Measurement and Numerical Study of Vertical Mixing Microstructure in the Bohai Strait

Zhang, Y.; Liang, S., and Sun, Z., 2017. Measurement and numerical study of vertical mixing microstructure in the Bohai Strait. Vertical mixing plays an important role in three-dimensional hydrodynamic and water quality models. The purpose of this study is to analyze the characteristics of vertical mixing in strong tidal waters and the simulation capability of turbulence closure models under the influence of flow velocity variations and temperature distribution. The distribution characteristics of thermocline and vertical mixing in the Bohai Strait is analyzed through field observations conducted using the Turbulence Ocean Microstructure Acquisition Profiler in summer. Based on the measured temperature and mixing data, three-dimensional numerical models are established with various temperature distribution and turbulence closure schemes. It is concluded that the vertical eddy viscosity Km and the thermal-diffusion coefficient Kh, using the Mellor and Yamada level 2.5 (MY-2.5) and the k-ϵ turbulence closure models, exhibit a significant change with change in flow velocity and a time delay compared with the flow velocity process disregarding the effect of wind. The vertical mixing coefficients due to changing flow velocity under isothermality or temperature stratification could vary by one or two orders of magnitude, respectively. Simulated mixing coefficients using two different turbulence closure models exhibit different variations under isothermality and temperature stratification and have a significant discrepancy with the measurement data. Thermocline evolution is compared using three different methods for Km and Kh. It is found that the temperature mixes gradually using constant mixing coefficients in the vertical direction. When the mixing coefficients are computed with the MY-2.5 and k-ϵ models, the temperature mixes rapidly in the shallow-water zone. The mixing of temperature is even faster in the lower layer than in the upper layer in the deepwater zone, leading to the thermocline moving toward the upper layer. Moreover, the temperature mixes faster in the k-ϵ model than in the MY-2.5 model.