Atmospheric vortex structures retrieved from single-Doppler radar observations. Part II: Objective method to deduce mean wind vector using the GVTD technique

In Part I of this paper, the formulation of the Generalized Velocity Track Display (GVTD) technique (Jou et al. 2007) and its advantages over the Ground-based velocity track display (GBVTD) technique in deducing kinematic structures of atmospheric vortices have been discussed. It has been shown that the GVTD technique is particularly effective in identifying the environmental mean wind as a set of parallel lines away from the vortex itself. This is a significant advantage over the GBVTD technique where only one component of the mean wind vector can be deduced. Being able to accurately deduce the vertical profile of the environmental winds is critical not only to improve the accuracy of the GVTD-retrieved kinematic vortex structure but also to aid the interpretation of vortex structure and dynamics under the influence of vertical wind shear. In this paper, an automatic iterative method, based on the GVTD technique, is developed to solve the mean wind from the observed VdD/RT display data. Experiment with an ideal Rankine vortex embedded within a pre-assigned uniform mean wind environment is used for error analysis. The results show that the error of retrieved mean wind speed is less than 1 m/s and the mean wind direction is less than 100. These errors are sensitive to the distance between the vortex center and the radar, the noise of the data, the mean tangential wind speed, and the mean radial wind speed. The mean wind solver has been applied to a real typhoon case, Nockten (2004), using Hwa-Lien (HL) and Wu-Feng-Shan (WFS) radar data of Taiwan. The maximum mean tangential wind speeds of Nockten, retrieved using GBVTD method from these two radars respectively, are different with a magnitude of 14m/s. This large inconsistency has been suspected due to significant mean wind component perpendicular to the line connecting the radar and the typhoon center. It is demonstrated that after applying the mean wind solver, the newly retrieved maximum mean tangential wind speed difference from these two radars has been reduced to 1.2 m/s and is significantly less than what was obtained previously. The importance of the mean wind solver is also demonstrated by discussing the retrieved mean vertical wind shear on determining the precipitation structure change during landfall and the possible influence on the track of the storm while it is very close to Taiwan topography.

Oral presentation is preferred. Please arrange the Part I paper preceding this paper in the same session.