Evaluation of Tropical Cyclone Structure in the GFDL fvGFS Model

Prediction of tropical cyclone structure and intensity is a critical component of numerical weather prediction.  In this study, the ability of GFDL’s new global model fvGFS (the Finite Volume Cubed Sphere (FV3) dynamical core with GFS physics) to represent tropical cyclones is assessed.  The fvGFS is a global model that can be used for both weather and climate prediction and research.

The composite structure of 25 simulated TCs from the Atlantic and Pacific basins during the 2015 hurricane season, using a version of the model with 13-km resolution, is compared with observational composites of TC structure.  In particular, the structure of TCs relative to the 850-200 hPa shear vector is evaluated.  The model correctly shows a maximum in vertical velocity in the downshear region, and this asymmetry becomes more pronounced as shear increases.  An evaluation of high-shear (shear > 12 m/s) cases that were intensifying vs. high-shear cases that were weakening indicates that the direction of the shear, especially relative to the storm motion, may play a role in regulating TC intensity change in high-shear scenarios.  High-shear intensifying cases experienced more northerly shear that tended to be closer to perpendicular to storm motion, while high-shear weakening cases tended to experience shear that was antiparallel to the storm motion, with a stronger westerly component. 

The ability of the model to simulate TCs at high-resolution was assessed by comparing 3-km nested simulations of TCs with 3-dimensional, high-resolution Doppler Radar data from the NOAA Hurricane Research Division.  Preliminary results indicate that the model is able to reasonably simulate track, intensity, and some of the observed structural features such as the eye size and slope of the eyewall.  Further work will add more cases to this evaluation of the high-resolution simulations, and potentially evaluate the impact of grid spacing on simulated TC structure.