2006年3月末河南一次沙尘暴过程的天气成因分析

利用常规观测资料、地面自动站加密观测资料以及NCEP1°×1°格点再分析资料,对2006年3月27日发生在河南省的一次沙尘暴过程的天气成因进行了分析。结果表明,这是一次冷锋南下时产生的沙尘暴天气,高空影响系统是携带冷空气东南移并发展的小槽,此短波槽最终替代了原有的东亚大槽。前期降水少导致地表干燥,河南省北部、东部的黄泛平原沙土土层为沙尘暴的发生提供了丰富的沙源。冷锋影响时,强烈的温度平流作用使得风力加大,垂直运动加强。锋生作用使冷锋导致的天气现象更为剧烈。冷锋不仅是静力不稳定能量的触发系统,锋区内还存在对称不稳定,有利于垂直运动进一步加强,将沙尘输送至高空。地面中尺度滤波表明,中尺度低压的形成和发展不仅使得湍流混合加强,同时也加剧了不稳定能量的释放。沙尘在强风、强湍流混合以及强上升气流作用下最终形成沙尘暴。     

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.     

Large-eddy simulation of turbulent flow above and within a forest

A large-eddy simulation has been performed of an atmospheric surface layer in which the lower third of the domain is occupied by a drag layer and heat sources to represent a forest. Subgridscale processes are treated using second-order closure techniques. Lateral boundaries are periodic, while the upper boundary is a frictionless fixed lid. Mean vertical profiles of wind velocity derived from the output are realistic in their shape and response to forest density. Similarly, vertical profiles of Reynolds stress, turbulent kinetic energy and velocity skewness match observations, at least in a qualitative sense. The limited vertical extent of the domain and the artificial upper boundary, however, cause some departures from measured turbulence profiles in real forests. Instantaneous turbulent velocity and scalar fields are presented which show some of the features obtained by tower instrumentation in the field and in wind tunnels, such as the vertical coherence of vertical velocity and the slope of structures revealed by temperature patterns.     

Some Characteristics of the Surface Boundary Layer of a Strong Cold Air Process over Southern China

In southern China,cold air is a common weather process during the winter season;it can cause strong wind,sharp temperature decreases,and even the snow or freezing rain events.However,the features of the atmospheric boundary layer during cold air passage are not clearly understood due to the lack of comprehensive observation data,especially regarding turbulence.In this study,four-layer gradient meteorological observation data and one-layer,10-Hz ultrasonic anemometer-thermometer monitoring data from the northern side of Poyang Lake were employed to study the main features of the surface boundary layer during a strong cold-air passage over southern China.The results show that,with the passage of a cold air front,the wind speed exhibits low-frequency variations and that the wind systematically descends.During the strong wind period,the wind speed increases with height in the surface layer.Regular gust packets are superimposed on the basic strong wind flow.Before the passage of cold air,the wind gusts exhibit a coherent structure.The wind and turbulent momentum fluxes are small,although the gusty wind momentum flux is slightly larger than the turbulent momentum flux.However,during the invasion of cold air,both the gusty wind and turbulent momentum fluxes increase rapidly with wind speed,and the turbulent momentum flux is larger than the gusty wind momentum flux during the strong wind period.After the cold air invasion,this structure almost disappears.     

Horizontal and Vertical Turbulent Fluxes Forced by a Gravity Wave Event in the Nocturnal Atmospheric Surface Layer Over the Amazon Forest

A nocturnal gravity wave was detected over a south-western Amazon forest during the Large-Scale Biosphere–Atmosphere experiment in Amazonia (LBA) in the course of the dry-to-wet season campaign on October 2002. The atmospheric surface layer was stably stratified and had low turbulence activity, based on friction velocity values. However, the passage of the wave, an event with a period of about 180–300 s, caused negative turbulent fluxes of carbon dioxide (CO₂) and positive sensible heat fluxes, as measured by the eddy-covariance system at 60 m (≈30 m above the tree tops). The evolution of vertical profiles of air temperature, specific humidity and wind speed during the wave movement revealed that cold and drier air occupied the sub-canopy space while high wind speeds were measured above the vegetation. The analysis of wind speed and scalars high frequency data was performed using the wavelet technique, which enables the decomposition of signals in several frequencies allowed by the data sampling conditions. The results showed that the time series of vertical velocity and air temperature were −90° out of phase during the passage of the wave, implying no direct vertical transport of heat. Similarly, the time series of vertical velocity and CO₂ concentration were 90° out of phase. The wave was not directly associated with vertical fluxes of this variable but the mixing induced by its passage resulted in significant exchanges in smaller scales as measured by the eddy-covariance system. The phase differences between horizontal velocity and both air temperature and CO₂ concentration were, respectively, zero and 180°, implying phase and anti-phase relationships. As a result, the wave contributed to positive horizontal fluxes of heat and negative horizontal fluxes of carbon dioxide. Such results have to be considered in nocturnal boundary-layer surface-atmosphere exchange schemes for modelling purposes.     

冷锋天气大气边界层内臭氧及 氮氧化物的观测研究

利用北京325 m气象塔作为高空平台,于1997年10月观测到一次冷锋过程大气边界层内O3及NOx体积分数的变化,研究了O3及NOx体积分数与气象要素之间的关系,着重讨论了冷锋过境前后O3及NOx的体积分数变化及其与输送过程的关系。研究表明:北京大气边界层中下层存在明显的O3体积分数垂直梯度,O3的垂直输送与风速及温度梯度密切相关。冷锋过程有利于高层O3向低层输送,使O3体积分数垂直梯度明显减小,并使NOx体积分数显著降低。     

A case study of the nocturnal boundary layer over a complex terrain

A case study of the structure of the nocturnal boundary layer (NBL) over complex terrain is presented. Observations were made during the third night of Project STABLE (Weber and Kurzeja, 1991), whose main goal was to study turbulence and diffusion over the complex terrain of the Savannah River Site (SRS) near Augusta, Georgia.The passage of a mesoscale phenomenon, defined as a turbulent meso-flow (TMF) with an explanation of the nomenclature used, and a composite structure of the lowest few hundred meters over complex terrain are presented. The spatial extent of the TMF was at least 30–50 km, but the forcing is not well understood. The TMF occurred without the presence of a synoptic-scale cold front, under clear conditions, and with no discernible discontinuity in a microbarograph pressure trace. The structure of the NBL over the complex terrain at SRS differed from the expected homogeneous terrain NBL. The vertical structure exhibited dual low level wind maxima, dual inversions, and a persistent elevated turbulent layer.The persistent elevated turbulent layer, with a spatial extent of at least 30 km, was observed for the entire night. The persistent adiabatic layer may have resulted from turbulence induced by shear instability.     

Boundary layer structure in two fronts passing a tower

Summary The atmospheric boundary layer in an ana-cold front and in a kata-cold front is investigated using measurements at a 200 m high tower near Karlsruhe, Germany. In the ana-cold front the wind speed decreased, whereas the passage of the kata-cold front was accompanied by strong gusts. The frontogenesis function of potential temperature was only weak. Turbulence increased at the fronts as expressed by the turbulent kinetic energy. However, the turbulent vertical momentum flux was relatively inefficient at the passage of the fronts. Also it is attempted to calculated flux profiles and friction in the cold air.With 15 Figures     

北京北郊冬季大风过程湍流通量演变特征的分析研究

利用中国科学院大气物理研究所325 m气象观测塔1993年12月～1994年1月大气边界层实验资料, 计算分析了大风过境过程中47 m和120 m高度湍流通量演变特征及其影响因子, 以及与风速、 稳定度等参数的关系。结果表明: 大风过程对近地面层的物质能量输送有着重要影响, 大风之前出现短时间动量上传和热量下传; 大风过程中的湍流通量数值明显高于过境后, 水平方向湍流通量数值和能量增加幅度大于垂直方向; 当风速大于临界值5 m/s时, 湍流通量与风速、 湍流动能的相关迅速增大; 湍流谱特征表现为湍流能量的低频部分增加、 湍流谱曲线变宽; 大风能强烈影响近地面层的能量收支。     

The turbulent characteristics in the lowest part of the sea breeze front in the atmospheric surface layer

Tower measurements for the sea breeze front in the surface layer were carried out over the Kochi plain about 2 km inland from Tosa Bay in Shikoku, Japan during the period from August 1986 to October 1987. The study shows that the penetration time of the sea breeze has an annual variation, which is around 0830 JST in summer and 12 JST in winter, and that the width of the sea breeze front depends on the ratio of the sea breeze speed and the opposing flow speed. Moreover, the frontal width also shows a seasonal variation.The characteristics of the vertical winds (w) found just before and just after the passage of the sea breeze front lead to remarkable downdrafts and updrafts, respectively, with relatively large vertical velocities. Such behaviour ofw is shown to be consistent with the flow relative to the head of the front as reviewed by Simpson (1987), influencing the magnitude of the turbulence scale and the turbulent energy dissipation near the ground surface.     

A wind tunnel study of air flow in waving wheat: Single-point velocity statistics

We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer.     

寒潮冷锋过境期间湍流特征量及其谱分析

利用两台超声风速温度仪于1992年12月至1993年1月间,在中国科学院大气物理研究所气象观测塔47 m和120 m两个高度观测到的风速三个正交分量以及温度的湍流脉动资料,计算分析寒潮冷锋过境时局地湍流特征量的日变化以及风速分量和温度的归一化湍流能谱。它们都在不同程度上反应出较大尺度湍流的特征。     

Observations and Modelling of the Atmospheric Boundary Layer Over Sea-Ice in a Svalbard Fjord

Sonic anemometer and profile mast measurements made in Wahlenbergfjorden, Svalbard Arctic archipelago, in May 2006 and April 2007 were employed to study the atmospheric boundary layer over sea-ice. The turbulent surface fluxes of momentum and sensible heat were calculated using eddy correlation and gradient methods. The results showed that the literature-based universal functions underestimated turbulent mixing in strongly stable conditions. The validity of the Monin-Obukhov similarity theory was questionable for cross-fjord flow directions and in the presence of mesoscale variability or topographic effects. The aerodynamic roughness length showed a dependence on the wind direction. The mean roughness length for along-fjord wind directions was (2.4 ± 2.6) × 10⁻⁴ m, whereas that for cross-fjord directions was (5.4 ± 2.8) × 10⁻³ m. The thermal stratification and turbulent fluxes were affected by the synoptic situation with large differences between the 2 years. Channelling effects and drainage flows occurred especially during a weak large-scale flow. The study periods were simulated applying the Weather Research and Forecasting (WRF) model with 1-km horizontal resolution in the finest domain. The results for the 2-m air temperature and friction velocity were good, but the model failed to reproduce the spatial variability in wind direction between measurement sites 3 km apart. The model suggested that wind shear above the stable boundary layer provided a non-local source for the turbulence observed.     

2010年春季北京地区强沙尘暴过程的微气象学特征

利用北京大学校园地区PM10质量浓度观测资料、中国科学院大气物理研究所325m气象塔气象要素梯度和湍流观测资料,分析了北京地区2010年3月20～22日两次强沙尘暴过程微气象学要素和沙尘参量的时空演变以及湍流输送特征,为理解北京地区强沙尘暴天气沙尘输送规律和微气象学特征提供参考。结果表明:3月20～22日强沙尘暴过程前后不同高度温度先升后降,气压和相对湿度则相反。强沙尘暴来临时,高层风速先迅速增大,低层风速增加略有滞后,风切变明显加强,PM10浓度最大值和风速极大值出现时间较吻合。强沙尘暴过境时,不同高度向下的湍流动量输送、向上的湍流热量输送和湍流动能明显加强。与3月21日非沙尘暴日相比,强沙尘暴过程湍流动量通量增加,有利于沙尘粒子的水平和垂直输送过程;由于冷锋过境,水平热通量增大;垂直热通量因白天温度垂直梯度减小而减小,夜间因逆温层被破坏而增加;水平湍流动能对湍流动能占主要贡献,垂直湍流动能仅占水平湍流动能的10%～25%。     

浪花和飞沫水滴在海面大风边界层中的垂直传输

对我们在南海海域建立的大气边界层观测站的资料进行分析表明,在冷涌和热带气旋（包括强台风）过境的大风期间,在边界层底层10 min平均的水平风速u基本不随高度而变,甚至大都伴随有明显的上升气流w。而且风场脉动中含有强相干性的阵风扰动（ v _g,频率位于1/60~1/600 Hz频段）,以及近于随机性的高频湍流脉动（ v _t,频率大于1/60 Hz）,它们的特性以及w都可以很好地用水平风速u来参数化表示。取实测的（u,w）和脉动 v '= v _g+ v _t,或取实测的u与参数化的 v _g、 v _t和w,应用拉格朗日随机模式作数值模拟,结果表明:由破头浪发射出来的浪花和飞沫水滴（半径r_p为10~500 μm）有相当大的一部分可以飞离大气底层而进入100 m高以上的大气中,继而对进入大气中的海盐气溶胶通量有重要贡献,不可以被忽略。在水滴的垂直传输过程中,阵风扰动起了极重要的作用,而在w>0且较显著时w更起重要作用。我们对上扬率（可上升至100 m以上高度的水滴数与由海面发射出的水滴数之比）作出了初步的参数化公式,有很高的精度,主要的参量是无量纲量u2/（r_qg）,其中r_p和g分别是水滴半径和重力加速度。     

Characteristics of canopy turbulence over a deciduous forest on complex terrain

Canopy turbulence plays an important role in mass and energy exchanges at the canopy-atmosphere interface. Despite extensive studies on canopy turbulence over a flat terrain, less attention has been given to canopy turbulence in a complex terrain. The purpose of this study is to scrutinize characteristics of canopy turbulence in roughness sublayer over a hilly forest terrain. We investigated basic turbulence statistics, conditionally sampled statistics, and turbulence spectrum in terms of different atmospheric stabilities, wind direction and vertical structures of momentum fluxes. Similarly to canopy turbulence over a homogeneous terrain, turbulence statistics showed coherent structure. Both quadrant and spectrum analysis corroborated the role of intermittent and energetic eddies with length scale of the order of canopy height, regardless of wind direction except for shift of peak in vertical wind spectrum to relatively high frequency in the down-valley wind. However, the magnitude of the momentum correlation coefficient in a neutral condition was smaller than typical value over a flat terrain. Further scrutiny manifested that, in the up-valley flow, temperature skewness was larger and the contribution of ejection to both momentum and heat fluxes was larger compared to the downvalley flow, indicating that thermal instability and weaker wind shear in up-valley flow asymmetrically affect turbulent transport within the canopy.     

Turbulence structure in a deciduous forest

Three-dimensional wind velocity components were measured at two levels above and at six levels within a fully-leafed deciduous forest. Greatest shear occurs in the upper 20% of the canopy, where over 70% of the foliage is concentrated. The turbulence structure inside the canopy is characterized as non-Gaussian, intermittant and highly turbulent. This feature is supported by large turbulence intensities, skewness and kurtosis values and by the large infrequent sweeps and ejections that dominate tangential momentum transfer. Considerable day/night differences were observed in the vertical profiles of the mean streamwise wind velocity and turbulence intensities since the stability of the nocturnal boundary layer dampens turbulence above and within the canopy.     

Effects of Thermal Stability and Incoming Boundary-Layer Flow Characteristics on Wind-Turbine Wakes: A Wind-Tunnel Study

Wind-tunnel experiments were carried out to study turbulence statistics in the wake of a model wind turbine placed in a boundary-layer flow under both neutral and stably stratified conditions. High-resolution velocity and temperature measurements, obtained using a customized triple wire (cross-wire and cold wire) anemometer, were used to characterize the mean velocity, turbulence intensity, turbulent fluxes, and spectra at different locations in the wake. The effect of the wake on the turbulence statistics is found to extend as far as 20 rotor diameters downwind of the turbine. The velocity deficit has a nearly axisymmetric shape, which can be approximated by a Gaussian distribution and a power-law decay with distance. This decay in the near-wake region is found to be faster in the stable case. Turbulence intensity distribution is clearly non-axisymmetric due to the non-uniform distribution of the incoming velocity in the boundary layer. In the neutral case, the maximum turbulence intensity is located above the hub height, around the rotor tip location and at a distance of about 4–5.5 rotor diameters, which are common separations between wind turbines in wind farms. The enhancement of turbulence intensity is associated with strong shear and turbulent kinetic energy production in that region. In the stable case, the stronger shear in the incoming flow leads to a slightly stronger and larger region of enhanced turbulence intensity, which extends between 3 and 6 rotor diameters downwind of the turbine location. Power spectra of the streamwise and vertical velocities show a strong signature of the turbine blade tip vortices at the top tip height up to a distance of about 1–2 rotor diameters. This spectral signature is stronger in the vertical velocity component. At longer downwind distances, tip vortices are not evident and the von Kármán formulation agrees well with the measured velocity spectra.     

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.     

Numerical modeling of the nocturnal PBL over the urban heat island in Changzhou city

A two-dimensional primitive equation model has been used to describe the changes of the fields of horizontal wind, temperature and vertical eddy exchange coefficient and the development and evolution of urban heat island circulation for the thermal and dynamical effects of urbanlization, while the air flows over a city in the nighttime. Because the turbulence and average fluid are in a condition of unbalance to each other, the turbulent energy varies with time. So the equations are closed by introducing the equation of the variable rate of turbulent energy in the model. We are able to obtain the distribution and change of the turbulent energy over the urban area in this way. Finally we examine the effect of urbanization by using the differences of the actual ground pollutant concentration and the vertical distribution of concentration emitted by an elevated pollutant source.In order to verify this model, the profiles of horizontal velocity and temperature that are predicted by this model are compared with observed data for Changzhou city.