NUMERICAL ASSESSMENT OF THE IMPACT OF STRONG WIND ON THERMAL STRATIFICATION IN LAKE BIWA, JAPAN

Abstract

Water temperature near the surface of a lake increases with increasing air temperature, whichresults in stratification. The strength of stratification substantially influences the transport of dissolved oxygenfrom the surface to the bottom water of a lake. In recent years, the decrease in dissolved oxygen at the bottom ofthe northern part of the Lake Biwa, the largest freshwater lake in Japan, has been observed. The main cause ofthis is considered to be the change in stratification, which depends on weather and climate conditions. In thepresent study, numerical simulations were carried out to investigate the effect of strong wind on the structure ofstratification in Lake Biwa. The baseline simulation was conducted using actual meteorological data, andexperimental simulations were conducted using meteorological data with modified wind speed and direction.The numerical experiments showed that if the magnitude of the wind is strong enough, the stratificationcollapses and the wind can enhance the vertical mixing in the bottom layer even in the summer season withstrong thermal stratification. In a stratified season, when the strong wind blows predominantly in one direction,the rate of the vertical mixing changes by the wind direction. Moreover, as the duration of the strong windextends, vertical mixing easily occurs.