P49     WRF model resolution impacts on a hydrodynamic model in near-shore environments.

Watson, Campbell D., Guillaume Auger, Harry R. Kolar, and Lloyd A. Treinish, IBM Research, TJ Watson Research Center

The increasingly urgent problem of harmful algal blooms (HABs) in lake and near-shore coastal environments has placed renewed focus on the coupled simulation of weather, hydrology and hydrodynamics. To date, modeling efforts at the Great Lakes and elsewhere have provided results generally consistent with observations, however, hydrodynamics closer to the shoreline – where HABs commonly originate – are often not well simulated. The poor near-shore hydrodynamic simulation can often be attributed to a weather model running at a resolution too coarse to properly resolve airflow dynamics at the land-water interface. To interrogate this issue, the present study uses Lake George, NY, a medium sized lake (55 x 3 km) in the eastern Adirondacks, as a testbed to quantify and understand the impact of weather model resolution on near-shore hydrodynamics. Lake George is the subject of The Jefferson Project, a collaborative effort between IBM Research, Rensselaer Polytechnic Institute and The FUND for Lake George.

The WRF-ARW V3 model is used to simulate the weather down to 0.33 km, and the SUNTANS Community Model is used to simulate the hydrodynamics with a variable horizontal grid of around 40 m. WRF and SUNTANS are coupled one-way. Results from a one-month case study in June 2017 examine the impact of WRF resolution on the simulated hydrodynamics of Lake George across three experiments: WRF at 3 km, 1 km and 0.33 km. Model results are compared to a lake-wide observation network. It reveals SUNTANS driven by WRF at 0.33 km provides much better representation of the lake temperature field and dynamics. For example, at the north end of the lake, the depth of the thermocline is more accurately simulated when using WRF at 0.33 km across the entire month, which results in a thicker mixed layer and thereby larger volume of water that is more conducive to phytoplankton growth. An observed downwelling event at the south of the lake is also better captured when using WRF at 0.33 km; this event is of importance to HABs-related research because immediately after the relaxation of the downwelling, the observed chlorophyll-a profiles show an increased concentration within the mixed layer. Reasons why these differences emerge across experiments is explained via an energy budget that shows increasing the resolution of WRF modifies the transfer of energy between the atmosphere and lake surface.