The turbulent flow in shallow-water coastal regions controls substance transport and mixing within these areas. These turbulent processes get highly involved in processes such as air-sea exchanges, diapycnal transport, and sediment resuspension in the coastal ocean, which are key focuses of the marine environment and climate. In coastal regions, turbulence mixing is mostly active in the wind-wave-driven surface boundary layer (SBL) and tide-current-driven bottom boundary layer (BBL). Recent studies have shown that the SBL and BBL in shallow water can have strong interactions and may merge into one layer. However, the physical mechanisms behind such a process are not fully resolved, particularly the part associated with wave-current interactions. In this study, we use the large eddy simulation (LES) to investigate the evolution and merging of coastal SBL and BBL. LES can reproduce high-fidelity turbulent flow with extremely high spatial and temporal resolutions, directly resolving interactions between two boundary layers and the turbulent mixing processes. The relation between the merging and the diurnal external forcings is characterized, and the different regimes of vertical mixing before and after merging are further identified.
 

coastal ocean, boundary layer, large eddy simulation, diurnal forcing, Langmuir turbulence