Water temperature dynamics and heat transfer beneath the ice cover of a lake

Water temperature profiles were measured every 2 min in a small and shallow urban Minnesota lake from 8 February 1989 until ice‐out on 11 April. The temperature record showed two distinct segments. Before 24 March, the temperature profile remained relatively constant, but later (when air temperatures were mostly above freezing) temperatures were dynamic and rose. During the quiescent period, the upper 3 m of the water column cooled slowly while the lower water remained virtually isothermal. The calculated thermal diffusion coefficient approached molecular thermal diffusivity near the ice cover and was roughly 3 times molecular diffusivity through much of the region of the thermocline (the upper 2 m). The average computed heat flux through the ice (50–55 cm thick) and snow (10– 30 cm deep) cover during this period was 2.8 W m−2. With the onset of above‐freezing air temperatures, temperatures began to change rapidly. Observed intrusions of warm water were most likely the results of advected inflows of warm water from the periphery of the lake.