Ice-covered Lake Onega: effects of radiation on convection and internal waves

Early-spring under-ice convection in the Petrozavodsk Bay of Lake Onega (Russia) was investigated as part of an interdisciplinary research project conducted during March 2015. Measurements performed using a thermistor chain and vertical profiling sensors were used to examine temperature dynamics in the convectively mixed and stratified layers of the lake. Radiative transfer through the ice was high leading to a large convective mixed layer (up to 20 m deep) during daytime. Convective velocity was evaluated using two different methods. It is shown that convective velocity (a maximum value of ~7.4 mm s−1, and daytime average of 3.9 mm s−1) is completely damped during the restratifying night hours. We observed internal waves in the thermocline below the convective mixed layer with intriguing variations between night and day. Maximum of internal wave energy was found to start in the afternoon and continue long after the end of solar radiation forcing. Our analysis indicates that local convective processes are key forcing mechanisms for the generation of internal waves in ice-covered lakes. We also hypothesize that spatial differential heating between the nearshore regions and the centre of the bay (e.g. density current intruding the thermocline) could be a source of internal waves in ice-covered lakes.