Journal of Tropical Oceanography >
Summertime stratification and turbulent mixing in the Yangtze Estuary
Received date: 2017-12-12
Request revised date: 2018-01-26
Online published: 2018-10-13
Supported by
National Basic Research Program of China (2013CB956502)
National Natural Science Foundation of China (41576091)
Copyright
Hydrological and turbulent microstructural observations were conducted in summer 2016 to analyze the summertime stratification and turbulent mixing in the Yangtze Estuary. The temporal and spatial distributions of salinity, temperature, stratification, and turbulent mixing showed highly-stratified structures in the estuary. The vertical distributions of density vertical gradient in different layers demonstrated bottom mixed layer and upper pycnocline, where a division of the stratification was approximately along the contour line of lgN 2 = - 4.0. The bottom mixed layer had a stronger dissipation rate and unstably-stratified structure, while the upper pycnocline had a weaker dissipation rate and stably-stratified structure, where the formation and spread of internal waves would occur. Under the condition of stratified structure, the dissipation rate, turbulent shear production and buoyancy flux were found to satisfy the local turbulent kinetic energy balance. Turbulent Reynolds number Ret and turbulent Froude number Frt in the bottom mixed layer and the upper pycnocline were distributed in different regions of the Ret-Frt space, indicating that the stratification and mixing in the estuary was in line with the classical theory of
LIN Yaokun , BAO Yun , CHEN Qicheng , WU Jiaxue . Summertime stratification and turbulent mixing in the Yangtze Estuary[J]. Journal of Tropical Oceanography, 2018 , 37(5) : 33 -39 . DOI: 10.11978/2017129
Fig. 1 Locations of field observations in the Yangtze Estuary. Filled squares represent the survey sites along transect M, and open square indicates the mooring site图1 长江河口现场观测站位分布 |
Tab. 1 Instruments and parameters used in the field observation表1 观测仪器及参数设置 |
站位 | 测量 方式 | 仪器 | 采样频率/Hz | 距底高度 | 测量要素 |
---|---|---|---|---|---|
走航 剖面 | 剖面 观测 | MSS-90L 微结构剖面仪 | 1024 | — | 温度、电导率、浊度、流速剪切 |
定点 站位 | 剖面 观测 | MSS-90L 微结构剖面仪 | 1024 | — | 温度、电导率、浊度、流速剪切 |
座底 观测 | RDI-ADCP 流速剖面仪 | 1 | 1.6m | 剖面速度 | |
ADV(两层) 三维点式流速仪 | 32 | 0.25m、 0.7m | 底层流速 |
Fig. 2 Calculation of dissipation rate at the mooring site A. (a) Shear spectrum and the Nasmyth universal spectrum against the wave numbers at 12th hour between 9~10 m of water depth; (b) the energy spectrum of turbulence at 6th hour at 0.25 m above the bottom, where the sloping solid line corresponds to an idealized fall-off rate of f-5/3 in inertial subrange图2 水体中的耗散率计算 |
Fig. 3 Vertical distributions along transect M: temperature (a), salinity (b) and turbidity (c); and vertical profiles of temperature, salinity and turbidity at sites M3, M5 and M7 (d)图 3 走航断面M的(a)温度、(b)盐度、(c)浊度断面分布和(d)M3、M5、M7站点剖面特性 |
Fig. 4 Vertical profiles at the mooring site A: turbidity (a), temperature (b) and salinity (c)图 4 定点站位A处的(a)浊度、(b)温度和(c)盐度断面分布 |
Fig. 5 Tidal variation at the mooring site A: buoyant frequency N2 (a) and gradient Richardson number Rig (b)图 5 定点站位A处的浮力频率N2 (a)与梯度理查森数Rig (b)的潮汐变化 |
Fig. 6 Tidal variation at the mooring site A: dissipation rate ε (a) and turbulent shear production P (b)图 6 定点站位A处的耗散率ε (a)与湍动能剪切生成P (b)的潮汐变化 |
Fig. 7 Comparison between turbulent shear production and dissipation rate at 0.48 m above the bottom (a), bottom mixed layer (b) and upper pycnocline layer (c)图 7 近底层0.48 m(a)、底部混合层(b)和上层密度跃层(c)中湍流动能剪切生成P和耗散率ε关系比较 |
Fig. 8 The Ret-Frt space in the bottom mixed layer and upper pycnocline图 8 底部混合层和上层密度跃层中 Ret-Frt平面分区图 |
The authors have declared that no competing interests exist.
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