对流边界层顶部特性的对流槽实验模拟研究

利用对流槽研究对流边界层的顶部特性.实验结果表明,产生于混合层的上冲热泡可在对流边界层顶上的覆盖逆温层中激发出重力波;夹卷层的湍流结构表现出各向异性,水平尺度大于垂直尺度,与混合层中的湍流结构明显不同;利用可视图像直观显示了夹卷过程,夹卷层的温度谱表现出独特的结构特征,湍流能谱有明显的分区现象,谱幂率与现有理论分析结果有较大偏离.在实验结果的基础上,提出了关于夹卷速率的新参数化方案,夹卷速率可由地面热通量、混合层高度和覆盖逆温强度确定,方案中的系数C由实验测量数据拟合得出:C=0.95,与Deardroff等的对流槽实验数据的拟合结果(C=1.11)非常接近.     

白天混合层顶部夹卷层厚度的特征研究

本文首先对Deardorff的一阶模型给予解释，在此基础上分析对流边界层湍流动能方程，分析机械湍流和对流湍流对边界层发展的贡献，提出一个新的速度尺度，混合层顶速度尺度，定义了全理查森数，给出夹卷层厚度的参数化方案，并用Boers和Elotanta的雷达观测数据进行验证。参数化方案与实验数据符合得很好。当夹卷层厚度表示为夹卷速度或夹卷理查森数的函数时，该函数曲线随边界层发展通常表现为磁滞回线形状现象，利用本文的理论进行了解释。     

湍流湿度脉动特性研究——根据淮河流域实验资料

利用淮河流域实验得到的湍流湿度脉动资料 ,应用统计理论、Fourier分析和小波分析方法 ,对湍流湿度脉动的统计特征、谱特征和间歇性特征作了初步分析 .结果表明 :(1)在淮河流域这样的高湿地区 ,水汽对其地表能量平衡 ,能量输送与循环具有重要作用 ;(2)湍流湿度脉动的能谱和互谱在惯性副区基本符合“ - 2 /3”次律和“ - 4/3”次律 ;(3)淮河流域湍流湿度脉动场存在很强的间歇性 ,不符合Kolmogorov湍流理论 .利用离散正交小波变换将湍流湿度脉动场中的耗散事件分离出来 ,通过引入限制因子 ,将湍流脉动场中的耗散事件滤去后 ,能较好地抑制湍流湿度的间歇性 .但与风速脉动相比 ,湍流湿度场间歇性的抑制效果较差 .     

大气湍流能谱的精细结构及能量级串

利用小波变换和傅里叶分析对近地层大气湍流脉动资料进行了分析,发现波数空间能谱著名 的“-5/3”标度律成立的区间中存在突变点;还发现对应于小波变换时间尺度2j,j=1, 2,…,j0,高频分量按照2(j-2)-1(j>1)的方式级串,这符合同步级串的物理图象 ;在标度区间内高频分量作用于幂律局部特征的效果是平均的,不存在影响标度指数的特征 频谱. 利用不同高度大气湍流资料和不同小波基函数作变换,结果是一致的. 我们还对H= 1/3的分形布朗运动产生的随机序列进行了对比实验,发现从能谱角度,实际发达大气湍流 偏离高斯分布的程度很小,二者的差别只在高阶标度律时明显.      

黄土高原复杂地形条件下湍流观测的各态历经性检验

为了揭示黄土高原复杂地形条件下湍流涡旋的分布特征,提高湍流观测的精度,湍流的各态历经性研究随即成为实验研究的首要问题.文章利用甘肃省平凉市白庙塬地形条件下单点湍流观测的结果,对比分析并揭示了从稳定层结到不稳定层结,除小于10min尺度的湍流易于满足各态历经性外,地形易造成周期性存在的大尺度湍流相干结构.与平坦下垫面湍流相比,塬区10~40min的大尺度湍流定常性更好,也易于满足各态历经性.在极不稳定层结条件下,风速较小,并具有明显的大尺度周期性波动,同时伴随平稳的增温趋势,因此造成大尺度风速湍流和温度湍流都较易于满足各态历经性;但在极稳定层结条件下,湍流间歇性则造成大尺度风速和温度波动的非周期性较强,湍流不易于满足各态历经性.     

边界层参数化方案对边界层热力和动力结构特征影响的比较

本文利用高分辨率中尺度WRF模式,通过改变边界层参数化方案进行多组试验,评估该模式对美国北部森林地区边界层结构的模拟能力,同时比较了五种不同边界层参数化方案模拟得出的边界层热力和动力结构.结果表明：除个别方案外,配合不同边界层方案的WRF模式都能成功模拟出白天对流边界层强湍流混合特征和夜间稳定边界层内强逆温、逆湿和低空急流等热力和动力结构.非局地YSU、ACM2方案在白天表现出强的湍流混合和卷夹,相比于局地MYJ、UW方案,模拟的对流边界层温度更高、湿度更低、混合层高度更高、感热通量更大,更接近实际观测,这表明在不稳定层结下考虑非局地大涡输送更为合理,但局地方案在风速和风向的预报上存在一定优势.TEMF方案得到的白天局地湍流混合强度为所有方案中最弱,混合层难以发展,无法体现对流边界层内气象要素垂直分布均匀的特点.对于夜间稳定边界层的模拟,不同参数化方案之间的差异较小,但是YSU方案在一定程度上高估了机械湍流,导致局地湍流混合偏强,从而影响了其对稳定边界层的模拟能力.     

层结海洋中小振幅内行进波的演变和破碎

采用高精度的拟谱方法，数值模拟了层结海洋中小振幅内行进波的演变和破碎过程.在演变过程中，导致内波破碎的PSI不稳定机制在共振相互作用中逐渐占据主导地位，能量从初级波向低频、高波数运动缓慢传递并形成一次级波包，随即破碎发生.破碎后产生的层化湍流引起的强烈混合以及湍流间歇性可从总能量和涡度峰度随时间的变化趋势看出.我们分析了层化湍流的一些统计特性，包括动能和有效位能沿垂向波数ky的功率谱.结果表明，动能和有效位能谱都存在一个谱段满足k-3y律，且分别可表示为01N4k-3y和02N4k-3y(N为Brunt Visl频率)，通常称其为浮力子区.另外，我们分析了Cox数(湍流扩散系数与分子扩散系数之比)，在层化湍流维持在一定强度时，计算结果和由海洋内区观测(远离内波强生成源和复杂地形)所推测的结论较为吻合.     

华北地区冬小麦田水热、二氧化碳和甲烷湍流输送特征的实验研究

本文利用2012年4月30日至5月10日华北地区大气湍流实验资料,分析了冬小麦田下垫面温度、湿度、二氧化碳(CO2)和甲烷(CH4)的湍流统计和输送特征,利用涡旋相关法计算的CH4通量值确定了松弛涡旋累积(REA)法计算CH4通量的经验系数.结果表明,不稳定层结下,温度、湿度、CO2和CH4的归一化标准差随稳定度参数z/L的关系满足-1/3幂次关系.热量、水汽和CO2水平方向的湍流输送和垂直方向的比值与稳定度参数z/L存在一定的相关关系,但CH4没有类似特征.实验期间,感热通量数值较低,潜热通量较高;CO2在夜间表现出微弱的向上输送,其余时段为向下输送,可以认为实验站所在地区是碳汇;CH4的湍流输送整体为向下输送,无明显的日变化规律,可以认为是CH4汇.利用松弛涡旋累积法获取CO2和CH4通量的参数取值分别为0.61和0.30.     

我国西部高原地区近地层湍流特征的研究

通过对青藏高原第二次大气科学试验(TIPEX)得到的当雄湍流资料的分析,讨论了青藏高原地区的近地层湍流特征.结果显示:速度分量、温度和湿度谱大多满足相似理论的-2/3次方律;高原上无因次垂直速度方差在中性时与前人在平原地区得到的结果比较接近,但高原上无因次水平风速方差值大于平原地区值.在强不稳定层结时,高原观测结果显示无因次垂直速度方差和无因次水平风速方差符合-1/3次方规律.在稳定层结时,风速三个分量的方差都随稳定度z/L的增大有所增大.无因次温度和湿度方差在强不稳定条件下服从-1/3次方规率.稳定条件下无因次温度方差随z/L略有下降趋势,而后趋于常数,无因次湿度方差无明显的规律.在干季以感热通量为主要地面热源,湿季两者贡献相当,潜热通量略大于感热通量.得出了不稳定层结、稳定层结情况下高原的无因次温度结构参数和湿度结构参数与z/L之间的关系,得到了中性层结时整体输送系数的实测结果.     

云覆盖对流边界层顶部湍流结构参数的研究

应用飞机探测资料分析研究云覆盖对流边界层顶部温度和湿度湍流结构,在考虑对流边界层顶部夹卷过程的基础上得到计算温度和湿度结构参数的公式。应用实际观测资料分析了云覆盖对流边界层顶部的湍流特征．资料分析表明,云外晴空区温度和湿度结构函数值明显高于云内的值．云顶边界清晰,通过界面温度和湿度具有明显的跃变特征．应用观测资料检验了温度和湿度结构参数计算公式,计算结果与观测结果符合较好．     

Turbulence structure of the boundary layer below marine clouds in the SOFIA experiment

The SOFIA (Surface of the Ocean: Flux and Interaction with the Atmosphere) experiment, included in the ASTEX (Atlantic Stratocumulus Transition EXperiment) field program, was conducted in June 1992 in the Azores region in order to investigate air-sea exchanges, as well as the structure of the atmospheric boundary layer and its capping low-level cloud cover. We present an analysis of the vertical structure of the marine atmospheric boundary layer (MABL), and especially of its turbulence characteristics, deduced from the aircraft missions performed during SOFIA. The meteorological situations were characteristic of a temperate latitude under anticyclonic conditions, i.e., with weak to moderate winds, weak surface sensible heat flux, and broken capping low-altitude cloud cover topped by a strong trade inversion. We show that the mixed layer, driven by the surface fluxes, is decoupled from the above cloud layer. Although weak, the surface buoyancy flux, and the convective velocity scale deduced from it, are relevant for scaling the turbulence moments. The mixed layer then follows the behavior of a continental convective boundary layer, with the exception of the entrainment process, which is weak in the SOFIA data. These results are confirmed by conditional sampling analysis, which shows that the major turbulence source lies in the buoyant moist updrafts at the surface.     

Effects of shear in the convective boundary layer: analysis of the turbulent kinetic energy budget

Effects of convective and mechanical turbulence at the entrainment zone are studied through the use of systematic Large-Eddy Simulation (LES) experiments. Five LES experiments with different shear characteristics in the quasi-steady barotropic boundary layer were conducted by increasing the value of the constant geostrophic wind by 5 m s^-1 until the geostrophic wind was equal to 20 m s^-1. The main result of this sensitivity analysis is that the convective boundary layer deepens with increasing wind speed due to the enhancement of the entrainment heat flux by the presence of shear. Regarding the evolution of the turbulence kinetic energy (TKE) budget for the studied cases, the following conclusions are drawn: (i) dissipation increases with shear, (ii) the transport and pressure terms decrease with increasing shear and can become a destruction term at the entrainment zone, and (iii) the time tendency of TKE remains small in all analyzed cases. Convective and local scaling arguments are applied to parameterize the TKE budget terms. Depending on the physical properties of each TKE budget contribution, two types of scaling parameters have been identified. For the processes influenced by mixed-layer properties, boundary layer depth and convective velocity have been used as scaling variables. On the contrary, if the physical processes are restricted to the entrainment zone, the inversion layer depth, the modulus of the horizontal velocity jump and the momentum fluxes at the inversion appear to be the natural choices for scaling these processes. A good fit of the TKE budget terms is obtained with the scaling, especially for shear contribution.     

Turbulent mixing in a rotating,stratified fluid

Abstract

An experimental study was carried out to investigate the effect of rotation on turbulent mixing in a stratified fluid when the turbulence in the mixed layer is generated by an oscillating grid. Two types of experiments were carried out: one of them is concerned with the deepening of the upper mixed layer in a stable, two-fluid system, and the other deals with the interaction between a stabilizing buoyancy flux and turbulence.

In the first type of experiments, it was found that rotation suppresses entrainment at larger Rossby numbers. As the Rossby number becomes smaller (Ro 0.1), the entrainment rate increases with rotation—the onset of this phenomenon, however, was found to coincide with the appearance of coherent vortices within the mixed layer. The radiation of energy from the mixed layer to the lower non-turbulent layer was found to occur and the magnitude of the energy flux was found to be increased with the rotational frequency. It was also observed that vortices are generated, rather abruptly, in the lower layer as the mixed layer deepens.

In the second set of experiments a quasi-steady mixed layer was found to develop of which the thickness varies with rotation in a fashion that is consistent with the result of the first experiment. Also the rotation was found to delay the formation of a pycnocline.     

Impact of soil moisture heterogeneity length scale and gradients on daytime coupled land‐cloudy boundary layer interactions

Using a coupled large‐eddy simulation–land surface model framework, the impact of two‐dimensional soil moisture heterogeneity on the cloudy boundary layer under varied free‐atmosphere stabilities is investigated. Specifically, the impacts of soil moisture heterogeneity length scale and heterogeneity in terms of soil moisture gradients on micrometeorological states, surface fluxes, boundary layer characteristics, and cloud development are examined. The results show that mesoscale circulations due to surface heterogeneity in soil moisture play an important role in transferring water vapour within the boundary layer and in regulating cloud distribution at the entrainment zone, which, in turn, provides feedbacks on boundary layer/surface energy budgets. The initial domain‐averaged soil moisture is identical for all homogenous and heterogeneous cases; however, the soil moisture heterogeneity in gradient and length scale between dry and wet regions has a significant impact on the estimates of near‐surface micrometeorological properties and surface fluxes, which further affect the boundary layer states and characteristics. Both liquid water potential temperature and liquid water mixing ratio increase with an increasing soil moisture gradient, whereas the amount of specific humidity decreases. Heterogeneity length scale and free atmosphere stability also amplify these impacts on the boundary layer structure and cloud formation. In a low atmospheric stability condition that potentially allows for a deeper boundary layer and a higher entrainment rate, cloud base height and cloud thickness significantly increase as the soil moisture gradient and length scale increase. Analysis to differentiate the influences of surface heterogeneity type (i.e. length scale vs gradient) shows that in general soil moisture gradient provides a larger impact than heterogeneity length scale, although the heterogeneity length scale is large enough to initiate circulation features responsible for differences in the coupled system between homogeneous and heterogeneous soil moisture cases. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamics behind warming of the southeastern Arabian Sea and its interruption based on in situ measurements

A study of the inter-annual variability of the warming of the southeastern Arabian Sea (SEAS) during the spring transition months was carried out from 2013 to 2015 based on in situ data from moored buoys. An attempt was made to identify the roles of the different variables in the warming of the SEAS (e.g., net heat flux, advection, entrainment, and thickness of the barrier layer during the previous northeast monsoon season). The intense freshening of the SEAS (approximately 2 PSU) occurring in each December, together with the presence of a downwelling Rossby wave, supports the formation of a thick barrier layer during the northeast monsoon season. It is known that the barrier layer thickness, varying each year, plays a major role in the spring warming of the SEAS. Interestingly, an anomalously thick barrier layer occurred during the northeast monsoon season of 2012–2013. However, the highest sea surface temperature (31 °C) was recorded during the last week of April 2015, while the lowest sea surface temperature (29.7 °C) was recorded during the last week of May 2013. The mixed layer heat budget analysis during the spring transition months proved that the intense warming has been mainly supported by the net heat flux, not by other factors like advection and entrainment. The inter-annual variability analysis of the net heat flux and its components, averaged over a box region of the SEAS, showed a substantial latent heat flux release and a reduction in net shortwave radiation in 2013. Both factors contributed to the negative net heat flux. Strong breaks in the warming were also observed in May due to the entrainment of cold sub-surface waters. These events are associated with the cyclonic eddy persisting over the SEAS during the same time. The entrainment term, favoring the cooling, was stronger in 2015 than that in 2013 and 2014. The surface temperatures measured in 2013 were lower than those in 2014 and 2015 despite the presence of a thick barrier layer. The substantial decrease in net heat flux along with entrainment cooling has been identified as causes for this behavior.     

A note on Stokes production of turbulence kinetic energy in the oceanic mixed layer: observations in the Baltic Sea

Shear- and convection-driven turbulence coexists with wind-generated surface gravity waves in the upper ocean. The turbulent Reynolds stresses in the oceanic mixed layer can therefore interact with the shear of the wave-generated Stokes drift velocity to extract energy from the surface waves and inject it into turbulence, thus augmenting the mean shear-driven turbulence. Stokes production of turbulence kinetic energy (TKE) is difficult to measure in the field, since it requires simultaneous measurement of the turbulent stress and the Stokes drift profiles in the water column. However, it is readily inferred using second moment closure models of the oceanic mixed layer provided: (1) wave properties are available, along with the usual water mass properties, and radiative and air–sea fluxes needed to drive the mixed layer model and (2) the model skill can be assessed by comparing the model results against the observed dissipation rates of TKE. Comprehensive measurements made during the Reynolds 2002 campaign in the Baltic Sea have made the estimation of Stokes production possible, and in this paper, we report on the effort and the conclusions reached. Measurements of air–sea exchange parameters and water mass properties during the campaign allowed a mixed layer model to be run and the turbulent stress in the water column to be inferred. Simultaneous wave spectrum measurements enabled Stokes drift profile to be deduced and wave breaking to be included in the model run, and the Stokes production of TKE in the water column estimated. Direct measurements of the TKE dissipation rate from an upward traversing microstructure profiler were used to assure that the model could reproduce the turbulent dissipation rate in the water column. The model results indicate that the Stokes production of TKE in the mixed layer is of the same order of magnitude as the shear production and must therefore be included in mixed layer models.     

Turbulence characteristics of flows passing through a tetrahedron frame in a smooth open-channel

The turbulence characteristics of flows passing through a tetrahedron frame were investigated by using a 2-dimensional fiber-optic laser Doppler velocimeter (2-D FLDV). Experiments for uniform flows with different bed slopes under both submerged and un-submerged conditions were carried out in a re-circulating flume with glass side walls. The experimental bed was a smooth fixed bed. It was observed that with the tetrahedron frame the mean longitudinal velocity decrease in the retardation zone. However, both the longitudinal and the vertical turbulence intensities are larger than those for the undisturbed approach flow. The tetrahedron frame may reduce the probability of sediment entrainment by retarding the flow and reducing the boundary shear stress. In addition, it may induce sediment deposition in a sediment laden flow by changing the flow direction and increasing the energy dissipation.     

Turbulent entrainment at an inversion

Three separate conditions controlling the structure of the inversion are discussed. The rate of change of boundary layer temperature and the vertical gradient of potential temperature in the overlying air set the maximum limit to possible rise rates. In transient conditions, the rise of the inversion layer is sometimes dominated by turbulent transport and erosion, however this cannot continue indefinitely. The horizontal advection above the inversion can also change the relative temperature increase at the inversion, and so may partly determine inversion history.In this paper the theory of the plume-connective boundary layer has been extended to allow the plumes to erode material from the overlying stable air, and so permit study of how the boundary layer could deepen by eroding upward into the capping inversion. Only clear-air processes without condensation are included in the model described here.When the mechanism of convection is examined, it is demonstrated that, because of energetic considerations, the rate of erosion, by buoyancy generated turbulence of the overlying air down across the inversion, will not usually determine the rate of rise of the inversion, except briefly. Other effects often dominate in determining the average erosion rate. Furthermore, because of the limitations set by the buoyancy of the entrained air, sharp inversions, exceeding about 1°C, cannot be formed without the action of latent heat from evaporating droplets. An intermediate buffer layer forms in the inversion zone whenever the erosion rate is high enough to produce a temperature increase above the boundary layer, sufficient to inhibit direct entrainment.     

The surface roughness and planetary boundary layer

Applications of the entrainment process to layers at the boundary, which meet the self similarity requirements of the logarithmic profile, have been studied. By accepting that turbulence has dominating scales related in scale length to the height above the surface, a layer structure is postulated wherein exchange is rapid enough to keep the layers internally uniform. The diffusion rate is then controlled by entrainment between layers. It has been shown that theoretical relationships derived on the basis of using a single layer of this type give quantitatively correct factors relating the turbulence, wind and shear stress for very rough surface conditions. For less rough surfaces, the surface boundary layer can be divided into several layers interacting by entrainment across each interface. This analysis leads to the following quantitatively correct formula compared to published measurements. 1 $$\begin{gathered} \frac{{\sigma _w }}{{u^* }} = \left( {\frac{2}{{9Aa}}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \left( {1 - 3^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \frac{a}{k}\frac{{d_n }}{z}\frac{{\sigma _w }}{{u^* }}\frac{z}{L}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \hfill \\ = 1.28(1 - 0.945({{\sigma _w } \mathord{\left/ {\vphantom {{\sigma _w } {u^* }}} \right. \kern-\nulldelimiterspace} {u^* }})({z \mathord{\left/ {\vphantom {z L}} \right. \kern-\nulldelimiterspace} L})^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \hfill \\ \end{gathered} $$ where \(u^* = \left( {{\tau \mathord{\left/ {\vphantom {\tau \rho }} \right. \kern-0em} \rho }} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}} \) , σ _w is the standard deviation of the vertical velocity,z is the height andL is the Obukhov scale lenght. The constantsa, A, k andd _n are the entrainment constant, the turbulence decay constant, Von Karman's constant, and the layer depth derived from the theory. Of these,a andA, are universal constants and not empirically determined for the boundary layer. Thus the turbulence needed for the plume model of convection, which resides above these layers and reaches to the inversion, is determined by the shear stress and the heat flux in the surface layers. This model applies to convection in cool air over a warm sea. The whole field is now determined except for the temperature of the air relative to the water, and the wind, which need a further parameter describing sea surface roughness. As a first stop to describing a surface where roughness elements of widely varying sizes are combined this paper shows how the surface roughness parameter,z ₀, can be calculated for an ideal case of a random distribution of vertical cylinders of the same height. To treat a water surface, with various sized waves, such an approach modified to treat the surface by the superposition of various sized roughness elements, is likely to be helpful. Such a theory is particularly desirable when such a surface is changing, as the ocean does when the wind varies. The formula, 2 $$\frac{{0.118}}{{a_s C_D }}< z_0< \frac{{0.463}}{{a_s C_D (u^* )}}$$ is the result derived here. It applies to cylinders of radius,r, and number,m, per unit boundary area, wherea _s =2rm, is the area of the roughness elements, per unit area perpendicular to the wind, per unit distance downwind. The drag coefficient of the cylinders isC _D . The smaller value ofz _o is for large Reynolds numbers where the larger scale turbulence at the surface dominates, and the drag coefficient is about constant. Here the flow between the cylinders is intermittent. When the Reynolds number is small enough then the intermittent nature of the turbulence is reduced and this results in the average velocity at each level determining the drag. In this second case the larger limit forz ₀ is more appropriate.