VERTICAL OVERTURNS - A COMPARISON OF THORPE AND OZMIDOV LENGTH SCALES
VERTICAL OVERTURNS - A COMPARISON OF THORPE AND OZMIDOV LENGTH SCALES
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DOI：
10.1029/jc087ic12p09601
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发表时间：
1982-01-01
影响因子：
3.6
通讯作者：
DILLON, TM
中科院分区：
文献类型：
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作者：
DILLON, TM
湍流倾覆事件长度尺度的客观测量，索普尺度，LT，与Ozmidov尺度L0 =（ε/N3）1/2进行比较，其中N是浮力频率，ε是动能耗散率。在远离地面的风强迫混合层和季节性温跃层中，L0和LT的量级相同，但在靠近混合层表面处，L0明显大于LT。比值L0/LT的变化归因于表面附近高能区梯度Richardson数的减小。另一个长度尺度LB=（DCx/N）1/2，其中C是考克斯数，D是温度的分子扩散系数，在混合层表面附近以及在层内和季节性温跃层中与LT是同一数量级。结果表明，只要涡动粘滞系数与涡动扩散系数之比为常数，湍流动能收支对梯度Richardson数的依赖性很弱。将温度变化耗散率与浮力频率和现有温度变化的乘积进行比较。温度波动被定义为观测到的温度分布与索普分布（如果翻转斑块在没有耗散的情况下重力坍塌，则会产生的温度分布）之间的温度差。它示出的温度方差方程中的主要平衡之间的变化率的产生和它被耗散的速率，温度方差的变化率可以是一个重要的修改，只有当方差衰减的时间远小于浮力周期的平衡。
An objective measure of the length scale of turbulent overturning events, the Thorpe scale,LT, is compared to the Ozmidov scaleL0= (ε/N3)1/2, whereNis the buoyancy frequency and ε is the kinetic energy dissipation rate. Far from the surface in wind‐forced mixing layers and in the seasonal thermocline,L0andLTare of the same order, but near the surface of a mixing layer,L0is significantly larger thanLT. The change in the ratioL0/LTis attributed to a decrease in the gradient Richardson number in the highly energetic zone near the surface. Another length scale,LB= (DCx/N)1/2, whereCxis the Cox number andDis the molecular diffusivity of temperature, is the same order asLTnear the surface of a mixing layer as well as in the layer interior and in the seasonal thermocline. It is shown, by using the turbulent kinetic energy budget, thatLB/LTis only weakly dependent on the gradient Richardson number as long as the ratio of eddy viscosity to eddy diffusivity is constant. The temperature variance dissipation rate is compared to the product of the buoyancy frequency and the existing temperature variance. Temperature fluctuations are defined as the temperature difference between the observed temperature profile and the Thorpe profile (the temperature profile which would result if an overturning patch gravitationally collapsed without dissipation). It is shown that the major balance in the temperature variance equation is between the rate at which variance is produced and the rate at which it is dissipated and that the rate of change of temperature variance can be an important modification to this balance only if the variance decays in a time much smaller than a buoyancy period.