Annual heat balance and equilibrium temperature of Lake Aegeri, Switzerland

Abstract

The mean heat budget of Lake Aegeri, Switzerland, is 950 MJ·m−2, comparable to that of neighbouring lakes. The annual variation in the net heat flux can be adequately described using a six-term heat balance equation based on 12 years of monthly mean meteorological and surface temperature data. Although the magnitude of the net heat flux is dominated by the radiative terms of the equation, the one-month backward shift of the net flux and total heat content extrema from the solstices and equinoxes respectively is due to the phase shift of the non-radiative with respect to the radiative terms. A linear approximation was used to express the net heat flux in terms of a heat exchange coefficient and an equilibrium temperature. The former varies from 17 to 28 W·m−2·K−1 in the course of a year; fluctuations in the latter are found to depend mainly on fluctuations in cloud cover and relative humidity, whilst the effect of fluctuations in air temperature and wind speed is slight.