Sunlight penetration dominates the thermal regime and energetics of a shallow ice-covered lake in arid climate

Abstract

The Mongolian Plateau is characterized by cold and arid winters with very little precipitation (snowfall), strong solar insolation, and dry air, but little is known about the thermal regimes of the ice and ice-covered lakes and their response to the distinct weather and climate in this region. In a typical large, shallow lake, ice and snow processes (cover) and under-ice thermodynamics were monitored for four winters in 2015–2019. Heat transfer at the ice–water interface and lake heat budget were investigated. The results revealed that persistent bare ice of 35–50 cm thickness transmits 20 %–35 % of the incident solar radiation into the water below. This is a dominant source for under-ice energy flows and causes/maintains high water temperature (up to 6–8 ∘C) and high heat flux from water to ice (averages of 20–45 W m−2) in mid-winter, as well as higher heat conduction in the ice interior during freezing. The heat balance shows that the transmitted radiation and the heat flux from water to ice are the dominant and highly correlated heat flows in the lake. Both bulk water temperature and temperature structure are sensitive to solar transmittance and occasional snow events. Under-ice convective mixing does not necessarily occur because of stratification of salinity in the water body. In particular, salt exclusion during freezing changes both the bulk salinity and the salinity profile, which plays a major role in the stability and mixing of the water column in this shallow lake.