During spring and autumn a special thermohydrodynamic phenomenon, called a thermal bar, can be observed in dimictic lakes. [A dimictic lake is a lake where a complete turnover of the water occurs twice a year (in spring and autumn).] The thermal bar is a zone of mixing that separates waters with temperatures above and below 4° C. Its practical importance is due to the possibility that it might isolate the nearshore region from the offshore region (as seen from the thermal bar), which may cause a serious pollution problem along the beach. This article presents a study of the thermal bar and its temporal behavior in Lake Ladoga, Russia, using NOAA-AVHRR CH4/CH5 data. As the thermal bar is associated with a rather characteristic temperature pattern, that is, relatively large temperature changes from the shore to the bar and weak gradients beyond implying almost isothermal conditions, satellite data can provide a good means of detecting the existence of a thermal bar. The satellite-derived water surface temperature distributions, calibrated with some in situ temperature data, show that the thermal bar progresses from the shallow southern region to the deep northern part of the lake. The thermal bar progression during spring, obtained from sequences of satellite images, is also compared with predictions from a simple theoretical model. The model assumes that the heat entering through the surface of a water column remains within the column (i.e., horizontal heat fluxes are neglected) and that complete vertical mixing takes place by convective water movements. The comparison shows a good agreement between satellite-derived and model-predicted thermal bar locations (although there are uncertainties as to certain inputs to the model). This indicates that advective and diffusive horizontal heat fluxes are not of primary importance for the thermal bar displacement and that the model is a good first approximation for predictions of the thermal bar location.
Subject classification (UKÄ)
- Oceanography, Hydrology, Water Resources