Characteristics of oceanic mesoscale variabilities associated with the inverse kinetic energy cascade

Oceanic geostrophic turbulence theory predicts significant inverse kinetic energy(KE) cascades at scales larger than the energy injection wavelength. However, the characteristics of the mesoscale variabilities associated with the inverse KE cascade in the real oceans have not been clear enough up to now. To further examine this problem,we analyzed the spectral characteristics of the oceanic mesoscale motions over the scales of inverse KE cascades based on high-resolution gridded altimeter data. The applicability of the quasigeostrophic(QG) turbulence theory and the surface quasigeostrophic(SQG) turbulence theory in real oceans is further explored. The results show that the sea surface height(SSH) spectral slope is linearly related to the eddy-kinetic-energy(EKE) level with a high correlation coefficient value of 0.67. The findings also suggest that the QG turbulence theory is an appropriate dynamic framework at the edge of high-EKE regions and that the SQG theory is more suitable in tropical regions and low-EKE regions at mid-high latitudes. New anisotropic characteristics of the inverse KE cascade are also provided. These results indicate that the along-track spectrum used by previous studies cannot reveal the dynamics of the mesoscale variabilities well.     

The wind-field structure in a stably stratified atmospheric boundary layer over a rough surface

Both wind turning with height and ageostrophic flow in a stably stratified atmospheric boundary layer are analyzed using a three-parameter turbulence model. For a quasi-steady state of the boundary layer, the cross-isobaric flow is determined only by turbulent stress at the surface in the direction of geostrophic wind. The “operative” prediction models, in which the first-order turbulence closure schemes are used, tend to overestimate the boundary-layer depth and underestimate the angle between the surface and geostrophic winds when compared to “research” models (schemes of high-level turbulence closure). The true value of the angle between the surface and geostrophic winds is significant for the presentation of a large-scale flow. A nocturnal low-level jet is a mesoscale phenomenon reflected in data obtained from measurements in a stably stratified atmospheric boundary layer. It is found that such jets are of great importance in transporting humidity, momentum, and air pollution. In this study, the difference between jet flows over a homogeneous underlying surface and over a spatially localized large-scale aerodynamic roughness is shown.     

挡潮闸矩形孔口式鱼道紊流结构及放鱼试验研究

对挡潮闸枢纽中矩形中孔、底孔鱼道中紊流结构进行了较为系统的试验研究,并做了放鱼试验。选择了一种鱼类偏爱流速所对应的流量作为典型流量,考虑了不同的孔口位置(中孔和底孔),用声学多普勒测速仪(ADV)量测了测点的三维瞬时流速及流向,分析了矩形孔口鱼道的三维时均流速分布特征、断面最大流速沿程变化规律、流速矢量场、紊动强度分布及雷诺应力分布。此外,还通过放鱼试验,利用在鱼体植入T形标签和高速摄影机观察了过鱼对象对中孔、底孔的反应情况,分析了过鱼对象与矩形孔口鱼道紊流结构的关系。试验结果表明:水流经中孔形成三维紊动自由射流,经底孔形成三维壁面射流,中孔纵向流速呈高斯分布,而底孔纵向流速则近似为高斯分布,流速由孔口向两侧逐渐减小;中孔和底孔横向流速在位于孔口范围内的纵剖面上沿程减小,孔口之外则变化较小;中孔和底孔垂向流速分布特征表现为在铅垂方向上均存在旋涡;在中孔水平面和纵剖面上,纵向最大流速均沿程衰减;中孔和底孔情形孔口处紊动强度和雷诺应力比非孔口处大得多,而非孔口处不同水深平面上紊动强度和雷诺应力变化趋于平缓;过鱼对象喜爱在紊动强度分布的峰值区和雷诺应力较大变幅区溯游。     

Investigation into wave properties of the atlantic surface temperature spatial variability on interannual scales

The results of investigations into the spatial variability of the Atlantic Ocean surface temperature field on interannual scales are presented. The analysis is based on monthly mean satellite data of the AVHRR Pathfinder Data JPL NOAA/NASA over 1985–2001. Specific features of the structure of the sea surface temperature (SST) fields averaged over 17 years, as well as fields of the gradient and variance of the SST time series, are described for each node of the data grid. It is shown with the use of the rhythmodynamic approach that spatial heat waves exist in the interannual variability of the SST field in two directions: zonal and meridional.     

Alternating zonal flows in a two-layer wind-driven ocean

Alternating zonal flows in an idealized wind-driven double-gyre ocean circulation have been investigated using a two-layer shallow-water eddy-permitting numerical model. While the alternating zonal flows are found almost everywhere in the time-mean zonal velocity field, their meridional scales differ from region to region. In the subpolar western boundary region, where the energetic eddy activity induces quasi two-dimensional turbulence, the alternating zonal flows are generated by the inverse energy cascade and its arrest by Rossby waves, and the meridional scale of the flows corresponds well to the Rhines scale. In the eastern part of the basin, where barotropic basin modes are dominant, the zonal structure is formed through the nonlinear effect of the basin modes and is wider than the Rhines scale. Both effects are likely to form zonal structure between the two regions. These results show that Rossby basin modes become an important factor in the formation of alternating zonal flows in a closed basin in addition to the arrest of the inverse energy cascade by Rossby waves. The wind-driven general circulation associated with eddy activities plays an essential role in determining which mechanism of the alternating zonal flows is possible in each region.     

Local advective mechanism for interdecadal variability in circulations driven by constant surface heat fluxes in idealized basins

Idealized numerical experiments with a depth level coordinate ocean circulation model (GFDL MOM3) have been conducted to investigate the structure of interdecadal variability from thermally driven circulations. The model oceans are driven by steady surface heat fluxes in the absence of surface wind stresses. Interdecadal variability is observed, with characteristics similar to those reported in many previous studies. To explain the nature of the variability we propose a new mechanism based on two local horizontal advective processes. This overcomes the limitations in previous theories based on the interplay between global properties such as zonal and meridional temperature gradients and overturning. One of the two advective processes is a zonal flow anomaly induced by a temperature anomaly along the northern wall through geostrophy southward of the temperature anomaly. A cold (warm) anomaly along the northern wall produces a positive (negative) zonal flow anomaly that induces a warm (cold) temperature anomaly by enhancing (weakening) warm advection from the western boundary along the path of the zonal flow anomaly. The temperature and flow anomalies are transported toward the eastern boundary by the mean eastward zonal flow. When the positive (negative) zonal flow anomaly that accompanies the warm (cold) temperature anomaly encounters the eastern wall, a downwelling (upwelling) anomaly is produced. To dissipate the vorticity due to this downwelling (upwelling) anomaly, a northward (southward) flow anomaly, which is another advective process governing the variability, is generated within a frictional boundary layer next to the eastern wall. The northward (southward) flow anomaly circulates cyclonically along the perimeter of the basin while enhancing (reducing) warm advection. So does the warm (cold) temperature anomaly carried to the eastern wall by the mean zonal flow while pushing the cold (warm) anomaly that produced the positive (negative) zonal flow anomaly westward and initiating the other half cycle of the variability. During the anomalous downwelling or upwelling, the available potential energy stored in the anomalous density field is released to maintain the variability. Thus, neither barotropic nor baroclinic instability supplies energy for the variability. The anomalous vertical velocity is stronger along the northern boundary and the northern part of the eastern boundary. A shallow continental slope added along those boundaries prohibits the anomalous vertical motion and weakens variability very effectively, while one along the western boundary does not.     

Large-scale turbulence under a solitary wave

The structure of large-scale turbulence under a broken solitary wave on a 1 in 50 plane slope was studied. Three-component velocity measurements were taken at different heights above a smooth bed in the middle surf zone using an acoustic Doppler velocimeter. The measured data showed that turbulent velocity components were well correlated in the middle part of the water column. The velocity correlations could be produced by an oblique vortex similar to the obliquely descending eddy observed previously by other investigators. The vertical distributions of the relative values of the components of the Reynolds stress tensor showed that the structure of turbulence evolved continuously between the free surface and the bottom. The evolution was related to transition from two-dimensional to three-dimensional flow structures and the effect of the solid bottom on flow structures. Time histories of measured turbulent kinetic energy and turbulence stresses showed episodic turbulent events near the free surface but more sporadic turbulence in the lower layer. Large or intense turbulent events were found to have short duration and time lag relative to the wave crest point. These events also maintained good correlations between the turbulence velocity components close to the bottom.Instantaneous turbulent velocity fields were measured near the bottom at the same cross-shore location by using a stereoscopic particle image velocimetry system. These measurements showed that the near-bed flow field was characterized by large-scale, coherent flow structures that were the sources of most of the turbulent kinetic energy and turbulence stresses. The types of organized flow structures observed included vortices and downbursts of turbulence descending directly from above, lateral spreading of turbulent fluid along the bed, and formation of vortices in shear layers between fluid streams. A common feature of the organized flow structures near the bed was the large turbulence velocities in the longitudinal and transverse directions, which reflected the influence of a solid bottom on the breaking-wave-generated turbulence arriving at the bed.     

Topographic effect of a marine ridge on the spin-down of a cyclonic eddy with mean flow

The topographic effect of a meridional marine ridge on the spin-down of a cyclonic eddy, which is embedded in a zonal mean flow, is examined by use of a two layer numerical model. It is shown that the cyclonic eddy initially given on the eastern flank of the marine ridge decays in a short time. This result is common to all cases with the different volume transports of the mean flow (3070 Sv) and of the cyclonic eddy (1535 Sv). During the decay process, the cyclonic eddy shifts mainly northward into the shallower region, which is different from the dominant westward shift of the isolated cyclonic eddy. If the mean flow across over the marine ridge at the more northern latitude, the cyclonic eddy spins down more rapidly. A mean flow shifts zonal or south-eastward over a western side of the ridge, while it deflects north-eastward over an eastern side. The deflection angle of mean flow over the ridge depends on the intensity of lower layer velocity and density stratification. It is suggested that the topographic effect of the meridional marine ridge on the cyclonic eddy with mean flow is influenced both by the global phenomena that controls the inclination of the mean flow from zonal direction and by the local phenomena that controls the intensity of the topographic effect of the marine ridge.     

Numerical simulation of micro-bubble drag reduction using population balance model

The phenomenon of drag reduction by the injection of micro-bubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Multiple-size group (MUSIG) based on population balance models, which resolve a wide range of bubble sizes taking into account the bubble break-up and coalescence have been used for this purpose. The simulated results are compared against the experimental findings of Madavan et al. [1984. Reduction of turbulent skin friction by micro-bubbles. Physics of Fluids 27, 356-363] and also other numerical studies explaining the sophisticated phenomena of drag reduction. For the two Reynolds number cases considered, the buoyancy with the plate on the bottom configuration is investigated, as from the experiments it is seen that buoyancy seem to play a role in the drag reduction. Numerical model employed in the investigation comprises of a micro-bubble laden flow wherein two independent sets of Reynolds averaged Navier-Stokes (RANS) transport equations were used to describe both the phases of the flow. The shear stress transport (SST) turbulence model is used as the turbulent closure for the primary phase and a zero equation turbulence model is used for the micro-bubbles. Change in the mean streamwise velocity profiles, void fraction, turbulence modification and other results are presented and discussed with corresponding change in the gas injection rates. The complex mechanism of drag reduction are scrutinised and explained in context to our numerical findings. Special attentions have been also devoted to divulge the effect of bubble coalescence and break-up caused by random collision and turbulent impact. Numerical results showed good agreement for the skin-friction coefficients against experimental data throughout various air injection rates. The MUSIG model was found to be one of the best candidates to resolve the bubble dynamics in micro-bubble-induced drag reduction problems.     

南海海盆中尺度涡移动速度统计分析

基于1993-2017年卫星高度计数据得到的中尺度涡追踪产品,分析了1000 m以深南海海盆中尺度涡移动速度C的时空分布特征。结果表明,南海海盆气候态平均的中尺度涡纬向移动速度c_x均为西向,经向移动速度c_y在海盆西北侧为南向,东南侧为北向。c_y随经度的变化与背景经向流的变化一致,相关系数达0.96,而c_x的变化与背景纬向流和β效应有关。c_x和C存在明显季节变化,夏季最慢,冬季最快。年际变化上,c_x和c_y的大值多发生在太平洋年代际涛动（PDO）负位相期的La Nina年。中尺度涡在其生命周期的开始和结束阶段（即生成和耗散阶段）移速较快,而在稳定的“中期”阶段移动缓慢。该趋势与涡旋转速呈反相关,相关系数为-0.93。以移速小于1.5 cm/s和大于15.4 cm/s定义的极慢和极快涡旋,分别占总涡旋数量的1.5%和1.9%。移速极慢的涡多出现在海盆的中部,且主要发生在夏季;移速极快的涡多出现在海盆的边缘,且主要发生在冬季。机制分析显示,南海海盆中尺度涡移动速度的时空分布受到大尺度背景流场调制。     

Zonal flows,Rossby waves,and vortex transport in laboratory experiments with rotating annular channel

Results of experiments are considered for flows generated by different sources-sinks of mass in the rotating annular channel with beta-effect simulation using the inclined bottom. Diagrams of regimes are presented in parameters of the dimensionless angular velocity of the zonal flow averaged over the channel width and the dimensionless angular velocity of transport of vortex perturbations of cyclonic and anticyclonic types. In experiments and the simplest linear theories, most attention is paid to diagram regions with a slow motion of vortices relative to the rotating coordinate system near the parameters for stationary Rossby waves.     

The Mathematical and Parameterized Expressions for Wave Induced Excess Flow of Momentum

Wave induced excess flow of momentum(WIEFM)is the averaged flow of momentum over a wave period due to wave presence,which may also be called 3-D radiation stress.In this paper,the 3-D current equations with WIEFM are derived from the averaged Navier-Stokes equations over a wave period,in which the velocity is separated into the large-scale background velocity,the wave particle velocity and the turbulent fluctuation velocity.A concept of wave fluctuating layer(WFL)is put forward,which is the vertical column from the wave trough to wave ridge.The mathematical expressions of WIEFM in WFL and below WFL are given separately.The parameterized expressions of WIEFM are set up according to the linear wave theory.The integration of WIEFM in the vertical direction equals the traditional radiation stress(namely 2-D radiation stress)given by Longuet-Higgins and Stewart.     

江苏近海潮流湍流切应力研究

潮流的流速分布和湍流切应力的分布密切相关。为了研究潮流湍流切应力的特征和变化规律,本文从潮流运动方程推导出随潮流呈周期性变化的湍流切应力随深度的分布表达式。从江苏近海现场实测流速剖面中计算得到一个潮周期内的湍流切应力的变化过程。通过将实测值与本文理论解的比较发现,计算值能很好地反映出实测值的变化特征,尤其是能正确地反映潮流在加速和减速过程中,湍流切应力偏离线性分布呈现上凹和下凹的现象,以及只有在加减速转换的时候切应力才呈现线性分布的特点。最后,通过分析湍流切应力振幅及相位沿水深的变化情况,认为反映潮流周期、涡粘性和水深之间关系的参数Ri是决定湍流切应力弯曲程度及分布形态的重要参数。对于近海潮流,水深是影响湍流切应力偏离线性程度的主要因素。     

On the effect that the direction of geostrophic wind has on turbulence and quasiordered large-scale structures in the atmospheric boundary layer

The dependence that the structure and intensity of turbulent and large-scale quasiordered eddies in the atmospheric boundary layer (ABL) have on the direction of geostrophic wind has been studied on the basis of a series of numerical experiments with a three-dimensional nonstationary model of high spatial resolution. The presence of the meridional component of the angular velocity of the Earth’s rotation results in a significant intensification of velocity fluctuations in a neutrally stratified turbulent flow during the easterly and northeasterly winds and in their decay during the westerly and southwesterly winds. This, in turn, results in significant variations in the mean velocity profile. It is shown that these variations are associated with the largest scale fluctuations and are comparable (in scale) to the depth of Ekman’s turbulent layer. It is found that, in the neutrally stratified ABL bounded in height and under stable stratification inside the ABL, the wind-direction dependence significantly decreases. The possibilities of parameterizing these effects in locally one-dimensional ABL models are discussed.     

不同湍流模式下钱塘江涌潮水流三维模拟

钱塘江涌潮具有动力作用强和流速变化快等特点。涌潮水流紊动复杂,流速的垂向分布和紊动强度息息相关。通过涌潮水流实测资料的分析可以发现,涌潮作用下流速垂向分布在底部和上层存在差异。为研究涌潮作用下流速垂向分布的特征,应用基于非结构网格下有限体积法模型FVCOM对钱塘江涌潮河段水流运动进行三维数值模拟。考虑到涌潮紊动作用复杂且对流速的垂向分布起着重要影响,采用不同的湍流模式对涌潮传播过程中水流的运动特征开展研究。通过与涌潮河段实测资料的验证,复演涌潮到达前后水流运动特征,给出涌潮水流湍动能的变化过程。研究成果有助于深入认识涌潮水流紊动特征和流速的分布规律,为涌潮作用下物质输运的研究提供基础。     

Long-term variability of oceanic frontal zones associated with large-scale atmospheric forcing

Global satellite sea surface temperature (SST) measurements and NCEP/NCAR reanalysis wind data for the period of 1982–2009 have been used to study the relationship between long-term variability of oceanic frontal zones (OFZ) and large-scale atmospheric forcing. Statistically significant positive correlations between the maximum magnitude of the meridional gradient of zonally averaged SST and meridional shear of zonal wind (which is an estimate of the Ekman convergence intensity) were found for all subpolar and subtropical OFZ of the World Ocean. Variability of the latitudinal position of OFZ cores may be associated with Ekman advection variability due to zonal wind variations. Strengthening of zonal wind results in a shift of subpolar OFZ cores to the south/north in the Northern/Southern hemispheres.     

Reynolds number variation in oscillatory boundary layers: Part I. Purely oscillatory motion

A numerical model based upon a low Reynolds number turbulence closure is proposed to study Reynolds number variation in reciprocating oscillatory boundary layers. The model is used to compute the boundary layer for flow regimes ranging from smooth laminar to rough turbulent. Criteria for fully developed turbulence are derived for walls of the smooth and rough types. In particular, a new criterion to identify the rough turbulent regime is determined based on the time-averaged turbulence intensity. The reliability of the present model is assessed through comparisons with detailed experimental data collected by other investigators. The model globally improves upon standard high Reynolds number closures. Variation through the wave cycle of the main flow variables (ensemble-averaged velocity, shear stress, turbulent kinetic energy) is remarkably well-predicted for smooth walls. Predictions are satisfactory for rough walls as well. Yet, the turbulence level in the rough turbulent regime is overpredicted in the vicinity of the bed.     

海气通量涡相关法计算中的时间尺度分析

基于黄海上连续14 d 的浮标观测资料, 采用多尺度分解法确定了海气通量涡相关法计算中的截断时间尺度, 并分析了该截断时间尺度的特征及其对感热通量计算的影响。研究结果是： 由多尺度分解法获得的湍通量截断时间尺度可将总通量中湍通量和中尺度通量分离开来, 截断时间尺度随着湍流强度或水平风速的增加而增加, 且感热通量的截断时...     

Laboratory measurements of large-scale near-bed turbulent flow structures under spilling regular waves

The instantaneous turbulent velocity field created by the breaking of spilling regular waves on a plane slope was measured in a plane running parallel to the slope using particle image velocimetry. The measurement plane was located at a height of about 1 mm above the bed. The measurement area encompassed the region where the large eddies generated at incipient wave breaking impinged on the bottom inside the surf zone. A total of 30 trials were conducted under identical experimental conditions. In each trial, six consecutive wave cycles were recorded. The measured velocity fields were separated into a mean flow and a turbulence component by ensemble averaging. The instantaneous turbulent velocity fields were analyzed to determine the occurrence frequency, location, geometry and evolution of the large eddies, and their contributions to instantaneous shear stresses, turbulent kinetic energy and turbulence energy fluxes. The motion of single glass spheres along the bed was also investigated. The two-phase flow measurements showed that the velocity and displacement of large solid particles on a smooth bed were significantly affected by the magnitude and direction of turbulence velocities. Overall, this study has examined the kinematic and dynamic properties of large eddies impinging on the bed and the interaction of these large-scale turbulent flow structures with the mean flow. The study has also highlighted the important role of large eddies in sediment transport.     

基于Argo历史观测的南海海盆尺度中层流场研究

针对南海海域海流环境复杂、中层实测数据量少的现状,本文基于2006-2016年布放在南海海域的114个Argo剖面浮标的卫星定位等信息,采用基于背景流和惯性流外推的最小二乘方法,获取南海海域1 200 m深中层流场信息,并采用Divand变分插值的方法形成网格化季节流场。结果表明:（1）针对南海中层（1 200 m）流场,单个Argo浮标可以刻画出具体的中尺度结构,如越南沿岸的反气旋涡,半径约为120 km,最大流速约为9.6 cm/s,平均流速为5.3 cm/s;（2） Argo网格化流场表明海盆尺度中层流场南海南部为反气旋环流结构,北部为气旋式环流,同时在吕宋海峡口存在从南海至太平洋的水体交换;（3）将该流场信息与HYCOM和YoMaHa'07两种资料对比,吻合度较高,与HYCOM再分析资料的偏差分布趋近于正态分布,海流的东西向分量的均方根误差为3.28 cm/s,南北向分量的均方根误差为3.26 cm/s。总体而言,利用Argo轨迹资料能够有效地反演出南海地区海盆尺度的中层环流特征。