Analysis Of The Characteristics Of The Vertical Turbulent Diffusion Coefficient In Taihu

The stratification and mixing of lakes are the key physical process of heat,dissolved oxygen or nutrient transport and diffusion.The turbulent diffusion coefficient Kv defines the degree of turbulence in the water,which is a physical quantity of describing the exchange of heat,momentum,nutrients and gases.Based on temperature time series at depths of 0.2m and 1.5m,the changes in phase and amplitude of daily water temperature were explored by the harmonic analysis method.We estimated the vertical turbulent diffusion coefficients(Kv)of daily mean at the MLW and PTS stations in 2015.They were also compared with the result calculated by the Hostetler method.Then,the variation characteristics and influencing factors of Kv were analyzed.Finally,the potential effects of turbulence on heat transfer in the water column were explored.(1)The range of the Kv at the MLW and PTS stations calculated by the amplitude and phase methods was mostly between 10-2 and 10-6 m2/s,and the complex correlation coefficients between the results estimated by the two methods were 0.65 and 0.74(P<0.001).The complex correlation coefficients between the Kv calculated by the Hostetler method and those calculated by the amplitude or phase method were between 0.30 and 0.56(P<0.001).(2)The number of Kv in winter,which was higher than 10-4m2/s or 10-5m2/s,was more than those in the other seasons,so winter is more susceptible to the influence of turbulent mixing.Due to the influence of typhoon or strong wind,the high vertical turbulent diffusion coefficients may not follow this trend.The Kv at the PTS station,located in the center of the lake,was significantly higher than those at the MLW station located near the lake shore.(3)The net flux and wind speed,which are thermodynamic and dynamic conditions to drive the mixing or stratification,are the two key factors related to the vertical turbulent diffusion coefficients.The response of water body to net heat flux is slower than to wind speed on the half hourly scale,so the wind speed is the mainly meteorological factor affecting Kv,while the net flux is only a secondary factor,which is dirrerent from the results on the diurnal scale.(4)The overall intensity of turbulence is high in spring and winter,but it is low in summer.In most cases,the thermal changes inside the water column are divided into three stages in one day.In the stage of strong radiation,the thermal effect is dominated by radiation and supplemented by turbulence,which makes the temperature of the water column rise.In the stage of weakening radiation,with the significant turbulence,the upper layer begins to cool down and the lower layer keeps rising.In the stage of no radiation and evenly mixed water temperature,it is dominated by turbulence.Turbulence causes heat to be transferred from the bottom to the top,and the temperature of the water column decreases as a whole.In the cloudy and rainy days,turbulence plays a more significant role,and it is the dominant factor in the thermal changes of the water column during the day.