Exploring the Impact of Tropical Cyclone Relocation in the Operational NCEP GFS/GDAS

As the resolution of operational global numerical weather prediction systems continues to increase, tropical cyclone (TC) vortex initialization becomes increasingly important for yielding continuing improvements in the forecast of TC track and especially intensity.  The current operational GDAS/GFS utilizes a combination of vortex relocation, bogus wind assimilation, and assimilation of minimum sea-level pressure information driven by real-time information from operational TC forecast agencies. 

The vortex relocation scheme operates on the background forecasts prior to the assimilation of observations, and performs a procedure that extracts a vortex from the model large-scale environment flow and relocates it at the observed location.  While the scheme was demonstrated to reduce track errors in older versions of the GFS, recent sensitivity experiments show that for newer generations of the GFS this mechanical relocation process may produce increased track forecast errors at longer lead times despite reducing the initial position uncertainty.  

A sensitivity experiment performed for Hurricane Joaquin (2015) using the 2015 hybrid 3D EnVar-based GDAS system to initialize the operational resolution GFS was compared to an experiment that excludes the vortex relocation.  When the vortex relocation was turned off in the GDAS, the GFS track forecasts were improved by 35-45%.  These forecasts correctly showed more of a threat from Joaquin to the Bahamas and showed much less of an incorrect threat from Joaquin to the mainland United States.  Here we will present the results from a more comprehensive sensitivity study covering much of the 2015 hurricane season to examine the impact of the vortex relocation scheme on GFS track forecasts for a larger sample of systems across global TC basins.