2:00 PM - 2:15 PM
[AHW28-13] Mechanisms of internal waves propagation and turbulent mixing in a lake
Keywords:freshwater lake , turbulent mixing , internal wave propagation, fluid dynamics , Fukushima , Lake Inawashiro
Lake Inawashiro, located in central Fukushima Prefecture, is a relatively large lake with a diameter of approximately 10 km and a maximum depth of about 100 m. The lake is known for its acidic tendency, which is influenced by volcanic materials flowing in from rivers. However, recent reports have indicated that this characteristic is changing due to variations in river discharge, and internal lake circulation and mixing, as a consequence of the gross change of water mass in the lake.
In this study, we conducted field observations to investigate the mixing processes of the lake from the perspective of flow characteristics, aiming to achieve both qualitative and quantitative understanding.
An analysis of mooring observation data collected near the lake center in 2020 revealed a prominent periodicity of approximately 19.7 hours during the summer (Panel a). A significant structural difference in periodic variations was observed between summer and winter: in summer, the phase structure of horizontal velocity propagated vertically, suggesting that wave energy was transferred from the upper layer to the lower layer (Panel b). In contrast, in winter, short-term periodic variations weakened, the vertical phase structure became nearly upright, and energy transfer was minimal.
Turbulence measurements across the lake's diagonal in June 2023 during the stratified period indicated a notable increase in the turbulent energy dissipation rate (ε) at the depth of the thermocline (Panels c,d). This increase is interpreted as the result of mixing energy being converted by the internal wave propagation observed in 2020. On the other hand, observations in December of the same year showed that ε levels at the sub-surface depth around 10-20 m were reduced to approximately one-quarter of the June values, likely due to differences in recent wind conditions.
Furthermore, a three-dimensional numerical simulation was conducted to analyze the lake's response to strong wind events during the summer stratified period (Panel e). In experiments with northward wind forcing, surface water was pushed southward, followed by the propagation of high-temperature patches along the coast to the right, indicating the presence of internal Kelvin waves. Near the Nagase River estuary, however, the propagation path of these high-temperature patches deviated, and signals were transmitted toward the opposite shore. This phenomenon suggests that the embayment structure near the river mouth is unsuitable for internal Kelvin wave propagation, causing a transformation into internal gravity waves.
These findings indicate that the generation of internal Kelvin waves induced by wind-driven surface water displacement, combined with the propagation of internal gravity waves through the lake interior regardless of topography, may maintain the overall distribution of turbulent mixing across the lake. Future work will focus on a more detailed investigation of the turbulent energy distribution, particularly in the vicinity of the Nagase River estuary.
In this study, we conducted field observations to investigate the mixing processes of the lake from the perspective of flow characteristics, aiming to achieve both qualitative and quantitative understanding.
An analysis of mooring observation data collected near the lake center in 2020 revealed a prominent periodicity of approximately 19.7 hours during the summer (Panel a). A significant structural difference in periodic variations was observed between summer and winter: in summer, the phase structure of horizontal velocity propagated vertically, suggesting that wave energy was transferred from the upper layer to the lower layer (Panel b). In contrast, in winter, short-term periodic variations weakened, the vertical phase structure became nearly upright, and energy transfer was minimal.
Turbulence measurements across the lake's diagonal in June 2023 during the stratified period indicated a notable increase in the turbulent energy dissipation rate (ε) at the depth of the thermocline (Panels c,d). This increase is interpreted as the result of mixing energy being converted by the internal wave propagation observed in 2020. On the other hand, observations in December of the same year showed that ε levels at the sub-surface depth around 10-20 m were reduced to approximately one-quarter of the June values, likely due to differences in recent wind conditions.
Furthermore, a three-dimensional numerical simulation was conducted to analyze the lake's response to strong wind events during the summer stratified period (Panel e). In experiments with northward wind forcing, surface water was pushed southward, followed by the propagation of high-temperature patches along the coast to the right, indicating the presence of internal Kelvin waves. Near the Nagase River estuary, however, the propagation path of these high-temperature patches deviated, and signals were transmitted toward the opposite shore. This phenomenon suggests that the embayment structure near the river mouth is unsuitable for internal Kelvin wave propagation, causing a transformation into internal gravity waves.
These findings indicate that the generation of internal Kelvin waves induced by wind-driven surface water displacement, combined with the propagation of internal gravity waves through the lake interior regardless of topography, may maintain the overall distribution of turbulent mixing across the lake. Future work will focus on a more detailed investigation of the turbulent energy distribution, particularly in the vicinity of the Nagase River estuary.