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TAO, Vol. 17, No. 2, June 2006 430 can be seen that when the phase of the wavenumber two V T is ahead of V R by π/4 with same amplitude, GBVTD coefficients A3 and B3 are near zero; hence, GBVTD technique cannot retrieve the wavenumber two amplitude in these conditions. However, by knowing the elliptical shape of the vortex as a priori these two mechanisms can still be distinguished by the characteristics of the corresponding GBVTD coefficients.

By using the observational data collected by the C-band Doppler radar which was located at the Green Island off the southeast coast of Taiwan, as well as the offshore island airport and ground weather stations, this article focuses on the mesoscale analysis of inner and outer rainband features of Typhoon Otto (1998), before and after affected by the Central Mountain Range (CMR) which exceeds 3000 m in elevation while the storm was approaching Taiwan in the northwestward movement.While the typhoon was over the open ocean and moved north-northwestward in speed of 15 km/h, its eyewall was not well organized. The rainbands, separated from the inner core region and located at the first and second quadrants relative to the moving direction of typhoon, were embedded with active convections. The vertical cross sections along the radial showed that the outer rainbands tilted outward and were more intense than the inner ones. As the typhoon system gradually propagated to the offshore area near the southeast coast of Taiwan, the semi-elliptic eyewall was built up at the second and third quadrants. Moreover, the strength of the eyewall became more intense compared with the outer rainbands, and the maximum wind axis was quite parallel to the vertical orientation of radar reflectivity in the eyewall. After the detailed streamline analysis, it indicated that the eyewall was enhanced by the confluence between the westerly flow, triggered by the farther outer circulation of the storm around the Taiwan Island, and the northwesterly flow near the inner circulation of the storm itself. Also, the left quadrant in the lower portion (below 2 km in altitude) possessed stronger Doppler velocity than that in the right quadrant, and the upper portion (above 2.0 km) had the opposite mode. This reverse phenomenon of Doppler wind in the lower portion of the typhoon became more pronounced while the storm was getting closer to the mountain. The estimated typhoon center below 1.5 km in altitude had a slower

In order to investigate the mesoscale structures and appropriate mechanisms for their maintenance in the Mei-yu front leading edge during the TAMEX (Taiwan Area Mesoscale EXperiment) TOP 9 (15 June 1987) off the east coast of Taiwan, we measured the fine-scale in-situ data by different sensors and sensed radar data by two airborne radars mounted on the NOAA P-3 research aircraft traversing the frontal system at six different altitudes. Based upon the sudden increase of the turbulence intensities, the position of the frontal leading edge at each flight level was identified exactly and, after deducting the propagating distance of the frontal system, the composited vertical cross sections of the system in kinematics, dynamics and thermodynamics was feasible.The significant findings are: the frontal edge was parallel to the isolines of horizontal component of wind and perturbed air density and the frontal edge demonstrated a well-defined density current, the variation of thermodynamic parameters was not obvious, but the pattern of potential temperature revealed a cold core in the lower level behind the frontal edge, which coincided with the position of the heaviest air density. The probable mechanisms for the propagation of the density current and the maintenance of the frontal system were proposed to be the intense horizontal pressure gradient force from rear to front in the cold core region and the moderate convective instability at the head of the system as well as the kinetic energy transport from the mean flow.