TURBULENT MEASUREMENTS OF THE COASTAL SHALLOW WATERS WITH A FREE-RISING MICRO-SCALE PROFILER

摘要

While there have been many studies conducted to investigate relation between vertical strength of density stratification and tidal mixing, there is no consensus that explains actual vertical mixing in semi-enclosed sea by reason of difficulty in measuring the mixing directly in the field. Hence we developed a free-rising Micro-Scale Profiler (MSP). This MSP rises to the sea surface from the bottom by its self-buoyancy at a speed of 50 cm s~(-1). On this ascending way, it measures vertical micro-structures of vertical velocity shear, water temperature, conductivity and depth at a sampling rate of 100 Hz. We collected data of micro-structures with this MSP, water quality and current in Suo-nada sea that is the western part of Seto Inland Sea of Japan. As a result, in the study area a typical two-layer structure was formed: high-temperature low salinity in the upper layer and low-temperature high-salinity in the lower layer. However, the magnitude of the tidal current ranged from 13 to 33 cm s~(-1) in the surface layer and was considerable large. Moreover, results indicated that the value of vertical eddy diffusivity (K_(z)) just above the pycnocline was as large as 5×10~(0) cm~(2) s~(-1); however the value inside the pycnocline was very small, more than two orders of magnitude smaller than the values just above or just below the pycnocline. This fact suggests that the pycnocline functions as a wall and restricts the water exchange between the upper and lower layers. Moreover, the large value of K_(z) just above the pycnocline was caused by the increase in the turbulent energy dissipation (ε) that was induced by the turbulent velocity shear generated by the stress on the pycnocline wall.