强风条件下近海浅水区海-气动量通量特征研究

Typhoons are one of the most impressive manifestations of air-sea interaction.Knowledge of the air-sea interaction at typhoon wind speeds is of primary importance for its intensity forcasts, risk assessments, wave and surge forcasts, marine engineering and transport safety. The momentum exchange at the air-sea interface is one of the major physical processes governing hurricane intensity. However,the extreme high winds, intense rainfall, large ocean waves, and copious sea spray in tropical cyclones push the surface-exchange parameters for momentum into untested regimes..After three decades of air-sea turbulence flux measurements, there is a general consensus on the behavior of the parameterizations of momentum at moderate wind speeds,e.g., the bulk drag coefficient increases roughly linearly with wind speed , and the aerodynamical roughness length is expressed as a function of viscous stress, friction velocity, wave age,and/or wave steepness, known as the modified Charnock relationships.There are relatively few air-sea momentum flux measurements in the wind regime higher than 22 m/s. But the change in the coefficient at higher wind speeds is not well understood because of a paucity of measurements. There remains the need for flux measurements in the high wind regime..This project will focus on the air-sea momentum exchange at high wind speeds, based on the wind profile and turbulent air flow measurements in the marine surface layer, and wave parameters, which represent the marine surface properties in the coastal waters of South China Sea. The convective component of the moment flux, which is ignored in most similar studies in the low pressure zone of a tropical cyclone, will be investigated firstly. The statistical characteristics and tendence of the friction velocity, drag coefficient and aerodynamic roughness length will be examined under high wind speeds and high sea state. The underlying mechanisms, which may have great influences on the air-sea momentum exchange, will be inspected. Further more, based on the comparizon of the results from different water zones(including others' published results), the shoaling effects will be investigated numberically.

准确理解与估计强风时的海-气动量通量，对热带气旋的强度与海浪预报有重要意义。根据已经获取的热带气旋环境中、代表华南近海下垫面的梯度风与湍流风速观测资料，利用廓线法与湍谱惯性区耗散法及涡动相关法直测的动量通量结果，重点区分对流分量与湍流分量，系统分析不同风速和波浪条件下拖曳系数及粗糙度长度等量的统计特征与变化规律，揭示强风下海-气动量交换特征及其与有关控制参数的关系。进一步，比较不同水域的结果差异并通过数值试验分析浅水效应造成的影响，推动强风海-气动量通量参数化的改进。

台风环境中海-气界面动量交换特征是海-气耦合机制研究中重要的物理量，并在台风，海浪与风暴潮等模式中具有广泛应用价值。基于南海近海一个海洋气象平台观测的近海面多层风速和大气湍流、海浪等数据，研究了9次登陆台风影响时段近海浅水区（该海洋气象平台点的无量纲水深为0.9，符合浅水区的定义）的摩擦速度u*和拖曳系数Cd特征，对应10 m高度风速U10范围3-42 m/s, 并与国外大洋深水区、中等水深(无量纲水深值为2.23)的观测研究结果做了对比，发现如下特征：在3 m/s≤U10≤30 m/s时，u*单调递增，u*最大值约1.8 m/s，U10>30 m/s时，u*随U10的增大而衰减；3 m/s≤U10≤10 m/s时，浅水区u*与深水区u*结果非常接近，随着风速的增大，浅水区u*开始显著高于深水区结果，当U10>30 m/s时，浅水区u*开始减小后又变得接近或小于深水区u*。Cd随U10的变化特征为：U10≤~10 m/s时，Cd近似维持常值1.5×10-3，之后随U10的增大而增大，在U10~24 m/s达到极大值约4×10-3，之后开始随 U10的增大而衰减，U10≈40 m/s时，达到极小值约1.3×10-3。通过与中等水深、深水区Cd对比，我们发现如下规律：在横轴U10-纵轴Cd坐标系中，Cd 依赖U10的关系曲线，随水深由深水到中等水深再到浅水，从坐标的右下角向左上角移动，即：随水深变浅，Cd极大值对应的U10减小，数据点整体量值增大。基于这个新的现象，我们创建了一种依赖水深和风速的Cd新方案，并利用GRAPES台风模式对台风南玛都（2011）个例进行了后报测试，试验结果显示，相对旧的Cd方案，路径和强度后报效果都有改进。