Improving the estimate of wind energy input into the Ekman layer within the Antarctic Circumpolar Current

Based on the data and method offered by Liu et al. （2009）, the direct wind and Stokes drift-induced energy inputs into the Ekman layer within the Antarctic Circumpolar Current （ACC） area are reestimated since the results of the former have been proved to be underestimated. And the result shows that the total rate of energy input into the Ekman-Stokes layer within the ACC area is 852.41 GW, including 649.75 GW of direct wind energy input （76%） and 202.66 GW of Stoke drift-induced energy input （24%）. Total increased energy input, due to wave-induced Coriolis-Stokes forcing added to the classical Ekman model, is 52.05 GW, accounting for 6.5% of the wind energy input into the classical Ekman layer. The long-term variability of direct wind and Stokes drift-induced energy inputs into the Ekman layer within the ACC is also investigated, and the result shows that the Stokes drift hinders the decadal increasing trend of direct wind energy input. Meanwhile, there is a period of 4-5 a in the energy spectrums, as same as the Antarctic circumpolar wave.

Wave energy input into the Ekman layer

This paper is concerned with the wave energy input into the Ekman layer, based on 3 observational facts that surface waves could significantly affect the profile of the Ekman layer. Under the assumption of constant vertical diffusivity, the analytical form of wave energy input into the Ekman layer is derived. Analysis of the energy balance shows that the energy input to the Ekman layer through the wind stress and the interaction of the Stokes-drift with planetary vorticity can be divided into two kinds. One is the wind energy input, and the other is the wave energy input which is dependent on wind speed, wave characteristics and the wind direction relative to the wave direction. Estimates of wave energy input show that wave energy input can be up to 10% in high-latitude and high-wind speed areas and higher than 20% in the Antarctic Circumpolar Current, compared with the wind energy input into the classical Ekman layer. Results of this paper are of significance to the study of wave-induced large scale effects.

Nonlinear Ekman Layer Theories and Their Applications

Based on the classical Ekman theory, a series of intermediate boundary layer models, which retain the nonlinear advective process while discard embellishments, have been proposed with the intention to understand the complex nonlinear features of the atmospheric boundary layer and its interaction with the free atmosphere. In this paper, the recent advances in the intermediate boundary-layer dynamic models are reviewed. Several intermediate models such as the boundary-layer models incorporating geostrophic momentum approximation, Ekman momentum approximation, and the weak nonlinear Ekman-layer model are a major theme. With inspection of the theoretical frameworks, the physical meaning and the limitations of each intermediate model axe discussed. It is found that the qualitative descriptions of the nonlinear nature in Ekman layer made by the intermediate models are fairly consistent though the details may be different. As the application of the intermediate models is concerned, the application of the intermediate models to the study of the topographic boundary layer, frontogenesis, low-level frontal structure, and low-level jet are especially summarized in this paper. It is shown that the intermediate boundary-layer models have great potential in illustrating the low-level structures of the weather and climate systems as they are coupled with the free atmospheric models. In addition, the important remaining scientific challenges and a prospectus for future research on the intermediate model are also discussed.

Wind Structure in an Intermediate Boundary Layer Model Based on Ekman Momentum Approximation

A quasi three–dimensional, intermediate planetary boundary layer (PBL) model is developed by including inertial acceleration with the Ekman momentum approximation, but a nonlinear eddy viscosity based on Blackadar’s scheme was included to improve the theoretical model proposed by Tan and Wu (1993). The model could keep the same complexity as the classical Ekman model in numerical, but extends the conventional Ekman model to include the horizontal accelerated flow with the Ekman momentum approximation. A comparison between this modified Ekman model and other simplified accelerating PBL models is made. Results show that the Ekman model overestimates (underestimates) the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow due to the neglect of the acceleration flow, however, the semi–geostrophic Ekman model overestimates the acceleration effects resulting from the underestimating (overestimating) of the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow. The Ekman momentum approximation boundary layer model could be applied to the baroclinic atmosphere. The baroclinic Ekman momentum approximation boundary layer solution has both features of classical baroclinic Ekman layer and the Ekman momentum approximate boundary lager.

A Dynamic Study of Ekman Characteristics by Using 1998 SCSMEX and TIPEX Boundary Layer Data

A dynamic study on Ekman characteristics by using 1998 SCSMEX and TIPEX boundary layer data is made. The results are as follows: (1) Similar dynamical Ekman characteristics are observed in the Tibetan Plateau and in the South China Sea and its surrounding area. (2) The thickness of the boundary layer is about 2250 m over the Tibetan Plateau, and considering its variation, the thickness could be up to 2250–2750 m. In the tropical southwest Pacific, the thickness of the boundary layer is about 2000 m, and the variation is smaller; a smaller thickness of the boundary layer is in the plain area of the Bohai Sea. (3) Because of the difference in elevation between the Tibetan Plateau and the tropical ocean area, the influence of the boundary layer on the atmosphere is quite different although the two areas have almost the same thickness for the boundary layer, the height where the friction forcing occurs is quite different. (4) The vertical structure of turbulence friction is quite different in the Plateau and in the tropical ocean area. Calculations by 1998 SCSMEX and TIPEX boundary layer data indicate that even in the lowest levels, eddy viscosity in the Tibetan Plateauan can be 2.3 times than in the tropical ocean area.

自由汇流旋涡Ekman抽吸演化机理

自由汇流旋涡形成过程中有抽吸现象发生,是一个比较复杂的气液两相耦合过程,其中所涉及的Ekman层耦合及演化机理具有重要的科研价值与实际意义.针对上述问题,提出了一种自由汇流旋涡Ekman抽吸演化机理建模与分析方法.基于多相流体体积VOF模型与湍动能—耗散(k-ε)模型,建立了面向汇流旋涡Ekman抽吸演化的两相动力学模型.基于上述模型,分析初始转动速度分量、排流量与Ekman抽吸过程的内在联系,并揭示相关流场分布规律.研究结果表明:初始扰动不同,汇流旋涡的吸气孔、抽气孔距离容器底面边界的高度保持不变;初始扰动加强,吸气阶段转速增加,Ekman边界层厚度及抽吸高度增加,抽吸、贯穿阶段Ekman抽吸现象减弱;初始扰动恒定,Ekman抽吸高度保持不变,与排流量变化无关.研究结果可为自由汇流旋涡形成机理方面的研究提供有益参考,也可为冶金、化工领域的旋涡抑制控制提供技术支持.

Ekman边界层动力学的理论研究

大气边界层及其与自由大气之间的相互作用具有明显的非线性特征,而这些特征是经典Ekman理论所不能描述的,因此,发展中等复杂程度(介于完全模式与经典Ekman模型之间)的大气边界层动力学模式,简称中间模式,对人们从理论上认识大气边界层动力学过程的非线性特征具有重要意义。本文对目前最具代表性的几个中间边界层模型:地转动量近似边界层模型、Ekman动量近似边界层模型以及弱非线性边界层模型进行了总结和分析,阐述了Ekman层主要动力学特征。通过分析上述各模型的理论框架,揭示了各模型的物理意义及其在描述Ekman边界层基本动力特征上的优点和局限性,并指出尽管在细节定量描述上有差异,但各中间模型对Ekman层动力学特征的定性描述具有很好的一致性。对于这些Ekman边界层近似理论模型的进一步应用问题,主要回顾和总结了利用上述模型探讨地形边界层结构、大气锋生过程、低层锋面结构和环流以及边界层日变化、低空急流形成等动力学问题的研究,并对这些研究所揭示的Ekman层动力学特征及其对自由大气低层运动的影响进行了分析,结果表明,这些Ekman边界层近似模型可以较好地揭示大气边界层动力学特征,在大气边界层动力学及其与自由大气相互作用的研究上具有重要价值。另外,还对目前Ekman边界层理论研究中存在的问题进行了一些分析,提出了有待进一步研究的科学问题。

西南低涡Ekman层流场特征分析

本文利用大气边界层模式,对1979年7月24—25日的一次西南低涡暴雨进行了Ekman层的流场分析。在理论计算的前提下,通过加入有限的实测风资料进行动力学调整,不仅在Ekman层中能较好地模拟出高层(850,700hPa)气旋环流特征,且发现西南低涡行星边界层具有中尺度扰动的特征。在整个气旋环流中有局部的反气旋环流出现,这是850,700hPa天气图上得不到的,且与边界层诊断分析相符合。

经典Ekman流,Ekman层的平衡风场及其扰动稳定性

本文首先利用变分方法,考察了边界层运动能量的变化,指出经典Ekman流是在不可压缩条件下,能量积分达最小值时的一种平衡运动。这对Ekman层运动的物理本质有了进一步的认识。其次,讨论了Ekman动量近似下的Ekman层的平衡风场特征,研究了该平衡风场附近扰动的稳定性问题,结果表明,在自由大气气压场不发生扰动条件下,Ekman层中存在一类新的与惯性不稳定相类似的动力不稳定,且其不稳定性可与Ekman抽吸相联系,还讨论了一般性条件下的扰动不稳定性问题。

海表Ekman流不稳定性致新型次级环流

海洋上混合层中的次级环流可通过物质和能量的垂直输运和混合过程把海洋表层的热量、动量与物质携带到次表层,对海洋上层次级环流生成机制的研究可以丰富对上层海洋的理解和认识。文中利用线性稳定性理论讨论了经典海表Ekman流的不稳定性,提出Ekman流的不稳定性可生成一种新型的次级环流。这种次级环流的空间尺度与雷诺数、Ekman流的垂向衰减速率、水平湍黏性系数与垂向湍黏性系数比值等密切相关,尺度范围从数十米到数千米。数十米量级的次级环流其垂向结构以及次级环流流轴与主流场偏角都与Langmuir环流的特征极为相似,是Langmuir环流形成机制的一种新解释。千米量级的次级环流能够解释黄海浒苔的条带分布。此外,所得次级环流的流轴与主流之间的偏角与科氏力有显著关系,北半球次级环流流轴偏向主流左侧,南半球反之。

Ekman层风速随高度分布方程的数值解试验

根据大气水平运动方程推导出Ekman层风速随高度分布方程,该分布可用二阶线性微分方程表示,用有限差分法求解该二阶线性微分方程的边值问题的数值解,并给出计算程序,输出结果。将结果与经典解析解比较,讨论了经典解的正确性和适用范围。

非线性大气Ekman层三维动力学特征

本文采用Boussinesq近似，利用地转动量近似（GMA）方法，研究了大气Ekman层的三维非线性动力学特征，得到了Ekman层中三维风速分布的解析解，着重讨论了热成风对Ekman层水平风速分布的影响，对传统的求垂直速度的方法进行了改进。

影响北极冰下海洋Ekman漂流垂直结构与深度因素的研究

Ekman漂流是上层海洋普遍存在的一种运动形式。本文提出了用实测温盐数据计算冰下Ekman流速的计算方法,与2010年北极考察期间同步获取的海流剖面数据进行比较,获得了满意的结果。基于这个结果,可以通过比较容易获得的温盐数据,计算Ekman漂流垂直结构。海水层化的存在导致在跃层处湍流黏性系数减小,强烈抑制了流速的向下传播,致使Ekman漂流在跃层处完全消失。结果表明,冬季上层海洋漂流会发生在较大的深度上,而夏季海冰拖曳引起的漂流只能达到20～30m的深度。Ekman漂流的深度只与跃层的深度有关,与海冰的漂移速度无关。漂流层变浅意味着海冰拖曳做功产生的能量不能进入海洋深处,而是在很浅的表层水体内积聚,有利于加剧海冰的底部融化。计算湍流黏性系数通常需要密度剖面和流速剖面的观测结果,在只有密度剖面的情况下,可以采用本文的方法计算Ekman漂流,获得上层海洋的湍流黏性系数剖面。

一个新的Ekman泵公式在大气边界层Ekman垂直速度计算中的应用

利用GCM模式资料和Tan最新得到的具有复杂边界的层结大气边界层的Ekman泵公式 ,计算了大气边界层顶的Ekman垂直速度。结果表明 ,除青藏高原外 ,一般地区的大气边界层顶垂直速度的量级为每秒零点几厘米。在青藏高原地区 ,大气边界层顶的垂直速度可达每秒几厘米的量级 ,其分布类型在冬季和夏季显著不同 ,与该地区的盛行风系紧密相关。将新的Ekman泵公式的结果与经典的Ekman泵公式的计算结果进行比较 。

基于伴随同化方法的Ekman模型垂向涡动黏性系数时间变化研究

为了得到一种有效的计算海水涡动黏性的途径,本文基于伴随同化方法,研究了Ekman模型中垂向涡动黏性系数(verticaleddyviscositycoefficient,VEVC)的时间变化。本文推导了时间变化VEVC的优化关系式,并利用理想实验对三个影响因素进行了探究,包括优化算法、初始猜测、观测深度,其主要结论是:(1)梯度下降法的优化效果优于共轭梯度法和有限记忆BFGS(limited-memoryBroyden–Fletcher–Goldfarb–Shanno)法;(2)初始猜测值与实际值接近时,收敛速度更快,反演误差更小;(3)反演结果对表层和次表层的流速更为敏感。本文从百慕大试验站锚泊系统(Bermuda Testbed Mooring, BTM)的数据中提取了Ekman流速,并反演了VEVC的时间变化,实际实验结果表明:(1)对于实测数据而言,仍是梯度下降法优化效果最好;(2)将VEVC设置为时间变化型的反演策略,反演效果优于常数型和深度变化型;(3)该地区VEVC在0.01m^2/s左右。该研究为Ekman流的数值模拟提供了一种确定VEVC时间变化的有效方法,对于其他动力机制的研究具有一定的参考价值。

Ekman模型时间变化风应力系数的伴随参数估计

本文基于时间分布参数设置,利用伴随同化方法,反演了Ekman模型中随时间变化的风应力拖曳系数,并在孪生实验和实际实验中对该方法进行了验证。在孪生实验中,研究了参数反演结果对不同影响因素的响应,包括:风速分布、风应力系数分布、风应力系数初始猜测值、风应力系数独立变量个数、观测数据误差和观测的深度。孪生实验结果验证了伴随同化方法反演Ekman模型中时变风应力系数的有效性,具体包括如下五个方面结论:1)不同风速分布下均能成功反演出不同风应力拖曳系数分布;2)反演结果对初始猜测值较为敏感,风应力系数初始猜测值越接近给定值,反演结果越好;3)风应力系数独立点个数的选取会显著影响反演结果,合理的选择有利于提高反演效率及减小观测数据误差;4)观测误差能够影响反演结果,观测数据误差在20%以下时能取得合理的反演结果;5)反演结果对观测数据的表层和次表层流速更为敏感,这是由Ekman流的物理性质决定的。实际实验,利用百慕大锚系试验平台的风速和流速数据,去除周期性潮流和地转流成分后得到Ekman流成分,并作为观测输入到该同化模型,反演出了适用于该区域和该时段的随时间变化的风应力系数。通过比较模拟流速和观测流速,证明利用伴随同化方法能从实测数据中反演出合理的时变风应力系数,对于海洋模型风应力系数的确定是一项有益的尝试。

SIMULATION OF BOUNDARY LAYER EFFECTS ON A HEAVY RAINFALL EVENT CAUSED BY A MESOSCALE CONVECTIVE SYSTEM OVER THE YELLOW RIVER MIDSTREAM AREA

A heavy rainfall event caused by a mesoscale convective system(MCS),which occurred over the Yellow River midstream area during 7-9 July 2016,was analyzed using observational,high-resolution satellite,NCEP/NCAR reanalysis,and numerical simulation data.This heavy rainfall event was caused by one mesoscale convective complex(MCC)and five MCSs successively.The MCC rainstorm occurred when southwesterly winds strengthened into a jet.The MCS rainstorms occurred when low-level wind fields weakened,but their easterly components in the lower and boundary layers increased continuously.Numerical analysis revealed that there were obvious differences between the MCC and MCS rainstorms,including their three-dimensional airflow structure,disturbances in wind fields and vapor distributions,and characteristics of energy conversion and propagation.Formation of the MCC was related to southerly conveyed water vapor and energy to the north,with obvious water vapor exchange between the free atmosphere and the boundary layer.Continuous regeneration and development of the MCSs mainly relied on maintenance of an upward extension of a positive water vapor disturbance.The MCC rainstorm was triggered by large range of convergent ascending motion caused by a southerly jet,and easterly disturbance within the boundary layer.While a southerly fluctuation and easterly disturbance in the boundary layer were important triggers of the MCS rainstorms.Maintenance and development of the MCC and MCSs were linked to secondary circulation,resulting from convergence of Ekman non-equilibrium flow in the boundary layer.Both intensity and motion of the convergence centers in MCC and MCS cases were different.Clearly,sub-synoptic scale systems in the middle troposphere played a leading role in determining precipitation distribution during this event.Although mesoscale systems triggered by the sub-synoptic scale system induced the heavy rainfall,small-scale disturbances within the boundary layer determined its intensity and location.

南海中部海域障碍层特征及其形成机制

南海中部海域（10°～19°N，108°～122°E）存在显著的季节变化的障碍层．障碍层发生概率夏季最大（52．8%），秋季次之（41．0%），春季最小（10．5％）．夏季（2000年8～9月）障碍层最显著，平均厚度约为14．2m；除114°E以东、吕宋岛以西海域为障碍层的多发区外，中南半岛东南海域（12°～14°N，110°～114°E）也存在显著的障碍层；春季（1998年4～6月）和秋季（1998年12月）障碍层平均厚度分别为6．8和11．2m，障碍层多位于114。E以东、吕宋岛以西海域．此外，吕宋岛以西海域（12。～16。N，116。～120。E）及中沙和西沙群岛附近（16。～18°N，110°～116°E）海域障碍层年发生几率超过20%，相对而言，其他海域障碍层年发生几率偏小．降水机制和层结机制分别是南海中部海域春、夏季和秋季障碍层形成的主要原因．其中，降水机制及东南向的Ekman平流较好的解释了春、夏季吕宋岛以西附近海域成为障碍层多发区的原因；此外，强降水是夏季中南半岛东南海域（12°～14°N，110°～114°E）障碍层产生的关键，反气旋涡（暖涡）有助于形成更强的障碍层，上升流对障碍层的影响有待进一步研究．

β-GMA与边界层动力学特征

本文研究了β平面中的地转动量近似(β-GMA)及其成立条件.利用β-GMA得到了一个描述Ekman动力学特征的动力学方程组.研究了β项和地转动量(GM)对Ekman层动力学的共同作用.结果表明,β-GMA下的解由两部分组成,一部分即为f-GMA下的解,另一部分为β项与GM的相互作用解.这样,一方面,β-GMA解由于β项存在,对一个定常纬向对称的高度场具有纬向非对称结构,而f-GMA解是纬向对称的.利用一个定常、轴对称的圆形涡旋例子计算了Ekman层的水平风速和边界层顶部的垂直速度,并与经典解、f-GMA解作了详细的分析比较.

台风影响南海上层环流的统计分析研究

利用温州台风网台风信息以及卫星遥感海表风场数据,研究了南海上层环流对台风的响应特征。合成分析发现,台风以Ekman输运的形式,能够诱发上层海洋十几厘米每秒的流速变化,并且主要集中在南海东部海盆。虽然台风引起的上层环流异常比较小,但是其方差非常显著。另一方面台风通过Ekman抽吸,引发上层海洋垂向运动异常,并且能够诱发强烈的等效混合。基于历史温盐数据发现,这种强的等效混合主要分布在吕宋海峡以西、越南以东以及台湾海峡,并且能够达到10^-3 m^2/s的量级。