Turbulence Anisotropy in the Near-Surface Atmosphere and the Evaluation of Multiple Outer Length Scales

Length scales determined by maximum turbulent kinetic energy (TKE), the integral scale, and two length scales based on Reynolds stress-tensor anisotropy are compared to the often stated outer length scales of boundary-layer depth and distance from the earth’s surface, $z$ . The scales are calculated using sonic anemometer data from two elevations, 5 and 50 m above the ground at the main tower site of the CASES-99 field campaign. In general, none of these scales agrees with the other, although the scale of maximum TKE is often similar to the boundary-layer depth during daytime hours, and the length scales derived from anisotropy characteristics are sometimes similar to $\kappa \!z, z$ , and $2z$ depending on scale definition and thermal stability. Except for the scale with the strictest isotropy threshold, the turbulence is anisotropic for each of the various candidates for the outer scale. Length scales for maximum buoyancy flux and temperature variance are evaluated and the turbulence characteristics at these scales are almost always found to be anisotropic.     

水平非均匀层结大气中重力波的时空结构

用WKBJ方法结合特征线法求得了重力波波包在水平非均匀层结和时变层结大气中演变的渐近解，结果表明层结水平非均匀性除引起重力波波幅的变化外，还引起波长和包络宽度的变化，当波包由层结大值区移向层结小值区时，水平波长变短，包络宽度变窄，同时振幅增加。层结随时间的变化不会引起波包波长和包络的宽度的变化，但层结随时间减小时，波包振幅增加。     

“尤特”特大暴雨过程的热力条件分析

1311号强台风尤特登陆后给广东带来持续性大范围强降水,对流降水特征十分显著。文章分析了"尤特"影响期间大尺度环流背景,重点讨论了此次持续性强降水过程中大气层结问题。发现低空急流向广东输送强的暖平流,是广东大气层结不稳定得以持续维持的根本原因。进一步分析发现,低空急流本身并不是"暖"的,当"尤特"趋向陆地时,陆地上的暖气团在"尤特"环流强迫下向南传播扩散,低空急流穿越这一暖区时温度升高才具备"暖"的特性。温度诊断方程结果进一步证实这一点。通过个例反查,在许多登陆后造成连续强降水的台风过程中均发现了这一特征。因此,台风登陆引起环境温度场的演变以及与低空急流的配置需引起业务预报上的重视。     

Effects of Updated RegCM4 Land Use Data on Near-Surface Temperature Simulation in China

Biogeophysical effects of land use and land cover (LULC) changes play a significant role in modulating climate on various spatial scales. In this study, a set of recent LULC products with a spatial resolution of 500 m was developed in China for update in RegCM4 (regional climate model version 4). Two sets of comparative numerical experiments were conducted to study the effects of LULC changes on near-surface temperature simulation. The results show that after LULC changes, areas of crops and mixed woodlands as well as urban areas increase over entire China, accompanied with greatly expanded mixed farming and forests/field mosaics in southern China, and reduced areas of 1) irrigated crops and short grasses in northern China and the Tibetan Plateau, and 2) semi-desert and desert in northwestern China. Improvements in the LULC data clearly result in more accurate simulations of the near-surface temperature. Specifically, increasing latent heat and longwave albedo due to enhanced LULC in certain areas lead to reduction in land surface temperature (LST), while changes in shortwave albedo and sensible heat also exert a great influence on the LST. Overall, these parameter adjustments reduce the biases in near-surface temperature simulation.     

中纬度夏季一次深厚对流过程的数值模拟研究:高空热力层结和风切变影响

针对一次发生在中纬度夏季的深厚对流过程进行的数值模拟研究的结果,引入一个较详细描述深厚对流状态下云微物理过程的参数化方案,对一个有限差分三维非静力弹性大气数值模式进行了改进.数值模拟得到了与天气雷达观测一致的结果.结果还显示,对于这次穿透高度超过了对流层顶的深厚对流过程,与高空斜压扰动有关的上对流层弱稳定热力层结对系统的维持起到了重要的作用.另外,还发现强烈的对流发展到对流层顶的高度时,受上曳体的驱动,在对流层顶附近的稳定层结中产生浮力性的扰动;它们与风切变相结合,进而对系统的水平移动产生控制作用.     

Comparison of Different Generation Mechanisms of Free Convection between Two Stations on the Tibetan Plateau

Based on high-quality data from eddy covariance measurements at the Qomolangma Monitoring and Research Station for Atmosphere and Environment(QOMS) and the Southeast Tibet Monitoring and Research Station for Environment(SETS),near-ground free convection conditions(FCCs) and their characteristics are investigated. At QOMS, strong thermal effects accompanied by lower wind speeds can easily trigger the occurrence of FCCs. The change of circulation from prevailing katabatic glacier winds to prevailing upslope winds and the oscillation of upslope winds due to cloud cover are the two main causes of decreases in wind speed at QOMS. The analysis of results from SETS shows that the most important trigger mechanism of FCCs is strong solar heating. Turbulence structural analysis using wavelet transform indicates that lowerfrequency turbulence near the ground emerges from the detected FCCs both at QOMS and at SETS. It should be noted that the heterogeneous underlying surface at SETS creates large-scale turbulence during periods without the occurrence of FCCs. Regarding datasets of all seasons, the distribution of FCCs presents different characteristics during monsoonal and non-monsoonal periods.     

Anisotropy of Unstably Stratified Near-Surface Turbulence

Boundary-Layer Meteorology - Classic Monin–Obukov similarity scaling states that in a stationary, horizontally homogeneous flow, in the absence of subsidence, turbulence is dictated by the...     

The Effects of Thermal Stratification on Clustering Properties of Canopy Turbulence

The intermittent structure of turbulence within the canopy sublayer (CSL) is sensitive to the presence of foliage and to the atmospheric stability regime. How much of this intermittency originates from amplitude variability or clustering properties remains a vexing research problem for CSL flows. Using a five-level set of measurements collected within a dense hardwood canopy, the clustering properties of CSL turbulence and their dependence on atmospheric stability are explored using the telegraphic approximation (TA). The binary structure of the TA removes any amplitude variability from turbulent excursions but retains their zero-crossing behaviour, and thereby isolating the role of clustering in intermittency. A relationship between the spectral exponents of the actual and the TA series is derived across a wide range of atmospheric stability regimes and for several flow variables. This relationship is shown to be consistent with a relationship derived for long-memory and monofractal processes such as fractional Brownian motion (fBm). Moreover, it is demonstrated that for the longitudinal and vertical velocity components, the vegetation does not appreciably alter fine-scale clustering but atmospheric stability does. Stable atmospheric stability conditions is characterized by more fine scale clustering when compared to other atmospheric stability regimes. For scalars, fine-scale clustering above the canopy is similar to its velocity counterpart but is significantly increased inside the canopy, especially under stable stratification. Using simplified scaling analysis, it is demonstrated that clustering is much more connected to space than to time within the CSL. When comparing intermittency for flow variables and their TA series, it is shown that for velocity, amplitude variations modulate intermittency for all stability regimes. However, amplitude variations play only a minor role in scalar intermittency. Within the crown region of the canopy, a ‘double regime’ emerges in the inter-pulse duration probability distributions not observed in classical turbulence studies away from boundaries. The double regime is characterized by a power-law distribution for shorter inter-pulse periods and a log-normal distribution for large inter-pulse periods. The co-existence of these two regimes is shown to be consistent with near-field/far-field scaling arguments. In the near-field, short inter-pulse periods are controlled by the source strength, while in the far-field long inter-pulse periods are less affected by the precise source strength details and more affected by the transport properties of the background turbulence.     

Thermal Stratification Effects on Flow Over a Generic Urban Canopy

The influence of local surface heating and cooling on flow over urban-like roughness is investigated using large-eddy simulations. By adjusting the incoming or outgoing heat flux from the ground surface, various degrees of local thermal stratification, represented by a Richardson number \((Ri_\tau )\) , were attained. Drag and heat transfer coefficients, turbulence structure, integral length scales, and the strength of quadrant events that contribute to momentum and heat fluxes were obtained and are compared with locally stable, neutral and unstable flows. With increasing \(Ri_\tau \) , or equivalently as the flow characteristics change from local thermal instability to stability, a gradual decline in the drag and heat transfer coefficients is observed. These values are found to be fairly independent of the type of thermal boundary condition (constant heat flux or constant temperature) and domain size. The maps of anisotropy invariants showed that for the values of \(Ri_\tau \) considered, turbulence structures are almost the same in shape for neutral and unstable cases but differ slightly from those in the stable case. The degree of anisotropy is found to decrease as \(Ri_\tau \) increases from \(-2\) to 2.5. Compared to the neutral case, the integral length scales are shortened in the streamwise and vertical direction by ground cooling, but enhanced in the vertical direction with ground heating. Quadrant analysis showed that an increase in floor heating increases the strength of ejections above the canopy. However, the contributions of updrafts or downdrafts to the heat flux are found not to be significantly influenced by the type of local thermal stratification for the values of \(Ri_\tau \) considered. From the octant analysis, the transport mechanisms of momentum and heat above the canopy are found to be very similar in both locally unstable and stable flows.     

Effects of Topographical Slope Angle and Atmospheric Stratification on Surface-Layer Turbulence

The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u _* /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ _w /U) is the smallest over the downslope surface whereas that of the u component (σ _u /U) has a minimum over the upslope surface. Consequently, σ _w /u _* has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ _u /u _* has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface.     

Near-Surface Vertical Flux Divergence in the Stable Boundary Layer

Flow in the stable boundary layer is examined at four contrasting sites with greater upwind surface roughness. The surface heterogeneity is disorganized and in some cases weak as commonly occurs. With low wind speeds, the vertical divergence (or convergence) of the momentum and heat fluxes can be large near the surface in what is normally assumed to be the surface layer where such divergence is neglected. For the two most heterogeneous sites, a shallow “new” boundary layer is captured by the tower observations, analogous to an internal boundary layer but more complex. Above the new boundary layer, the magnitudes of the downward fluxes of heat and momentum increase with height in a transition layer, reach a maximum, and then decrease with height in an overlying regional boundary layer. Similar structure is observed at the site with rolling terrain where the shallow new boundary layer at the surface is identified as cold-air drainage generated by the local slope above which the flow undergoes transition to an overlying regional flow. Significant flux divergence near the surface is generated even over an ice floe for low wind speeds and in a shallow Ekman layer that forms during the polar night. For higher wind speeds, the magnitude of the downward fluxes decreases gradually with height at all levels as in a traditional boundary layer.     

Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-eddy Simulations. Part 1: Free Convection

Convection in a quasi-steady, cloud-free, shear-free atmospheric boundary layer is investigated based on a large-eddy simulation model. The performed tests indicate that the characteristic (peak) values of statistical moments at the top of the mixed layer are proportional to the interfacial scales (from gradients of scalars in the interfacial layer). Based on this finding a parameterization is proposed for profiles of scalar variances. The parameterization employs two, semi-empirical similarity functions F_m(z/z_i) andF_i(z/z_i), multiplied by a combination of the mixed-layer scales and the interfacial scales.     

夏季白天中国中东部不同类型云分布特征及其对近地表气温的影响

本文利用2001～2017年ERA5再分析资料以及CERES卫星资料,探究夏季白天中国中东部不同类型云的云量及其光学厚度的时空变化特征,并利用一维辐射对流模式定量分析不同类型云对近地表气温的影响。观测结果表明:夏季白天中国中东部总云量及其光学厚度整体呈由南向北逐渐减小的分布特征,且中高云量占主导地位。总云量整体呈?0.3% a?1显著减少趋势,其中低云的贡献（?0.27% a?1）最大;总云光学厚度为0～0.1 a?1增加趋势,其中低云光学厚度（0.06 a?1）和中低云光学厚度（0.03 a?1）呈增加趋势,而中高云光学厚度（?0.08 a?1）和高云光学厚度（?0.03 a?1）呈减少趋势。模式结果表明:四种不同类型云的温度效应（Cloud Effect Temperature, CET）均为负值,表现为降温效应。低云、中低云、中高云和高云的年均CET值分别为?2.9°C、?2.7°C、?2.2°C和?1.7°C。其中,低云在华北平原降温可达?5°C;中低云和中高云在四川盆地和云贵高原降温可达?7.8°C。不同类型云温度效应与近地表气温的年际变化具有较好的一致性,具体表现为:2004年前（后）近地表气温呈现下降（上升）趋势,不同类型云的CET在此期间呈下降（上升）趋势,表现为云的降温效应增强（减弱）与近地表气温下降（上升）相对应,体现了夏季白天中国中东部4种不同类型云温度效应与近地表气温都呈正相关关系。特别地,夏季白天中国中东部中高云量占主导地位,其CET与近地表气温的相关系数高达0.63。综上,夏季白天中国中东部不同类型云温度效应对近地表气温的影响不同,但均呈正相关关系。定量分析不同类型云对近地表气温的影响可以为定量研究云反馈对区域增暖的作用以及合理预估未来区域增暖情景提供必要的科学参考。     

大气层结和天气尺度位温扰动对锋生的作用

文中利用半地转锋生模式考虑了三种不同大气层结廓线和两种不同水平温度对比的天气尺度位温扰动场的作用。计算结果表明,层结对锋生速度影响较小,对锋区垂直运动影响却很大;天气尺度位温扰动对锋生速度和锋区垂直运动都有相当大的影响。     

RS485总线在近地层通量观测系统中的应用

RS485接口具有良好的抗噪声干扰性、长传输距离和接收多站数据能力等优点,使其成为分布式测量控制系统首选的串行接口。介绍了RS485总线的特点、工作方式、组网及在张掖近地层通量观测系统中的具体应用,同时探讨了RS485总线使用中的一些问题及采取的应对措施。     

Convection of heat

Summary Several aspects of the formulation and physical meaning of heat flux by convection are discussed. Convective heat flux, is uniquely determined only when the flux of each constituent substance vanishes. For uniform composition, the physically satisfactory formulation of eddy flux of heat is in terms of the velocity fluctuation about the weighted mean introduced byHesselberg but in some important problems this formulation is practically the same asSwinbank's. It is suggested that convective heat flux be interpreted to include the flux of not only enthalpy but also potential energy.

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Zusammenfassung Es werden mehrere Möglichkeiten der Formulierung und der physikalischen Interpretation des Wärmeflusses durch Konvektion diskutiert. Der konvektive Wärmefluß ist nur dann eindeutig bestimmt, wenn der Fluß der einzelnen Komponenten verschwindet. Eine physikalisch befriedigende Formulierung der Wärmescheinleitung im Falle eines homogenen Mediums läßt sich mittels Geschwindigkeitsschwankungen um die ausgeglichene Bewegung gewinnen, wie sie vonHesselberg eingeführt wurden; doch ist diese Formulierung bei einigen wichtigen Problemen praktisch identisch mit der vonSwinbank. Es wird vorgeschlagen, unter konvektivem Wärmefluß nicht nur den Fluß von Enthalpie, sondern auch den von potentieller Energie zu verstehen.

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Résumé Cet exposé discute les divers aspects du sens physique du flux calorifique dû à la convection. Le flux convectif de chaleur n'est déterminé absolument que lorsque de flux le chaque constituant est égal à zéro. Pour une composition uniforme, la formule physiquement satisfaisante du flux calorifique dû à la turbulence est en fonction de la fluctuation de la vitesse par rapport à la moyenne pondérée introduite parHesselberg. Dans certains problèmes importants, cette formule est cependant pratiquement la même que celle deSwinbank. L'auteur propose d'interpréter le flux de chaleur convectif de manière à y inclure non seulement le flux d'enthalpie, mais aussi le flux d'énergie potentielle.

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This research was sponsored by the Office of Naval Research, U. S. Department of the Navy, under contract with Brown University.Contribution No. 701 from the Woods Hole, Oceanographic Institution.     

帕米尔高原近地层气象要素垂直变化特征

利用中国与巴基斯坦交界的红其拉甫口岸帕米尔高原陆气相互作用观测站 32 m 梯度铁塔资料,分析了帕米尔高原在不同天气下近地层气温、湿度和风速等气象要素廓线日变化特点和脉动特征,结果表明：（1）晴天,日间风速随高度升高而增大,夜间风速随高度升高呈波动减小;阴雨天,风速整体随高度升高而增大;不同天气下各高度层脉动风速概率分布均呈现高度越高,脉动风速分布越集中的特征。（2）气温廓线在晴天表现为夜间辐射型、早上过渡型、白天日射型和傍晚过渡型四种类型,气温脉动随着高度的降低而增大;阴雨天气时气温廓线均为白天日射型,各高度层的气温脉动变化特征与晴天相反。（3）不同天气的比湿廓线分布均随高度增加而减小,比湿脉动范围晴天大于阴雨天,且在时间序列上波动性均具有一致性。     

Effects of Unstable Thermal Stratification on Vertical Fluxes of Heat and Momentum in Urban Areas

A Reynolds-averaged Navier–Stokes microscale model is used for the simulation of the effect of unstable thermal stratification on the flow within an aligned configuration of building-like cubes as used in Santiago et al. (Urban Clim 9:115–133, 2014). The spatially-averaged results show increased dispersive fluxes, turbulent length scales and sectional drag coefficient. An extension of K-theory is presented to parametrize the sum of the turbulent and dispersive fluxes, and the length scale and drag coefficient increases are parametrized as functions of the ratio of buoyant and inertial forces. This approach improves the results of urban canopy parametrization simulations inside and above the urban canyon and represents the first attempt to account for the dispersive fluxes and the effect of solar radiation on the flow.