Ice-covered Lake Onega: effects of radiation on convection and internal waves
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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.
KeywordsIce-covered lake Solar radiation Radiative heating Convective mixed layer Under-ice convection Internal waves
The present study was supported by the FEEL Foundation, “Fondation pour l’Etude des Eaux du Léman”. The authors would like to thank Andrey Mitrokhov, Nikolay Palshin, Andrey Georgiev, Alexey Tolstikov, Andrey Balagansky and Maksim Potakhin (Northern Water Problems Institute, Karelian Research Centre, Russ. Acad. Sci.) for their efforts in collecting observational data. We also would like to thank Vasili Kovalenko the head of the expedition and Nikolay Filatov. First author thanks Marie-Elodie Perga for fruitful discussions and help in the field, and all scientists from the Life Under the Ice project (http://wiki.epfl.ch/ladoga). We finally thank the two anonymous reviewers for their very valuable comments.
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