable boundary layer. These similarity formulas can be used to simulate meandering enhanced diffusion of passive scalars in a low wind speed stable ABL.     

Thermal sounding of the atmospheric boundary layer in the center of an oxygen absorption line

Radiophysics Scientific Research Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 2, pp. 130–136, February, 1992.     

大气边界层各向异性的室内模拟研究

边界层的各向异性研究对于了解实际大气的湍流特性和经典理论的适用范围均具有较大的意义。利用室内水槽实验模拟了实际大气边界层,使用小波变换方法分析了对数光强起伏谱的特征,将获得的标度指数与理论值进行比较,结果表明混合层在水平方向较为接近各向同性,而垂直方向则呈现一定的各向异性。     

Accurate temperature profiling of the atmospheric boundary layer using an ultraviolet rotational Raman lidar

An ultraviolet rotational Raman lidar (RRL) system at an eye-safe wavelength of 354.7 nm is demonstrated for profiling the vertical temperature of the atmospheric layer. Two sets of narrow-band interference filters, which central wavelengths are located at 353.9 and 352.5 nm, respectively, are employed to block the elastic Mie- and Rayleigh-scattering signals and to separate two required rotational Raman signals for temperature retrieval. Experiments were carried out for verifying the feasibility of the prototype of the ultraviolet RRL from May to November in 2008, and the system calibrations were performed with the radiosonde data which were obtain from the local meteorological bureau, and good agreements were got in the experiment results. The observation results show that a statistical temperature error of less then 1 K was obtained up to the heights of 2.5 km for nighttime and 2.0 km for daytime with 300 mJ laser energy and ∼8 min observation time.     

Control of boundary layer on a flat plate by means of cavity flow

This paper describes the control of boundary layer on a flat plate by means of cavity flow. In this study, classifying the shapes of cavities into circular arc, rectangle and triangle makes the discussion, and the depths of the cavities are changed systematically. It is made clear, by numerical calculations and experiments, what states of flow are shown in the internal parts of cavities and what kinds of influence are exerted to the boundary layers in the upstream and downstream flows. As a result, the following facts are made clear. By taking up properly the depth ratios (k/c) of cavities complying with the individual cavity shapes, the boundary layer thickness in the flow upstream and downstream of the cavities can be controlled. Meanwhile with any of the cavities, the existence of the minimum boundary layer thickness (δ/δ _m ) _min min is seen at a depth ratio.     

Lunar tides in the electric field of the atmospheric boundary layer

Investigations are aimed at a search for and an estimation of a correlation between the electric field in the atmospheric boundary layer (ABL) and the lunar tides. The lunar tides and their correlation with the electric field in the ABL can be studied either by accumulation of a large experimental data arrays or by the method of signal reception at spatially separated points. The problem of investigating lunar tide manifestations in the electric field in the ABL is solved using a network of stations for monitoring the vertical electric field strength component, specially constructed models and their consequences, and procedures developed for experimental data processing.     

Laboratory simulation of rotating atmospheric boundary layer flows over obstacles

Summary The present study fits in the frame of a research program concerning in general the dynamics of airflow in the atmospheric boundary layer and in particular the influence of terrestrial rotation on the movements of air masses interacting with natural extended obstacles (mountains). The experiment has been performed by the method of hydraulic simulation, using schematic models at reduced scale in a channel placed on a rotating platform. We only considered the case of a neutral atmosphere and studied the wake of an obstacle with semi-circular section and the reciprocal interaction of two obstacles of this kind placed perpendicularly to the flow. In this last case we investigated the influence of the distance between the obstacles on their wake length and on the vorticity conditions inside the wakes. Among the various results we obtained, the modifications of the velocity profiles over the reliefs and their dependence on the rotation velocity are particularly interesting. We discuss here our results from two different points of view, namely the purely hydraulic one (which includes the effects of different rotation velocities) and the atmospheric one, according to which the model simulates—with given reduction scales—an actual situation characterized by a fixed value of the Coriolis parameter. As to the first approach to the problem, we found that: 1) The roll with horizontal axis, which is observed behind an obstacle, becomes narrower and narrower as the rotation velocity of the platform increases, while its stability in time and its definition in space increase. In general, it may be said that rotation plays a stabilizing role for vortex dimensions and velocity profiles. 2) The transversal velocity behind the obstacles may attain values about twice the mean longitudinal velocity of the flow. 3) When rotating, the roll is thicker at its left edge than on the right one, due to the transversal flux which provides fluid supply. 4) When the thickness of the boundary layer is increased by making the channel bottom rough, the above-mentioned phenomena are emphasized; moreover, with a second obstacle placed in the flow (and not too far from the first) the transversal velocity components increase, due to a canalization effect. 5) The accelerations of the low layers over the obstacles are strongly amplified by rotation, due to the action of Coriolis' force. As for the second approach, we checked the extent to which the simulation procedures adopted for our laboratory flows, and for their boundary conditions, can be representative of the features of atmospheric phenomena. In order to do that, we compared the dynamical structure of the low flow layers over the obstacle with the analogous structure observed in the field and in wind tunnel by other authors, as well as with the predictions of a few theoretical models. Inside the lowest part of the planetary boundary layer, where the overshooting due to the relief is confined, a good consistency was found among all these results, in particular for what concerns the maximum of overflow velocity. This work has been carried out during 1981–1982 on the rotating platform of Institut de Mécanique, Université de Grenoble, within the ambit of a research convention with Istituto di Cosmogeofisica of C.N.R., Torino.     

Sound propagation in an unstable atmospheric layer

The equation of sound propagation in an unstable medium produced by the presence of supersaturated water vapor in it, which can appear in a hurricane area, is derived. This equation takes into account the effects of sound velocity dispersion, amplification, damping, and nonlinear effects. Some solutions to this equation are presented, illustrating the specific features of sound propagation in an unstable medium. Published in Akusticheskiĭ Zhurnal, 2007, Vol. 53, No. 3, pp. 477–480.     

激光雷达探测北京城区夏季大气边界层

为了研究北京城区夏季大气边界层结构变化特征及大气边界层内气溶胶消光特性,2004年8月利用便携式米散射激光雷达对北京城区夏季大气边界层进行了系统观测。反演了观测站上空大气气溶胶的消光特性垂直分布以及大气边界层的高度。分析了气象条件和人类活动对大气边界层结构的影响。观测数据表明： 北京城区夏季大气边界层有明显的日变化特征,早晚比较低,日间有一个从低变高再变低的过程,中午前后达到最高。结合气象参数对测量数据进行的统计分析表明：北京城区夏季大气边界层高度相对稳定,多分布在1.8km以下,平均值为0.68km;大气边界层内存在浓度较高的气溶胶粒子,平均光学厚度（3km以内）在0.30左右。     

Influence of ionization-radiation power on electric effects in the atmospheric boundary layer

We consider the mechanism of fluctuations of the vertical component of near-ground electric field of the atmosphere. We propose the mechanism of appearance of VHF electromagnetic radiation in the atmosphere resulting from rotationally-rotary transitions of dipole quasi-molecules consisting of large hydrated cluster ions. A molecular kinetic model of water vapor condensation on the atmospheric charged drops is developed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 9–19, January 2007.