Tracking Lagrangian transport in Lake Geneva: A 3D numerical modeling investigation

Abstract

Lake Geneva, the largest freshwater lake in Western Europe, is subject to important environmental pressures from its densely populated shores and watershed. To maintain and improve water quality in this lake, as well as in other enclosed or semi-enclosed basins, it is essential to understand and be able to predict how nutrients and pollutants are transported within it. A 3D numerical modeling study of Lagrangian transport in Lake Geneva is presented, showing the dispersion of water (based on tracking inert water particles) inflowing from the lake's main tributary, the Rhône River. The relation between dominant winds, circulation patterns, and transport was analyzed. The results demonstrated that transport within the lake is highly inhomogeneous in space and intermittent in time, because water mass movements are controlled by the wind-induced formation of large-scale gyres and their subsequent breakdown into smaller ones. Particle spreading was shown to be sensitive to the depth of the initial particle release, and to the mean depth of the particles' trajectory. However, several preferential pathways could be identified. Some water particles rapidly (days) traveled across the entire lake, through the near-shore region in the upper layer, while others remained trapped for months, particularly in the central region of the lake at depth. Deeper particles tended to remain longer in the lake, due to the insulating effect of stratification, bathymetry obstacles, and slower currents at greater depth.

Wintertime Coastal Upwelling in Lake Geneva: An Efficient Transport Process for Deepwater Renewal in a Large, Deep Lake

“…The inertial period at the latitude of Lake Geneva is approximately 16.5 hr. Numerous observational and numerical studies have shown that Coriolis force effects play an important role in determining the lake's hydrodynamics (e.g., Bauer et al, 1981; Bouffard & Lemmin, 2013; Cimatoribus et al, 2018, 2019; Lemmin, 2020; Lemmin et al, 2005).…”

“…The particle tracking results also suggest that most of the water masses upwelled in the nearshore area descended back to a maximum depth O(200 m) after the wind stress had ceased (Figures 9d and S5b). The trajectories of these descending water masses were found to be strongly affected by the dynamics of the large-scale motions in the lake, often characterized by large gyres (Cimatoribus et al, 2019), as seen in Figures 9c, 9d, and S5b. Forward particle tracking shows that due to the presence of these large-scale gyres, descending particles rapidly spread over a wide area in the Grand Lac basin, thus demonstrating that coastal upwelling during wintertime, even in a limited nearshore area, is an effective contributor to deepwater renewal over a much larger area.…”

“…To simulate the hydrodynamics of Lake Geneva, we employed a hydrostatic version of the MITgcm, which solves an incompressible Boussinesq form of the 3-D Navier-Stokes equations (Marshall et al, 1997). This code is widely used in the oceanographic community and has successfully been applied to lakes (Djoumna et al, 2014;Dorostkar et al, 2017;Dorostkar & Boegman, 2013), and recently to Lake Geneva (Cimatoribus et al, 2019(Cimatoribus et al, , 2018. Following Cimatoribus et al (2018), we ran a low-resolution (LR) and a high-resolution (HR) version of the 3-D model.…”

“…The inertial period at the latitude of Lake Geneva is approximately 16.5 hr. Numerous observational and numerical studies have shown that Coriolis force effects play an important role in determining the lake's hydrodynamics (e.g., Bauer et al, 1981;Bouffard & Lemmin, 2013;Cimatoribus et al, 2018Cimatoribus et al, , 2019Lemmin, 2020;Lemmin et al, 2005). Lake Geneva is subject to a variety of winds (local fishermen distinguish over 20 different winds by name) generated by differential heating, pressure differences across the Alps, and/or large-scale atmospheric pressure gradients (Bohle-Carbonell, 1991).…”

“…The model configurations were adapted from Cimatoribus et al (2018) and Cimatoribus et al (2019). The former study describes the configurations in detail and provides an in-depth model validation using (i) temperature and current velocity time series, and vertical temperature profiles, both recorded at a number of monitoring stations and moorings located around the entire lake, and (ii) temperature variance and kinetic energy spectra.…”