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。       

南海台风边界层湍流小波分析

利用小波变换（WT）对香港天文台飞机观测台风“妮妲”（1604）资料进行分析,研究在不稳定、不均匀的台风边界层中湍流涡旋的垂直传输作用。在0.1~5 Hz惯性子区内横风和顺风分量功率谱密度能较好符合-5/3幂律。小波分析显示:横风的小波功率谱峰值集中在1 km之下,顺风分量的小波功率谱峰值集中在1~6 km之间;眼区动量通量的主要贡献尺度为2.3 km,眼区外主要贡献尺度在1~2 km,中低层为较小尺度（< 1.0 km）;湍流功能(TKE)的生成尺度主要集中在4 km之下。这项研究定量描述了南海北部台风边界层各个区域湍流结构的差异特征,讨论了对台风边界层通量参数化的可能影响。      

巴丹吉林沙漠北缘拐子湖近地层湍流能谱特征分析

利用2013年6月巴丹吉林沙漠拐子湖地区流沙下垫面的陆气通量观测资料,计算并分析了该研究区不同大气稳定层结条件下的湍流速度各分量谱和温度谱及湍流的局地各向同性特征。结果表明:该研究区风速各分量的湍流强度随平均风速的增加而逐渐减小,当平均风速2 m/s时,风速各分量的湍流强度逐渐稳定且基本限定在0.5以内。在不同大气稳定度下,湍流速度和温度能谱曲线在惯性副区内逐渐有合并趋势且遵循Kolmogorov提出的-2/3定律,速度各分量谱在高频段均满足各向同性且符合低频限制理论。随着稳定度的增加,风速分量的能谱曲线逐渐降低且向高频端移动,风速分量和温度能谱对应的谱峰长度则逐渐减小。该研究区水平湍流尺度范围为9.0~600 m,垂直湍流谱峰波长为10.79~75 m。该结果介于草地和森林下垫面之间,与塔克拉玛干沙漠腹地的试验结果较为接近。     

广州市近地层大气的湍流微结构和谱特征

本文利用UVW脉动风速仪资料分析了广州市区近地层大气的湍流强度、相关系数、尺度和速度谱,并获得了不同稳定度条件下的速度谱模式.结果表明,城市近地层大气湍流在惯性副区接近局地各向同性、速度谱符合Kolmogorov相似理论;气流方向上下垫面粗糙度的增加,使沿海城市近地层大气湍流能量(特别是铅直方向)比平坦、均匀下垫面上的增加.     

长江三角洲常熟地区近地层湍流特征的研究

利用1999年5～9月长江三角洲常熟地区的湍流脉动观测资料,分析了该地区感热、潜热和动量通量的日变化以及湍流强度和湍流谱特征,结果表明该地区各能量的日变化与常见情况相同,白天潜热通量的输送占主导地位,感热通量比潜热小得多,且湍流的日变化与天气的关系非常密切,晴天比阴天要大得多.湍流强度和湍流谱的分析结果表明,在近中性条件下,常熟地区各方向湍流强度接近典型平坦下垫面的值.非中性条件下,垂直方向规一化标准差与稳定度的关系都较好地满足1/3次幂定律,水平方向没有垂直方向上的规律好,特别是稳定条件下,数据点分散性大;各风速分量谱在高频段满足理论上的-2/3次幂定律,纵向和垂直方向速度谱的峰值频率与典型平坦下垫面上的结果一致.总体输送系数CD、CH与稳定度的关系密切,随稳定度增加有减小的趋势.     

不同强风天气系统风特性的个例分析

利用广东省东莞市的80 m气象塔在台风莫拉菲、强对流和强冷空气期间的观测数据,分析不同强风天气系统近地面边界层的平均风和脉动风特性参数。研究发现:(1)1号强对流的风速变化最为剧烈,风速的10 min升幅可分别达台风和强冷空气大风的1.8和3. 7倍。强对流的10 min平均风向变幅也最剧烈,可达到120°·(10 min)~(-1)。(2)台风强风廓线的幂指数值为0. 177,要大于建筑结构荷载规范(GB 5009—2012)给出的B类下垫面0.15的参考值,而强对流的幂指数值接近规范给出的A类下垫面的参考值0. 12,强冷空气的幂指数值则仅为0. 10。(3)1号强对流强风平均湍流强度分别是台风和强冷空气大风平均湍流强度的1.4～2.1和1.7～2.0倍。(4)台风和强对流的风攻角在-3°～3°范围内,强冷空气大风的风攻角在0°附近变动。(5)强风影响期间湍流空间积分尺度均有增大的现象,台风强风经过时,水平(纵向和横向)湍流空间积分尺度明显增大了一个量级。(6)台风强风的湍流功率谱不满足在惯性子区湍流能谱密度的-5/3次方与频率成正比的规律和各向同性假设。台风强风的湍流能谱值要显著高于强对流和强冷空气,其中,在桥梁敏感频域台风的平均湍流能谱值约为强对流和强冷空气的3倍。     

长江三角洲常熟地区近地层湍流特征的研究

利用1999年5－9月长江三角洲常熟地区的湍流脉动观测资料，分析了该地区感热，潜热和动量通量的日变化以及湍流强度和湍流谱特征，结果表明：该地区各能量的日变化与常见情况相同，白天潜热通量的输送占主导地位，感热通量比潜热小得多，且湍流的日变化与天气的关系非常密切，晴天比阴天要大得多。湍流强度和湍流谱的分析结果表明，在近中性条件下，常熟地区各方向湍流强度接近典型平坦下垫面的值。非中性条件下，垂直方向规一化标准差与稳定度的关系都较好地满足1／3次幂定律，水平方向没有垂直方向上的规律好，特别是稳定条件下，数据点分散性大；各风速分量变在高频段满足理论上的－2／3次幂定律，纵向和垂直方向速度谱的峰值频率与典型平坦下垫面上的结果一致，总体输送系数CD，CH与稳定度的关系密切，随稳定度增加有减小的趋势。     

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。     

城市冠层上下大气湍流特征分析

利用兰州市榆中县城市冠层架设的3台涡动相关仪(EC)观测大气湍流资料,分析了城市冠层上下不同下垫面湍流通量和不同风向范围内湍流动能变化特征,之后对城市冠层上下3台EC观测湍流动能最小、最大分布方向上分别对应的最小最大湍流动能的风速谱进行了研究,进一步检验了局地相似理论在城市冠层上下的适用性。结果表明:(1)城市冠层之上水泥、砖石等构成下垫面和城市冠层之内草坪下垫面观测感热通量、摩擦速度较为接近,观测潜热通量、CO2通量在白天差异明显。(2)城市冠层之上的湍流动能总体上大于冠层之内,冠层之上气流来向的上风向较为开阔时湍流动能较大,而冠层之内气流来向的上风向为街道口时湍流动能较大。(3)城市冠层之上的湍涡尺度大于冠层之内,城市冠层小尺度湍涡风速谱在惯性副区基本符合-2/3次方关系,且准各向同性,大尺度湍涡风速谱在惯性副区不符合-2/3次方关系,且各向异性。(4)不稳定层结下,城市冠层上下无量纲速度方差与稳定度基本满足1/3次方局地相似关系,稳定层结下不满足;城市冠层上下无量纲温度、湿度、CO2浓度方差在所有层结下均不满足-1/3次方局地相似关系。(5)近中性层结下,城市冠层上下u、v、w方向无量纲速度方差分别为3.52,3.03,1.49和2.62,2.22,1.50。     

小兴安岭近地层湍流能谱特征

利用小兴安岭南麓五营地区涡动相关系统的观测资料,分析不同大气稳定度条件的湍流速度谱、温度谱、湿度谱、协谱及局地各向同性特征。结果表明:三维速度(u、v、w)谱和垂直速度(w)与水平纵向速度(u)、温度(θ)的协谱(uw、θw)的谱峰均随稳定度的增加而向高频端移动。湍流速度(u、v、w)谱、温度(θ)谱和湿度(q)谱在惯性副区均满足-2/3定律。uw协谱、θw协谱和垂直速度(w)与湿度(q)的协谱(qw)在惯性副区并不完全遵循-4/3定律,尤其是uw协谱的拟合斜率更接近-1。水平方向湍流谱峰波长范围为130～1820 m,垂直范围为49～113 m。温度谱谱峰波长范围为149～260 m,湿度谱谱峰波长范围为198～455 m。uw协谱谱峰波长范围为228～455 m,θw协谱谱峰波长范围为172～260 m,qw协谱谱峰波长范围为172～346 m。v谱在惯性副区基本满足局地各向同性,w谱在惯性副区不满足局地各向同性,可能与森林下垫面对垂直方向湍流大涡的破碎作用有关。     

The spatial structure of turbulence at production wavenumbers using orthonormal wavelets

Orthonormal wavelet expansions are applied to surface-layer measurements of vertical wind speed under various atmospheric, stability conditions. The orthonormal wavelet transform allows for the unfolding of these measurements into space and scale simultaneously to reveal the large intermittent behavior in space for the turbulent production wavenumbers. Both Fourier and wavelet power spectra indicated the existence of a –1 power law for the vertical velocity measurements at the production wavenumbers. The –1 power law in the turbulent production range was derived from surface-layer similarity theory. A dimensionless skewness, structure function is applied to the wavelet decomposed vertical velocity field to trace the destruction of the shear-or buoyancy-induced anisotropy under various stability conditions. The structure skewness function revealed shear- or buoyancy-induced eddy asymmetry dependence on stability at each scale within the –1 power-law wavenumber range with more isotropy during propagation from smaller to larger wavenumbers. The asymmetry of these events at the turbulent production wavenumbers appeared very localized in space, as well as in scale, and could be described with a simple eddy-overturning model. It is demonstrated that the wavelet transform is suitable for such analysis.     

稳定条件下盆地上空大气边界层湍谱特征

本文利用100m~3系留气艇携带超声风温仪在复杂的盆地地形上空对1000m以下大气进行观测所获得的资料,研究小风稳定条件下大气边界层湍流结构特征。结果表明,在双对数坐标中,纵向速度u谱,垂直速度w谱,温度T谱在惯性区均遵循Kolmogorov的-2/3次律;横向速度V谱有其特殊的情形;协谱uw,wT及近地层的uT协谱在惯性区服从-4/3次律。和平坦、均一、开阔下垫面不同的是谱的峰值频率向高频移动,且没有发现近地层具有的谱峰随高度的明显变化关系。     

Long-term Correlations and Extreme Wind Speed Estimations

In this paper, we use fluctuation analysis to study statistical correlations in wind speed time series. Each time series used here was recorded hourly over 40 years. The fluctuation functions of wind speed time series were found to scale with a universal exponent approximating to 0.7, which means that the wind speed time series are long-term correlated. In the classical method of extreme estimations, data are commonly assumed to be independent (without correlations). This assumption will lead to an overestimation if data are long-term correlated. We thus propose a simple method to improve extreme wind speed estimations based on correlation analysis. In our method, extreme wind speeds are obtained by simply scaling the mean return period in the classical method. The scaling ratio is an analytic function of the scaling exponent in the fluctuation analysis.     

Multiscaling properties of concentration fluctuations in dispersing plumes revealed using an orthonormal wavelet decomposition

The dynamical characteristics of concentration fluctuations in a dispersing plume over the energetic and inertial-convective range of scales of turbulent motion are studied using a multiscale analysis technique that is based on an orthonormal wavelet representation. It is shown that the Haar wavelet concentration spectrum is similar to the Fourier concentration spectrum in that both spectra exhibit an extensive inertial-convective subrange spanning about two decades in frequency, with a scaling exponent of -5/3. Analysis of the statistical properties (e.g., fluctuation intensity, skewness, and kurtosis) of the concentration wavelet coefficients (i.e., the concentration discrete detailed signal) suggests that the small scales are always more intermittent than the large scales. The degree of intermittency increases monotonically with decreasing scale within the inertial-convective subrange, reaching a plateau at the very small scales associated with the beginning of the near-dissipation subrange. The probability density function (pdf) of the concentration discrete detailed signal displays stretched exponential tails with an intermittency exponent (tail slope) q that increases as ^a , where is the scale or dilation and a is a power-law exponent that is dependent on downwind distance, plume height, and stratification strength with typical values in the range from about 0.25 to 0.35. It is shown that the concentration variance cascade process requires a phase coherency of eddies between different scales at the small-scale end of the inertial-convective subrange.The variation of the concentration wavelet statistics with height above the ground is investigated. The increased mean shear near the ground smooths the fine-scale plume structure for scales within the inertial-convective subrange, producing a weaker spatiotemporal intermittency in the concentration field compared to that measured higher up in the plume. The pdf of the concentration detailed signal at a fixed scale possesses less elongated tails with decreasing height z. The intermittency exponent q is found to decrease roughly linearly with increasing z.Finally, the results of the wavelet decomposition are combined to provide a conceptual model of the turbulent transport, stirring, and mixing regimes in a dispersing plume. The implications of the results for contaminant texture in a plume are discussed.     

湍流不同尺度间的相互作用

湍流是由各种不同尺度的涡旋构成的。从整体上讲,存在Ｋｏｌｍｏｇｏｒｏｖ导得的惯性区结构函数的２／３次方定律和能谱的－５／３次方定律。这种幂律相关的湍流是由每种尺度上指数相关的局部相似的Ｂｒｏｗｎ运动所构成,从而从理论上解释了不同尺度之间的相互作用以及湍流系数Ｋ随尺度变化的规律,初步论证了大气不同层结下湍流系数的差别。结果对研究大气、海洋不同尺度间的相互作用有重要意义。     

Isotropic and anisotropic correlations in turbulent wind speed data

The correlations in turbulent wind speed data from the open ocean are studied. They are calculated as Fourier transforms of the spectraldensities, which are computed from Fourier transforms of the measured windspeed components. All correlations are found to decay within a few seconds.It is shown that the spectra are most efficiently calculated using data sections whose length does not exceed the observed correlation decay time.Longer runs include more random noise without containing more information.The ratio of the along-wind to transverse spectral levels is investigatedand found to be in excellent agreement with the assumption of isotropy inthe inertial wave number range.     

自适应阈值方法去除风廓线雷达地物杂波

风廓线雷达探测过程中电磁波传输会受到各类杂波的干扰,其中,地物是主要来源。从功率谱数据上看,地物杂波主要集中在零频附近,且幅度较高,不加以抑制会影响气象回波的识别。针对目前常用的小波阈值滤波法在处理近零频回波被杂波覆盖时效果不佳的情况,该文结合风廓线雷达特点,提出一种根据小波分解高频系数自适应确定阈值的方法,并通过模拟数据与风廓线雷达实测数据进行检验,结果表明:即便信号靠近零频,且被杂波覆盖,该方法也能快速准确识别信号回波。同时,该算法原理简单、计算量小、易于实现,在实际应用中能够增加谱峰识别准确率,可为改善风廓线雷达产品质量提供参考。     

Turbulence spectra in the near-neutral surface layer over the Loess Plateau in China

We present the power spectra of wind velocity and the cospectra of momentum and heat fluxes observed for different wind directions over flat terrain and a large valley on the Loess Plateau. The power spectra of longitudinal (u) and lateral (v) wind speeds satisfy the −5/3 power law in the inertial subrange, but do not vary as observed in previous studies within the low frequency range. The u spectrum measured at 32 m height for flow from the valley shows a power deficit at intermediate frequencies, while the v spectrum at 32 m downwind of the valley reaches another peak in the low frequency range at the same frequency as the u spectrum. The corresponding peak wavelength is consistent with the observed length scale of the convective outer layer at the site. The v spectrum for flat terrain shows a spectral gap at mid frequencies while obeying inner layer scaling in its inertial subrange, suggesting two sources of turbulence in the surface layer. All the spectra and cospectra from the valley direction show a height dependency over the three levels.     

An Investigation into Ridge-Top Turbulence Characteristics During Neutral and Weakly Stable Conditions: Velocity Spectra and Isotropy

An investigation into high Reynolds number turbulent flow over a ridge top in New Zealand is described based on high-resolution in-situ measurements, using ultrasonic anemometers for two separate locations on the same ridge with differing upwind terrain complexity. Twelve 5-h periods during neutrally stratified and weakly stable atmospheric conditions with strong wind speeds were sampled at 20 Hz. Large (and small) turbulent length scales were recorded for both vertical and longitudinal velocity components in the range of 7–23 m (0.7–3.3 m) for the vertical direction and 628–1111 m (10.5–14.5 m) for the longitudinal direction. Large-scale eddy sizes scaled to the WRF (Weather Research and Forecasting) numerical model simulated boundary-layer thickness for both sites, while small-scale turbulent features were a function of the complexity of the upwind terrain. Evidence of a multi-scale turbulent structure was obtained at the more complex terrain site, while an assessment of the three-dimensional isotropy assumption in the inertial subrange of the spectrum showed anisotropic turbulence at the less complex site and evidence of isotropic turbulence at the more complex site, with a spectral ratio convergence deviating from the 4/3 or unity values suggested by previous theory and practice. Existing neutral spectral models can represent locations along the ridge top with simple upwind complexity, especially for the vertical wind spectra, but sites with more orographic complexity and strong vertical wind speeds are often poorly represented using these models. Measured spectra for the two sites exhibited no significant diurnal variation and very similar large-scale and small-scale turbulent length scales for each site, but the turbulence energy measured by the variances revealed a strong diurnal difference.