用连续子波变换提取城市冠层大气湍流的相干结构

切变湍流的相干结构是湍流研究中的重大发现,它表明湍流在表面上看来不规则运动中具有可检测的有序运动,这种相干结构在切变湍流的脉动生成和发展中起着主宰作用.因此识别和提取相干结构对于认识和研究湍流是非常重要的.用数字滤波法将包含相干结构的大尺度信号提取出来以后,再用子波分析,根据子波能量极大值的判别方法,分别确定出大气湍流三个方向上的速度脉动信号相干结构的频率或时间尺度,然后由确定尺度上的连续子波反演公式,提取出大气湍流三个方向上的速度脉动信号相干结构所对应的波形.     

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.     

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。     

Long-term study of coherent structures in the atmospheric surface layer

A long-term study of coherent turbulence structures in the atmospheric surface layer has been carried out using 10 months of turbulence data taken on a 30-m tower under varying meteorological conditions. We use an objective detection technique based on wavelet transforms. The applied technique permits the isolation of the coherent structures from small-scale background fluctuations which is necessary for the development of dynamical models describing the evolution and properties of these phenomena. It was observed that coherent structures occupied 36% of the total time with mean turbulent flux contributions of 44% for momentum and 48% for heat. The calculation of a transport efficiency parameter indicates that coherent structures transport heat more efficiently than momentum. Furthermore, the transport efficiency increases with increasing contribution of the structures to the overall transport.     

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。     

大气边界层和大气环境研究进展

大气边界层物理和大气环境是大气科学的重要领域,中国科学院大气物理研究所自成立以来在这一研究领域取导了丰硕的成果.作者重点介绍最近十多年来在大气边界层探测、大气边界层结构特征、大气湍流理论、城市和区域大气污染预测预报模式研究等方面取得的重要进展,并对大气边界层和大气环境研究的未来发展作了展望.     

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.     

Large-Scale Turbulence Structures and Their Contributions to the Momentum Flux and Turbulence in the Near-Neutral Atmospheric Boundary Layer Observed from a 213-m Tall Meteorological Tower

Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels.     

Analysis of turbulent exchange and coherent structures in the stable atmospheric boundary layer based on tower observations

Using data collected at the Spanish low troposphere research centre CIBA (Centro de Investigación de la Baja Atmósfera) and at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands, we analyzed the most significant features of different coherent structures occurring in the stable atmospheric boundary layer. In particular, we used both the Reynolds and wavelet methods to analyze a solitary wave, a gravity wave, a density current and a low-level jet. For each of these structures, we found that wavelet analysis had the capacity to distinguish the different scales involved in these events due to the different timing and heights of the thermal instabilities and downdrafts associated with the disturbances. In addition, the wavelet method highlights the different roles of turbulence and coherent structures in the transfer of heat, moisture and CO₂ in the nocturnal boundary layer.     

1961—2017年华北区域大气自净能力变化特征及其影响因素分析

基于华北区域80个国家地面气象观测站资料,采用大气自净能力指数(ASI)分析了1961—2017年华北区域大气自净能力的气候分布特征及长期变率;用MK和MT法及Morlet小波对ASI进行了突变检验和能量谱分析;结合ASI的REOF空间载荷场分布特征,将华北区域进行分区,并研究了气象要素与华北区域及各分区的可能联系;对于大气自净能力较低的Ⅳ区,选择近年污染相对严重的部分城市（石家庄、保定和唐山）,对比了ASI与污染气象条件及不同级别AQI天数占比情况。结果表明:年、季尺度的大气自净能力在京津冀和山西大部、内蒙古东北部等区域呈现出较低的气候值,在内蒙古中部等其他地区呈现出相对较高的气候值,大气自净能力在季节尺度上呈现出从春季到冬季逐渐下降的变化趋势;区域变暖背景下,冬季风环流减弱,天气过程频率和强度减弱,地面风速随之降低,导致ASI降低;研究区域的ASI与混合层高度、地表通风系数、良和轻度级别天数占比均呈现出同相月变化,而与静稳天气指数、重度和严重级别天数占比呈现出反相月变化规律。根据风速和气温对大气自净能力的影响规律,推测未来大气自净能力很可能进一步降低,因此建议地方政府进行适当的大气污染减排和区域产业升级,有利于控制未来的空气污染形势。      

大气边界层湍流涡旋结构的小波分解

利用离散正交小波在若干物理判别准则下对边界层湍流脉动信号进行去噪和多尺度分解,从而有效地区分出均匀各向同性小涡成份和大尺度含能涡旋成份。能谱分析发现,小涡的谱动力学行为具有非常好的标度不变性,标度关系满足Kolmogorov的“-5／3”律。       

微波波段大气折射率结构常数的仿真研究

大气折射率结构常数描述了大气湍流起伏的强弱,它表征了大气折射率随机不均匀性的剧烈程度。光波和无线电波在大气中传播时会受到大气湍流的影响而产生各种不良效应,如:光斑漂移、闪烁、相位起伏等。因此,对大气折射率结构常数的研究具有重要的意义。本文利用常规探空资料对微波波段大气折射率结构常数进行了仿真研究,结果表明:在低空,特别是大气边界层之内,大气折射率结构常数主要为湿度所贡献;在高空,大气折射率结构常数主要为温度所贡献。在微波波段,影响大气折射率结构常数最大的气象因子并不是温湿压的大小,而是它们梯度的大小,其中湿度梯度的大小对其影响最大,在实际的低空测量大气折射率结构常数时,主要考虑湿度梯度与温度梯度的影响。     

The Use of Weather Forecasts to Characterise Near-Surface Optical Turbulence

The propagation of optical and electromagnetic waves is affected by small-scale atmospheric turbulence, quantified by the structure parameter of the refractive index. In the atmospheric surface layer, the mean structure parameter C_n²{C_{n}^{2}} , as averaged over the large-scale turbulence, relates to meteorological forcings through well-documented relationships. Present-day numerical weather forecast models routinely produce these forcings at the global scale. This study introduces a method where the products of such a model are used to calculate the mean optical turbulence near the surface. The method is evaluated against scintillometry measurements over climatologically distinct sites in Western Europe. The diurnal cycle modulation, and regional and seasonal contrasts, are all reproduced by our predictions. Hence, the present method explains and predicts some essential aspects of the meteorological variability of C_n²{C_{n}^{2}} near the surface. The noted discrepancies combine instrumental limitations, site peculiarities, differences related to distinct averaging procedures, and model errors, notably from weather forecasts. The minute-scale fluctuations of the measured scintillation rate are also analysed in the light of the forecast weather conditions. Fair-weather daytime periods consistently show a small short-term variability compared to the nighttime and perturbed weather periods. Thus, this short-term variability appears to have a predictable component.     

北京地区严重大气污染的气象和化学因子

利用北京３２５ ｍ 气象塔进行了大气污染物臭氧、氮氧化物、一氧化碳和气象要素的同步观测, 对影响大气污染的气象和化学因子进行了分析,探讨大气污染物变化的规律, 为城市空气污染预报提供理论基础。     

Structure Function Analysis of Two-Scale Scalar Ramps. Part I: Theory and Modelling

Structure functions are used to study the dissipation and inertial range scales of turbulent energy, to parametrize remote turbulence measurements, and to characterize ramp features in the turbulent field. Ramp features are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. The analysis of structure functions to identify ramp characteristics is used in surface renewal methods for estimating fluxes. It is unclear how commonly observed different scales of ramp-like shapes (i.e., smaller ramps and spikes embedded in larger ramps) influence structure function analysis. Here, we examine the impact of two ramp-like scales on structure function analysis using artificially generated data. The range of time lags in structure function analysis was extended to include time lags typically associated with isotropic turbulence to those larger than the ramp durations. The Van Atta procedure (Arch Mech 29:161–171, 1977) has been expanded here to resolve the characteristics of two-scale ramp models. This new method accurately, and in some cases, exactly determines the amplitude and duration of both ramp scales. Spectral analysis was applied to the structure functions for a broad range of time lags to provide qualitative support for the expanded Van Atta procedure results. The theory reported here forms the foundation for novel methods of analyzing turbulent coherent structures.     

Surface Temperature and Surface-Layer Turbulence in a Convective Boundary Layer

Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared imagery and sonic anemometer measurements were obtained during the boundary layer late afternoon and sunset turbulence (BLLAST) experimental campaign. Temporal turbulence data in the surface-layer are then analyzed jointly with spatial surface-temperature imagery. The surface-temperature structures (identified using surface-temperature fluctuations) are strongly linked to atmospheric turbulence as manifested in several findings. The surface-temperature coherent structures move at an advection speed similar to the upper surface-layer or mixed-layer wind speed, with a decreasing trend with increase in stability. Also, with increasing instability the streamwise surface-temperature structure size decreases and the structures become more circular. The sequencing of surface- and air-temperature patterns is further examined through conditional averaging. Surface heating causes the initiation of warm ejection events followed by cold sweep events that result in surface cooling. The ejection events occur about 25 % of the time, but account for 60–70 % of the total sensible heat flux and cause fluctuations of up to 30 % in the ground heat flux. Cross-correlation analysis between air and surface temperature confirms the validity of a scalar footprint model.     

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.     

Structure Function Analysis of Two-Scale Scalar Ramps. Part II: Ramp Characteristics and Surface Renewal Flux Estimation

Ramp features in the turbulent scalar field are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. Although finer scale ramp-like shapes embedded within larger scale ramp-like shapes can readily be perceived in turbulent scalar traces, their presence has largely been overlooked in the literature. We demonstrate the signature of more than one ramp scale in structure functions of the turbulent scalar field measured from above bare ground and two types of short plant canopies, using structure-function time lags ranging in scale from isotropic to larger than the characteristic coherent structures. Spectral analysis of structure functions was used to characterize different scales of turbulent structures. By expanding structure function analysis to include two ramp scales, we characterized the intermittency, duration, and surface renewal flux contribution of the smallest (i.e., Scale One) and the dominant (i.e., Scale Two) coherent structure scales. The frequencies of the coherent structure scales increase with mean wind shear, implying that both Scale One and Scale Two are shear-driven. The embedded Scale One turbulent structure scale is ineffectual in the surface-layer energy and mass transport process. The new method reported here for obtaining surface renewal-based scalar exchange works well over bare ground and short canopies under unstable conditions, effectively eliminating the α calibration for these conditions and forming the foundation for analysis over taller and more complex surfaces.     

Detection and analysis of coherent structures in urban turbulence

Summary The continuous wavelet transform provides a suitable tool to visualize the vertical structure of turbulence and to detect coherent structures in turbulent time series. This is demonstrated with a simple example of an artificially ramp structured time series. In this study turbulence data, i.e. the fluctuations of the horizontal wind components u′ and v′, the vertical component w′ and temperature T′, sampled with 20.83 Hz and measured simultaneously at three levels (z/h=1.5, 2.1 and 3.2, with z as the sensor height and h the height of the roughness elements) over an urban canopy in the inner city of Basel, Switzerland, are analyzed. The detection of the coherent structures was performed using the Mexican hat wavelet and the zero-crossing method. The analysis for unstable conditions shows that organized structures (ejection-sweep cycles) cover about 45% of the total run time. A conditional average from a total of 116 detected ejection-sweep sequences during 7 hours was calculated over a time window of 100 seconds. This dominating time scale was derived from peak frequencies of the wavelet spectra as well as from the Fourier spectra. It is shown that the normalized amplitudes of fluctuations of temperature and longitudinal wind speed during the events are largest at the lowest measurement level just above the canopy and decrease with increasing distance from the roughness elements. A comparison of related studies over different non-urban surfaces (mainly forests) shows that the shape of conditionally averaged ejection-sweep sequences is very similar for all canopies, however, the dominating time scale in general increases the rougher the surface is and the higher the roughness elements are.     

雾霾天气下DAML空间相干光通信系统的研究

大气湍流引起的折射率起伏会导致光信号的振幅和相位发生随机波动,同时日益严重的城市雾霾现象带来的系统性能降低问题也不可忽视.基于湍流相位波动和激光器相位噪声的缓慢时变特性,本文提出了将DAML相位估计算法应用到空间相干光通信系统中来提高系统性能,并推导了在大气湍流、激光器相位噪声以及各种天气状况衰减效应影响下,针对DQPSK信号的DAML相位估计系统的误码率,最后重点分析了在雾、霾天气条件下该系统的传输距离限制和波长选择特性.仿真结果表明：湍流相位波动造成的信噪比损失低于0.1 dB;在雾和霾天气下,相较于传统的OOK IM／DD FSO通信系统,DAML相位估计系统在不同的大气湍流环境中至少有500 m的传输距离优势;在霾和薄雾天气,传输波长为1 550 nm的DAML系统比波长为850 nm的系统的性能更优,但在能见度低于200 m的浓雾天气,系统性能基本与波长选择无关.