16:45 〜 17:00
[AOS14-06] 霞ヶ浦における鉛直混合と物質輸送
キーワード：混合、熱輸送、沿岸域、溶存酸素
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 (Kr) 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 Kr 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) KrI18 = 0.09LE2S, where LE is the Ellison scale and S is the vertical velocity shear (Ivey et al., 2018); (2) a method with an assumption of LO ~ LE, where LO is the Ozmidov length scale, viz., e = LO2N3 ~ LE2N3, KrLE = G LO2N ~ G LE2N, where G is the mixing efficiency 0.2. The relationship between LO and LE were estimated from the microstructure surveys and KrLE was computed from high frequency sampled temperature. KrLE showed reasonable agreements with Kr and KrI18. We analyzed time series of the vertical mixing using KrLE. 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.