Heat Exchange at the Water–Ice Interface and Ice Growth in a Small Boreal Lake

A new technique has been developed for estimating the thickness of black ice and calculating the heat flux at the water–ice interface. An analysis of data from an autonomous device consisting of three thermistor chains measuring the temperature of black ice, water, and the upper layer of bottom sediments is presented. The device was installed in a small lake during six winter seasons. The vertical temperature distributions were averaged for each day and approximated by smooth functions separately for water and ice. The obtained functions were used to calculate the thickness of black ice, the temperature in the under-ice buffer layer (up to 1.0 m thick) and in the thin laminar water layer adjacent to the ice (from 5 mm to 1–2 cm thick) for each day, and to estimate the heat flux from water to ice. A change in the thickness of the gradient and laminar layers over the lake area and during the winter and spring seasons was revealed. During the first weeks of freeze-up at shallow-water stations, heat fluxes from water to ice were 0.5–1.5 W/m² and reached 3 W/m² above the deep-water basin in the zone of warm water upwelling. In the deep-water region, the heat flux into ice decreased during the winter as geostrophic circulation weakened and currents that generated warm water upwelling diverged. At shallow-water stations, the heat flux into ice increased during the winter as water temperature increased due to heat influx from bottom sediments. In the spring, as radiation-generated convection developed, the thickness of the subglacial gradient layer decreased in all areas of the lake to 30–50 cm, while the heat flux into ice increased to 8–25 W/m².