Spatial and Temporal Characteristics of Horizontal Rolls and Cells in the Atmospheric Boundary Layer Based on Radar and in Situ Observations

This paper is the second of a series devoted to the observation and analysis of coherent structures in the cloudy Atmospheric Boundary Layer (ABL) such as horizontal rolls or thermal cells. In the first paper, the TRAC (Turbulence Radar Aircraft Cells) experiment which is the observational support of this investigation based on coupled radar-aircraft measurements, was presented along with an overview of the main results of this campaign held in June 1993 in France. Here the analysis is focused on the spatial characteristics (length-scale, orientation ... ) of the coherent structures, their temporal evolution and vertical distribution deduced from the radar reflectivity fields acquired in clear air at several levels for five different ABLs. For that, an original and efficient image processing method able to extract the major mode of the organisation was developed. These characteristics are examined in relation to the dynamic and thermodynamic state of the ABL using mean and turbulent information as observed by the in-situ aircraft. These experimental results are the basis of a future Large Eddy Simulation modeling of an organised ABL which is in progress and will be the concern of the third paper in the series.     

Experimental Study on the Wake Meandering Within a Scale Model Wind Farm Subject to a Wind-Tunnel Flow Simulating an Atmospheric Boundary Layer

The phenomenon of meandering of the wind-turbine wake comprises the motion of the wake as a whole in both horizontal and vertical directions as it is advected downstream. The oscillatory motion of the wake is a crucial factor in wind farms, because it increases the fatigue loads, and, in particular, the yaw loads on downstream turbines. To address this phenomenon, experimental investigations are carried out in a wind-tunnel flow simulating an atmospheric boundary layer with the Coriolis effect neglected. A \(3 \times 3\) scaled wind farm composed of three-bladed rotating wind-turbine models is subject to a neutral boundary layer over a slightly-rough surface, i.e. corresponding to offshore conditions. Particle-image-velocimetry measurements are performed in a horizontal plane at hub height in the wakes of the three wind turbines occupying the wind-farm centreline. These measurements allow determination of the wake centrelines, with spectral analysis indicating the characteristic wavelength of the wake-meandering phenomenon. In addition, measurements with hot-wire anemometry are performed along a vertical line in the wakes of the same wind turbines, with both techniques revealing the presence of wake meandering behind all three turbines. The spectral analysis performed with the spatial and temporal signals obtained from these two measurement techniques indicates a Strouhal number of \(\approx 0.20 - 0.22\) based on the characteristic wake-meandering frequency, the rotor diameter and the flow speed at hub height.     

济南章丘大气边界层风温场特征研究

基于章丘气象站2004—2009年实测资料,研究了当地大气边界层风向、风速、气温的时空分布特征,并利用SPSS 17.0数据统计软件的曲线参数估计法,探讨了符合章丘实际的风速随高度变化的拟合曲线及其数学函数关系。结果显示:研究地区大气边界层盛行风向为SSW,且呈现随高度增加沿顺时针方向偏转的特征;风速变化特征在10—300 m和350—1500 m不同高度层存在显著差异,春、夏、秋季和年平均风速随高度变化的指数曲线拟合结果总体好于幂函数曲线,而冬季则相反;气温变化特征四季基本一致,近地面层存在逆温现象,冬季逆温日数最多;逆温强度冬半年的大于夏半年的。     

北京冬季大风过程大气边界层特征分析

利用北京中国科学院大气物理研究所325 m气象观测塔的气象梯度资料和湍流资料,分析了2014年11月29日至12月5日北京两次大风过程中气象要素和湍流输送特征的变化。第一次大风过程的强度和持续时间均高于第二次大风过程。强烈的风速垂直切变主要集中在距地面100 m高度范围内,最强风速垂直切变达到0.31 s~(-1)。大风过程中,阵风系数呈现随高度减小的趋势,越接近地面,阵风系数愈大。阵风强度的变化与阵风系数相似,100 m以下高度时,阵风强度随高度增大而减小。大风过程自上而下改变边界层结构,平均动能、湍流动能和摩擦速度最先从上层(280 m)发生变化且迅速增加。近地层由于风速垂直梯度的显著差异,近地层垂直方向的湍流强度最大。大风时各功率谱在低频区(0.01 s~(-1))达到峰值,大风过后各高度的能量都有所下降。     

An Idealized Mean Wind Profile for the Atmospheric Boundary Layer

We test a flexible, idealized mean wind profile for the loweratmosphere that can easily be matched to whatever windobservations may be available. Its intended function is to providea `best guess' wind profile from limited observations, e.g., foruse in dispersion models, and to this end, following earlierauthors, we have matched a Monin–Obukhov layer to a baroclinic Ekman layer.To demonstrate the flexibility of the two-layer wind profile, weoptimize its free parameters to provide best interpolative fits toa sample of multi-level wind profiles. These include model windprofiles extracted from the Canadian Global EnvironmentalMulti-scale weather model (GEM), as well as experimental profilesfrom the Wangara experiment, and from an over-ocean dispersionexperiment (LROD). In most cases the two-layer profile fit issatisfactory.     

大气边界层强风的阵性和相干结构

我国北方春季冷锋过境后,常骤发强风,甚至起沙扬尘,持续数小时甚至一二天,通过对边界层超声风温仪的资料分析,可知大风常叠加有周期为3～6 min的阵风,较有规律,且有明显的相干结构:阵风风速峰期有下沉运动,谷期有上升运动;阵风扰动以沿平均流的顺风方向分量为主,横向和垂直方向的分量都较小,其本质是低频次声波和重力波的混合;阵风沿顺风向且向下传播.周期小于1 min的脉动在水平面上基本是各向同性的不规则的湍涡.大风期间,无论是平均流、阵风和湍流脉动,至少在120 m高度以下,主要都有西风和北风动量下传,感热上传.平均流的动量下传强于由脉动下传的量,与一般天气情况不同,而且阵风与湍流的动量下传的量值差不多.平均流和阵风在动量传送上起相当大的作用.     

大气边界层风速脉动的分形模拟

应用曲线分数维计算方法计算大气边界层实际风速观测资料的分数维值,并根据分形理论构造出一个理论模型来模拟真实的大气边界层风速时间序列。将模拟和真实数据的一些重要的统计特征,例如方差、风速概率分布、谱密度函数和自相关函数等,进行了比较,结果表明二者具有很好的一致性。     

Flux-profile Relationships for Wind Speed and Temperature in the Stable Atmospheric Boundary Layer

Wind and temperature profiles in the stable boundary layer were analyzed in the context of MoninObukhov similarity. The measurements were made on a 60-m tower in Kansas during October 1999 (CASES-99). Fluxprofile relationships, obtained from these measurements in their integral forms, were established for wind speed and temperature. Use of the integral forms eliminates the uncertainty and accuracy issues resulting from gradient computations. The corresponding stability functions, which were nearly the same for momentum and virtual sensible heat, were found to exhibit different features under weakly stable conditions compared to those under strongly stable conditions. The gradient stability functions were found to be linear, namely _m = 1+ 5.8 and _h = 1 + 5.4 up to a limit of the MoninObukhov stability parameter = 0.8; this is consistent with earlier findings. However, for stronger stabilities beyond a transition range, both functions were observed gradually to approach a constant, with a value of approximately 7. To link these two distinct regimes, a general but pliable functional form with only two parameters is proposed for the stability functions, covering the entire stability range from neutral to very stable conditions.     

Measurements and Modelling of the Wind Speed Profile in the Marine Atmospheric Boundary Layer

We present measurements from 2006 of the marine wind speed profile at a site located 18 km from the west coast of Denmark in the North Sea. Measurements from mast-mounted cup anemometers up to a height of 45 m are extended to 161 m using LiDAR observations. Atmospheric turbulent flux measurements performed in 2004 with a sonic anemometer are compared to a bulk Richardson number formulation of the atmospheric stability. This is used to classify the LiDAR/cup wind speed profiles into atmospheric stability classes. The observations are compared to a simplified model for the wind speed profile that accounts for the effect of the boundary-layer height. For unstable and neutral atmospheric conditions the boundary-layer height could be neglected, whereas for stable conditions it is comparable to the measuring heights and therefore essential to include. It is interesting to note that, although it is derived from a different physical approach, the simplified wind speed profile conforms to the traditional expressions of the surface layer when the effect of the boundary-layer height is neglected.     

北京城市化发展对大气边界层特性的影响

利用中国科学院大气物理研究所大气边界层物理与大气化学国家重点实验室的北京325 m气象塔1993年～2003年夏季 (7月～9月) 的观测资料, 统计分析了各年的风速与温度廓线分布特征。统计分析结果表明, 随着城市化的发展, 相对风速有逐年减小的趋势, 并且越靠近地面, 相对风速的减小越明显, 这反映了城市建筑对近地面层空气流动的摩擦作用。对风速廓线进行线性拟合得到风速随高度的垂直递增率, 发现无论是100 m以下的近地面层还是较高层, 风速的垂直递增率都随城市化发展存在逐年增大的趋势, 表明粗糙下垫面的影响已经向高层扩展。根据温度廓线计算了各年的温度垂直递减率, 发现其有增大的趋势, 这表明城市化发展对边界层热力结构同样有显著影响。本文还依据统计整理得到的近中性层结下的风速廓线资料, 利用莫宁-奥布霍夫相似理论计算了下垫面的空气动力学参数, 结果表明, 地表粗糙度、 零平面位移随着城市化发展皆有明显增加的趋势。同时, 分析了各空气动力学参数与平均风速及无量纲风速的关系。其中, 摩擦速度和平均风速二者基本成正相关, 且摩擦速度随平均风速的增大而增大的趋势越发明显。本文研究结果对研究城市化发展对区域大气边界层结构、 气候和环境影响有参考意义, 可为城市大气边界层模式和区域气候模式提供参数化依据。     

彭州边界层铁塔风梯度特征研究

利用2018年10月~2020年12月彭州边界层铁塔的风速风向观测资料,分析了彭州地区近地面各季节平均风速日变化、盛行风向及污染系数特征,在此基础上,选取彭州大风天气个例,对按月分类法和风速区间法的反演结果进行了有效性验证。结果表明：除夏季10m高度盛行西风以外,其余三季各高度均盛行东北风,秋冬两季各高度东北风出现频率大于春夏两季,每个季节随着高度增加,东北风出现频率均逐渐增大,对应平均风速均逐渐增大。春夏季10m高度的最大污染系数在偏西方向,其他各高度最大污染系数均在东北方向,各层高度最小污染系数多在东南或偏南方向;秋冬季各高度污染系数均大于春夏季,最大均为东北方向,10~20m最小污染系数多在偏南或偏东南方向,60~90m最小污染系数为偏西北方向;彭州铁塔偏西和东北方向不宜布设污染源。两种风速反演方法均能对近地面风速进行较为有效的反演,风速区间法在大风区间的反演效果优于按月分类法。     

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。     

大气边界层气象学研究综述

文中回顾了大气边界层气象学的发展历史，总结了目前大气边界层气象学的主要进展，并指出国内外在未来大气边界层气象学研究方面面临的一些主要科学问题，以及对未来大气边界层气象学的发展方向提出若干建议，同时还指出了大气边界层气象学在思想上和方法上应该注意的一些相关问题。     

On Modelling the One-Dimensional Atmospheric Boundary Layer

Several commonly used turbulence closure schemes forthe atmospheric boundarylayer (ABL) are applied to simulate neutral, nocturnal and diurnal cyclesituations in a one-dimensional ABL. Results obtained with the differentschemes, E-, E- and its modified versions, and twoversions ofthe q ² Level 2.5, are compared and discussed.     

Turbulent Helicity in the Atmospheric Boundary Layer

We consider the assumption postulated by Deusebio and Lindborg (J Fluid Mech 755:654–671, 2014) that the helicity injected into the Ekman boundary layer undergoes a cascade, with preservation of its sign (right- or alternatively left-handedness), which is a signature of the system rotation, from large to small scales, down to the Kolmogorov microscale of turbulence. At the same time, recent direct field measurements of turbulent helicity in the steppe region of southern Russia near Tsimlyansk Reservoir show the opposite sign of helicity from that expected. A possible explanation for this phenomenon may be the joint action of different scales of atmospheric flows within the boundary layer, including the sea-breeze circulation over the test site. In this regard, we consider a superposition of the classic Ekman spiral solution and Prandtl’s jet-like slope-wind profile to describe the planetary boundary-layer wind structure. The latter solution mimics a hydrostatic shallow breeze circulation over a non-uniformly heated surface. A 180°-wide sector on the hodograph plane exists, within which the relative orientation of the Ekman and Prandtl velocity profiles favours the left rotation with height of the resulting wind velocity vector in the lowermost part of the boundary layer. This explains the negative (left-handed) helicity cascade toward small-scale turbulent motions, which agrees with the direct field measurements of turbulent helicity in Tsimlyansk. A simple turbulent relaxation model is proposed that explains the measured positive values of the relatively minor contribution to turbulent helicity from the vertical components of velocity and vorticity.     

The Stable Atmospheric Boundary Layer over an Antarctic ice Sheet

Turbulence measurements up to 11-m height and longterm profile measurements up to 45-m height performed at the German Neumayer Station in Antarctica are used to investigate different components of turbulence closure schemes of the stable boundary layer. The results confirm the linear relationships for the universal functions of momentum and heat exchange in the stability range z/L < 0.8 ... 1, whereas the local scaling approach should be used above the surface layer. Furthermore, boundary-layer heights below 50 m are frequently observed at this coastal Antarctic site, mainly due to the influence of stability above the boundary layer. It is shown that the inclusion of this stability into parametrization relations is necessary to provide realistic equilibrium heights of the stable boundary layer. Two relations, based on different physical approaches, were successfully applied for the parametrization of the equilibrium height.     

夏季青海湖局地环流及大气边界层特征的数值模拟

使用美国NCAR新版MM5V3.6非静力模式,采用两重嵌套方法,模拟了青海湖区域的局地环流及大气边界层特征,并且与无湖试验进行了比较。结果表明:白天由于青海湖的存在有很好的降温作用,夜晚则有保温效应,表现出明显的冷(暖)湖效应;青海湖对感热和潜热的影响有很强的日变化,白天湖面感热、潜热都小,夜间情况相反,这使得白天青海湖是冷干岛,夜间是暖湿岛;青海湖使得白天湖面边界层顶低,陆面边界层顶高,夜间相反。这样的边界层顶高度和温度、地面能量通量相配合,形成了一个很好的保护机制,对青海湖的水土保持和生态环境的维持有正效应;青海湖使得湖面上空大气下沉,陆面上空大气上升,从而产生了湖面上空大气冷干,陆面上空大气暖湿的边界层特征;青海湖边缘的陆面形成的较大的湿气柱围绕着湖面,起到了保护湖面的作用;青海湖低空白天有明显的湖面向四周的辐散气流,而夜间则为从北偏东方向来的陆风。     

金沙江下游局地大气边界层风场变化特征

利用2008年在金沙江下游溪洛渡水电站坝区及向家坝水电站库区获得GPS低空探空资料以及同步地面观测资料,统计分析了坝区从地面开始到大气边界层2000 m高度四季不同高度的风场变化特征.结果表明:(1)春季溪洛渡坝区大气边界层以偏西风为主导风向,1500m高度层以下静风和小风出现频率大,是四个季节中地面静风、小风出现频率的最大值;(2)夏季地面静风、小风出现频率为四个季节中最小,夏季大气边界层中低层主要盛行西风和西北偏西风;(3)秋季溪洛渡坝区大气边界层中低层主要盛行偏西风,到高层则逐渐转变为偏北风;(4)冬季溪洛渡坝区大气边界层低空盛行以西风和西北偏西风为主导的偏西风;中高层主要风向是西风、西南偏西风、东风和东北偏东风;(5)溪洛渡坝区秋、冬季大气边界层西风、东北偏北风、东北偏东风风速最大值均出现在2000m高度层.     

不同下垫面的大气边界层湍流扩散特征

利用海洋、戈壁、城郊和城市4种不同下垫面上的湍流观测资料,运用多尺度湍流概念和方法计算分析了风速谱和扩散参数等湍流特征量。结果表明:下垫面的粗糙度对大气边界层湍流特征影响很大,随着粗糙度的增大,速度谱的峰值频率向高频方向移动,扩散参数增大。