【作者】 刘志宇；

【导师】 魏皓；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2009， 博士

【摘要】 占全球海洋总面积8％的陆架海是海洋中物理、生物活动最为密集的区域．在陆架海中，湍流混合是众多物理、生化过程的核心，它是水体结构及营养盐通量的关键控制因子，并控制着初级生产率及颗粒物的沉降、再悬浮、絮凝与解凝过程．利用在黄海与Clyde海两典型强潮驱陆架海中湍流混合的直接观测，本文对强潮驱陆架海中的湍流混合特征及其机理进行了较为深入与系统的研究．通过对弱层化季节黄海两对比性站位(潮流类型分别为往复型与旋转型)处潮流结构与湍流混合特征的研究，本文发现往复型与旋转型潮流中潮混合特征显著不同：1、在往复型潮流中，水体不同深度处水平流速垂直剪切的位相基本一致，而在旋转型潮流中，由海底摩擦作用于潮流而产生的强流速剪切由海底缓慢向上传播；2、在往复型潮流中，由平均流速的对数分布所估计出的海底摩擦速度与由实测高频流速直接计算的结果基本相当，而在旋转型潮流中，前者是后者的二倍．分析表明，潮流的旋转、由局地地形造成的型阻以及水体的弱层化可能是上述差异产生的原因；3、经典的壁湍流理论以及湍封闭模型中经常采用的湍动能与其耗散率间的关系对于弱层化的往复型潮流是成立的，而在旋转型潮流中，壁湍流理论需要进行修正．对强层化季节黄海两对比性站位(分别位于中央海盆区与局地陆坡区)处层化、内波以及湍流混合特征的研究结果表明：1、强层化季节的陆架海水体一般呈现显著的三层热盐结构，在水体近乎混合均匀的上混合层与潮流底边界层之间为强跃层；2、近表层水体的湍流混合强度主要由海表浮力通量的日变化与海表风强迫控制，而在潮流底边界层内，潮混合是水体热量、物质、动量与能量垂直交换的主要机制；3、潮混合影响的深度由潮流大小决定，在黄海，一般可达10-15 m，因此，在水深较深的区域，在跃层与潮混合所至深度范围的上界之间存在湍流混合非常弱的区域，这显著抑制水体内物质的垂直通量，为物质垂直交换的瓶颈，而在水深较浅的区域，潮混合影响范围可至跃层底部，因此物质在跃层以下整个水体中混合非常均匀，当跃层内间歇性强混合发生时，可以产生显著的跨跃层物质输运；4、近底潮致强湍流耗散缓慢地向上传播，底上不同深度处垂直湍扩散系数也具有显著的位相差异，且二者均随时间呈现四分之一周日周期的变化；5、在地形较为平坦的中央海盆区，内波活动非常微弱，因此跃层内湍流混合非常弱，垂直扩散系数为分子扩散水平，跨跃层物质通量受到显著抑制，而在地形变化较为显著的局地陆坡区，内波活动非常活跃，除内潮的影响外，高频内波与内孤立波的影响也很显著，因此跃层内存在很强的间歇性强混合，内孤立波存在的区域，水体湍流混合显著增强．基于剪切不稳定的线性理论，本文研究了强层化强潮驱陆架海中水体的动力稳定性．结果表明：1、一般来讲水体并不处于一些研究者所认为的临界稳定性状态，而可能是很不稳定或很稳定；2、虽然强潮驱陆架海中实际流动所对应的临界梯度Richardson数有时非常接近Miles-Howard临界值0．25，但在很多情况下其显著小于0．25，因此在实际应用中简单地取0．25作为剪切不稳定发生的临界值是不合理的；3、水体中最不稳定扰动生长的e-折周期与最不稳定扰动最显著处的浮性周期基本相当，水体中小振幅扰动的生长率与湍流混合强度显著地受控于水体的层结状况；4、水体湍流混合特征与剪切不稳定性之间存在着机制性联系，可以由不稳定扰动的特征量来参数化湍动能耗散率，本文给出了相应的参数化公式．更多还原

【Abstract】 The shelf seas have an importance that is out of proportion to the relativelysmall fraction of the area of the global ocean they occupy (～8%).It is area ofintense physical and biological activity.In shelf seas,turbulent mixing is central tomany physical and biological processes,as it is often the key determinant of watercolumn structure and nutrient fluxes,and hence the rate of primary production,and also the settling,resuspension,aggregation and disaggregation of particulatematter.In this dissertation,turbulence and mixing in typical tidally energeticshelf seas are investigated by using direct measurements of turbulence parametersin the Yellow Sea and Clyde Sea.By analyzing the characteristics of turbulence at two comparative stations(the fows are reversing and rotating tidal currents,respectively) in the YellowSea,it is found that the reversing and rotating tidal flows affect small-scale near-bottom dynamics differently.In reversing flows,the near-bottom shear and theshear at the upper levels are almost in phase,while in rotating flows,the shear gen-crated near the scafloor propagates slowly to the water interior.The log-layer andskin-layer estimates of the bottom friction velocity show close correspondence forthe reversing tidal flows,but when the tidal vector rotates over a sloping bottomthe log-layer estimate is approximately twice of the skin-layer one.The rotation,form drag due to local topographic inhomogeneities,and weak but appreciablestratification are suggested to be possible sources for this discrepancy.The clas-sical wall-layer parameterization of the turbulent dissipation rate is found to holdwell for reversing flows,while modifications are needed for rotating flows.Therelationship between the turbulent kinetic energy and its dissipation rate,whichis widely used to parameterize the dissipation rate in turbulence closure models,is found to hold well for both reversing and rotating flows,but with differentcoefficients.Microstructure profiling measurements at two comparative stations (a deepercentral basin and a local shelf break) in the stratified Yellow Sea are analyzed,with emphasis on tidal and internal-wave induced turbulence near the bottomand in the pyenocline.The water column has a distinct three-layer thermohaline structure,consisting of weakly stratified surface and bottom boundary layers anda narrow sharp pycnocline.Turbulence in the surface layer is controlled by thediurnal cycle of buoyancy flux and wind forcing at the sea surface.while thebottom stress induced by barotropic tidal eurrents dominates turbulence in thebottom boundary layer.The maximum level at which the tidally enhanced mixingcan affect generally depends on the magnitude of the tidal current,and it canbe up to 10-15 m in the Yellow Sea.This suggests that,in the deeper regionsof the shelf seas,turbulent dissipation and mixing are very weak at the levelsbetween the near-bottom tidally enhanced layer and the pycnocline.Therefore,these levels provide a significant bottle neck for the vertical exchanges.In theshallow regions,however,the tidally-induced turbulence can occupy the wholewater colum below the pycnocline.A quarter-diurnal periodicities of the turbulentdissipation rate and eddy diffusivity are found at different heights with evidenttime lag.In the relatively flat central basin,the pycnocline is essentially non-turbulent and internal-wave activity is very weak.Therefore,vertical fluxes acrossthe pycnocline decreased to molecular levels.In contrast,internal waves of variousperiods can be always found near the local shelf break.Particularly,on the passageof internal solitary waves,turbulence in pycnocline can be increased by orders.The linear theory of dynamic instability is used to investigate the stability ofbaroclinic tidal flows in shelf seas.It is found that the flows are not generally ina state of marginally stable,but either very unstable or very stable.Although forsome flows the critical gradient Richardson number is close to the Miles-Howardlimit of 0.25,for others it is substantially less.The e-folding period of the mostunstable disturbances is found to be generally close to the buoyancy period at thelevels where the disturbance has the largest amplitude.This suggests that thegrowth rate of small disturbances and the consequent turbulence in the flow arelargely under a physical control of the st ratification.A mechanistic link betweenthe turbulent dissipation and the instability of the flow is reveale d,based on whicha new paramerization formula for turbulent dissipation is devised.更多还原