Insights on Simulating Summer Warming of the Great Lakes: Understanding the Behavior of a Newly Developed Coupled Lake-Atmosphere Modeling System

The Laurentian Great Lakes are the world's largest freshwater system and regulate the climate of the Great Lakes region, which has been increasingly experiencing climatic, hydrological, and ecological changes. An accurate mechanistic representation of the Great Lakes thermal structure in Regional Climate Models (RCMs) is paramount to studying the climate of this region. Currently, RCMs have primarily represented the Great Lakes through coupled one-dimensional (1D) column lake models; this approach works well for small inland lakes but is unable to resolve the realistic hydrodynamics of the Great Lakes and leads to inaccurate representations of lake surface temperature (LST) that influence regional climate and weather patterns. This work overcomes this limitation by developing a fully two-way coupled modeling system using the Weather Research and Forecasting model and a three-dimensional (3D) hydrodynamic model. The coupled model system resolves the interactive physical processes between the atmosphere, lake, and surrounding watersheds; and validated against a range of observational data. The model is then used to investigate the potential impacts of lake-atmosphere coupling on the simulated summer LST of Lake Superior. By evaluating the difference between our two-way coupled modeling system and our observation-driven modeling system, we find that coupled-lake atmosphere dynamics can lead to a higher LST during June-September through higher net surface heat flux entering the lake in June and July and a lower net surface heat flux entering the lake in August and September. The unstratified water in June distributes the entering surface heat flux throughout the water column leading to a minor LST increase, while the stratified waters of July create a conducive thermal structure for the water surface to warm rapidly under the higher incoming surface heat flux. This research provides insight into the coupled modeling system behavior, which is critical for enhancing our predictive understanding of the Great Lakes climate system. Plain Language Summary The Laurentian Great Lakes, a tremendously large body of freshwater, have a profound effect on the regional climate. Therefore, climate simulations for the Great Lakes region, including climate projections, rely on an accurate simulation of the Great Lakes. However, traditionally, the Great Lakes within Regional Climate Models have been simulated using a one-dimensional (1D) lake model. This 1D approach leads to a poor simulation of the lake surface temperature (LST) and eventually affects the climate simulation. In this study, we introduce a newly developed regional climate modeling system that allows for a two-way exchange of information between the Weather Research and Forecasting (WRF) model and a three-dimensional (3D) model of the Great Lakes. This system is able to accurately reproduce the historical conditions of the Great Lakes. Additionally, to improve our understanding of coupled model behavior, we investigated how the two-way interaction between WRF and the lake model affects the simulation of summer LST in Lake Superior. We discovered that the two-way interaction between the models can lead to higher estimates of summer LST through changes in the surface heat fluxes.