Radiation transfer and heat budget during the ice season in Lake Pääjärvi, Finland

Lake Pääjärvi, a boreal Finnish lake, was investigated in winter for weather conditions, structure and thickness of ice and snow, solar radiation, and under-ice current and temperature. Heat budget of Lake Pääjärvi in January–March was governed by terrestrial radiation losses of 20–35 W m−2 recompensed by ice growth of 0.5–1.0 cm day−1. In April, snow melted, albedo decreased from 0.8 to <0.1, and the mean ice melt rate was 1.5 cm day−1. Internal melting and surface melting were about equal. The mean turbulent heat loss was small. The heat flux from the water to ice was about 5 W m−2 in winter, increasing to 12 W m−2 in the melting season. The light attenuation coefficient was 1.1 m−1 for the congelation ice (black ice) in winter, compared with 1.5 m−1 for the lake water, and it was up to 3 m−1 for candled congelation ice in spring, and about 10 m−1 for superimposed ice (white ice) and snow. Gas bubbles were the main factor that reduced the transparency of ice. The radiation penetrating the ice heated the water body causing convective currents and horizontal heat transfer. This increased the temperature of the water body to about 3°C before the ice break-up. After the snow had melted, the euphotic depth (the depth of 1% surface irradiance) was estimated as 2.0 m, only two-thirds that in summer.