山地中尺度环流中的大气边界层湍流摩擦效应

建立了一个三维原始方程数值模式，模拟在光滑下边界和无滑脱下边界两种情况下，山区中尺度环流的演变过程，指出在弱层结条件下，边界层湍流摩擦效应有利于流场的分离和涡旋偶极子环流的发展；在强层结条件下，表面摩擦效应削弱了分离气流和涡旋性环流的强度，但使其范围扩大。边界层湍流摩擦效应引起的分离流场和尾流区环流与非粘性动力过程引起的流场分离现象，虽然从其表现形式上看完全相同，但是其形成则是由完全不同的物理机制所控制。边界层湍流摩擦效应在上述两种分离流场的演变过程中表现出完全相反的作用。地转科氏力的调整作用在山地背风坡上促进了气旋性辐合环流的发展，而抑制了反气旋性环流的发展。     

热岛环流的动力学分析

文中根据线性化的大气方程组求得了城市热岛环流的理论模式。利用此模式分析了温度场和流场的三维结构,并和观测结果做了对比。在解析解中,研究了大气条件诸如风速、湍流扩散系数、Ragleigh摩擦以及层结的影响,同时讨论了重力波对温度分布垂直结构的效应。     

一个强冷岛的数值试验结果

本文利用考虑了流场水平非均匀切变的湍流涡旋系统和水平湍流交换的二维定常行星边界层数值模式,计算了一个发展较强的冷岛结构。     

南亚高压结构及其时频特征

前言随着季节转换,北半球夏季各月,在亚洲南部上空对流层上部出现的南亚高压,是一个直接影响北半球大气环流演变的重要环流系统。对于它,人们已从某些角度,比如说:它的位置、形状,流场特征,振荡特性,及其与我国天气的关系等方面作了研究。而关于它的结构及其在时域和频域上的特征却缺乏探讨。我们认为,对于这样一个既直接影响到整个北半球环流的变化,又与我国东部夏季大范围旱涝有重大影响     

重庆城市热岛环流结构和湍流特征的数值模拟

利用中尺度模式WRF（V3.9）对2016年8月17～18日重庆一次城市热岛环流个例进行了数值模拟，探讨了山地城市热岛环流的三维结构和演变特征，分析了热岛环流期间湍流动能和各项湍流通量的特征。结果表明:15:00（北京时，下同）乡村风开始出现，随着热岛强度增强乡村风增大，18:00热岛环流结构最显著，次日02:00热岛环流结构被破坏，仅低层存在微弱的乡村风。其中，重庆市城市热岛环流最强时，水平尺度约城市尺度的1.5～2倍，垂直厚度约1.3 km，水平风速约2～4 m s?1，最大上升速度约0.5 m s?1。受地形、河流以及背景风的影响，环流呈现非对称的结构，且强度较弱。湍流特征分析结果表明，城市区域的湍流动能明显大于其它区域。此外，城市热岛环流通过湍流运动将郊区的水汽输向城市；高层湍流动量补充边界层中因热岛环流发展而造成的动量耗散。     

多时间尺度环流对热带气旋海棠(0505)路径的影响

利用滤波方法将NCEP/NCAR提供的FNL风场资料分离出天气尺度和低频环流场,研究不同时间尺度环流对台风海棠（0505）路径的影响。热带气旋海棠路径的特征可以分两个主要阶段,在第一阶段,海棠西侧的天气尺度反气旋和低频流场副高南侧气流共同引导海棠向西南运动;第二阶段初期,天气尺度环流抑制海棠向北转向,但海棠西侧的QBW气旋和MJO尺度的气流共同引导海棠向北运动。海棠运动后期,海棠与低频气旋的相互作用导致了天气尺度流场中波列的产生和发展,形成了热带气旋的传播分量,令海棠产生了向东北转向的趋势,但是由于MJO环流场中副高的北抬和QBW气旋的引导,最终导致海棠向西运动。     

大连区域海陆风环流的数值模拟

本文基于海陆风环流的形成机制,在研究分析海陆风环流形成的物理模型基础上,建立了海陆风环流的数学模型。根据此基础,以大连地区海陆风环流为计算实例,模拟了海陆风形成的压力场、速度场、温度场和湍流动能场的日变化及太阳辐射日变化的过程地面的能量变化及导致的湍流动能的变化,预测出海陆风环流的水平湍流扩散系数和动量、温度和湍流动能的垂直湍流扩散系数,为求解海陆风中的污染物扩散浓度以进行环境污染损失评价提供参考。应用此模型,对大连地区的海陆风环流进行了数值模拟,定性与定量地给出了海陆风场中的速度、压力、温度及湍流动能分布情况和主要参数值。结果表明,海陆风环流的大气压力场局地日变化较小,温度场变化较明显。在中午前后,动量、温度和湍流动能的垂直扩散系数达到了最大值。模拟结果与其他文献模拟结果的对比表明,本文建立的模型模拟与实验的结果相符,但预测精度仍需要进一步检验。     

梅雨涡旋与环境场动能的相互作用

在过渡湍流理论基础上建立二维非局地闭合模式，并用所建模式研究沿海地区中尺度海陆风环流和内国界层结构，得到了合理的平均场和湍流场。     

夏季太平洋低纬地区季风环流与信风环流一种可能的连接机制

北半球夏季太平洋低纬地区的平均经向环流,西部(150°E以西)为季风环流;中部和东部(170°W以东)为信风区的Hadley环流;150°E—170°W之间为季风环流与信风环流的连接区或过渡区。连接季风环流与信风环流的水平环流系统,在高层为太平洋中部热带对流层高层槽(TUTT),低层为强大的太平洋副热带高压。太平洋中部高空槽区就是季风环流与信风环流的连接区或过渡区。本文分析了高空槽的流场结构,并根据各层水平环流和各经度带的垂直环流给出了太平洋低纬地区的三维气流分布示意图。      

三维山体过山气流流场特征的数值模拟

采用高阶矩湍流闭合方案，建立了一个细网格、高分辨率、三维非静力ＰＢＬ数值模式，并由其模拟了三维山体条件下的流场结构和湍流场特征。为反映数值模拟结果的可靠性，对中性条件下三维山体流场进行了风洞试验。与数值模拟结果对比分析表明，数值计算与风洞试验结果有较好的一致性；使用该模式模拟山体条件下的流场结构能取得较好的结果；将模拟结果作为随机游动扩散模式的三维风场及湍流场资料输入，为复杂地形条件下大气污染的研     

Temporal Evolution of Low-Level Winds Induced by Two-dimensional Mesoscale Surface Heat-Flux Heterogeneity

Using large-eddy simulation (LES), the effects of mesoscale local surface heterogeneity on the temporal evolution of low-level flows in the convective boundary layer driven by two-dimensional surface heat-flux variations are investigated at a height of about 100 m over flat terrain. The surface variations are prescribed with sinusoids of wavelength 32 km and varying amplitudes of 0, 50, 100, and 200 W m $^{-2}$ . The Weather Research and Forecasting numerical model is used as a mesoscale-domain LES model that has a grid spacing fine enough to explicitly resolve energy-containing turbulent eddies and a model domain large enough to include mesoscale circulations. Mesoscale circulations induced by the two-dimensional surface heterogeneity may undergo a flow transition and an associated spectral energy cascade, which has been found previously but only with one-dimensional surface heat-flux variations. Over a strongly heterogeneous surface prescribed with a two-dimensional sinusoid of amplitude 200 W m $^{-2}$ , the domain-averaged variance of the horizontal wind component initially grows rapidly, then undergoes a flow transition and subsequently rapidly decays. With a background wind, the induced mesoscale circulations are inhibited in the streamwise direction. However in the spanwise direction, somewhat stronger mesoscale circulations are induced, compared with those with no background wind. The background wind attenuates the significant reduction of the low-level temperature gradient by the fully-developed mesoscale horizontal flow. Spectral decomposition reveals that this rapid transition also exists in the mesoscale horizontal flows induced by the intermediate surface heterogeneity prescribed with a sinusoid of amplitude 100 W m $^{-2}$ . However the transition is masked by continuously growing turbulence.     

The Influence of Thermally-Induced Mesoscale Circulations on Turbulence Statistics Over an Idealized Urban Area Under a Zero Background Wind

The influence of mesoscale circulations induced by urban-rural differential surface sensible heat flux and roughness on convective boundary-layer (CBL) flow statistics over an isolated urban area has been examined using large-eddy simulation (LES). Results are analyzed when the circulations influence the entire urban area under a zero background wind. For comparison, the CBL flow over an infinite urban area with identical urban surface characteristics under the same background meteorological conditions is generated as a control case (without circulations). The turbulent flow over the isolated urban area exhibits a mix of streaky structure and cellular pattern, while the cellular pattern dominates in the control case. The mixed-layer height varies significantly over the isolated urban area, and can be lower near the edge of the urban area than over the rural area. The vertical profiles of turbulence statistics over the isolated urban area vary horizontally and are dramatically different from the control case. The turbulent kinetic energy (TKE) sources include wind shear, convergence, and buoyancy productions, compared to only buoyancy production in the control case. The normalized vertical velocity variance is reduced compared to the control case except in the central urban area where it is little affected. The low-level flow convergence is mainly responsible for the enhanced horizontal velocity variance in the central urban area, while wind shear is responsible for the additional local maximum of the horizontal velocity variance near the middle of the CBL outside the central area. Parameterizations in the prognostic equation for TKE used in mesoscale models are evaluated against the LES results over the isolated urban area. We also discuss conditions under which the urban-induced circulations occur and when they may affect the entire urban area. Given that urban-induced circulations can influence the entire urban area within hours for an urban area of a realistic size, it is inappropriate to directly apply empirical relations of turbulence statistics derived under horizontally-homogenous flow conditions to an urban area.     

A New Scheme for the Simulation of Microscale Flow and Dispersion in Urban Areas by Coupling Large-Eddy Simulation with Mesoscale Models

A coupling scheme is proposed for the simulation of microscale flow and dispersion in which both the mesoscale field and small-scale turbulence are specified at the boundary of a microscale model. The small-scale turbulence is obtained individually in the inner and outer layers by the transformation of pre-computed databases, and then combined in a weighted sum. Validation of the results of a flow over a cluster of model buildings shows that the inner- and outer-layer transition height should be located in the roughness sublayer. Both the new scheme and the previous scheme are applied in the simulation of the flow over the central business district of Oklahoma City (a point source during intensive observation period 3 of the Joint Urban 2003 experimental campaign), with results showing that the wind speed is well predicted in the canopy layer. Compared with the previous scheme, the new scheme improves the prediction of the wind direction and turbulent kinetic energy (TKE) in the canopy layer. The flow field influences the scalar plume in two ways, i.e. the averaged flow field determines the advective flux and the TKE field determines the turbulent flux. Thus, the mean, root-mean-square and maximum of the concentration agree better with the observations with the new scheme. These results indicate that the new scheme is an effective means of simulating the complex flow and dispersion in urban canopies.     

延庆-张家口地区复杂地形冬季山谷风特征分析

基于2016年12月—2017年2月和2017年12月—2018年2月两年冬季的近地面自动气象站逐时观测数据以及张家口探空数据分析延庆-张家口一带（包括张家口崇礼、赤城、海坨、小五台山区,延怀、怀涿、洋河、蔚县盆地以及北京延庆、昌平、怀柔部分平原地区）复杂地形的风场精细化时、空分布特征,揭示不同复杂地形下局地风场的时、空变化规律,加深对复杂地形动力、热力作用对近地面风场影响的认识,为冬季山区风场预报以及复杂地形数值模式改进提供参考。结果表明:晴朗小风天风持续性作为矢量平均风速和标量平均风速的比值,可以作为研究风场变化规律的重要参数。根据风持续性的日变化特征,可以将研究区域内所有站点分为10种类型,分别代表不同局地地形特征的影响,风持续与风向变化的相关也很强。研究区域主要有3种类型的地形风:斜坡风、峡谷风以及较大尺度的山区平原风。不同地形特征下的风场、风持续性存在明显不同的日变化特征,山风和谷风相互转化的时间也不同,山区最早,盆地次之,平原区最晚;山风时段持续时间较谷风时段长,风速小;晴朗小风天实测风反映了实际风场的特征,而排除环境背景风场,弱化地形动力作用后整个冬季的局地风作为理论山谷风,更能反映热力作用下的山谷风特征。      

Large-Eddy Simulations of Surface Heterogeneity Effects on the Convective Boundary Layer During the LITFASS-2003 Experiment

We investigate the impact of observed surface heterogeneities during the LITFASS-2003 experiment on the convective boundary layer (CBL). Large-eddy simulations (LES), driven by observed near-surface sensible and latent heat fluxes, were performed for the diurnal cycle and compare well with observations. As in former studies of idealized one- and two-dimensional heterogeneities, secondary circulations developed that are superimposed on the turbulent field and that partly take over the vertical transport of heat and moisture. The secondary circulation patterns vary between local and roll-like structures, depending on the background wind conditions. For higher background wind speeds, the flow feels an effective surface heat-flux pattern that derives from the original pattern by streamwise averaging. This effective pattern generates a roll-like secondary circulation with roll axes along the mean boundary-layer wind direction. Mainly the upstream surface conditions control the secondary circulation pattern, where the fetch increases with increasing background wind speed. Unlike the entrainment flux that appears to be slightly decreased compared to the homogeneously-heated CBL, the vertical flux of sensible heat appears not to be modified in the mixed layer, while the vertical flux of latent heat shows different responses to secondary circulations. The study illustrates that sufficient time averaging and ensemble averaging is required to separate the heterogeneity-induced signals from the raw LES turbulence data. This might be an important reason why experiments over heterogeneous terrain in the past did not give any clear evidence of heterogeneity-induced effects.     

Multi-scale Transport Processes Observed in the Boundary Layer over a Mountainous Island

Over complex terrain, convection and thermally-driven circulations simultaneously occur under fair weather conditions during the day. To investigate these processes on the basis of observations, simultaneous measurements on different scales are necessary. Comprehensive measurements with the mobile observation platform KITcube were performed on the mountainous island of Corsica during the HYdrological cycle in Mediterranean EXperiment (HyMeX) field campaign in late summer and autumn 2012. Using a case study, the benefit of integrated measurement systems and coordinated scan strategies was demonstrated, and experimental evidence of, and new insights into, convective and advective transport processes in a valley were obtained. Convection, thermally-driven circulations and topographic and advective venting led to the diurnal cycle of temperature, humidity and wind over complex terrain in the mountain atmospheric boundary layer (mountain ABL), which was deeper than an ABL over homogeneous terrain under equal surface forcing. Due to the combined transport processes on different scales, the mountain ABL in a valley also extended beyond the convection layer, which was characterized by surface-based, buoyancy-driven turbulent mixing. Strong subsidence, with a vertical velocity of about 1 m s \(^{-1}\) , was present within the mountain ABL for several hours around noon and suppressed the convection-layer growth. Above the layer with subsidence, elevated vertical motions, consisting of alternating updrafts and downdrafts, occurred. Once the convection layer grew to the bottom of the layer with elevated vertical motions, surface-based convective cells occasionally coupled to the elevated updrafts, as a result of which the convection layer rapidly deepened.     

Geochemical behaviour of the Tunisian background aerosols in Sirocco wind circulations

This study examines spatial and time evolutions of the principal constituents of the Tunisian background aerosols under Sirocco wind circulations. Aerosols coming from the Sahara Desert were found to be loaded with particulate matter, especially silicon. The aerosols were shown to have varying geochemical behaviour along the ``South-North" displacement of the Saharan plumes, depending on the wind flow characteristics, geomorphologic features and the nature of soils swept by the wind. In the south and the center part of the country, the transfer of aerosol constituents to the soil (by gravity and/or impaction) was probably predominated by localized enrichment phenomena. The latter are reinforced by the effect of turbulent winds over bare soils, wind wakes and probably selective disintegration, especially in the vicinity of the geomorphologic features of central Tunisia. These relatively high features, extending over important distances, appear to be of paramount importance for the phenomena of redistribution of aerosol constituents even during periods without Sirocco wind circulations. In the northern section of the country, aerosol constituent concentrations dropped to almost 50%, in spite of the abundance of localized turbulent winds. This may be explained by the effect of forests and the relatively dense vegetation cover, which clearly reinforces the transfer phenomena to the soil and the attenuate of dust entrainment.     

Theoretical considerations of meandering winds in simplified conditions

The influence of turbulence on the meandering phenomenon is investigated. The study, based on the three-dimensional Navier–Stokes equations, shows that when the turbulent fluxes can be neglected an asymptotic solution results. This solution reproduces a horizontal wind oscillation with an infinite relaxation time. When there is turbulent forcing, on the other hand, a transition occurs to a new order, characterized by a spatial reorganization, leading to a wind field with a well-defined direction.     

多时间尺度环流对台风“天鹅”(1515)突变路径的影响

利用NCL滤波方法将NCEP提供的FNL风场资料分离出天气尺度,准双周振荡(QBWO,Quasi-Biweekly Oscillation)和热带季节内振荡(MJO,Madden-Julian Oscillation)环流场,研究不同时间尺度环流对台风"天鹅"(1515)突变路径的影响。台风路径的特征能够分3个阶段,其中第二阶段台风发生突然转折。第一阶段,天气尺度上台风东侧的反气旋和QBWO环流场中的波列共同引导台风向西偏北方向运动,而MJO环流场中的引导气流作用较小;第二阶段,天气尺度上台风东侧的反气旋和低频环流场中台风附近的气旋共同促进了"天鹅"近90°的突然转向,其中,高、低频分量分别促使台风突然向北、向东转向;第三阶段,天气尺度上的气旋与反气旋、QBWO环流场中的反气旋以及MJO环流场中的脊共同引导"天鹅"向东北方向运动,其中MJO环流场中气旋附近的偏东风促使"天鹅"向西运动,但由于它被天气尺度上强烈的偏西风所抵消,故"天鹅"仍向东运动。     

Boundary-layer effects on mountain waves: a new look at some historical studies

Early studies of mountain waves reported various results that have rarely been investigated since. These include: large-amplitude mountain waves above an unstable boundary layer much higher than the mountains; a repeated downwind drift and upwind jump of mountain waves; and larger vertical wind magnitude near sunrise and/or sunset. These are investigated using over 3,000 radiosondes and meso-strato-troposphere (MST) radar. Superadiabatic temperature gradients are found beneath mountain waves, explainable by convection which appears to raise the mountain-wave launching height. Movement of mountain-wave patterns is studied by a new method using height–time vertical wind data. A swaying motion of mountain waves, with period of a few minutes, appears to be equally upwind and downwind, rather than asymmetric at the heights measurable. Also, vertical wind shows no change in mean, variance or extreme values near sunrise and sunset, despite the expected diurnal changes of boundary-layer structure. An explanation for differences between MST radar and other measurements and models of mountain waves is suggested in terms of more than one variety of mountain wave. Type 1 has stable air near the ground; type 2 is above a convective/turbulent boundary layer of significant height as compared to the mountains.