Numerical Analysis of Sensitivity of Structure of the Stratification in Lake Biwa, Japan by Changing Meteorological Elements

Jinichi Koue,Tomohito Matsuo,Akira Kondo,Hikari Shimadera

doi:10.3390/w10101492

Abstract

Climatic factors such as air temperature and wind speed can affect the structure of stratification in Lake Biwa. In general, the rise in air temperature and the decrease in wind speed weaken the vertical mixing and strengthen the structure of the stratification, which interrupts the transport of the substances. However, how much the change of each climate element can influence the structure of the stratification is not clarified. Therefore, it is important to evaluate the effects of each element on the stratification quantitatively. In the present study, we investigated the effect of the change in air temperature and wind speed on the seasonal change of stratification in Lake Biwa by using a three-dimensional hydrodynamic model. Numerical simulations were carried out for a baseline case using realistic meteorological data from 2007 to 2012 and hypothetical cases using meteorological data with modified air temperature or wind speed for sensitivity analysis. The analysis showed that the increase and decrease in air temperature changed the vertical water temperature uniformly in almost all layers. Thus, the strength of the stratification is hardly changed. The increase and decrease in wind speed, however, altered the water temperature near the surface of the lake, so that it significantly influenced the stratification. The increase in wind speed made the water parcels of the surface layer well mixed, and the decrease in wind speed made the mixed layer thinner.

Highlights

Thermal structure and dynamics of mixing in lakes are mainly forced by energy exchanges between the atmosphere and water
The simulation results of the change in air temperature and wind speed can be compared with the baseline case each other to investigate the difference in strength of stratification and the period of stratification
The impacts of meteorological elements such as air temperature and wind speed on the structure of stratification in Lake Biwa, Japan were investigated by using the simulation outputs of the three-dimensional hydrodynamic model

Summary

Introduction

Thermal structure and dynamics of mixing in lakes are mainly forced by energy exchanges between the atmosphere and water. Madeline [27] reported that increasing the air temperatures and decreasing wind speeds may be large drivers to thermal structures in three morphometrically different lakes in the US by using the dynamic reservoir simulation model for water quality adopting a one-dimensional model. These one-dimensional models cannot express the horizontal distribution of stratification; there are a few papers that analyze the response of the thermal structure to meteorological elements by using the three-dimensional model. The results of numerical simulation of the flow field in Lake Biwa during the five years from 2007 to 2012 and the hypothetical cases using the meteorological data with changing the average temperature and wind speed were compared

Calculation Domain

Hydrodynamic Model

Initial Conditions

Boundary Conditions

Baseline Simulation

Results and Discussion

Evaluation of the Reproducibility of the Three-Dimensional Hydrodynamic Model

Heat Flux on the Water Surface at Imazu-Oki

Influence on the on Period of Existence of of theChange

Conclusions