Title: Evaluating and improving modeled turbulent heat fluxes across the North American Great Lakes

Abstract Turbulent fluxes of latent and sensible heat are important physical processes that influence the energy and water budgets of the North American Great Lakes. These fluxes can be measured in situ using eddy covariance techniques, and are regularly included as a component of lake-atmosphere models. To help ensure accurate projections of lake temperature and circulation and regional meteorology, here we validate the output of five algorithms used in three popular models to calculate surface heat fluxes; the Finite-Volume Community Ocean Model (FVCOM, with three different options for heat flux algorithm), the Weather Research and Forecasting (WRF) model, and the Large Lake Thermodynamics Model. These models are used in research and operational environments and concentrate on different aspects of the Great Lakes’ physical system. We isolated only the code for the heat flux algorithms from each model and drove them using meteorological data from four over-lake stations within the Great Lakes Evaporation Network (GLEN), where eddy covariance measurements were also made, enabling co-located comparison. All algorithms reasonably reproduced the seasonal cycle of the turbulent heat fluxes, but all of the algorithms except for the Coupled Ocean Atmosphere Response Experiment (COARE) algorithm showed notable overestimation of the fluxes in fall and winter. Overall, COARE had the best agreement with eddy covariance measurements. The four algorithms other than COARE were altered by updating the parameterization of roughness length scales for air temperature and humidity to match those used in COARE, yielding sensible and latent heat fluxes with improved agreement with observations.