Effects of salinity stratification on turbulence in the South Passage of the Changjiang River Estuary
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摘要: 本文基于2020年秋季长江口南槽实测高频湍流和盐度剖面数据，通过定量分析水体盐度层化对湍流垂向黏滞系数、拖曳系数和垂向流速能谱的影响，探讨盐度层化对水体湍流的影响。研究结果表明：观测站点总体上呈现涨潮流期逐渐层化、落潮流期逐渐混合的周期性层化特征。盐度层化显著抑制底层湍流强度，导致底层拖曳系数和垂向黏滞系数降低，且随着距底床高度增加，层化引起的湍流参数的减小越明显。垂向流速的谱分析结果显示：盐度层化对低频大尺度涡旋具有明显抑制作用，且距离底床越远湍流受到的抑制作用越强。盐度层化使湍流垂向黏滞系数的垂向结构出现以下变化：峰值和均值减小，峰值出现位置降低，峰值以上位置快速衰减，整体湍流强度减弱。盐度层化对湍流的抑制程度与层化区的高度成负相关，与层化的强度成正相关。Abstract: Based on the measured high-frequency turbulence and salinity profile data from the South Passage of the Changjiang River Estuary in autumn 2020, how water salinity stratification modulates the turbulence viscosity coefficient, drag coefficient, and vertical velocity energy spectrum is quantified, to assess its influence on water-column turbulence. The observation station generally exhibits periodic stratification, with gradual transitions from mixing to stratification during the flood current, and from stratification to mixing during the ebb current. Salinity stratification significantly suppresses the intensity of turbulence in the bottom layer, leading to a decrease in the drag coefficient and viscosity coefficient of the bottom layer. Furthermore, as the height above the bed increases, the reduction in turbulence parameters caused by stratification becomes more pronounced. Spectral analyses of vertical velocity time series at multiple elevations indicate that stratification disproportionately damps low-frequency, large-scale eddies, and that the suppression strengthens away from the bed. Vertically, stratification reshapes the vertical eddy viscosity structure by lowering both the peak and the mean values, shifting the position where the peak occurs downward, and accelerating the decay above the peak, leading to an overall reduction in turbulence. The degree of turbulence suppression is negatively correlated with the height of the stratified region and positively correlated with the intensity of stratification.
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Key words:
- mixing and stratification /
- turbulence /
- eddy viscosity /
- drag coefficient
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图 1 研究区域概况和观测点站位
Fig. 1 Sketch of the study area and location of the observational station
图 2 流速垂向剖面随时间变化过程
图中E区域表示落潮流期，F区域表示涨潮流期
Fig. 2 Vertical profiles of current velocity from spring tide to neap tide
E marks ebb current and F marks flood current
图 5 潮周期平均盐度、盐度差值与分层系数随时间变化过程
Fig. 5 Time series of tidal mean salinity, surface-bottom salinity difference, and stratification index
图 6 表底层盐度差与拖曳系数（a）、垂向黏滞系数（b）随时间变化过程
Fig. 6 Time series of surface–bottom salinity difference, drag coefficient (a), and vertical eddy viscosity (b)
图 7 底层特定位置的拖曳系数（a）、垂向黏滞系数（b）与表底层盐度差的关系
Fig. 7 Comparison of elevation-specific drag coefficient (a) and vertical eddy viscosity (b) to the surface–bottom salinity difference
图 8 混合和层化时刻的流速、盐度剖面（a）和底层垂向流速功率谱（b）对比
Fig. 8 Velocity and salinity profiles (a) as well as power spectra of near-bed vertical velocity (b) corresponding to mixed and stratified conditions
图 9 弱水动力情况下典型时刻流速、盐度和Az剖面对比
Fig. 9 Profiles of current velocity, salinity, and vertical eddy viscosity at representative times under weak hydrodynamic conditions
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