Particle Dynamics in the Nearshore of Lake Michigan Revealed by an Observation-Modeling System

Given that few drifter experiments combined with a wave-current coupled model system had been conducted in the complex nearshore area, this work was motivated to reveal the nearshore dynamics by applying an observation-modeling system to Lake Michigan. Analysis of 11 surface drifters, wind, and current observations along the lake's eastern coast indicates that their trajectories are synergistically controlled by winds and initial releasing sites. Additionally, strong winds significantly impact nearshore dynamics, and the highly sensitive nearshore and offshore drifters are stranded in distinct regions. Simulations indicate that the model reproduces drifter trajectories and endpoints reasonably and that particle fates are mainly dominated by winds, while effects from heat flux and waves are also important. Further analysis of wave effects on particle dynamics indicates that both the wave-induced sea surface roughness and Stokes drift advection are crucial to the simulated particle trajectories during wind events. Finally, virtual experiments confirm that particle dynamics are evidently susceptible to winds and initial locations. Overall, both the inclusion of physics effects (e.g., adding winds, heat fluxes, and waves) and diminishing the model uncertainties (e.g., from various wind data sources, wind drag coefficient formulations, model grids, and vertical turbulent mixing parameterizations) are important methods to improve the particle simulations. The successful application of this nearshore observation-modeling system to Lake Michigan can be beneficial to the understanding of nearshore-offshore transports and larval and fisheries recruitment success in similar freshwater and estuarine environments. Plain Language Summary We combine surface drifter observations and computer simulations to understand movements of particles in the nearshore of Lake Michigan. Analysis of drifters' moving paths, wind, and current measurements in the summers of 2014 and 2015 indicates that drifter movements are strongly related to winds and initial releasing sites. Multiple computer programs were run to simulate the designed scenarios by artificially removing one of the influencing factors at each simulation from the model. We find that particle movements are collectively affected by winds, heat flux transfer between the air and lake water, and surface gravity waves. Further investigations demonstrate that both wave-induced sea surface roughness and Stokes drift advection are important to the simulated particle trajectories during wind events. After doing "what-if" scenarios in the computer model, distinct particle routes driven by wind-induced surface flows were simulated for virtual particles hypothetically released at various locations near the lake's southeastern coast. For example, nearshore group primarily shows longshore movements, while offshore one drifts into the deep basin. Overall, both the inclusion of physics effects (e.g., adding winds, heat fluxes, and waves) and diminishing the model uncertainties (e.g., from various wind data sources, wind drag coefficient formulations, model grids, and vertical turbulent mixing parameterizations) are important methods to improve the particle simulations. The outcome from this study will enhance the understanding of larval transport from nearshore to offshore areas in Lake Michigan and similar freshwater and estuarine environments.