Impact of Coupled Ocean-Atmosphere Model Simulations on the Mesoscale Environment of Six Nor'easter Cases

An investigation into how ocean-atmosphere coupling impacted Weather Research and Forecasting (WRF) model simulations of mesoscale features of six intense, winter time cyclones was completed. Model simulations included an uncoupled WRF run, a coupled 1D mixed layer ocean model, and a coupled, 3D full physics, ocean model simulation. Model simulations were conducted for 180 hours, starting roughly 72 hours prior to the first precipitation impacts in the highly populated Mid-Atlantic US and associated cyclogenesis. Simulation accuracy was evaluated by comparing each model run to Global Forecasting System model operational analysis.

Over the entire simulation period, WRF-ocean coupling altered the magnitude of sea-level pressure, 10 m winds, and 500-hPa geopotential height by up to 6.0 hPa, 14.5 m s^-1, and 45 m, respectively, relative to uncoupled WRF simulations. Similar to coupled ocean-atmosphere hurricane studies, model coupling during intense, wintertime cyclones led to no notable alterations in simulated storm track. Despite this result, simulated cyclone intensity more closely matched model analysis due to more realistic simulations of mid-tropospheric latent heating. Ocean-atmosphere coupling reduced local-storm environment simulation errors in 56.5% of all analyzed time periods versus uncoupled WRF simulations. Analysis over the entire WRF model domain (i.e., differences in dry energy norms, RMSEs, and various atmospheric fields) however indicated only subtle impacts to both simulation outcome and accuracy. Overall atmospheric impacts from ocean-atmosphere coupling were primarily limited to regions from the Appalachian Mountains and points eastward, below 500 hPa, and impacted mesoscale features such as the low-level jet (up to 10 m s^-1 stronger), and precipitation patterns (up to 40 mm total). Consistent with the RMSE results, these variations however did not lead to wide-scale alterations to WRF-simulated dynamical fields.