DENSITY STRUCTURE OF THE FINE SURFACE-WATER LAYER OF LARGE LAKES AND REMOTE MEASUREMENT OF THE TEMPERATURE

Remote sensing of surface temperatures in large lakes differs in some critical. respects from that in marine conditions. Firstly, the atmosp heric structure over large lakes has some continental features, which makes the calculation of its influence more complicated. Secondly, the vertical temperature structure in the fine surface layer of a lake is more complex than in marine waters. A prominent feature of the therma l structure of spring heating of large lakes (e.g. in Lake Ladoga from May to July) is the existence of a thermal front, which is manifested on the lake surface as a tine strip with steep horizontal temperature gradient. The existence of a thermal front makes it possible to calib rate infra-red space survey data without using in situ surface tempera ture measurements. The calibration of arbitrary heat brightness values for +4 degrees C can be obtained by plotting a frequency distribution histogram of the heat brightness values obtained for the IR-image pix els. While the thermal front is in existence, the heat brightness freq uency distribution appears bimodal, and the brightness value at the mi nimum between the peaks is assigned the temperature of +4 degrees C. L aboratory and field experiments have enabled us to separate two types of near-surface density structures. Under conditions of density instab ility in the fine surface layer, microconvection limits the magnitude of vertical temperature difference in this layer to a constant range o f 0.6-0.8 degrees C. However, when a stable density structure prevails during calm conditions, much greater vertical temperature differences may exist.