The velocity track display (VTD) technique can be used to deduce tropical cyclone (TC) primary circulation from single Doppler radar data. In the original formulation, the VTD equations were closed by assuming that the asymmetric radial wind components were small so they could be ignored. When combining the VTD technique with a simplex algorithm, the vorticity center and the radius of maximum wind (RMW) of a TC can be objectively identified. Satisfactory vorticity centers and primary circulations were derived in wavenumber one TCs such as Typhoon Alex (1987) and Hurricane Danny (1997). However, the VTD-simplex technique encountered difficulties in deducing TC centers for wavenumber two TCs, for example, in Typhoon Herb. This difficulty is likely caused by the bias in the mean tangential wind from the wavenumber two radial wind, not from the wavenumber one radial wind. In wavenumber two TCs, this bias is significant enough to affect the results.

The purpose of this paper is to present an improved VTD formulation in an attempt to resolve the asymmetric radial winds. Recent theoretical studies on wavenumber two TCs and observational studies on the characteristics of Typhoon Herb suggested that the wavenumber two disturbance can be represented using the streamfunction derived in the textbook "Hydrodynamics" (Lamb 1932). Tangential and radial velocities can be derived from the streamfunction. Therefore, the phase and amplitude relationship between the tangential and radial winds at each radius can be derived and be used as additional constraints in the VTD formulation. Significant improvement on the vorticity center and primary circulation has been made when applying this improved VTD formulation to Typhoon Herb. Storm structures of Typhoon Herb will be presented in the conference.