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, which
results in stratification. The strength of stratification substantially influences the transport of dissolved oxygen
from the surface to the bottom water of a lake. In recent years, the decrease in dissolved oxygen at the bottom of
the northern part of the Lake Biwa, the largest freshwater lake in Japan, has been observed. The main cause of
this is considered to be the change in stratification, which depends on weather and climate conditions. In the
present study, numerical simulations were carried out to investigate the effect of strong wind on the structure of
stratification in Lake Biwa. The baseline simulation was conducted using actual meteorological data, and
experimental 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 stratification
collapses and the wind can enhance the vertical mixing in the bottom layer even in the summer season with
strong 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 wind
extends, vertical mixing easily occurs.