Turbulent flux calculation and shortwave radiation influences on depth-temperature profiles of Lake Superior

To further improve Great Lakes hydrodynamic-ice modeling, depth-temperature profiles for Lake Superior were analyzed using the observations and the unstructured grid Finite Volume Community Ocean Model (FVCOM) with the internally-coupled Los Alamos Sea Ice Model (CICE). FVCOM-CICE simulations were conducted across a series of turbulent flux algorithms and shortwave radiation data from the National Oceanic and Atmospheric Administration’s (NOAA) High Resolution Rapid Refresh (HRRR). The key values for comparison were timing of stratification, surface temperature, and depth of the thermocline. These three factors inform other physical properties of the lakes, such as ice cover. Overall model advancement is aimed toward the improvement of use-cases for marine navigation. In this investigation, those improvements are compared through the onset and offset of ice, total water level, and coastal storm surge. Input of turbulent flux algorithms included a calculation with a wind-speed parameterized roughness term from the Coupled Ocean-Atmosphere Response Experiment (COARE), and a legacy Great Lakes algorithm. Downward shortwave radiation with dynamic albedo and a pre-calculated net shortwave radiation from HRRR were compared as input for the shortwave radiation forcing. The model produced high-resolution, spatial data for the entirety of Lake Superior from 2018 to 2019. The output from the model was compared to depth-temperature measurements from two year-round mooring stations, one in Western Lake Superior and the other in central. The investigation shows higher agreement when using the latest COARE iteration as the heat flux algorithm and downward radiation with dynamic albedo calculations. This combination shows the greatest agreement with the in-situ temperature measurements in summer months, which show a clear depth and date of stratification. While measurements show well mixed winter profiles, the model simulates inverse stratification occurring from February to March 2018. Despite the difference of depth between the two mooring stations, there is not a significant amount of variation between the results of the model at the Western and central stations. The model advancement in this study will inform future versions of NOAA’s Great Lakes Operational Forecast System (GLOFS).