Single-Doppler Radar Analysis of a Mesocyclone in the Taiwan Strait

In this study, the kinematic and precipitation structure of a mesocyclone associated with a hook echo was analyzed by single Doppler radar data. The mesocyclone was embedded in a mesoscale convective rainband near northern Taiwan coast on 10 September 2004. The synoptic environment was characterized by a moderate CAPE and weak ambient vertical shear from surface to 5 km. In addition, a pronounced low-level mesoscale shear/convergence zone, which might result from the interaction of two tropical depressions, was also identified in the northwest coast of Taiwan.

The mesocyclone was retrieved by analyzing the dipole signature revealed from the single Doppler radar radial velocity data. GBVTD method proposed by Lee et al. (1999) was applied. The analysis shows that this mesocyclone formed initially at low level, then deepened and strengthened rapidly into mature stage with the vertical depth deeper than 8km and later decayed rapidly. The mesocyclone lasted for about 2 hour. The GBVTD-derived primary circulation shows the radius of maximum wind (RMW) of the mesocyclone was about 5~6 km and varied from inward tilting to outward tilting with time. The mean radial wind field was initially characterized with a low-level inflow inside RMW and outflow outside RMW respectively. The strongest reflectivity was associated with stronger updraft near RMW, and weak downdraft was located at the center of the cyclone. Subsequently the downdraft and reflectivity near the mesocyclone center strengthened obviously, accompanied with the low-level outflow, strong updraft as well as high reflectivity extending outside RMW. The relative tangential wind initially exhibited a wavenumber 1 asymmetric structure with the maximum wind region at the left portion of the cyclone and shifted counterclockwise with height. The axisymmetric tangential wind strengthened and reached its maximum intensity with a value about 20 ms-1 at z=1km. After that the wave-1 asymmetric structure redeveloped with the maximum wind at the left-front of motion. In summary, the evolution and structure of the mesocyclone is similar to that observed within a non-supercell thunderstorm previously observed. It is worth to mention that the axisymmetric circulation characteristics of the mesocyclone at its mature stage resemble a feature very similar to that observed in a mature typhoon. However, there are significant differences, i.e., the size is much smaller, the life time is much shorter, and the downdraft in the center is produced by precipitation instead of compensating subsidence.