TEMPERATURE DISTRIBUTION AND CURRENT SYSTEM IN A CONVECTIVELY MIXED LAKE

During spring and autumn, many lakes in temperate latitudes experience intensive convective mixing in the vertical, which leads to almost is othermal conditions with depth. Thus the regime of turbulence appears to be similar with that characteristic of convective boundary layers i n the atmosphere. In the present paper a simple analytical approach, b ased on boundary-layer theory, is applied to convective conditions in lakes. The aims of the paper are firstly to analyze in detail the temp erature distribution during these periods, and secondly to investigate the current system, created by the horizontal temperature gradient an d wind action. For these purposes, simple analytical solutions for the current velocities are derived under the assumption of depth-constant temperatures. The density-induced current velocities are shown to be small, in the order of a few mm/sec. The analytical model of wind-driv en currents is compared with field data. The solution is in good quali tative agreement with observed current velocities under the condition that the wind field is steady for a relatively long time and that resi dual effects from former wind events are negligible. The effect of the current system on an approximately depth-constant temperature distrib ution is then checked by using the obtained current velocity fields in the heat transfer equation and deriving an analytical solution for th e corrected temperature field. These temperature corrections are shown to be small, which indicates that it is reasonable to describe the te mperature distribution with vertical isotherms.