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This study investigates the mesoscale interaction of typhoon circulation and southwesterly flow during the passage of Typhoon Morakot (2009) over Taiwan using radar observations. Single Doppler radar analysis characterized typhoon features with remarkably distant rainbands, and identified the strong southwesterly monsoonal flow with a maximum speed of more than 45 m s -1 in the southern flank of distant rainbands. Dual-Doppler synthesized winds elucidated a confluent mechanism on the northern side of the rainbands with a maximum convergence over 1.5 × 10 -3 s -1 . Velocity azimuth display (VAD) winds showed the intensification of southwesterly flow at low levels. The southwesterly monsoonal flow initiated about 6 hrs before typhoon landfall, and then became prominently involved with typhoon circulation. Also, the radial component with respect to the typhoon center was enhanced and became comparable with the tangential one (~30 m s -1 ) about 7.5 hrs after landfall. The variation in intensity of radial components can be regarded as a unique precursor for the extension of influence of the southwesterly monsoonal flow onto the typhoon circulation. Furthermore, the strong convergence showed that the interaction between the southwesterly flow and typhoon circulation might contribute to the development of rainbands, as well as the intensification of the inward radial flow embedded within typhoon.

Through the analysis of Doppler radar data, this study focuses on the characteristics and evolution of convection embedded within the principal band in Typhoon Morakot (2009) under the impingement of the intense southwesterly (SW) monsoonal flow. The intensity of the SW flow is comparable with the typhoon circulation at the third quadrant. The kinematic analysis shows that the northward component of the SW flow decelerates while approaching the rainband, creating significant convergence zones which results in the initiation and development of convective cells within the principal band.The vertical kinematic characteristics of the rainband reveal two types of downdrafts namely inner-edge and low-level downdrafts. The inner-edge downdrafts coupled by the radially inward tilting convection were initiated by the precipitation drag. Dynamically, the existence of the perturbed high at 1.5 km altitude in the inner-edge downdrafts supported the finding. Furthermore, it is evident that the distribution of two perturbation highs in the vicinity of the rainband could lead to SW flow deformation locally and fortify the mechanism of convergence, resulting in the merging of convective cells into the rainband. The maximum vertical vorticity coupled with the horizontal wind maximum at the middle levels of the rainband was also observed.

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