[A-OS14] Coastal physical processes associated with mixing, eddies, internal waves

[AOS14-06] Vertical mixing and mass transport in a shallow lake, Lake Kasumingaura, Japan

*Eiji Masunaga¹, Tatsumi Kitamura², Sachihiko Itoh³ (1.Ibaraki University, 2.Ibaraki Kasumigaura Environmental Science Center, 3.The University of Tokyo)

Keywords:Mixing, Heat flux, Coastal areas, Dissolved oxygen

This study presents vertical mixing and associated mass transport processes in a shallow lake, Kasumigaura, Japan. Field campaigns were carried out using a mooring system, microstructure profiler (VMP250) and high-resolution tow-yo instrument (YODA Profiler) in summer, 2020. The mooring system consisted of high and low frequency sampling thermistors (SBE56s and TidbiT v2s), and Aquadopp current profiler. Strong stratification and vertical mixing occurred with periods of several hours to several days. The Richardson number was less than 1/4 when complete vertical mixing occurred (temperature difference in the water column is less than 0.5 degrees). The rate of turbulent kinetic energy dissipation rate (e) and vertical eddy diffusivity (K_r) were 10^-10–10^-9 W/kg and 10^-7–10^-6 m²/s, respectively, near the bottom boundary layer under weak wind stratified conditions.On the other hand, when vertical mixing occurred due to wind stress, e and K_r were 10^-8–10^-5W/kg and 10^-5–10^-3 m²/s, respectively. In addition to microstructure surveys, the eddy diffusivity was also estimated by two independent methods using high-frequency sampled temperature data: (1) K_rI18 = 0.09L_E²S, where L_E is the Ellison scale and S is the vertical velocity shear (Ivey et al., 2018); (2) a method with an assumption of L_O ~ L_E, where L_O is the Ozmidov length scale, viz., e = L_O²N³ ~ L_E²N³, K_rLE = G L_O²N ~ G L_E²N, where G is the mixing efficiency 0.2. The relationship between L_O and L_E were estimated from the microstructure surveys and K_rLE was computed from high frequency sampled temperature. K_rLE showed reasonable agreements with K_r and K_rI18. We analyzed time series of the vertical mixing using K_rLE. Stratification and mixing were repeatedly generated by daily cycle shortwave radiation and sea-breeze, respectively. Surface stratification was increased by shortwave radiation from the sunrise to noon. The wind stress was enhanced by see breeze in the afternoon, which results in strong vertical mixing. Convective mixing due to night-time cooling was supposed to be weaker than wind-induced mixing. Vertical oxygen transport from the surface layer toward the bottom layer was generated by the wind-induced mixing in the afternoon. In addition, vertical mixing also contribute to nutrient supplies from sediments leading to eutrophication.