Asymptotic behaviour of a turbulent mixing path near a wave layer

The paper reports on a class of models for a turbulent boundary layer which are based on the turbulent energy balance equation and closure and use hypothesis of the type of Karman relations for turbulent mixing paths or complementary transport equations. It is shown that in the absence of a velocity shear near the kinetic turbulent energy flux all the models are encompassed by a unique hypothesis and satisfy with depth a linear dependence of the mixing path.Translated by Vladimir A. Puchkin.     

Energetics of wind-induced turbulent mixing in the ocean

The pattern and magnitude of the global ocean overturning circulation is believed to be strongly controlled by the distribution of diapycnal diffusivity below 1000 m depth. Although wind stress fluctuation is a candidate for the major energy sources of diapycnal mixing processes, the global distribution of wind-induced diapycnal diffusivity is still uncertain. It has been believed that internal waves generated by wind stress fluctuations at middle and high latitudes propagate equatorward until their frequency is twice the local inertial frequency and break down via parametric subharmonic instabilities, causing diapycnal mixing. In order to check the proposed scenario, we use a vertically two-dimensional primitive equation model to examine the spatial distribution of “mixing hotspots” caused by wind stress fluctuations. It is shown that most of the wind-induced energy fed into the ocean interior is dissipated within the top 1000 m depth in the wind-forced area and the energy dissipation rate at low latitudes is very small. Consequently, the energy supplied to diapycnal mixing processes below 1000 m depth falls short of the level required to sustain the global ocean overturning circulation.     

夏季长江河口层化与湍流混合特征分析

利用2016年夏季长江河口现场水文特性与湍流微结构观测资料, 分析了长江河口水体温盐结构、层化发育、湍流与混合特征。结果表明: 1)夏季长江河口水体密度层化结构明显, 根据各层水体密度梯度差异, 可将水体分为底部混合层和上层密度跃层, 两部分的密度层化界限与浮力频率等值线lg N ² = - 4.0接近。2)底部混合层湍动能耗散率大, 层化结构弱, 水体分层稳定性弱; 上层密度跃层湍动能耗散小, 层化结构强, 水体分层稳定性强, 这有利于河口内波的发育与传播。3)在密度层化的作用下, 水体的湍动能耗散率、湍动能剪切生成及浮力通量的能量关系在一定范围内符合湍动能局部能量平衡方程。不同层之间的湍流弗劳德数Fr_t和湍流雷诺数Re_t在Fr_t-Re_t平面上呈现明显的分区, 与经典的分层剪切流理论基本吻合。     

Quantitative evaluation of turbulent mixing in the Central Equatorial Pacific

Turbulent mixing in the central equatorial Pacific has been quantitatively evaluated by analyzing data from microstructure measurements and conductivity temperature depth profiler (CTD) observations in a meridionally and vertically large region. The result that strong turbulent mixing with dissipation rate ε (>O(10^?7) W kg^?1), continuing from sea-surface mixed layer to low Richardson number region below, in the area within 1° of the equator, shows that turbulent mixing has a close relationship to shear instability. ε > O(10^?7) W kg^?1 and turbulent diffusivity K _ρ  > O(10^?3) m² s^?1 were obtained from near-surface to 85 db at stations even southwardly beyond 3°S, where it is already far from the southern boundary (~2°S) of the Equatorial Undercurrent. Turbulence-induced heat flux and salinity flux were calculated, and both had their maxima in the equatorial upwelling region, though the former was downward and the latter was upward. Accordingly, vertical velocity in the upwelling region was estimated to be similar to the results derived by other methods. These fluxes and the vertical velocity suggest the critical importance of turbulent mixing in maintaining the well-mixed upper layer. Secondly, in the intermediate region (>500 db), turbulent eddies were investigated by applying Thorpe’s method to the CTD data. A large number of overturns were detected, with spatial-averaged K _ρ (700–1,000 db) being 3.3 × 10^?6 m² s^?1, and the corresponding K _ρ-max reaching to O(10^?4) m² s^?1 in the north (3°–13°N). The results suggest that, in the intermediate region, considerable turbulent mixing occurs and moderates the properties of the water masses.     

Estimates of the turbulent kinetic energy budget in the oceanic convective boundary layer

The terms of the steady-state turbulent kinetic energy (TKE) budget in the oceanic convective boundary layer (CBL) are estimated by use of microstructure data obtained over the continental shelf of the East China Sea. The dissipation term is calculated from the micro-scale vertical shear of horizontal velocity measured directly using a freely-falling microstructure profiler, whereas the buoyancy flux and shear production terms are estimated indirectly by integrating vertically the one-dimensional conservation equation of density and by applying similarity theory, respectively. The transport term, calculated as the residual of the other three terms, vertically redistributes the TKE from the upper half of the CBL to the lower half, consistent with the TKE budgets in the atmospheric CBL and in shear-free and slightly-sheared CBLs simulated by large eddy-simulation models. The relatively large contribution of the transport term to the TKE budget shows that a local equilibrium form of the TKE equation is not appropriate for the TKE budget in the oceanic CBL.     

Evaluation of spatial distribution of turbulent mixing in the central Pacific

A long-term mean turbulent mixing in the depth range of 200–1000 m produced by breaking of internal waves across the middle and low latitudes (40°S–40°N) of the Pacific between 160°W and 140°W is examined by applying fine-scale parameterization depending on strain variance to 8-year (2005–2012) Argo float data. Results show that elevated turbulent dissipation rate (ε) is related to significant topographic regions, along the equator, and on the northern side of 20°N spanning to 24°N throughout the depth range. Two patterns of latitudinal variations of ε and the corresponding diffusivity (K_ρ) for different depth ranges are confirmed: One is for 200–450 m with significant larger ε and K_ρ, and the maximum values are obtained between 4°N and 6°N, where eddy kinetic energy also reaches its maximum; The other is for 350–1000 m with smaller ε and K_ρ, and the maximum values are obtained near the equator, and between 18°S and 12°S in the southern hemisphere, 20°N and 22°N in the northern hemisphere. Most elevated turbulent dissipation in the depth range of 350–1000 m relates to rough bottom roughness (correlation coefficient?=?0.63), excluding the equatorial area. In the temporal mean field, energy flux from surface wind stress to inertial motions is not significant enough to account for the relatively intensified turbulent mixing in the upper layer.     

Chaotic transport and mixing of a passive admixture by vortex flows behind obstacles

A two-layer nonviscous model of chaotic advection in a unidirectional pulsating running current above a delta-shaped underwater elevation is considered. The property of local stability is used and a characteristic similar to the cumulative Lyapunov exponent is introduced that makes it possible to determine the range of regular and chaotic particle behavior. The estimates obtained using this characteristic are for clarifying passive admixture transport in analog model problems. Knowledge of the maximum chaotization region boundary is important for oceanology in view of interpreting point vortices as a model of distributed vortices. The criterion based on using the cubic Hamiltonian approximation for a nonlinear resonance model is introduced to estimate the limiting boundary of the regular region.     

The mixing efficiency and the processes of restratification in a stably stratified medium

The mechanism of the effect of a collapsing turbulent eddy on diapycnal transport in a stably stratified fluid is considered. It is shown that at small Richardson turbulent numbersRi ₀ the mixing efficiency increases asRi ₀, and at large numbers it decreases in proportion toRi ₀ ^–1/2 .Translated by Mikhail M. Trufanov. UDK 551.465.15.     

南海南部上层的湍流混合特征

A turbulent microstructure experiment was undertaken at a low latitude of 10°N in the South China Sea in late August 2012. The characteristics of the eddy diffusivity above 650 m were analyzed, which is one order of magnitude larger than that in the open ocean at that low latitude. Enhanced eddy diffusivities by strong shears and sharp changes in topography were observed. The strongest eddy diffusivity occurred in the mixed layer, and it reached O(10–2 m2/s). Strong stratification in the thermocline inhibited the penetration of surface eddy diffusivities through the thermocline, where the mixing was weakest. Below the thermocline, where the background eddy diffusivity was approximately O(10–6 m2/s), the eddy diffusivity increased with depth, and its largest value was O(10–3 m2/s).     

渤海弱层化期湍流混合观测特征分析

利用2017年9月在渤海共享航次中取得的湍流混合直接观测数据,本文研究了渤海海域湍流混合的空间分布特征及有关的影响因素。9月观测海区水体垂向层结较弱,莱州湾受黄河冲淡水影响出现高温低盐结构,位于渤海中央浅滩南北两侧洼地的双中心冷水结构依旧存在。湍流观测结果表明湍动能耗散率在10^-9～10^-5W/kg之间变化,统计上满足对数正态分布。耗散率强值区出现在辽东湾及渤海湾湾口近岸处,相应的垂向湍扩散系数约为10^-6～10^-2m²/s。垂向上,水体表、底层混合较强,进一步研究发现弱层化水体的平均湍动能耗散率〈ε〉与风速和正压潮流速的大小存在正相关关系。另一方面,耗散率ε与浮性频率N近似满足ε=2.0×10^-8+3.0×10^-7(N²/N₀²)^-5的拟合函数关系,反映了层化对水体垂向混合的抑制作用。     

Photoadaptation in a convective layer

Measurements of Lagrangian trajectories acquired with neutrally buoyant floats in an upper ocean mixing layer during a five-day period of nearly cloudless and calm conditions, provide an opportunity for investigating the implications of photo-adaptation of phytoplankton exposed to varying light intensities as they traverse the water column in convective plumes. The predominant convective motions develop over night and persist through the morning until giving way to thermal restratification. Despite the paucity of observed trajectories, their essential characteristics are extracted and used to generate a large number of pseudo-tracks having characteristics similar to those measured at any given depth and time of day. A Monte Carlo simulation is run for a sample set of biological variables to identify predominant features of the production. For each pseudo-trajectory the production is calculated from a photo-adaptive model in order to identify primary features of the instantaneous and accumulated production. The simulation illustrates the way in which photo-inhibition can reduce production near the surface during the morning, developing a subsurface layer of increased production that progressively deepens to approximately 12 m at noon. The surface production is suppressed during the afternoon, but subsequently recovers as light levels drop below the photo-inhibition threshold.     

Boltzmann-Jaynes variational method and the temperature distribution of thermals in the turbulent convective atmospheric surface layer

A statistical mechanism that explains the formation of probability distribution functions of thermals according to temperature fluctuations is considered. In the proposed approach based on the Boltzmann-Jaynes variational method, a statistical ensemble of convective thermals is characterized by a class of stationary probability densities that depend on temperature fluctuations. It is assumed that the probability density functions of this class may depend on the potential energy, as well as on the available potential energy. For a class of stationary probability density functions, the entropy functional is defined to be an analogue of the Boltzmann H-entropy. The equilibrium distributions of thermals according to temperature fluctuations correspond to the most probable distributions that yield a maximum of the entropy functional. The exponential and normal distributions of thermals according to temperature fluctuation that are constructed using the variational method quite adequately approximate field atmospheric observations, as well as the results of laboratory modeling.     

Observations of turbulent mixing and vertical diffusive salt flux in the Changjiang Diluted Water

Based on micro structure measurements from a repeated sampling station southwest of the Jeju Island during summer,we studied the hydrography,pycnocline turbulence,and vertical salt flux in the Changjiang Diluted Water(CD W).The water column was well stratified with the CDW occupied the surface～20 m.Most of the large turbulent kinetic energy dissipation rate(ε) were found in the bottom boundary layer.Interestingly,intermittent strong turbulence(ε>10^-6 W/kg) occurred in the pycnoclin...     

Observed characteristics of flow,water mass,and turbulent mixing in the Preparis Channel

Preparis Channel is the very important exchange path of energy and materials between the northern Bay of Bengal and Andaman Sea(AS). A set of hydrographic measurements, a microstructure profiler, and a deep mooring were used to determine the characteristics of water masses, turbulent mixing, and flows in the Preparis Channel. The unprecedented short-term mooring data reveal that a deep current in the deep narrow passage(below 400 m) of the Preparis Channel flows toward the Bay of Bengal(BoB) wit...     

Roles of vertical turbulent mixing in the ocean response to Typhoon Rex (1998)

How the role of vertical turbulent mixing (VTM) in sea surface cooling (SSC) varies with the moving speed of a tropical cyclone was examined for Typhoon Rex (1998) by using the Meteorological Research Institute Community Ocean Model (MRI.COM). The MRI.COM well reproduced TRMM/TMI three-day mean sea surface temperature (SST) fields along Rex’s track. During the fast-moving phase of Rex, SSC simulated by the MRI.COM was caused by shear-induced VTM on the right side of the track. During the slowly-moving phase, on the other hand, the Ekman-pumping area mostly overlapped the VTM area right behind Rex’s center. During the recurvature phase, cool water transported by the upwelling was more efficiently entrained into a mixed layer by the VTM for nearly a 1 near-inertial period after the passage of Rex. We then modified the entrainment formulation of Deardorff (1983), which was incorporated into a slab mixed-layer ocean model (SOM) so as to fit to the results simulated by the MRI.COM. The principal modifications are as follows: (1) consideration of turbulent kinetic energy (TKE) production caused by surface wave breaking; (2) increase in the coefficient for estimating dissipation to balance with TKE production due to turbulent transport; and (3) changing the initial guess for the critical Richardson number. These modifications led to an improvement of SST simulations by the SOM. The impact of the modifications on simulated SSTs turned out to be more significant than the impacts of initial mixed-layer depth and the difference between diurnally-varying and daily mean short-wave radiation.     

Microstructure measurements and finescale parameterization assessment of turbulent mixing in the northern South China Sea

Both microscale and finescale measurements were conducted along 20°N and 21°N in the northern South China Sea (SCS) during July 2007. Spatial variability of turbulent kinetic energy (TKE) dissipation rate was examined, and two finescale parameterizations were assessed and compared. TKE dissipation rates along the 21°N section were found to be much higher than those along 20°N; in particular, remarkably high TKE dissipation rates existed near the Luzon Strait and around the Dongsha Plateau, which were likely caused by internal tides and internal solitary waves, respectively. The Gregg–Henyey scaling does not work well in the northern SCS, while the MacKinnon–Gregg scaling with a modified parameter matches the observations in both magnitude and variability. One explanation is that the large-scale/low-mode shear mainly comes from low-frequency internal waves such as internal tides, which are not described well by the Garrett–Munk spectrum.     

南海中部上层海洋湍流混合的空间分布特征及参数化模型

通过对2010年5月南海16°N和14.5°N断面的湍流微结构剖面观测资料分析,给出了南海海盆上层湍流混合空间分布特征:在16°N断面上,上层10~400m垂向平均湍动能耗散率ερ在东侧略大于西侧;相反,在14.5°N断面上,西侧ερ均值约是东侧ερ的4倍,其中,西侧110.5°~111°E的ερ的平均值为2.6×10-6 W/m3,东侧118.5°E的ερ仅为5.89×10-7 W/m3。通过分析细结构剪切和湍流混合的相关性,发现剪切是南海中部上层强湍流混合的主要驱动力,揭示了高模态内波破碎可能是湍流混合的主要机制。另外,研究了大洋中的3种参数化模型,发现适用于大洋近海的参数化MacKinnon-Gregg(MG)模型能较好地用浮频和剪切估算南海中部深海区上层湍流耗散率。     

南海北部中尺度反气旋涡的湍流混合空间分布特征

文章利用GHP细结构参数化方法和Thorpe-scale方法,分析水下滑翔机于2015年5月在南海北部采集的数据,估算了南海北部中尺度反气旋涡的湍流混合空间分布特征。结果显示该反气旋涡的混合具有明显的空间非对称性,混合率在其运动方向的后侧边缘明显增强达到O(10^-3 m²/s)量级;而在其运动方向的前侧边缘,平均混合率要小一个量级。这一混合非对称特征与中尺度的涡动能密切相关性。中尺度涡后侧边缘处存在高流速剪切,容易引起垂向剪切不稳定,可能是引起该处混合增强的主要因素。另外,中尺度涡后侧边缘发展的次中尺度过程同样导致了该处强混合。本研究结果有助于人们进一步认识南海北部的混合过程。     

变性绕极深层水入侵时南极普里兹湾湍流混合特征研究

基于中国第28、29和31次南极科学考察中的CTD数据,利用Thorpe尺度方法计算了普里兹湾及其附近海域湍动能耗散率,分析了其分布特征,并对当地的水团结构进行研究.结果表明,普里兹湾及其附近海域中,前两个航次观测中次表层湍动能耗散率强度在陆架坡折区域达到最大.在水团分布方面,在第28和29航次中均观测到了变性绕极深层...