An Impact Study of NCEP GEFS TC Track Forecast

The Global Ensemble Forecast System (GEFS v11) has been upgraded in December 2^nd 2015 with following main changes: 1) Increasing horizontal and vertical resolutions; 2) Latest GFS model physics with semi-Lagrangian integration of advection instead of Euler integration; 3) Initial perturbations from EnKF 6-hour forecast instead of BV-ETR. The GEFS (v11) has best performance for large-scale circulation, summer precipitation forecast, surface temperature forecast, extreme weather event and TC track forecast overall. However, it degrades TC track forecast of day-6 and day-7 for Atlantic and East Pacific basins, and increases TC intensity forecast error (or weaker TC intensity) as well.

The study focuses on the extend-range (beyond day-5) TC track forecast for Atlantic basin. The four-way experiments have been designed for selected Hurricane cases those include: 1) GEFSv10 with BV-ETR initial perturbations; 2) GEFSv11 with EnKF-F06 initial perturbations; 3) GEFSv11 with BV-ETR initial perturbations; and 4) GEFS-10 with EnKF-F06 initial perturbations. Through the case study, we could identify the differences those are relatively contributed by initial condition/perturbations or numerical modeling. Meanwhile, an analysis of other atmospheric variables, such as lower level winds, surface pressure, water vapors and et. al. around tropical region, will help us to understand the impact from the changes of initial condition, model physics and numerical schemes.