Features of eddy kinetic energy and variations of upper circulation in the South China Sea

The features of eddy kinetic energy (EKE) and the variations of upper circulation in theSouth China Sea (SCS) are discussed in this paper using geostrophic currents estimated from Maps of Sea Level Anomalies of the TOPEX/Poseidon altimetry data. A high EKE center is identified in the southeast of Vietnam coast with the highest energy level 1 400 cm2 ·s^(-2) in both summer and autumn. This high EKE center is caused by the instability of the current axis leaving the coast of Vietnam in summer and the transition of seasonal circulation patterns in autumn. There exists another high EKE region in the northeastern SCS, southwest to Taiwan Island in winter. This high EKE region is generated from the eddy activities caused by the Kuroshio intrusion and accumulates more than one third of the annual EKE, which confirms that the eddies are most active in winter. The transition of upper circulation patterns is also evidenced by the directions of the major axises of velocity variance ellipses between 10?and 14.5°N, which supports the model results reported before.

Seasonal variation of eddy kinetic energy in the South China Sea

Mesoscale eddy activity and its modulation mechanism in the South China Sea （SCS） are inves- tigated with newly reprocessed satellite altimetry observations and hydrographic data. The eddy kinetic energy （EKE） level of basin-wide averages show a distinct seasonal cycle with the maximum in August-December and the minimum in February-May. Furthermore, the seasonal pattern of EKE in the basin is dominated by region offshore of central Vietnam （OCV）, southwest of Taiwan Island （SWT）, and southwest of Luzon （SWL）, which are also the breeding grounds of mesoscale eddies in the SCS. Instability theory analysis suggests that the seasonal cycle of EKE is modulated by the baroclinic instability of the mean flow. High eddy growth rate （EGR） is found in the active eddy regions. Vertical velocity shear in the upper 50-500 m is crucial for the growth of baroclinic instability, leading to seasonal EKE evolution in the SCS.

Eddy Kinetic Energy Study of the Snowstorm over Southern China in January 2008

The energetics of the third stage of a snowstorm over China was analyzed using ECWMF data.The analysis of the energy budget for the Middle East trough and the western Pacific trough that developed toward China on 25-28 January 2008 showed the advection of the geopotential by the ageostrophic wind to be both a crucial source and the primary sink of the eddy kinetic energy centers associated with the troughs.The magnitudes of the energy conversion terms,interaction kinetic energy conversion and baroclinic conversion,were too small to explain the development of the energy centers and the jet streaks.The energy centers gained energy at their entrance regions via the convergence of the ageostrophic geopotential fluxes,and then lost energy at their exit regions by the same fluxes.At the entrance regions,the fluxes converged,increasing the geopotential gradient,which generated a stronger geostrophic wind and higher kinetic energy,resulting in an ascending motion in this area.When the troughs moved to China,the ascending motion caused by the convergence of the fluxes at entrance region intensified the snowstorms over central and southern China.

Large eddy simulation of the rotation effect on the ocean turbulence kinetic energy budget in the surface mixed layer

A non-hydrostatic, Boussinesq, and three-dimensional large eddy simulation(LES) model was used to study the impact of the Earth's rotation on turbulence and the redistribution of energy in turbulence kinetic energy(TKE) budget. A set of numerical simulations was conducted,(1) with and without rotation,(2) at different latitudes(10°N, 30°N, 45°N, 60°N, and 80°N),(3) with wave breaking and with Langmuir circulation, and(4) under different wind speeds(5, 10, 20, and 30 m/s). The results show that eddy viscosity decreases when rotation is included, indicating that rotation weakens the turbulence strength. The TKE budget become tight with rotation and the effects of rotation grow with latitude. However, rotation become less important under Langmuir circulation since the transport term is strong in the vertical direction. Finally, simulations were conducted based on field data from the Boundary Layer and Air-Sea Transfer Low Wind(CBLAST-Low) experiment. The results, although more complex, are consistent with the results obtained from earlier simulations using ideal numerical conditions.

Seasonal variations of eddy kinetic energy fl ux in the South Indian Countercurrent region

The inverse cascade fl ux of kinetic energy(KE)and its length scale in the South Indian Countercurrent(SICC)region were investigated using 24-year satellite altimeter daily data from 1993–2016.The evolution of the eddy life cycle in the SICC was presented systemically.It was found that the arrest and inject scales of inverse cascade were within the baroclinic most unstable scale and observed energy-containing eddy scale.The seasonal cycle of the arrest scale,inject scale,and amplitude of the inverse cascade were compared with the eddy kinetic energy seasonal cycle,which revealed a 1.5-month lag of the eddy kinetic energy to the vertical shear of zonal velocity,implying the existence of nonlinear processes during the eddy growth phase.Meanwhile,the anisotropy of the inverse cascade indicated that kinetic energy might be transferred from meridional motions to zonal motions,which is probably caused byβeff ect.These results would be benefi cial for a better understanding of the KE transfer processes in the SICC region.

The variability of eddy ki/netic energy in the South China Sea deduced from satellite altimeter data

We used fifteen years （1993-2007） of altimetric data, combined from different missions （ERS-1/2, TOPEX/Poseidon, Jason-l, and Envisat）, to analyze the variability of the eddy kinetic energy （EKE） in the South China Sea （SCS）. We found that the EKE ranged from 64 cm2/s2 to 1 390 cm2/s2 with a mean value of 314 cm2/s2. The highest EKE center was observed to the east of Vietnam （with a mean value of 509 cm2/s2） and the second highest EKE region Was located to the southwest of Taiwan Island （with a mean value of 319 cm2/s2）. We also found that the EKE structure is the consequence of the superposition of different variability components. First, interannual variability is important in the SCS. Spectral analysis of the EKE interannual signal （IA-EKE） shows that the main periodicities of the IA-EKE to the east of Vietuam, to the southwest of Taiwan Island, and in the SCS are 3.75, 1.87, and 3.75 years, respectively. It is to the south of Taiwan Island that the IA-EKE signal has the most obvious impact on EKE variability. In addition, the IA-EKE exhibit different trends in different regions. An obvious positive trend is observed along the east coast of Vietnam, while a negative trend is found to the southwest of Taiwan Island and in the east basin of Vietnam. Correlation analysis shows that the IA-EKE has an obvious negative correlation with the SSTA in Nifio3 （5°S-5°N, 90°W-150°W）. E1 Nifio-Southem Oscillation （ENSO） affects the 1A-EKE variability in the SCS through an atmospheric bridge--wind stress curl over the SCS. Second, the seasonal cycle is the most obvious timescale affecting EKE variability. The locations of the most remarkable EKE seasonal variabilities in the SCS are to the east of Vietnam, to the southwest of Taiwan, and to the west of Philippines. To the east of Vietnam, the seasonal cycle is the dominant mechanism controlling EKE variability, which is attributed primarily to the annual cycle there of wind stress curl. In this area, the maximum EKE is observed in autumn. To the southwest of Taiwan Island, the EKE is enlarged by the stronger SCS circulation, which is caused by the intrusion branch from the Kuroshio in winter. Finally, intra-annual and mesoscale variability, although less important than the former, cannot be neglected. The most obvious intra-annual and mesoscale variability, which may be the result of baroclinic instability of the background flow, are observed to the southwest of Taiwan Island. Sporadic events can have an important effect on EKE variability.

Interannual variations in energy conversion and interaction between the mesoscale eddy field and mean flow in the Kuroshio south of Japan

Using 19-year satellite altimetric data, variations in the eddy kinetic energy, energy exchanges and interaction between the eddy fi eld and mean fl ow are discussed for the Kuroshio south of Japan. In the seasonal cycle, the eddy kinetic energy level is a minimum in December/January and a maximum in April/May. In addition to seasonal variations, the eddy kinetic energy undergoes interannual changes. The energy transfers mainly from the mean fl ow to the eddy fi eld in the Kuroshio south of Japan, and dominant energy exchanges mainly occur along the Kuroshio path south of Japan in each year from 1993 to 2011. In addition, there is often barotropic instability south of Honshu. Regarding interactions between the eddy fi eld and mean fl ow, cyclonic and anticyclonic accelerations are also found along the Kuroshio path and they fl ank each other. There is cyclonic acceleration always imposed on southeast of Kyushu, and anticyclonic acceleration dominates south of Honshu from 2001 to mid-2005. Reynolds stress is used to explain the dynamic process of energy exchange. Furthermore, lag-correlation and linear regression analysis show that variability of the energy conversion rate and Reynolds stress involve responses to eddy acceleration at two time scales. The enhanced eddy acceleration induces large Reynolds stress, and enhanced Reynolds stress or barotropic instability further enforces energy transfer from the mean fl ow to the eddy fi eld.

Eddy properties in the Pacific sector of the Southern Ocean from satellite altimetry data

Mesoscale eddies play a key role in the ocean dynamics of the Southern Ocean, and eddy response to the climate changes has also been widely noted. Both eddy kinetic energy（EKE） and eddy detection algorithm are used to study the eddy properties in the Pacific sector of the Southern Ocean. Consistent with previous works,the maps of the EKE illustrate that higher energy confines to the Antarctic Polar Frontal Zone（APFZ） and decreases progressively from west to east. It also shows that the most significant increase in the EKE occurs in the western and central parts of the Pacific sector, where the baroclinicity of the Antarctic Circumpolar Current（ACC） is much stronger. Statistical eddy properties reveal that both of the spatial pattern and interannual variation of the EKE are primarily due to the eddy amplitude and the eddy rotational speed, rather than the eddy number or the eddy radius. In general, these results furtherly confirm that anomalous westerly wind forcing associated with the positive Southern Annular Mode（SAM） index enhances the Southern Ocean eddy activity by strengthening the eddy properties.

Large Eddy Simulation and Study of the Urban Boundary Layer

Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind velocity near the canopy is decreased, turbulence is intensified, TKE, variance, and momentum flux are increased, the momentum and heat flux at the top of the PBL are increased, and the development of the PBL is quickened; (3) the height of the roughness sublayer (RS) of the actual city buildings is the maximum building height (1.5-3 times the mean building height), and a constant flux layer (CFL) exists in the lower part of the UBL.

Wavelet spectrum analysis on energy transfer of multi-scale structures in wall turbulence

The streamwise velocity components at different vertical heights in wall turbulence were measured. Wavelet transform was used to study the turbulent energy spectra, indicating that the global spectrum results from the weighted average of Fourier spectrum based on wavelet scales. W＇avelet transform with more vanishing moments can express the declining of turbulent spectrum. The local wavelet spectrum shows that the physical phenomena such as deformation position in the boundary layer, and the or breakup of eddies are related to the vertical energy-containing eddies exist in a multi-scale form. Moreover, the size of these eddies increases with the measured points moving out of the wall. In the buffer region, the small scale energy-containing eddies with higher frequency are excited. In the outer region, the maximal energy is concentrated in the low-frequency large-scale eddies, and the frequency domain of energy-containing eddies becomes narrower.

Regional Characteristics of Typhoon-Induced Ocean Eddies in the East China Sea

The asymmetrical structure of typhoon-induced ocean eddies（TIOEs） in the East China Sea（including the Yellow Sea）and the accompanying air–sea interaction are studied using reanalysis products. Thirteen TIOEs are analyzed and divided into three groups with the k-prototype method： Group A with typhoons passing through the central Yellow Sea; Group B with typhoons re-entering the sea from the western Yellow Sea after landing on continental China; and Group C with typhoons occurring across the eastern Yellow Sea near to the Korean Peninsula. The study region is divided into three zones（Zones Ⅰ, Ⅱ and Ⅲ） according to water depth and the Kuroshio position. The TIOEs in Group A are the strongest and could reverse part of the Kuroshio stream, while TIOEs in the other two groups are easily deformed by topography. The strong currents of the TIOEs impact on the latent heat flux distribution and upward transport, which facilitates the typhoon development. The strong divergence within the TIOEs favors an upwelling-induced cooling. A typical TIOE analysis shows that the intensity of the upwelling of TIOEs is proportional to the water depth, but its magnitude is weaker than the upwelling induced by the topography. In Zones Ⅰ and Ⅱ, the vertical dimensions of TIOEs and their strong currents are much less than the water depths.In shallow water Zone Ⅲ, a reversed circulation appears in the lower layer. The strong currents can lead to a greater, faster,and deeper energy transfer downwards than at the center of TIOEs.

Double-averaging analysis of turbulent kinetic energy fluxes and budget based on large-eddy simulation

The turbulent flow over a channel bed roughened by three layers of closely packed spheres with a Reynolds number of Re= 15 000 is investigated using the large eddy simulation（LES） and the double-averaging（DA） method. The DA velocity is compared with the results of the corresponding laboratory experiments to validate the LES results. The existence of the types of vortex structures is demonstrated by the Q-criterion above the permeable bed. The turbulent kinetic energy（TKE） fluxes and budget are quantified and discussed. The results show that the TKE fluxes are directed downward and downstream near the virtual bed level. In the TKE budget, the form-induced diffusion rate is significant in the vicinity of the crest bed level, and the TKE production rate and the dissipation rate attain their peaks at the crest bed level and decrease sharply below it.

Surface available gravitational potential energy in the world oceans

Satellite altimetry observations,including the upcoming Surface Water and Ocean Topography mission,provide snapshots of the global sea surface high anomaly field.The common practice in analyzing these surface elevation data is to convert them into surface velocity based on the geostrophic approximation.With increasing horizontal resolution in satellite observations,sea surface elevation data will contain many dynamical signals other than the geostrophic velocity.A new physical quantity,the available surface potential energy,is conceptually introduced in this study defined as the density multiplied by half of the squared deviation from the local mean reference surface elevation.This gravitational potential energy is an intrinsic property of the sea surface height field and it is an important component of ocean circulation energetics,especially near the sea surface.In connection with other energetic terms,this new variable may help us better understand the dynamics of oceanic circulation,in particular the processes in connection with the free surface data collected through satellite altimetry.The preliminary application of this concept to the numerically generated monthly mean Global Ocean Data Assimilation System data and Archiving,Validation,and Interpretation of Satellite Oceanographic altimeter data shows that the available surface potential energy is potentially linked to other dynamic variables,such as the total kinetic energy,eddy kinetic energy and available potential energy.

干湿对流大气边界层大涡模拟及模式收敛性分析

利用中尺度天气研究预报模式WRF中的大涡模拟(LES)版本对理想干对流边界层和浅对流云覆盖的海洋性大气边界层结构和湍流输送特征进行大涡模拟,并通过不同水平分辨率大涡模拟结果分析了大涡模式的收敛性。结果表明:不同分辨率大涡模式模拟的干对流边界层高度约为1.15 km,位温、水汽混合比和水平风速等平均变量在边界层内均匀分布,但在近地层和边界层顶垂直梯度较大。随模式分辨率的提高,可分辨出更精细的对流泡结构,夹卷层的位温方差增大,模拟的混合层平均物理量均匀分布更接近地面,与粗分辨率下大涡模拟的偏差主要集中在边界层下层和逆温层附近,解析热通量增加,但总热通量保持不变。不同于干对流边界层,在有浅对流云覆盖的边界层内,混合层高度较低,混合层内平均气象要素垂直廓线和方差或通量廓线与干对流边界层内相似,低层混合层内平均气象要素呈垂直均匀分布,但由于混合层以上存在条件不稳定云层,云层内有正热通量和正垂直湍流动能,而混合层顶到浅对流云底部为负热通量,反映了混合层顶夹卷过程作用。较粗分辨率的大涡模式增加了逆温层顶、近地层平均风速和通量的模拟偏差。湍流发展旺盛的干对流边界层,大涡模式在约40 m的水平分辨率上收敛,而对混合层高度较低的浅积云覆盖边界层,模式则在约30 m水平分辨率上收敛,具有较低混合层高度的大气边界层需要更高分辨率的大涡模式描述。

Spatial and spectral investigation of turbulent kinetic energy in cavitating flow generated by Clark-Y hydrofoil

In the present paper,the turbulent cavitating flow generated by a Clark-Y hydrofoil is investigated by large eddy simulation(LES)combined with Zwart-Gerber-Belamri(ZGB)cavitation model.In order to shed light on the influence of cavitation on turbulent energy distribution among scales,energy spectrum obtained from the three-dimensional velocity field is firstly applied to turbulent cavitating flow.Spatial and spectral distributions of turbulent kinetic energy are studied for both non-cavitating flow and cavitating flow.Cavitation is found to have a significant effect on the original turbulent flow by inducing more large-scale turbulent structures.The energy spectrum of cloud cavitating flow also experiences a periodic evolution as cavitation develops,and a large amount of turbulent kinetic energy is found to generate as the first shedding,cutoff and second shedding of cavities happen.

海洋中尺度涡的机械能及其源汇研究

由于观测手段的限制,海洋中尺度涡的能量输送及生成与耗散动力过程一直是世界大洋能量循环中悬而未决的一个问题,而中尺度涡在海洋中又广泛存在,它不仅占据海洋表层动能的绝大部分,而且是能量级串中连接大尺度和小尺度的中间环节,在大洋能量传输中扮演着重要的角色。文章以中尺度涡场机械能的大小、分布、源汇为主线,结合作者在这方面的最新研究成果,综述当前对中尺度涡机械能及其源汇的研究进展。

采用大涡PIV方法研究搅拌槽内湍流动能耗散率

在槽径为0.476m的六直叶涡轮桨搅拌槽内,采用粒子图像测速仪(PIV)对桨叶区的流场进行了实验研究,得到了桨叶区的平均流速和湍流动能(k)分布,采用大涡PIV方法对湍流动能耗散率(ε)分布进行了估算,计算了ε与k的相关系数.结果表明大涡PIV方法能有效地估算ε分布;桨叶区的射流向上倾斜,两尾涡分布于射流两侧,射流的倾角和两尾涡中心间距随射流向壁面运动而变化,射流倾角先增大再减小,相位角θ=40o时达到最大值13.2o,两尾涡中心间距先减小再增大,θ=20o时达到最小值0.0387(用槽径T无因次化);湍流动能和湍流动能耗散率峰值均位于尾涡靠近射流的区域;湍流动能和湍流动能耗散率的平均相关系数为0.363,射流核心区相关系数小于周边区域.

海洋中尺度涡旋源汇空间分布特征研究

借助卫星高度计数据,对中尺度涡进行识别和追踪;以16年内中尺度涡个数上的生消为判据,发现中尺度涡在除赤道外的全球大洋中生消频繁,但在海盆内区并没有明显的生成占优区或消亡占优区;而在中纬度近岸的狭窄东边界内中尺度涡生成居多,在另一侧近岸的狭窄西边界中尺度涡消亡居多。同时,我们以一阶斜压模态所对应的特征深度作为两层结构的内界面深度,并假定涡动能平均分配于正压模态和一阶斜压模态,计算得到了包含涡动能和涡有效重力位能的中尺度涡能量年平均净生成率和净耗散率,发现虽然海盆西边界是涡场能量耗散大于能量生成的区域,但强耗散过程实际上在海盆西侧内区的强流及其回流区均有发生。另外,中尺度涡生消个数差值的分布与中尺度涡能量净生成率和净耗散率的分布表明,虽然海盆东边界近岸区域内中尺度涡的生成居多,但该区域中尺度涡的能量偏弱,因此该区并非涡场能量的主要源区。

青藏高原高空流型对西太平洋台风路径影响的诊断分析

利用观测研究，动力诊断分析等手段，从上下游效应、中低纬相互作用的角度来探讨青藏高原高空天气系统的变化与西太平洋台风运动两者之间的关系．１９７０～１９９５ 年２５ 年间的统计结果表明，青藏高原高空流型与台风路径有如下关系：高原高空５００ ｈＰａ 为低值系统控制时，有利于台风西行；反之，高原高空５００ ｈＰａ 为高压时，近海台风往往转向．动力诊断分析的结果揭示了高原上空系统影响下游系统的物理机制，即高原上游扰动动能的传递使得下游的槽发展，并进一步影响台风的引导气流．高原脊的存在，使得涡动动能的输送通道偏北；高原上为槽时，涡动动能的输送通道偏南．高原槽前的南风和台风东侧南风将低纬度的低位涡输入副热带高压，有利于副热带高压的发展，影响台风运动，体现了中低纬相互作用对天气系统的影响．

代数亚格子湍动能模型及非黏性亚格子应力模型在喷雾大涡模拟中的应用

在KIVA3V中将考虑简化的代数亚格子湍动能模型的3种亚格子尺度模型和K方程湍动能模型的非黏性亚格子应力模型纳入程序中,对定容弹中柴油的喷射和雾化过程进行了数值研究.以燃油液相和蒸气相分布的试验数据为依据,揭示了代数亚格子尺度湍动能模型对燃油喷雾计算结果的影响,并且与K方程亚格子湍动能模型进行了对比,同时分析了加入的非黏性亚格子应力张量模型的适用性.结果显示3种亚格子应力模型及代数亚格子湍动能模型的组合可以有效地模拟流场中湍动能变化对液滴弥散速度的影响,从而使液相贯穿距及液相和气相浓度分布与试验结果更为接近.同时,该代数模型所计算的亚格子尺度湍动能与K方程模型计算结果相当吻合.非黏性亚格子模型可以计算相对准确的喷雾结果.