Intercomparison of Ground-Based Velocity Track Display (GBVTD)-retrieved circulation centers and structures of Hurricane Danny (1997) from two coastal WSR-88Ds

A plausible primary circulation and circulation center of a tropical cyclone (TC) can be deduced from a coastal Doppler radar using the ground-based velocity track display (GBVTD) technique and the GBVTD-simplex algorithm. The quality of the retrieved primary circulation is highly sensitive to the accuracy of the circulation center that can only be estimated from the degree of scattering of all possible centers obtained in GBVTD-simplex analyses from a single radar in real TCs. This study extends previous work to examine the uncertainties in the GBVTD-simplex-derived circulation centers and the GBVTD-derived primary circulations in Hurricane Danny (1997) sampled simultaneously from two Doppler radars [Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) in Mobile, Alabama, and Slidell, Louisiana] for 5 h. It is found that the mean difference between the individually computed GBVTD-simplex-derived centers is 2.13 km, similar to the estimates in previous studies. This value can be improved to 1.59 km by imposing time continuity in the radius of maximum wind, maximum mean tangential wind, and the center position in successive volumes. These additional physical criteria, not considered in previous work, stabilized the GBVTD-simplex algorithm and paved the way for automating the center finding and wind retrieval procedures in the future. Using the improved set of centers, Danny's axisymmetric tangential wind structures retrieved from each radar showed general agreement with systematic differences (up to 6 m s-1) in certain periods. The consistency in the wavenumber-1 tangential winds was not as good as their axisymmetric counterparts. It is suspected that the systematic differences in the axisymmetric tangential winds were caused by the unresolved wavenumber-2 sine components rather than from the relatively small cross-beam mean wind components in Danny.