Tuesday, 12 August 2003: 11:15 AM

Improved tropical cyclone circulation centers derived from the GBVTD-simplex algorithm

Michael M. Bell, NCAR, Boulder, CO; and W. C. Lee

Poster PDF
(166.4 kB)
The GBVTD-simplex algorithm can objectively estimate a tropical cyclone (TC) circulation center for a constant altitude PPI (CAPPI) by maximizing the GBVTD-retrieved mean tangential wind from a single Doppler radar velocity field. The algorithm finds the vorticity center by conducting a simplex search from multiple initial guesses. The mean center location and tangential wind for each radius are then calculated from the solutions, and points beyond one standard deviation from the mean are removed. The center, wind, and standard deviation are then recalculated with the final number of points. A subjective examination of the mean tangential wind for different radii can then provide a best estimate of the radius of maximum wind(RMW) and corresponding circulation center. Statistical agreement of solutions from different initial guesses may provide a confident estimate, but for some storms factors such as weak velocity gradients, radar data gaps, and strong asymmetries may result in a difficult selection of the RMW and the TC center. An objective method for selecting centers that provides a high confidence level and ensures continuity over time could therefore augment the simplex algorithm, especially in an operational setting.

This paper describes a method in which the GBVTD-simplex parameters of mean tangential wind, number of converging centers, and standard deviation of those centers, are used to objectively select a TC circulation center. The method first assigns a score to each parameter for each radius at a particular height. Different weighting schemes are then applied to get a combined score for each radius, from which one can select the RMW with the highest score at each height. Then independent least squares polynomial curve fits are applied to the RMW, x and y coordinates of the corresponding centers, and mean tangential wind over an arbitrary period of time. A set of membership functions is then obtained from each of the four curves. The circulation centers that have the highest total membership in the resulting functions are selected, thus striking a balance between confidence level and continuity over time. Results from Hurricane Danny (1997) have shown consistent center selections from WSR-88D radars in Mobile, Alabama (KMOB) and Slidell, Louisiana (KLIX), and good agreement with dual Doppler derived circulation centers.

Supplementary URL: