An Investigation of Metrics Used to Determine the Center of Model Tropical Cyclones

As computing power increases, so do the grid spacings of numerical weather prediction models. Recent modeling studies have increased grid spacings in the core of tropical cyclones to on the order of tens of meters – large eddy scale. What these studies, along with improved observing platforms, have shown is that the inner core of hurricanes is anything but symmetric, with constant breakdown and reformation of a ring of vorticity that exists just inside the radius of maximum winds. Local vorticity maxima and pressure minima (mesovortices) form and decay during this process. In some instances, these mesovortices can exist for extended periods of time, rotating around the eye multiple times. This complicates the process of locating the center of the storm, which has significant implications for research of the storm – particularly when transforming winds from rectangular to cylindrical grids. Additionally, finding the center of the vortex at multiple levels is important for determining vortex tilt when the vortex is under the duress of shear.

There have been multiple attempts at finding a way to calculate the center of a vortex accurately. These methods can be divided up into three basic categories: local extreme, centroid weighting, and minimization of azimuthal variance (MAV). A fourth hybrid method – a centroid of a mass surface outside the core – is presented to act as a benchmark against which these methods can be compared. Each method has its advantages and disadvantages. This study will quantify the differences among the three on an adiabatic, barotropically-unstable vortex in shear and on high-resolution, full-physics hurricanes from the GFDL model. The metrics for comparison are the barotropic energy conversion term from the linearized energy equation and shear metrics of tilt and precession. It is shown that when using localized center-finding algorithms, the transient mesovortices become false centers, and this noticeably alters the barotropic energy conversion term and the measure of the evolution of the tilt.