INVESTIGATION OF THE SPRING THERMAL REGIME IN LAKE LADOGA USING FIELDAND SATELLITE DATA

We use a combination of field and satellite data to investigate the th ermal regime in Lake Ladoga in spring 1992. The analysis concentrates mainly on the temperature structure in the convectively mixed region a nd the dynamics of a special thermo-hydrodynamic phenomenon called ''t hermal bar'' (a zone of descending water with the temperature of maxim um density equal to approximately 4-degrees-C). A theoretical model of the temperature distribution in the convectively mixed region (temper atures <4-degrees-C) and the thermal bar progression is also applied a nd compared with satellite data. The study shows that the temperature offshore the 4-degrees-C isotherm in the convectively mixed region of the lake is almost depth-constant because of strong convective mixing and that the horizontal heat flux seems to be of minor importance in t his region. The comparison between predictions from the simple theoret ical model describing the temperature evolution and movement of the th ermal bar with observations showed a fairly good agreement. However, c alculations of the change of heat content in water columns along secti ons indicate heat transport from the nearshore, stably stratified regi on toward the 4-degrees-C isotherm which will affect the movement of t he thermal bar. This effect has been incorporated in an improved model of the thermal bar progression. A better agreement between prediction s and observations was also obtained with this model.