Physical Mechanism and Model of Turbulent Cascades in a Barotropic Atmosphere

In a barotropic atmosphere,new Reynolds mean momentum equations including turbulent viscosity,dispersion,and instability are used not only to derive the KdV-Burgers-Kuramoto equation but also to analyze the physical mechanism of the cascades of energy and enstrophy.It shows that it is the effects of dispersion and instability that result in the inverse cascade.Then based on the conservation laws of the energy and enstrophy,a cascade model is put forward and the processes of the cascades are described.     

Numerical Physical Mechanism and Model of Turbulent Cascades in a Barotropic Atmosphere

In a barotropic atmosphere, new Reynolds mean momentum equauons including turbulent viscosit,dispersion, and instability are used not only to derive the KdV-Burgers-Kuramoto equation but also to analyze the physical mechanism of the cascades of energy and enstrophy. It shows that it is the effects of dispersion and instability that result in the inverse cascade. Then based on the conservation laws of the energy and enstrophy, a cascade model is put forward and the processes of the cascades are described.     

Changes in the normal mode energetics of the general atmospheric circulation in a warmer climate

Changes in the normal mode energetics of the general atmospheric circulation are assessed for the northern winter season (DJF) in a warmer climate, using the outputs of four climate models from the Coupled Model Intercomparison Project, Phase 3. The energetics changes are characterized by significant increases in both the zonal mean and eddy components for the barotropic and the deeper baroclinic modes, whereas for the shallower baroclinic modes both the zonal mean and eddy components decrease. Significant increases are predominant in the large-scale eddies, both barotropic and baroclinic, while the opposite is found in eddies of smaller scales. While the generation rate of zonal mean available potential energy has globally increased in the barotropic component, leading to an overall strengthening in the barotropic energetics terms, it has decreased in the baroclinic component, leading to a general weakening in the baroclinic energetics counterpart. These global changes, which indicate a strengthening of the energetics in the upper troposphere and lower stratosphere (UTLS), sustained by enhanced baroclinic eddies of large horizontal scales, and a weakening below, mostly driven by weaker baroclinic eddies of intermediate to small scales, appear together with an increased transfer rate of kinetic energy from the eddies to the zonal mean flow and a significant increase in the barotropic zonal mean kinetic energy. The conversion rates between available potential energy and kinetic energy, C, were further decomposed into the contributions by the rotational (Rossby) and divergent (gravity) components of the circulation field. The eddy component of C is due to the conversion of potential energy of the rotational adjusted mass field into kinetic energy by the work realized in the eddy divergent motion. The zonal mean component of C is accomplished by two terms which nearly cancel each other out. One is related to the Hadley cell and involves the divergent component of both wind and geopotential, while the other is associated to the Ferrel cell and incorporates the divergent wind with the rotationally adjusted mass field. Global magnitude increases were found in the zonal mean components of these two terms for the warmer climate, which could be the result of a strengthening and/or widening of both meridional cells. On the other hand, the results suggest a strengthening of these conversion rates in the UTLS and a weakening below, that is consistent with the rising of the tropopause in response to global warming.     

中国南海夏季风强、弱年多尺度相互作用能量学特征

中国南海夏季风为东亚季风的主要系统之一,其具有多重尺度特征,除季节平均环流场外,低频（季节内振荡）和高频（天气尺度）扰动也十分活跃,各尺度系统存在明显的年际变化。该研究使用ERA-Interim和NCEP/NCAR两套再分析资料,从季风平均动能（MKE）诊断的角度出发,探讨了1979-2010年中国南海夏季风环流年际变化的能量来源及其和扰动场的相互作用过程。结果表明:中国南海夏季风对流活跃年份,中国南海南部（12°N以南）及中南半岛一带为季风平均动能显著增强区,此与南亚季风区西风急流的增强并向东延伸有关;中国南海北部（12°N以北）及西太平洋为气旋性环流盘踞,季风槽加深。中国南海南部季风平均动能增强的能量源自于扰动动量通量与平均环流的相互作用,强季风年,平均环流失去较少的动能给扰动场（亦即平均环流保留较多的动能）。通过进一步探讨高频（<10 d）及低频（10-90 d）扰动场与平均环流不同分量的（散度、涡度、风垂直切变）相互作用过程,发现季风平均动能的增长主要来自于<10 d扰动与季风平均散度和涡度的相互作用。中国南海北部季风槽区季风平均动能的维持来自于大气热源和平均上升运动的相互作用,但同时有较多的季风平均动能向扰动动能转换,有利于扰动的成长。因此,强季风年,中国南海北部热带气旋生成数目增多,夏季北传的季节内振荡也增强,导致中国南部沿海及华南地区出现较多的灾害天气。      

湍流的粘性和频散效应

本文应用Prandtl混合长理论导得了既包含湍流粘性又包含湍流频散的新的Reynolds平均运动方程组.分析指出:(1)湍流不仅存在粘性效应,而且存在频散效应;(2)正是湍流的频散效应可能导致能量逆转(即常说的负粘性现象,实质为一定条件下的负频散现象),并给出了能量逆转的必要条件和充分条件;(3)给出了湍流的KdV-Burgers方程模型.     

Local kinetic energy budget of high-frequency and intermediate-frequency eddies: winter climatology and interannual variability

The local budget of eddy kinetic energy (EKE) for both high-frequency (HF, 2–6 days) and intermediate-frequency (IF, 7–29 days) eddies are evaluated for Northern Hemisphere boreal winter using the 31-year (1979/80–2010/11) NCEP-DOE reanalysis. A new form of EKE equation is used to isolate the kinetic energy generation/destruction due to interactions among eddies of different timescales. The main source of HF EKE is baroclinic conversion that is concentrated in the mid-lower troposphere. Barotropic conversion mainly damps HF EKE and shows positive contributions to IF EKE on the northern flank of the winter-mean tropospheric jet. Interaction between HF and IF eddies acts as a sink for HF EKE and a main source for IF EKE, especially over the eastern ocean basins, confirming the substantial role of synoptic-scale transients in the development of IF phenomena such as atmospheric blocking. Large interannual variability is found for various EKE budget terms. The HF EKE response to El Niño is characterized by a dipole (tri-pole) anomaly over the North Pacific (North Atlantic). Baroclinic conversion is the main driver of the observed changes in HF EKE while barotropic conversion, interaction between HF and IF eddies, and energy flux convergence all play non-negligible roles in determining the final meridional structure of the HF EKE anomalies. Associated with El Niño, IF EKE generally decreases over the North Pacific and increases over the North Atlantic, which mainly result from changes in baroclinic conversion and EKE conversion due to eddy–eddy interactions. The latter is dominated by interaction between IF and LF (low-frequency, 30–90 days) eddies over the North Pacific, and by interactions between HF and IF eddies, and between IF and LF eddies over the North Atlantic.     

登陆台风Matsa (麦莎) 中尺度扰动特征分析

地面中尺度自动站和多普勒雷达资料的分析都表明, 台风Matsa登陆后的低层螺旋云带中活跃着中尺度气旋性涡旋系统。本文使用新一代中尺度WRF模式对台风Matsa登陆后的变化特征进行了数值模拟, 使用四维变分多普勒雷达分析系统 (4D-VDRAS) 对台风Matsa多普勒雷达径向风进行了风场反演。在此基础上对台风Matsa登陆后中尺度扰动特性进行了初步探讨; 对台风Matsa与其螺旋云带的中尺度系统之间动能和涡度的相互转换进行了诊断分析。分析表明: (1) 数值模拟和雷达风场反演结果表明, 登陆台风Matsa的低层螺旋云带中活跃着中尺度气旋式涡旋系统, 与之相伴随的为较强的中尺度上升区, 而且, 中尺度垂直上升运动的强弱与雷达对流回波强度成正相关, 中尺度垂直上升运动越强, 雷达对流回波发展越旺盛。 (2) 台风Matsa与其中尺度系统动能转换的诊断分析说明, 低层中尺度系统从台风Matsa环流中获得动能而发展; Matsa在陆地上长久维持主要是从高层获得动能。 (3) 台风Matsa与其中尺度系统涡度转换的诊断分析说明, 低层中尺度系统向Matsa输送正涡度主要依靠中尺度垂直运动来完成; 高层正涡度的转换通过水平输送和垂直输送共同来完成。所以, 中尺度系统所产生的正涡度源源不断地向Matsa输送, 使Matsa的气旋性环流可以在陆地上长久维持。     

Comparative energetics analysis of CCM2 with different horizontal resolutions

 Comprehensive global energetics analysis is carried out for the NCAR CCM2 with different horizontal resolutions of R15, T42, T63, and T106 to assess the effect of various model truncations on the global energetics characteristics in climate models. Both the energy levels and energy transformations are examined over the zonal wave number domain during a northern winter and summer. In addition to the simulated atmosphere, the ECMWF global analysis during 1986 to 1990 is analyzed for comparison using the same diagnostic scheme. Previous studies have revealed that zonal kinetic energy is supplied by synoptic disturbances in terms of the zonal-wave interactions of kinetic energy. According to our result, however, such an energy flow from eddies to zonal motions is valid only for zonal wave numbers up to about 30. We find that the zonal-wave interactions of kinetic energy change sign beyond wave number 30 where the energy is transformed from zonal to eddies for both the ECMWF and CCM2-T106. The large-scale zonal motions are diffusive against the short waves beyond wave number 30, which may well be parameterized by various forms of the diffusion schemes. We suggest from this result that the atmospheric disturbances with wave numbers lower than 30 are necessary to represent accurately the two-way interactions between zonal and eddy motions, because these waves can actively influence the behavior of the zonal motions. Based on this finding, we suggest that the model resolution of R15 is inadequate for climate studies from the energetics point of view, and that resolution of T42 is the minimum requirement to represent the general circulation adequately. Some other discrepancies are discussed in detail for the coarse resolution climate models. Received: 15 July 1996/Accepted: 3 January 1997     

Influence of bottom topography roughness on the jet and inertial recirculation of a mid-latitude gyre

Several numerical experiments are conducted to examine the influence of mesoscale, bottom topography roughness on the inertial circulation of a wind-driven, mid-latitude ocean gyre. The ocean model is based on the quasi-geostrophic formulation, and is eddy-resolving as it features high vertical and horizontal resolutions (six layers and a 10 km grid). An antisymmetrical double-gyre wind stress curl forces the baroclinic modes and generates a strong surface jet. In the case of a flat bottom, inertia and inverse energy cascade force the barotropic mode, and the resulting circulation features strong, barotropic, inertial gyres. The sea-floor roughness inhibits the inertial circulation in the deep layers; the barotropic component of the flow is then forced by eddy-topography interactions, and its energy concentrates at the scales of the topography. As a result, the baroclinicity of the flow is intesified: the barotropic mode is reduced with regard to the baroclinic modes, and the bottom flow (constrained by the mesoscale sea-floor roughness) is decoupled from the surface flow (forced by the gyre-scale wind). Rectified, mesoscale bottom circulation induces an interfacial form stress at the thermocline, which enhances horizontal shear instability and opposes the eastward penetration of the jet. The mean jet is consequently shortened, but the instantaneous jet remains very turbulent, with meanders of large meridional extent. The sea-floor roughness modifies the energy pathways, and the eddies have an even more important role in the establishment of the mean circulation: below the thermocline, rectification processes are dominant, and eddies transfer energy toward permanent mesoscale circulations strongly correlated with topography, whereas above the thermocline mean flow and eddy generation are influenced by the mean bottom circulation through interfacial stress. The topography modifies the vorticity of the barotropic and highest baroclinic modes. Vorticity accumulates at the small topographic scales, and the vorticity content of the highest modes, which is very weak in the flat-bottom case, increases significantly. Few changes occur in surface-intensified modes. In the deep layers of the model, the inverse correlation between relative vorticity and topography at small scales ensures the homogenization of the potential vorticity, which mainly retains the largest scales of the bottom flow and the scale of β.     

Impact of the Nocturnal Low-Level Jet and Orographic Waves on Turbulent Motions and Energy Fluxes in the Lower Atmospheric Boundary Layer

The nocturnal low-level jet (LLJ) and orographic (gravity) waves play an important role in the generation of turbulence and pollutant dispersion and can affect the energy production by wind turbines. Additionally, gravity waves have an influence on the local mixing and turbulence within the surface layer and the vertical flux of mass into the lower atmosphere. On 25 September 2017, during a field campaign, a persistent easterly LLJ and gravity waves were observed simultaneously in a coastal area in the north of France. We explore the variability of the wind speed, turbulent eddies, and turbulence kinetic energy in the time–frequency and space domain using an ultrasonic anemometer and a scanning wind lidar. The results reveal a significant enhancement of the turbulence-kinetic-energy dissipation (by?50%) due to gravity waves in the LLJ shear layer (below the jet core) during the period of wave propagation. Large magnitudes of zonal and vertical components of the shear stress (approximately 0.4 and 1.5 m² s^?2, respectively) are found during that period. Large eddies (scales of 110 to 280 m) matching the high-wind-speed regime are found to propagate the momentum downwards, which enhances the mass transport from the LLJ shear layer to the roughness layer. Furthermore, these large-scale eddies are associated with the crests while comparatively small-scale eddies are associated with the troughs of the gravity wave.

     

非均匀风场与急流强迫的水体涡旋动力特征模拟

通过数值模拟有限区域水气界面由强迫作用驱动形成的水体涡旋及环流动力结构特征,分析非均匀风场、水体急流、两者叠加以及环境边界和地转偏向力等因子的综合影响,探讨此类水体涡旋结构和动力特征。风应力驱动的水体涡旋尺度大,相对深厚,正涡旋具有下凹表面,负涡旋具有上凸表面。水体急流驱动的涡旋形成在急流两侧,对应急流所在深度及厚度尺度相对较小,也较浅,但流速与强度均大于风场驱动的涡旋环流。地形阻挡起着引导涡旋环流走向的作用;同时在北半球地转偏向力对急流侧向负涡旋形成和强度增强更为有利。此外正涡旋对应的辐合辐散势函数强于负涡旋,有利于正涡旋区垂直上升运动强于负涡旋中垂直下沉运动。非均匀风场及水体急流两种强迫叠加作用下,涡旋数量增加、尺度减小,底层的流场形态及强度与表层差异增大。形成的水体涡旋结构呈现多种形态：深厚的整层一致;浅薄的仅维持在上层,或上下层环流相反等。风应力驱动的涡旋以正压性为主,水体急流驱动的涡旋因急流的垂直强切变而具有强的斜压性,在正斜压动能的转换中,正压性涡旋区有斜压动能向正压动能转换,斜压性涡旋区有正压动能向斜压动能转换,均有利于这两个区域正负涡旋的维持。     

2000/2001年冬季北欧异常流型形成机理及其对我国北方天气的影响

从Rossby波能量的频散和瞬变斜压扰动的正压反馈角度,探讨了2000/2001年冬季盛行的斯堪的纳维亚环流型(正位相)的维持机理,并讨论了其对我国北方地区天气气候的影响。该环流型上游部分,即北大西洋和斯堪的纳维亚半岛的两个活动中心主要由瞬变斜压扰动的正压反馈所形成,部分由Rossby波能量频散所维持。而下游部分,即贝加尔湖附近的气旋式环流异常主要由Rossby波能量从上游频散所形成和维持。与该环流型对应,冷空气在西、中西伯利亚堆积,并在其南侧的西风气流区中平流,使得我国华北部分地区和东北地区比常年冷。该环流型导致新疆北部地区、内蒙古东部及东北地区降雪量显著增强。大西洋急流的东伸及对应斜压扰动的显著加强是预报北疆和内蒙古东部及东北地区多雪的一个重要前兆因子。     

Energy budgets on the interactions between the mean and eddy flows during a persistent heavy rainfall event over the Yangtze River valley in summer 2010

In this study,a persistent heavy rainfall event(PHRE) that lasted for around 9 days(from 0000 UTC 17 to0000 UTC 26 June 2010) and caused accumulated precipitation above 600 mm over the Yangtze River valley,was reasonably reproduced by the advanced research WRF model.Based on the simulation,a set of energy budget equations that divided the real meteorological field into the mean and eddy flows were calculated so as to understand the interactions between the precipitation-related eddy flows and their background circulations(BCs).The results indicated that the precipitation-related eddy flows interacted with their BCs intensely during the PHRE.At different layers,the energy cycles showed distinct characteristics.In the upper troposphere,downscaled energy cascade processes appeared,which favored the maintenance of upper-level eddy flows;whereas,a baroclinic energy conversion,which reduced the upper-level jet,also occurred.In the middle troposphere,significant upscaled energy cascade processes,which reflect the eddy flows’ reactionary effects on their BCs,appeared.These effects cannot be ignored with respect to the BCs’ evolution,and the reactionary effects were stronger in the dynamical field than in the thermodynamical field.In the lower troposphere,a long-lived quasi-stationary lower-level shear line was the direct trigger for the PHRE.The corresponding eddy flows were sustained mainly through the baroclinic energy conversion associated with convection activities.Alongside this,the downscaled energy cascade processes of kinetic energy,which reflect the direct influences of BCs on the precipitation-related eddy flows,were also favorable.A downscaled energy cascade of exergy also appeared in the lower troposphere,which favored the precipitation-related eddy flow indirectly via the baroclinic energy conversion.     

Turbulence intermittency in the atmospheric surface layer over snow-covered sites

Wind component fluctuations have been decomposed into contributions of large and small-scale eddies to study turbulence structures in the atmospheric surface layer over snow-covered pack ice for a case with snow drift and another without. It was found that on average, the large eddies contribute little to the covariance, but significantly to the variance of the horizontal wind components. The turbulent kinetic energy and the friction velocity show clearly expressed energy and flux variations. The time series of turbulent statistics, calculated on the basis of large eddies only, show the same behavior. However, the time variations of the turbulent kinetic energy and the friction velocity, determined by the small eddies, are suppressed when there is drifting snow.The occurrence of different types of motions (ejections, sweeps and interactions) was detected by the quadrant-technique. It was found that events with strong drag reduction during snow drift correspond to a strong increase of contributions from the first and third quadrant. However, strong drag reduction events in the case without snow drift are caused by a general decrease of contributions from all four quadrants. It was also found that interactions are significantly less correlated with ejections and sweeps in the case with snow drift than in the case without.     

Investigations of the 500mb level in relation to the general circulation

Summary The redistribution of kinetic energy in the 500mbar level is studied for 1966 as well as for 1967. The total kinetic energy is split up in three parts: a) the energy due to the mean flow, b) the energy due to eddies with a period of time longer than a month and c) the energy due to eddies with a period of time less than a month but longer than one day.The transport of these energies takes place in formally three different ways, namely by the mean flow, by longperiod and shortperiod eddies (or monthly and daily eddies).Both the transports in zonal and meridional direction are studied. As will be shown in section 2, this yields 16 transportterms. It turns out that for the 500mbar level in the region from 100°W to 30°E and from 30°N to 65°N and based on geostrophic approximated winds: a) the longperiod eddies are important, b) the role of the longperiod eddies as well as the role of the shortperiod eddies decreases with increasing latitude and c) the zonal transports and the divergence of those transports are mostly larger than the meridional transports, and the divergence of the meridional transports.

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Untersuchungen der 500 mbar-Fläche in bezug auf die allgemeine Zirkulation,

Zusammenfassung Die Verteilung der kinetischen Energie im 500mb-Niveau wird für die Jahre 1966 und 1967 untersucht. Die gesamte kinetische Energie wird dabei in drei Teile zergliedert: a) die kinetische Energie der mittleren Strömung, b) die Energie aus länger als einen Monat andauernden Wirbeln und c) die Energie aus kürzer als einen Monat, aber länger als einen Tag andauernden Wirbelelementen.Der Transport dieser Energien erfolgt formal auf drei verschiedenen Wegen, nämlich durch die mittlere Strömung, durch lang- und kurzperiodige Wirbelelemente (Turbulenzelemente von Monatsdauer bzw. von Tagesdauer). Es werden die Transporte in zonaler und in meridionaler Richtung untersucht; daraus ergeben sich 16 Transportterme. Für die Region von 100°W bis 30°E und 30°N bis 65°N erweist sich unter Zugrundelegung geostrophisch approximierter Winde, a) daß langperiodige Wirbel von Bedeutung sind, b) daß die Bedeutung langperiodiger wie auch kurzperiodiger Wirbelelemente mit zunehmender geographischer Breite abnimmt und c) daß die zonalen Transporte und ihre Divergenz meistens größer sind als die meridionalen Transporte und die Divergenz meridionaler Transporte.

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With 25 Figures     

The influence of the coast on the dynamics of upwelling fronts: Part II. Numerical simulations

We investigated the dynamics of upwelling fronts near a coast. This work was first motivated by laboratory experiments [Bouruet-Aubertot, Linden, Dyn. Atmos. Oceans, 2002] in which the front is produced by the adjustment of a buoyant fluid initially confined within a bottomless cylinder. It was shown that cyclonic eddies consisting of coastal waters are enhanced when the front is unstable near the coast (the outer vertical boundary). The purpose of this paper is to provide further insights into this process. We reproduced the experimental configuration using a three-dimensional model of the primitive equations. We first show that for coastal fronts more potential energy, in terms of the maximum available potential energy, is released than for open-ocean fronts. Therefore, waves of larger amplitude are generated during the adjustment and the mean flow that establishes has a higher kinetic energy in the former case. Then as baroclinic instability starts and wave crests reach the boundary, cyclonic eddies are enhanced as in the laboratory experiments and in a similar way. However, in contrast to the laboratory experiments, offshore advection of cyclonic eddies can occur in two stages, depending on the spatial organization of the baroclinic wave. When the baroclinic wave consists of the sum of different modes and is thus highly asymmetric, the offshore advection of cyclonic eddies occurs just after their enhancement at the boundary, as in the laboratory experiments. By contrast, when a single-mode baroclinic wave develops, neighboring cyclonic eddies first merge before being advected offshore. Very different behavior is observed for open-ocean fronts. First a mixed baroclinic–barotropic instability grows. Then the eddies transfer their energy to the mean flow and the barotropic and baroclinic instabilities start again. An excellent agreement is obtained with the main result obtained in the laboratory experiments: the ratio between growth rates of surface cyclonic and anticyclonic vorticity increases as the instability develops nearer to the coast.     

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea(SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic(warm-core anticyclonic) eddies tend to cool(warm) the overlying atmosphere and cause surface winds to decelerate(accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce(enhance) the vertical momentum transport over the cold(warm) eddy, resulting in the decrease(increase) of sea surface wind. Diagnoses of the model's momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.     

北京地区一次PM2.5重污染过程的边界层特征分析

利用北京市环境保护监测中心和美国大使馆的细颗粒物（PM2.5）逐时监测数据,中国科学院大气物理研究所325 m气象梯度塔资料以及实况天气图和探空资料,对2015年11月27日至12月1日北京的PM2.5重污染过程的边界层特征进行了分析。研究发现:这次重污染过程持续时间长、强度大,其中PM2.5浓度超过75 μg/m3的时次共计126 h,超过150 μg/m3共计116 h,小时最高PM2.5浓度为522 μg/m3。在高低空环流场配置的影响下,近地面静风和多层逆温结构抑制了污染物在水平和垂直方向上的输送,加上边界层内的深厚湿层,使得其中气溶胶不断吸湿增长,高PM2.5浓度得以维持。在重污染期间,湍流动能较低,不利于污染物的水平和垂直扩散。垂直方向的湍流动能一直占水平方向的15%~20%左右,水平湍流动能占主要贡献。摩擦速度与湍流动能呈现出相似的变化趋势,不同高度之间的摩擦速度差别不大。超出前后时次一个数量级的湍流强度尖峰的出现是湍流场发生调整的一个信号,是PM2.5浓度发生剧烈转变的前兆,预示着污染状况更加糟糕。重污染过程中感热通量的输送方向为从地面向大气输送,感热通量和潜热通量都大幅减少,并且表现出明显的日变化特征。对湍流功率谱计算和分析表明,在重污染过程期间,时间尺度为5 min至6 h的中尺度过程对从地面到大气方向的动量和热量通量输送做出了重要贡献。     

用连续子波变换提取城市冠层大气湍流的相干结构

切变湍流的相干结构是湍流研究中的重大发现,它表明湍流在表面上看来不规则运动中具有可检测的有序运动,这种相干结构在切变湍流的脉动生成和发展中起着主宰作用.因此识别和提取相干结构对于认识和研究湍流是非常重要的.用数字滤波法将包含相干结构的大尺度信号提取出来以后,再用子波分析,根据子波能量极大值的判别方法,分别确定出大气湍流三个方向上的速度脉动信号相干结构的频率或时间尺度,然后由确定尺度上的连续子波反演公式,提取出大气湍流三个方向上的速度脉动信号相干结构所对应的波形.     

Coherent Structures Detected in Atmospheric Boundary-layer Turbulence using Wavelet transforms at Huaihe River Basin, China

The presence of coherent structures in turbulent shear flows suggests order in apparently random flows. These coherent structures play an important dynamical role in momentum and scalar transport. To develop dynamical models describing the evolution of such motion, it is necessary to detect and isolate the coherent structures from the background fluctuations. In this paper, we decomposed atmospheric turbulence time series into large-scale eddies, which include coherent structures and small eddies, which are stochastic by using Fourier digital filtering. The wavelet energy computed for the three components of the velocity fluctuations in the large-scale eddies appears to have local maximum values at certain time scales, which correspond to the scales or frequencies of coherent structures. We extract coherent signals from large-scale vortices at this scale by inverse wavelet transform formulae. This method provides an objective technique for examining the turbulence signal associated with coherent structures in the atmospheric boundary layer. The average duration of coherent structures in three directions based on Mexican hat wavelets are 33 s, 34 s and 25 s respectively. Symmetric andanti-symmetric wavelet basis functions give almost the same results. The main features of the structures during the day and night have little difference. The dimensionless durations for u, v and w have linear correlations with each other. These relationships are insensitive to the wavelet basis.