The application of the Extended-GBVTD method to derive the typhoon asymmetric structures

The Ground Based Velocity Track Display (GBVTD) technique is extended to two Doppler radars to retrieve the structure of a tropical cyclone's (TC) circulation. With this extension, it is found that the asymmetric part of the TC radial wind component can be derived up to its angular wave number one structure, and the accuracy of the retrieved TC tangential wind component can be further improved. Although two radar systems are used, a comparison with the traditional dual-Doppler synthesis indicates that using this Extended GBVTD (EGBVTD) approach, it is easier to estimate the mean flow over a larger area when there are missing data, and isolate the TC circulation. It is also suggested that the GBVTD method be applied to TCs over a flat surface (e.g. the ocean) where the assumption of relatively smaller asymmetric radial winds than asymmetric tangential winds is more likely to be true. When TCs can be observed by two radar systems, especially when the topographic effects are expected to be significant in the case of a landfalling typhoon/hurricane, the EGBVTD rather than the traditional dual-Doppler synthesis should be used. The feasibility of the proposed EGBVTD method is demonstrated by applying it to an idealized TC circulation model as well as a real case study. Finally, the possibility of combining EGBVTD with other observational instruments, such as dropsonde or wind profilers, to recover the asymmetric TC radial flow structures with even higher wave numbers is discussed.