华东沿海高标准农田大气湍流特征的观测分析

为评估Monin-Obukhov相似理论和涡度相关系统在高标准农田的适用性,利用南通高标准农田水稻下垫面涡度相关观测数据,分析了大气稳定度、湍流方差相似性、湍流谱特征、湍流强度和湍流动能等。结果表明：该高标准稻田上方大气呈现白天不稳定、夜晚稳定的昼夜变化特征;Monin-Obukhov相似理论在该高标准农田适用,三维风速归一化标准差随大气稳定度的变化符合“1/3”次方规律,温度、比湿、CO₂密度归一化标准差随大气稳定度的变化在大气不稳定时符合“1/3”次方规律;在惯性子区,三维风速的湍流能谱符合“-2/3”次方斜率,垂直风速与温度、湿度和CO₂密度的协谱符合“-4/3”次方斜率,可见涡度相关系统能够观测该高标准农田的水热和CO₂通量;三个方向上的湍流强度均随风速增大而减小,且水平方向湍流强度大于垂直方向;湍流动能随风速呈二次函数增大,呈现昼高夜低的日变化特征,大气中性时达到最大,且以动力湍流贡献为主。研究结果可为揭示华东地区高标准农田生态系统与大气之间的物质和能量交换特征以及机制提供参考。     

鼎湖山针阔叶混交林生态系统能量平衡分析

能量平衡分析作为评价涡度相关法通量观测数据可靠性方法，备受学界重视。应用OLS（Ordinary least squares）和EBR（Energy Balance Ratio）2种方法，系统分析了广东省鼎湖山针阔叶混交林生态系统能量平衡特点，并分析各种涡度通量修正方法对能量平衡的影响，结果表明鼎湖山通量站平均能力平衡不闭合度为33％-47％，略高于普遍报道的不闭合度范围（10％-30％）。WPL修正、μ＊订正和坐标转换，使得能量平衡闭合度有所提高，但夜间特别是冬季能量平衡较差问题依然没有得到根本解决，表明夜间弱湍流并不是导致夜间能量平衡闭合度差的主要原因。为客观评价本通量站以及ChinaFLUX能量平衡状况和通量数据质量，确定涡度相关法CO2通量数据分析方法和改进策略提供依据。     

麦田通量数据质量控制及能量闭合性分析

近地面层能量不闭合是涡度相关系统观测中普遍存在的难点问题。为了提高涡度相关系统通量观测数据的质量,选取郑州农业气象试验站2009年10月—2010年6月整个冬小麦生长季的通量观测资料,对30 min通量数据进行野点剔除、数据插补和Webb-Pearman-Leuning(WPL)校正等数据预处理。用两种方法(OLS和EBR)评价了该地区麦田生态系统能量平衡状况。结果表明:处理后的数据质量有明显提升,日平均湍流能增加7.09 W·m~(-2),日平均CO2通量减少0.0730 mg·m~(-2)·s~(-1)。农田涡度相关系统观测的能量平衡比率(EBR)日变化规律明显:早晨和傍晚昼夜交替时EBR波动最大,白天能量闭合状况优于夜间;暖季EBR大部分都在1左右浮动,能量平衡闭合程度较高,而冬季能量平衡闭合程度比暖季偏低。能量通量和湍流通量有较强的相关性,决定系数R2=0.8066,能量闭合度为88%,存在能量不闭合现象。     

“6·30”川渝特大暴雨过程中西南低涡发展机制模拟分析

利用地面加密自动站、NCEP(1°×1°)再分析及WRF-ARW数值模拟资料,对造成2013年6月29日至7月1日川渝地区特大暴雨过程的西南低涡的演变特征及热动力机制进行了诊断分析。此次特大暴雨过程分为三个阶段,其中与西南低涡发展相关的第二阶段为主要降雨时段。结果表明:(1)西南低涡生成后,呈现增强-减弱-再次增强的波动性演变特征,在6月30日上午和夜间分别经历了两次显著的涡度发展过程,并在第二次发展过程中达到最强。(2)低空辐合是西南低涡最主要的涡源,由低空辐合导致的正涡度增加近乎贯穿于整个西南低涡的生命史。在西南低涡的第二次发展过程中,中层辐合和涡度的垂直输送显著增强,也是西南低涡的重要涡源。(3)负值非平衡动力强迫激发了低空辐合的增长,在非平衡动力强迫的各项中,位势高度的拉普拉斯项为非平衡动力强迫提供了主要的负贡献来源。(4)非绝热加热先于西南低涡而增强,两者间的正反馈作用可能是西南低涡波动性发展的重要机制。在关闭了微物理过程中的潜热和地面潜热及感热通量的敏感性试验中,西南低涡及降雨的模拟均有不同程度的减弱。     

边界层抽吸引起的旋转减弱

本文在边界层顶垂直速度正比于地转涡度和地转风速,并与下垫面粗糙度有关的前提下,研究了边界层抽吸引起的涡度变化,在圆对称气压系统内得到了不同粗糙度情况下的涡度场和气压场的变化速率,修正了经典理论的结果。在湍流交换系数是地转风速及高度的函数的前提下,推导了地形存在时边界层顶垂直速度的公式,并用来讨论地形存在时的旋转减弱问题。     

山地复杂下垫面湍流特征观测分析

利用谱方法和能量闭合度分析等方法,对兰州市皋兰山2006年夏季和2005年冬季涡度相关系统测定的湍流数据质量进行了分析,结果表明:4个观测点测定的三维风速、空气温度的功率谱在惯性副区都符合"-2/3"定律,垂直风速和温度的协谱在惯性副区也基本符合"-4/3"定律.4个观测点都存在能量平衡不闭合现象,夏季观测点绿化地和裸地能量平衡比率(EBR)分别为66%和94%,冬季绿化地和裸地的EBR分别为62%和84%;裸地观测点的闭合程度明显好于绿化地.本文还讨论了能量闭合度的日变化特征及各个观测点的地表能量收支状况.     

青藏高原东部拉萨河下游地区大气湍流交换特征研究

地气之间物质和能量随湍流运动进行输送,涡度相关技术是研究地气交换过程和评估大气资源的重要手段,它对湍流特征和精确的通量观测研究具有重要的作用.本研究利用拉萨蔡公堂通量站的闭路涡度相关系统,观测了 2020年8-11月青藏高原东部拉萨河下游地区典型高寒草甸下垫面的通量特征,并分析了该区域生长季和非生长季不同大气稳定条件下...     

北京325米气象塔上CO2梯度观测数据质量控制与评价

近些年涡度相关系统在城市通量研究中得到了广泛应用,因城市下垫面的特殊性和复杂性,以及系统观测原理和仪器精度存在着局限性,使得结果存在10%以上偏差,故必须对原始观测数据进行质量控制和评价。本文对北京325 m气象塔七层高度上的CO₂(二氧化碳)、能量等通量进行了长期观测,研究了涡度相关技术在城市环境通量观测中的适用性,设计出了一套适合于复杂城市下垫面上的涡度相关系统资料前处理和质量控制方案,并对比了不同修正方案效果。分析结果表明,城市下垫面环境中计算CO₂通量的最优周期为30 min;二次坐标旋转法优于平面拟合法;频率响应修正后的感热通量提高5.21%,潜热通量和CO₂通量均提高9.42%。空气密度脉动修正Liu法(Liu,2005)优于WPL法(Webb et al.,1980);湍流谱在惯性副区满足-2/3次方定律,协方差谱满足-4/3次方定律。经过这套前处理和质量控制方案,原始数据中有79%能够用于基础研究,该质量控制与评价体系可为复杂城市下垫面通量研究提供参考。     

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。     

山地边界层中场变量的动力学研究

采用与实测较接近的二次函数来表达Ｅｋｍａｎ层中的湍流粘性系数Ｋ，在圆形气压场条件下，求得了山地上空边界层中的风速，进而求得散度、涡度和垂直速度等场变量随高度的分布。并作图分析了这些场变量的一些动力学特征。改进了以往在求解析解时，略去运动方程中湍流粘性力项中的关于高度的一阶导数项，以及取山坡面上风速为零作下边界条件等欠合理欠精确的做法。所求得的风速、散度、涡度和垂直速度均用简单的初等函数表示出来，有助于边界层参数化和深化对边界层动力学的认识。     

A study on the motion characteristics and their impact on the wind measurement post-processing of the GPS dropwindsonde Part I: effects of the wind-finding equations

Since Global Positioning System dropwindsonde wind measurements are taken in neither a conventional Eulerian framework nor a perfect Lagrangian framework, proper post-processing is necessary to derive conventional Eulerian wind statistics from dropwindsonde measurements. In this study, the equations governing the dropwindsonde motions were first linearized and then analytically solved to explore the effects of the wind-finding equations introduced by Hock and Franklin (Bull Amer Meteor Soc 80:407–420, 1999) when retrieving horizontal winds from dropwindsonde measurements. It is found that the wind-finding equations are essential for calculating turbulence statistics from dropwindsonde measurements. To confirm, numerical simulations of dropwindsonde motions in a pseudo-stochastic wind field were further conducted based on the full nonlinear dropwindsonde motion-governing equations. Based on the simulation results, the present paper showed the effects of the wind-finding equations and the companion paper discussed the impacts of various post-processing/composition schemes on both the mean and turbulent wind statistics composited from pseudo-dropwindsonde measurements.     

Approximate calculation of Lagrangian statistical functions of fluid particle pairs in isotropic turbulence

Using a hypothesis of an approximate relation between Eulerian and Lagrangian statistical functions of turbulence, calculations of one- and two-particle Lagrangian correlation tensors of fluid particle displacements and velocities in a field of isotropic and stationary turbulence are given.     

Estimation Of The Lagrangian Structure Function Constant C0 From Surface-Layer Wind Data

Eulerian turbulence observations, madein the surface layer under unstable conditions (z/L > 0),by a sonic anemometer were used to estimatethe Lagrangian structure function constant _O. Twomethods were considered. The first one makes use of arelationship, widely used in the Lagrangian stochasticdispersion models, relating _O to the turbulent kineticenergy dissipation rate , wind velocity variance andLagrangian decorrelation time. The second one employsa novel equation, connecting _O to the constant of thesecond-order Eulerian structure function. Beforeestimating _O, the measurements were processed in orderto discard non-stationary cases at least to a firstapproximation and cases in which local isotropy couldnot be assumed. The dissipation was estimated eitherfrom the best fit of the energy spectrum in theinertial subrange or from the best fit of the third-orderlongitudinal Eulerian structure function. Thefirst method was preferred and applied to the subsequentpart of the analysis. Both methods predict thepartitioning of _O in different spatial components as aconsequence of the directional dependence of theEulerian correlation functions due to the isotropy.The _O values computed by both methods are presented anddiscussed. In conclusion, both methods providerealistic estimates of _O that compare well withprevious estimations reported in the literature, evenif a preference is to be attributed to the second method.     

Horizontal coherence and Pasquill's beta

Previous work has suggested that horizontal coherence between longitudinal wind components increases with decreasing stability. Further analysis now suggests that this relation is valid only for separations of 50 m or larger. For separations of a few meters, the dependence of coherence on stability has the opposite sign. A hypothesis is suggested which accounts for the rather complex relationship between coherence, stability, roughness, and separation. The theory is extended to suggest an inverse relationship between the decay of coherence with time, and the ratio of Lagrangian and pseudo-Lagrangian turbulence scales to the longitudinal Eulerian scale.     

On the Lagrangian and Eulerian Time Scales of Turbulence Within a Two-Dimensional Array of Obstacles

Boundary-Layer Meteorology - Fields of Lagrangian ( $$T^{L}$$ ) and Eulerian ( $$T^{E}$$ ) time scales of the turbulence within a regular array of two-dimensional obstacles of unit aspect ratio...     

A Lagrangian Decorrelation Time Scale in the Convective Boundary Layer

A new method for deriving the Lagrangian decorrelation time scales for inhomogeneous turbulence is described. The expression for the time scales here derived for the convective boundary layer is compared to those estimated by Hanna during the Phoenix experiment. Then the values of C₀, the Lagrangian velocity structure function constant, and of B_i, the Lagrangian velocity spectrum constant, were evaluated from the Eulerian velocity spectra and from the Lagrangian time scales derived, under unstable conditions, from Taylor's statistical diffusion theory. The numerical coefficient of the lateral and vertical Lagrangian spectra in the inertial subrange was found equal to 0.21, in good agreement with previous experimental estimates.     

The Effect of Scale on the Applicability of Taylor’s Frozen Turbulence Hypothesis in the Atmospheric Boundary Layer

Taylor’s frozen turbulence hypothesis is the central assumption invoked in most experiments designed to investigate turbulence physics with time resolving sensors. It is also frequently used in theoretical discussions when linking Lagrangian to Eulerian flow formalisms. In this work we seek to quantify the effectiveness of Taylor’s hypothesis on the field scale using water vapour as a passive tracer. A horizontally orientated Raman lidar is used to capture the humidity field in space and time above an agricultural region in Switzerland. High resolution wind speed and direction measurements are conducted simultaneously allowing for a direct test of Taylor’s hypothesis at the field scale. Through a wavelet decomposition of the lidar humidity measurements we show that the scale of turbulent motions has a strong influence on the applicability of Taylor’s hypothesis. This dependency on scale is explained through the use of dimensional analysis. We identify a ‘persistency scale’ that can be used to quantify the effectiveness of Taylor’s hypothesis, and present the accuracy of the hypothesis as a function of this non-dimensional length scale. These results are further investigated and verified through the use of large-eddy simulations.     

A three-dimensional model for calculating the concentration distribution in inhomogeneous turbulence

A new approach for calculating the concentration distribution in inhomogeneous turbulence is suggested. The model is a 3-D model, constrained to describe incompressible flow. The model requires a knowledge of the covariance matrix of the Eulerian velocities and the two-point third moments. The model is applied for three types of turbulent field: homogeneous isotropic turbulence, constant flux neutral boundary layer and free convective turbulence. The required Eulerian moments are calculated using the eddy model of the turbulent field. Concentration moments are calculated and results are compared to experimental data. Other model predictions which have no experimental support can be compared to measurements when available.     

A spectral and lag-correlation analysis of turbulence in a deciduous forest canopy

The processes influencing turbulence in a deciduous forest and the relevant length and time scales are investigated with spectral and cross-correlation analysis. Wind velocity power spectra were computed from three-dimensional wind velocity measurements made at six levels inside the plant canopy and at one level above the canopy. Velocity spectra measured within the plant canopy differ from those measured in the surface boundary layer. Noted features associated with the within-canopy turbulence spectra are: (a) power spectra measured in the canopy crown peak at higher wavenumbers than do those measured in the subcanopy trunkspace and above the canopy; (b) peak spectral values collapse to a relatively universal value when scaled according to a non-dimensional frequency comprised of the product of the natural frequency and the Eulerian time scale for vertical velocity; (c) at wavenumbers exceeding the spectral peak, the slopes of the power spectra are more negative than those observed in the surface boundary layer; (d) Eulerian length scales decrease with depth into the canopy crown, then increase with further depth into the canopy; (e) turbulent events below crown closure are more correlated with turbulent events above the canopy than are those occurring in the canopy crown; and (f) Taylor's frozen eddy hypothesis is not valid in a plant canopy. Interactions between plant elements and the mean wind and turbulence alter the processes that produce, transport and remove turbulent kinetic energy and account for the noted observations.     

Modelling Mean and Turbulence Fields in the Dry Convective Boundary Layer with the Eddy-Diffusivity/Mass-Flux Approach

The treatment of turbulence closure in atmospheric models is examined in the context of the dry convective boundary layer (CBL) and the eddy-diffusivity/mass-flux (EDMF) approach. The EDMF approach is implemented into a model called TAPM to use a coupled two-equation prognostic turbulence closure and the mass-flux approach to represent turbulence in the CBL. This work also extends the range of turbulence variables that can be derived from the mass-flux component of the model and uses these along with their values from the prognostic scheme to provide total turbulence fields that can be used to compare to data and/or to feed into other components of TAPM, including those needed to drive Eulerian and Lagrangian air pollution dispersion modules. Model results are presented for the afternoon of a simulated summer day and are compared to both laboratory and field observations in a mixed-layer scaled framework. The results show that the EDMF approach works well within TAPM and can provide good predictions of mean and turbulence fields, including in the upper levels of the CBL. The EDMF approach has several attractive features, including the potential to be one approach to unify the treatment of turbulence and dry and moist convection in atmospheric models.