On similarity profiles predicted in the convective marine boundary layer

On the basis of the ideal of local scale similarity theory, the profile equations of wind, temperature and humidity for the eonvective marine boundary layer have been obtained. The marine boundary layer measurements were made over the western Pacific Ocean as past of the Tropical Ocean and Global Atmosphere (TOGA) Programme during Nov. 1986-Feb. 1987. The similarity profiles predicledfor wind. temperature and humidity in the MBL are in good agreement with the observational data.     

Full scale wake and boundary layer instrumentation feasibility study

Fluid-flow measuring devices intended for full scale use were designed and tested. One device consisted of a rake supporting different types of pitot tubes. This will be used to obtain a velocity profile of the boundary layer of a test ship. The other device was a five-hole pitot tube that was used in a wake survey. The pressure transport medium (i.e. the medium that carried the pressure signal from the pitot tube to the pressure transducer) was air instead of water. This proved to be successful, as the results indicate.     

On the vertical lifting of dust in a convective unstable atmospheric boundary layer

A model for the density Q of vertical mass flux of sand (dust) in the convective atmospheric boundary layer as a function of the number density N of convective elements (including vortices), friction velocity u _*, and vertical (turbulent) buoyancy flux B is proposed. It is shown that the flux Q is proportional to the product of the square root of B and the sixth power of u _*. This finding is consistent with empirical dependences Q(u _*) reported in the literature. We discuss two methods for experimentally determining density N when the lifting of dust occurs, mainly due to (terrestrial and Martian) dust devils.     

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.     

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.     

Electricity of the convective atmospheric boundary layer: Field observations and numerical simulation

The electric state of the middle-latitude convective atmospheric boundary layer (ABL) is investigated during a point in time when it is not disturbed by clouds, precipitation, mist, or industrial aerosols. A numerical model is developed that estimates the electroaerodynamic state of the convective ABL. The model is parameterized using results of field observations and laboratory experiments. According to the model, vertical profiles of atmospheric electric field strength, space charge density, electric conductivity, and atmospheric electric current density are calculated in the horizontally homogeneous approximation with a high space-time resolution.     

一次由气旋发展与边界层东北气流触发的强对流天气分析

利用多种探测资料及NCEP/NCAR FNL 1°×1°再分析资料,对2019年4月24日发生在山西的大范围强对流天气进行了分析。结果表明:1)此次过程是在弱天气尺度强迫背景下,地面锋面气旋发展和低层偏东北气流伸入河套地区,触发了1个持续拉长状对流系统(persistent elongated convective systems,PECS)和1个β中尺度持续拉长状对流系统(meso-β-scale PECS,MβECS)发生发展造成的。2)与MβECS对应,雷达回波上表现为涡旋状的回波中镶嵌着多个对流单体,PECS则表现为4个线状回波和1个强降水单体风暴。雷达产品能更精细刻画较小尺度系统特征,但分类强对流的某些典型特征并不明显。3)物理量诊断揭示,低层锋生作用不仅使暖锋加强触发MβECS发展造成北部强对流,且使得冷锋加强和气旋发展,此背景下形成的边界层急流和地面中尺度系统导致中南部对流单体合并、加强并高度组织化。强对流范围和强度与涡旋或辐合线尺度及风场辐合强度密切相关,气旋内温压风湿场的扰动特征能更好地解释较小尺度系统形成发展的物理机制,且这些特征较强对流提前1～3 h出现,对强对流临近预报具有很好的指示意义。4)低层东北气流是干冷与暖湿空气的一个倾斜交界面,该面上各种要素并不均匀,围绕该支气流形成一个气旋式次级环流圈,是中尺度对流系统的重要触发机制;气流两侧存在较大纬向风垂直切变,是造成对流风暴传播、持续时间长的重要原因之一。     

Study on the characteristics of the marine boundary layer in the Equatorial Pacific

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海面状态参数化对海上大气边界层数值模拟的影响研究

由于海上大气边界层的下垫面是复杂多变的海面,因此海面状态的参数化对海上大气边界层数值模拟起到十分关键的作用.采用高分辨率的大气边界层模式进行敏感性试验研究,讨论了不同海面参数化方案对海上大气边界层数值模拟产生的影响.结果表明:几种海面粗糙度参数化方案在中低风速下模拟垂直结构主要的差异出现在边界层和自由大气分界面上;考虑...     

Non-sea-salt sulfate in the marine boundary layer and its possible impact on chloride depletion

1Introduction Sulfurisoneofthemostimportantelements whosebiogeochemicalcyclesarepivotaltotheatmo- sphericenvironmentandtotheclimaticchanges.The predominant,stablechemicalspeciesofsulfurinthe atmosphereissulfuricacidorsulfatewhichunderor- dinaryconditionsexistsmainlyinsmallaerosolparti- cles(Chinetal.,1996;Kerminenetal.,2000;Restadet al.,1998).Sulfuricacidorsulfate-dominatedaerosols areubiquitousintheloweratmosphere.Inthemarine boundarylayer(MBL),non-sea-saltsulfuricacidorsulfate(nss-SO42-)…     

浅化波浪层流边界层流速分布特性的数值分析

建立了同时考虑波致雷诺应力和时均水平压强梯度影响的二阶波浪边界层数学模型,模型计算得到的浅化波浪层流边界层内瞬时流速剖面、振荡速度幅值和时均流速剖面均与水槽实验数据吻合较好,在此基础上探讨了浅化波浪边界层流速分布特性及其影响机制。随着波浪的浅化变形,边界层内时均流速剖面"底部向岸、上部离岸"的变化特征越来越明显。这是二阶对流项引起的波致雷诺应力和离岸回流引起的时均水平压强梯度共同作用的结果,在床面附近由波致雷诺应力占主导作用并趋于引起向岸流动,在上部区域由时均水平压强梯度占主导作用并趋于引起离岸流动。     

Spatial spectra and characteristic horizontal scales of temperature and velocity fluctuations in the convective boundary layer of the atmosphere

Using a high-resolution LES numerical model, we calculated the turbulent thermal convection for high ratios of horizontal and vertical sizes of the computational domain (26: 26: 1). The natural analog of the simulated process is a planetary boundary layer (PBL) of the atmosphere growing with height in the background of stably stratified overlying air layers over a horizontally homogeneous heated surface under a weak average wind. We obtained the spectral distributions of variances of fluctuations in potential temperature and velocity components in ranges corresponding to scales from a few tens of meters to a few tens of kilometers. We found energetically significant segments of the spectrum of large-scale fluctuations in the potential temperature for which the power dependences S ～ k ^?1/3 and S ～ k ^?4/3 are satisfied with good accuracy. We calculated the characteristic spatial scales of horizontal fluctuations in velocity and temperature. We obtained a dependence of these scales on the height of the growing convective PBL. We discuss the characteristic features of large-scale distributions in terms of the self-similarity of the growing boundary layer behavior.     

Mass transport in a two-layer wave boundary layer

The transport of a chemical species under the pure action of surface progressive waves in the benthic boundary layer which is loaded with dense suspended sediments is studied theoretically. The flow structure of the boundary layer is approximated by that of a two-layer Stokes boundary layer with a sharp interface between clear water and a heavy fluid. The simplest model of constant eddy diffusivities is adopted and the exchange of matter with the bed is ignored. For a thin layer of heavy fluid, whose thickness is comparable to the surface wave amplitude and the Stokes boundary layer thickness, effective transport equations are deduced using an averaging technique based on the method of homogenization. The effective advection velocity is found to be equal to the depth-averaged mass transport velocity, while the dispersion coefficient can be shown to be positive definite. Explicit expressions for the transport coefficients are obtained as functions of fluid properties and flow kinematics. Physical discussions on their relations are also presented.     

Numerical study on scale effect of form factor

The scale effect of form factor is investigated via a numerical approach in this paper, where the turbulent ship flow is computed by solving the steady and incompressible Reynolds-averaged Navier-Stokes and continuity equations. A wall function approach is employed to bridge the near-wall and outer turbulent flow region. The numerical scheme based on a finite-volume formulation is applied to discretize the coupled governing equation. For the sake of numerical stability, accuracy and economy, an identical grid is employed to compute ship flow at different Reynolds number, where the grid is optimized for the medium Reynolds number of the investigated range. Four surface ships and two sub-bodies with notably different geometrical characteristics are chosen as the investigated cases, where double-model flow without appendages is computed. The calculated total resistance coefficient shows a decreasing tendency against Reynolds number among all studied hulls. Similar to the calculated total resistance coefficient, the calculated friction resistance coefficient decreases with the Reynolds number and varies relatively little for a given Reynolds number among different hulls. The viscous pressure resistance coefficient is less insensitive to the Reynolds number but apparently depends on hull form. Compared with the form factor calculation based on empirical friction lines, the flat-plate friction prediction based on CFD approach clearly gives smaller Re-dependent form factor, which should more realistically reflect the scale effect of form factor. The form factor exhibits a near linear and increasing dependence on Reynolds number. The numerical results show that the dependence of r_P on Reynolds number mainly governs the scale effect of form factor.     

珠江河口底边界层湍流积分尺度研究

介绍湍流积分尺度的几种常用计算方法.基于三组珠江口崖门的湍流观测资料,分别计算了一个潮周期内珠江口底边界层湍流积分尺度的变化,并对比不同计算方法的稳定性.计算结果表明,基于Taylor假定的自相关函数积分法的计算结果稳定可靠.湍流积分尺度在一个潮周期内的变化很大,积分尺度的变化与平均流的流速变化有着很好的对应关系,三个测次纵向、横向和垂向积分尺度的平均值分别约为4.9～10.4 cm、3.3～6.2 cm和4.3～5.2 cm.     

太平洋涌浪分布特征及其对海-气边界层的影响

本文对海-气边界层波致风机制的相关理论进行了阐述,并利用ERA-40再分析资料给出了太平洋谱峰速度、波龄、波陡等描述涌浪和波致风机制物理量的年际和季节空间分布特征。分析表明:东太平洋赤道地区等海域涌浪速度最大且涌浪由南向北传播明显;太平洋波边界层高度基本呈现出东高西低的分布形势;波致风机制主要发生在赤道热带海域,北半球夏季波致风机制偏强,冬季偏弱,南半球反之;北半球北部海域夏季更易发生波致风机制,赤道附近海域相反;南海为风浪与涌浪组成的混合浪,对其波候等相关研究有必要分开进行讨论。     

The concentration and distribution of dimethyl sulfide in the marine atmospheric boundary layer near the equator

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Wave boundary layer over a stone-covered bed

This paper summarizes the results of an experimental investigation on wave boundary layers over a bed with large roughness, simulating stone/rock/armour block cover on the sea bottom. The roughness elements used in the experiments were stones the size of 1.4cm and 3.85cm in one group of experiments and regular ping-pong balls the size 3.6cm in the other. The orbital-motion-amplitude-to-roughness ratio at the bed was rather small, in the range a/k_s = 0.6–3. The mean and turbulence properties of the boundary-layer flow were measured. Various configurations of the roughness elements were used in the ping-pong ball experiments to study the influence of packing pattern, packing density, number of layers and surface roughness of the roughness elements. The results show that the friction factor seems to be not extremely sensitive to these factors. The results also show that the friction factor for small values of the parameter a/k_s does not seem to tend to a constant value as a/k_s → 0 (contrary to the suggestion made by some previous investigators). The present friction-factor data indicates that the friction factor constantly increases with decreasing a/k_s. An empirical expression is given for the friction factor for small values of a/k_s. The results further show that the phase lead of the bed friction velocity over the surface elevation does not seem to change radically with a/k_s, and found to be in the range 12°–23°. Furthermore the results show that the boundary-layer turbulence also is not extremely sensitive to the packing pattern, the packing density, the number of layers and the surface roughness of the roughness elements. There exists a steady streaming near the bed in the direction of wave propagation, in agreement with the existing work. The present data indicate that the steady streaming is markedly smaller in the case where the ping-pong balls are aligned at 45° to the wave direction than in the case with 90° alignment. Likewise, it is found that the steady streaming is relatively smaller in the case of the one-layer ping-pong-ball roughness than in the case of the two-layer situation.     

An experimental study of turbulent boundary layer with slit suction

Experiments are performed on a flat plate with a transverse suction slit in the Reynolds number range 5 × 10⁵ < R_e < 1.1 × 10⁶. Mean velocity profiles, RMS values are measured with hot wire anemometry. Friction velocity is numerically calculated. The experiments showed that a classical boundary layer parameter α is related to the suction coefficient S_c with an equation of the form: .The value of A seems to depend strongly on the relative location with respect to suction slit and possibly weakly on Reynolds number.