A new method for vortex relocation within a balanced flow field

During the summer of 2011, AirDat and NCAR implemented a version of WRFDA with tropically tuned physics. A new vortex relocation scheme was developed to reconstruct the initial conditions with a true vortex position and flow field in the first guess.

Conventional methods typically involve either directly introducing a new vortex, or using the bogus data assimilation (BDA) scheme. While these methods consider storm axis symmetry and asymmetry, they do not directly consider the flow field in the first guess, which has the potential to delay or negatively alter the storm development.

Here, we employ the direct introduction of a new vortex, and add the real storm structure in first-guess to the new vortex under a specifically derived weight to obtain proper asymmetric storm structure within the first guess.

The vortex relocation steps are as follows: 1) Locate the vortex corresponding to the storm in the first-guess with an inaccurate storm initial location by analyzing maximum near-surface relative vorticity within a prescribed radial distance from the best track location. 2) Remove the first-guess vorticity and divergence within a specific radius of the first-guess storm. 3) Take the difference between 1 and 2 to isolate the vortex. 4) Insert a bogus storm with axis symmetry at the best track based on 2, while at the same time, relocate the isolated vortex from 3 to the best track, and determine a weight between the bogus storm and the axis symmetry vortex. 5) Finally, calculate the first-guess initial conditions with the accurate storm location and real flow field.

Results from this new methodology will be presented using multiple tropical storm and hurricane cases from 2011.