Identifying and quantifying deepwater-renewal processes during winter cooling in a large, deep lake (Lake Geneva)

N. Peng*, U. Lemmin, F. Mettra, R. S. Reiss, D. A. Barry

Ecological Engineering Laboratory (ECOL), Institute of Environmental Engineering (IIE), Faculty of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland

2 PM, Thu April 6th room 178, Ancienne Ecole de Médecine 1205 Genève

Field measurements and 3D numerical modeling were combined to investigate heat content dynamics in Lake Geneva’s large basin (Grand Lac, maximum depth 309 m) during an exceptionally cold spell in early 2012. This combined approach permitted identification of the lake’s heat content, which is strongly impacted by different winter cooling and deepwater renewal processes. The short duration of the intense cold spell was insufficient for full lake turnover. Indeed, temperature measurements indicated that convective cooling only reached a depth of less than 200 m. Nevertheless, lower temperatures near bottom implied lateral advection of cold water into the deepest layers. Numerical modeling allowed for identification and quantification of components of the lake’s heat budget via separating potential sources of deepwater renewal. It was found that two different lateral cooling processes contributed to deepwater renewal: cold water density currents discharged from: (i) Lake Geneva’s smaller and shallower basin (Petit Lac, maximum depth 75 m), and (ii) shallow littoral zones of the Grand Lac. These results question the common 1D convective vertical cooling concept considered as the primary control on deepwater heat content in large, oligomictic lakes. Instead, 3D lateral intrusions must also be considered. As climate change results in warmer winters and subsequently reduces vertical convective cooling in large lakes, it will become increasingly important to understand and quantify the interplay of these different 3D processes contributing to deepwater renewal.