Typical near surface layer current patterns in Lake Geneva’s main basin (Grand Lac)

Tuesday, 16 December 2014
Amir Razmi, D. Andrew Barry, Damien Bouffard and Ulrich Lemmin, EPFL Swiss Federal Institute of Technology Lausanne, Lausanne, Switzerland
The Lake Geneva current field is controlled by a variable wind field and thermal stratification. Typical current patterns in the Grand Lac which is oriented roughly in the East-West direction were determined under real time wind conditions and thermal stratifications using the DELFT3D-FLOW hydrodynamic model. The model solves the Reynolds-Averaged Navier-Stokes equations, combined with a k-ε turbulence closure in σ (lakebed-following) coordinates. The model was forced by a space-time dependent wind stress and heat flux over a 2.2 km × 2.2 km horizontal grid (interpolated onto the model grid) provided by the Swiss Meteorological office (COSMO2, MeteoSwiss) in 2012. Modeling results revealed stronger circulation for the large scale gyres in the non-stratified season (winter), compared with the stratified season (summer). A clockwise circulation pattern in the western part and a counter-clockwise current pattern in the central part of the Grand Lac are identified under large scale north-easterly winds (Bise), which occur frequently. For large scale south-westerly winds (Vent), another frequent wind, a long-fetch (up to ~ 30 km) leads to a clockwise circulation pattern in the central part of the main basin while a counter-clockwise recirculating gyre is induced in the eastern part. Strong eastward currents are observed near the Grand Lac north-shore. The simulation results compared well with ADCP data and time series of temperature profiles from the middle of the Grand Lac.