Density structure of the fine surface water layer of large lakes and remote measurement of the temperature

Abstract

Remote sensing of surface temperatures in large lakes differs in some critical respects from that in marine conditions. Firstly, the atmospheric 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 thermal 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 fine strip with steep horizontal temperature gradient. The existence of a thermal front makes it possible to calibrate infra-red space survey data without using in situ surface temperature measurements. The calibration of arbitrary heat brightness values for +4 ºC can be obtained by plotting a frequency distribution histogram of the heat brightness values obtained for the IR-image pixels. While the thermal front is in existence, the heat brightness frequency distribution appears bimodal, and the brightness value at the minimum between the peaks is assigned the temperature of +4 ºC. Laboratory and field experiments have enabled us to separate two types of near-surface density structures. Under conditions of density instability in the fine surface layer, microconvection limits the magnitude of vertical temperature difference in this layer to a constant range of 0.6–0.8 ºC. However, when a stable density structure prevails during calm conditions, much greater vertical temperature differences may exist.