Numerical experiments to evaluate effect of external forces on temperature stratification in Lake Kasumigaura, Japan

Abstract

Lake Kasumigaura is the second largest lake in Japan, and it is shallow and highly eutrophic. Although variations in lake current have been revealed by field observations in previous research, numerical simulations should be adopted to further reveal the influence of external forces on the lake flow, and investigate the phenomena of temperature stratification and algal blooms. An unstructured grid, parallel computing coastal ocean model, SUNTANS, is utilized for modeling lake currents in this study. The horizontal resolution of the unstructured grid is approximately 200 m, and 30 layers are split vertically. The simulation results demonstrated that SUNTANS could reproduce observations in the lake and had a good performance for modeling the obvious diurnal stratification. Through a comparison of results of different cases, the difference in the temporal temperature change would indicate the driving effect of wind and the significant contribution of insolation to the formation of stratification. Particle-tracking results showed a strong relationship between the surface distribution of algal blooms and the wind condition. Wind driving combined with topographic conditions led to a tendency for particles to become trapped by a complicated shoreline and accumulate on the northern part of the lake. This trend was inferred to be one of the reasons why algal blooms often occur in those areas.