大气边界层湍流相干结构的识别

首先利用数字滤波方法对淮河流域试验的大气边界层湍流观测资料进行三项分解，将大气边界层湍流的风速信号分解为近似各项同性的小尺度涡和各向异性的大尺度涡。然后再将大尺度涡信号进行离散正交小波分解，寻求相干结构的主要特征尺度。对于大气边界层湍流垂直脉动风速来说，其相干结构的主要特征尺度为16s；对径向与纬向脉动来说，其相干结构的主要特征尺度为32－64s。在此基础上，利用小波的反变换提取出相干结构的信号与非相干结构的信号，并计算两间的相关系数，最大仅有0．02。此外，对原始大气湍流观测信号不进行数字滤波，直接利用本中子波分析法提取湍流相干结构所得结果作比较研究；并探讨了采用对称或似对称离散正交小波对此研究的影响。     

高风速相干结构对通量输送影响的实验研究

切变湍流的相干结构是湍流研究中的重大发现,它表明湍流运动并非完全随机,其中具有可检测的有序结构.本文通过处理南京浦口地区大气边界层观测数据,来分析不稳定层结中高风速相干结构特征.本次观测项目包括对场地中央的气象铁塔上2 m和40 m高度上超声风速仪的脉动速度、温度测量以及风廓线雷达对边界层风速廓线的测量.对超声水平风速时间序列数据进行小波变换 (时间尺度400 s),通过阈值来识别这种高风速相干结构.与多普勒风廓线雷达测量结果对比后发现,这种方法确定的相干结构符合常规的认识,具有较长的时间尺度和较大的垂直尺度 (接近边界层厚度).分析三天相干结构特性得到无量纲空间间隔约为6,即每隔6个边界层厚度的水平位置出现一个高速相干结构.通过与垂直风速小波系数的比较,发现高风速相干结构与向下垂直风速之间有较好相关,这与湍流中 “阵风” 现象的研究结论相似.使用四象限分析方法分类得到两种动量通量输送为负的运动:较小水平风速的上扬 (ejection) 运动 (简称为上扬运动) 和较大水平风速的下扫 (sweep) 运动 (简称为下扫运动),这两种运动在整个湍流活动中处于主导地位.高风速相干结构通过促进下扫运动和抑制上扬运动来影响动量通量的输送.     

强风天气下边界层结构特征

近地层观测的强风运动表明,叠加在平均流动之上的脉动通常有两种,一种是随机的湍流脉动,还有一种具有相干结构的阵风扰动。分析表明,上层强风的剪切运动产生阵风,并向下传递能量,对近地层的通量传输起到重要作用。本文利用北京325 m气象塔、位于海拔1257 m的妙峰山测风塔和位于海拔1688 m的灵山测风塔的资料,分析了强风天气下,边界层上层出现阵风并向下传递的过程,进一步证实无论在近地层还是边界层上层,强风期间,叠加在平均流动上除了高频湍流脉动之外,还有周期为1~10分钟的阵风,即相干结构。阵风峰期有下沉运动,阵风谷期有上升运动。这些相干结构在边界层上层产生,向下运动和传播过程中受到平均气流梯度的切变作用和地面摩擦,破碎为湍流结构。边界层上层的阵风和湍流产生的动量通量向下传递,使得强风期间,边界层中阵风和湍流对通量具有同样的输送能力,对边界层中沙尘、污染物等气溶胶的传输具有重要作用。本研究为模式中进行通量输送参数化方案的修正提供了观测和理论依据。     

有速度切变时,Lorenz系统中浑沌的特点

Lorenz方程中引进切变以后,对普朗特数为0.7的大气可以出现浑沌。 速度切变在一定情况下起阻尼作用,有切变时比无切变时湍流发生的临界莱雷数要高。 浑沌是一种非周期的现象,它在相空间中的轨迹是混乱的。 通向湍流的道路有很多种。切变较小时为普通湍流形态;切变较大时是间歇湍流形态;切变适中时为短暂层流状态。 有速度切变时的浑沌对大气湍流的研究有很大的实际意义。     

有关大气湍流的几个问题

浑沌(chaos)概念的提出,有关大气湍流的若干问题有深入讨论的必要。根据我们近几年研究对如下问题提出自己见解,供深入讨论:确定的动力系统可以描述大气湍流,大气湍流可以用有限维动力系统来研究,速度切变?u/?z有两重性,稳定层结下可以出现湍流,Lorenz方程的意义及局限性,湍流发生     

大气边界层阵风相干结构的产生条件

壁湍流相干结构的发现是近代湍流研究的重大进展之一,从20世纪50年代开始,在大气边界层湍流中也发现了相干结构——对流云街,并进行了系统的研究。近些年来,人们发现在近地层湍流中也存在相干结构。利用北京325 m气象塔对城市下垫面中大风和小风天气的风速分析,发现较有规律的周期3~6 min的阵风,且有明显的相干结构,而对不同下垫面的阵风研究,均发现存在这种相干结构,这种阵风相干结构对通量输送有不可忽视的作用。本文利用2012年4月甘肃省民勤县巴丹吉林沙漠观测塔的超声风速和平均场风速、温度观测资料,对阵风相干结构的产生条件进行了分析。采用傅立叶变换,将三维超声风速按频率分成基流(周期10分钟以上)、阵风扰动(周期1到10分钟)、湍流脉动(周期小于1分钟)三部分,结合平均场的资料分析发现:阵风相干结构出现在静力中性、不稳定甚至略微稳定的条件下,或者说机械作用主导的大气边界层,阵风区就会出现相干结构,热力作用对其有抑制和干扰的作用。从而,阵风的相干结构和壁面相干结构都出现在中性条件下,是机械湍流的现象,都主导着动量能量的输运。阵风区的相干结构并不等同于对流云街,他们出现在不同的大气稳定度条件下且尺度不同。     

线性热力学对大气系统的应用(I)大气系统的线性唯象关系和热力学性质(英文)

从一般的热力学原理或其它自然原理对唯象关系所强加的限制,能够演绎出大气系统的一系列热力学性质。利用非平衡态线性热力学导出了湍流K闭合理论中湍流交换系数同唯象系数的关系,从理论上证明大气系统热量湍流输送同水汽之间存在交叉耦合,还导出了湍流强度同速度和位温梯度的关系,从而证明速度和位温空间分布的非均匀性是湍流之源。并证明湍流强度定理,不可压缩气体和各向同性湍流大气中,湍流强度正比于速度与位温梯度的标积。进而证明大气涡旋定理,位温的切变将导致涡旋运动或各种环流运动,速度涡度等于速度同位温相对梯度的矢积。展现了线性热力学在大气系统的应用前景。     

Application of Linear Thermodynamics to the Atmospheric System. Part Ⅰ: Linear Phenomenological Relations and Thermodynamic Property of the Atmospheric System

从一般的热力学原理或其它自然原理对唯象关系所强加的限制，能够演绎出大气系统的一系列热力学性质。利用非平衡态线性热力学导出了湍流K闭合理论中湍流交换系数同唯象系数的关系，从理论上证明大气系统热量湍流输送同水泡之间存在交叉耦合，还导出了湍流强度同速度和位温梯度的关系，从而证明速度和位温空间分布的非均匀性是湍流之源。并证明湍流强度定理，不可压缩气体和各向同性湍流大气中，湍流强度正比于速度与位温梯度的标积。进而证明大气涡旋定理，位温的切变将导致涡旋运动或各种环流运动，速度涡度等于速度同位温相对梯度的矢积。展现了线性热力学在大气系统的应用前景。     

子波变换在大气科学中的应用研究

子波变换是在80年代初才开始发展起来的崭新的数学分析方法。它适宜于分析研究信号的局部性质。子波变换在地震资料分析、图像处理、声音信号分析、分形、抽样、湍流以及大气和海洋科学的诸多研究领域中得到了广泛的应用，与传统数学方法相比显示出巨大的优越性。本文在简要介绍子波变换的定义和一些基本概念的基础上，结合实例给出子波变换在大气科学中的应用研究。最后，讨论了使用子波变换时应注意的问题并展望了它的应用潜力。     

结合毫米波雷达提取降水条件下风廓线雷达大气垂直速度的研究

风廓线雷达主要是利用大气湍流对电磁波的散射作用,在晴空条件下对大气风场等进行探测。在降水天气下,风廓线雷达能同时接收到大气湍流回波和雨滴的散射回波信号,其探测到的回波功率谱中降水信号谱和大气湍流信号谱叠加在一起,使得大气的运动被雨滴的运动信息所掩盖,给后续的大气风场反演带来误差。而毫米波云雷达在降水天气下仅能探测到云雨粒子的回波而无法探测到大气湍流回波,基于这一差异结合毫米波云雷达资料对风廓线雷达功率谱数据进行订正,剔除其中的降水回波信息,进而获取正确的大气运动垂直速度。通过一次典型弱降水天气过程的雷达资料对该方法进行了可行性验证,并将计算得出的大气垂直速度与传统双峰法提取的大气运动垂直速度及原始风廓线雷达垂直速度进行了对比分析,显示在弱降水天气下该方法能有效消除降水对风廓线雷达垂直速度测量的影响,提高弱降水天气下测速准确率,并且在湍流谱极其微弱的情况下该方法也能准确地获取到大气运动垂直速度信息。但是云雷达回波在降水时会有衰减,虽然是弱降水也会导致在高层距离库上的订正效果变差,故目前只适用于弱降水时低距库处的降水订正。      

Coherent Structures Detected in Atmospheric Boundary-layer Turbulence using Wavelet transforms at Huaihe River Basin, China

The presence of coherent structures in turbulent shear flows suggests order in apparently random flows. These coherent structures play an important dynamical role in momentum and scalar transport. To develop dynamical models describing the evolution of such motion, it is necessary to detect and isolate the coherent structures from the background fluctuations. In this paper, we decomposed atmospheric turbulence time series into large-scale eddies, which include coherent structures and small eddies, which are stochastic by using Fourier digital filtering. The wavelet energy computed for the three components of the velocity fluctuations in the large-scale eddies appears to have local maximum values at certain time scales, which correspond to the scales or frequencies of coherent structures. We extract coherent signals from large-scale vortices at this scale by inverse wavelet transform formulae. This method provides an objective technique for examining the turbulence signal associated with coherent structures in the atmospheric boundary layer. The average duration of coherent structures in three directions based on Mexican hat wavelets are 33 s, 34 s and 25 s respectively. Symmetric andanti-symmetric wavelet basis functions give almost the same results. The main features of the structures during the day and night have little difference. The dimensionless durations for u, v and w have linear correlations with each other. These relationships are insensitive to the wavelet basis.     

Detecting Near-Surface Coherent Structure Characteristics Using Wavelet Transform with Different Meteorological Elements

In the present study, three wavelet basis functions (Mexican-hat, Morlet, and Wave) were used to analyze atmospheric turbulence data obtained from an eddy covariance system in order to determine effect of six meteorological elements (three-dimensional wind speed, temperature, and CO2 and H2O concentrations) on the time scale of coherent structures. First, we used the degree of correlation between original and reconstructed waveforms to test the three wavelets’ performance when determining the time scale of coherent structures. The Wave wavelet’s reconstructed coherent structure signal best matched the original signal; thus, it was used for further analysis of the time scale, number, and time cover of the meteorological elements. We found similar results for all elements, though there was some internal variation, suggesting that coherent structures are not inherently dependent on these elements. Our results provide a basis for proper coherent structure detection in atmospheric turbulence and improve the understanding of similarities and differences between coherent structure characteristics of different meteorological elements, which is helpful for further research into atmospheric turbulence and boundary layers.     

Experimental investigation of atmospheric boundary layer turbulence

This paper describes how to measure turbulence in the atmospheric boundary layer (ABL) in order to address certain problems in modern atmospheric physics. These problems mainly relate to the Earth's energy budget (including the hydrological cycle) and biogeochemical cycles. Starting from the main characteristic numbers and the basic equations of atmospheric turbulent flow, we show what turbulence parameters are important to measure. Special attention is given to the various methods used to compute the turbulent fluxes. We analyse the range of scales which has to be measured to properly capture the eddies contributing to the turbulent transfers. This range of scales determines what sensors can be used in the atmospheric surface layer and in the ABL. We describe the most widely used instruments and their performances. The principal platforms used to deploy these instruments are examined. Aircraft are described in more details, because they allow a thorough exploration of the ABL. In the last section, some examples of ABL turbulence signals measured in various conditions are presented. These examples illustrate horizontally homogeneous turbulence as well as inhomogeneous signals for which standard analysis techniques cannot be used. We show how some recent techniques, like wavelet transforms, can help to investigate this kind of signal. At the end, we present what would be interesting to do in the near future for the study of ABL turbulence.     

Assessing the shear-sheltering theory applied to low-level jets in the nocturnal stable boundary layer

This paper investigates the existence of shear sheltering on turbulence data over a quasi-ideal experimental site in Oklahoma, USA. Originally developed for engineering flows, the shear-sheltering theory is predicated upon the idea of low-level jets blocking large eddies aloft, preventing them from propagating to the surface. In this scenario, suppression of low-frequency turbulence energy and reduction of surface fluxes would be expected. Results from the Oklahoma experiment show instead an enhancement of surface turbulence intensity and of the relative contribution of large scales to total (co)variances for low-level jet cases with strong shear, thus suggesting the absence of shear sheltering at the site. The results underline the complexity of surface-atmosphere interactions in nocturnal stable conditions. Atmospheric modeling of exchange using various scenarios of surface characteristics, flow regimes, and low-level jet properties is suggested to further assess the potential applicability of the shear-sheltering theory to atmospheric flows.     

Detection of long-term coherent exchange over spruce forest using wavelet analysis

Summary This study presents a discussion of a method for automated and quasi-online analysis of coherent structures using wavelet transform. The method is optimised for rapid processing of vector and scalar variables obtained over tall vegetation. It has been designed to assess long-term statistics of coherent structures, as it is applicable over a wide range of atmospheric conditions. Data of artificial and real turbulent signals are used to perform the analysis and to evaluate the presented method.Different wavelet functions are used for filtering the original signals, determining characteristic time scales, and detecting individual coherent structures. On this basis, statistics of temporal separation of coherent structures and phase shift between different variables can be calculated.Background turbulence and spikes are found to be efficiently removed without changing the shape, particularly the sharp localised gradients, of coherent structures. The determined peak in the calculated wavelet variance spectrum is observed to correspond very well to characteristic event durations and to satisfy the definition of coherent structures present in vector and scalar variables. The detection algorithm was successful in analysing data covering a wide range of atmospheric conditions. Detected individual coherent structures provide a parallel temporal pattern for scalar variables, but a phase shift between scalar and vector components.     

Wavelet Analysis of Intermittent Turbulent Transport in the Atmospheric Surface Layer over a Monsoon Trough Region

The structure of the turbulence in the atmospheric surface layer over a monsoon trough region has been studied using structural analysis based on wavelet transform. The observational site is located at the eastern (wet) end of the monsoon trough region, characterized by high moisture in the atmospheric surface layer. On the average relative humidity varied from 70% to 100% during the experiment. The wind and temperature data, collected at Kharagpur (22°25' N, 87°18' E) at six observational hours of a day in June 1990 during the Monsoon Trough Boundary Layer Experiment (MONTBLEX), have been utilized in the study. The wind and instantaneous momentum flux time series were decomposed into 12 scales using the Haar wavelet transform. The eddies exhibited a large temporal variability generating intermittency in the energy and flux distributions. A criterion based on the isotropy has been suggested for separating the large eddies from the small eddies. At the separation scale the isotropy coefficient drops sharply. It is shown that the intermittency in the small eddies resulted from the spatial variation of energy, and deviation of velocity statistics from the Gaussian distribution known as flatness. The deviation from the -5/3 power law has been attributed to the increased mean values of, (i) the coefficient of variation of energy, and (ii) the flatness factor, in the inertial subrange. The decomposition of the instantaneous momentum flux time series reveals that the major contribution to the total flux arises from the large eddies. The quadrant analysis of the momentum flux shows that ejections and sweeps account for a substantial part of the total flux, and quantifies the relative importance of the various spatial scales that contribute to the transport of momentum.     

Attached Eddies and Production Spectra in the Atmospheric Logarithmic Layer

We investigate the production components of turbulent spectra within logarithmic layers over flat ground. This assumes that the turbulence giving rise to these spectra consists of active coherent structures (eddies) that are attached to the ground, and whose properties display perfect statistical self-similarity under inner scaling. That is, we take the extreme view that active coherent structures not only contribute to turbulence production spectra but explain the whole of them, so that neither detached eddies nor unstructured motions make any significant contribution. Perfect self-similarityis held to apply only to eddies that are themselves formed totally within the log layer, so the theory applies in the limit of spectra obtained at the hearts of very deep log layers. The model predicts that spectral variance and covariance should become independent of wavenumber at small wavenumbers. This asymptotic behaviour is observed in all neutral spectra from the Kansas experiment. The model also interprets the various positions of the spectral peaks observed at Kansas and in aircraft flights over the sea as consequences of the eddies being aggregated into files aligned with the wind. The observed spectra are therefore consistent with large-scale wedge-like structures being the principal component of active turbulence in the neutral atmospheric surface layer.