Author(s): Nobuaki Kimura; Akira Tai; Wen-Cheng Liu; Jeng-Wei Tsai; Chih-Yu Chiu
Linked Author(s): Akira Tai
Keywords: Mixing; Heat content; Schmidt stability; PCA; Subtropical; Small lake
Abstract: Studying a mixing process in a stratified lake is helpful to understand biological, chemical, and physical processes. Statistical analyses were performed in a stratified, small, shallow lake in a subtropical alpine region (Yuan-Yang Lake in Taiwan) to determine what kind of physical factor is dominant for heavy-rainfall-induced mixing. This study focused on vertical mixing in entire water column and surface layer mixing reaching to diurnal thermocline. The effects of meteorological driving forces, such as wind, heating/cooling, and inflow, on the vertical mixing and surface layer mixing were evaluated using the relations between each force and the change rate of thermal stability between pre-mixing and mixing periods. For the surface layer mixing, a comparison between penetrative convection related to heating/cooling and wind-related friction velocity was conducted for each heavy rainfall event. For further investigation of inflow effects on the vertical mixing for heavy rainfall events, heat content that measures thermal potential energy was introduced. In the results from these examinations, wind input on vertical mixing was more significant than the other meteorological forcing factors for storm-dominant events; wind energy input in the surface layer was more dominant than the energy of heating/cooling for surface layer mixing; and the inflow effect on vertical mixing for larger extreme weather events (corresponding to lower air-pressure events) was also crucial for vertical mixing. However, the forcing by heating/cooling was likely to less contribute to mixing because the process of heating/cooling was likely less dynamic than the processes of wind and inflow inputs to internal response of the lake. In addition, a principal component analysis (PCA) modified by partial correlation was performed to quantitatively verify the aforementioned results. The first and second components, which accounted for>78%of the total variance in the PCA, showed that the intensity of vertical mixing was affected primarily by wind-induced turbulence and inflow intrusion, whose impacts were approximately equivalent, and was weakly associated with the effect of net heat flux.
Year: 2016