Evaluation of the Coupled MOM6 Ocean Model Component during the 2023 HAFS Real-Time Experiments

The Hurricane Analysis and Forecast System (HAFS) is a community based and developed tropical cyclone (TC) modeling and data assimilation system that transitioned to operations in late June 2023. The operational, coupled HAFSv1 implements two-way coupling between the FV3 atmospheric model and the HYCOM ocean model. Exchanged variables like surface heat and momentum fluxes (atmosphere to ocean) and sea surface temperature (SST, ocean to atmosphere) are coupled at 6 minute intervals. Retrospective runs of HAFSv1 showed improvement in TC intensity forecasts compared to other coupled, regional hurricane models, HWRF and HMON.

For HAFS development under the Unified Forecasting System (UFS) framework, a new version of the system HAFSv1.1A implements MOM6 ocean coupling, along with a higher resolution moving nest, warm-cycling capability, Thompson microphysics, and other updates. For the 2023 hurricane season, HAFSv1.1A was run in real-time, parallel to the operational system.

The primary objective of this study is to evaluate the coupled ocean response from MOM6 during the 2023 real-time experiment in comparison to the operational HAFS configuration and available observations and analyses. The upper-ocean density and thermodynamic properties within coupled models are integral to improve TC intensity forecasts. Therefore, the ocean analysis will focus on upper-ocean metrics that affect air-sea heat fluxes, such as the simulated mixed layer depths, ocean heat content (OHC) and temperature and salinity vertical profiles.

The MOM6 ocean model evaluation will use satellite-based SST and OHC products to monitor large-scale ocean features. A point-wise analysis will leverage in situ observations such as gliders, Argo floats, buoys, XBTs and Saildrones that are available as part of ocean surveys during the 2023 hurricane season.