The secondary low and heavy rainfall associated with Typhoon Mindulle (2004)

This paper presents an observational and numerical study of Typhoon Mindulle (2004) as it affected Taiwan. Mindulle made landfall on the east coast of Taiwan at 1500 UTC 1 July 2004, and after 13 h, it exited Taiwan from the north coast. Severe rainfall (with a maximum amount of 787 mm) occurred over central-southwestern Taiwan on 2 July 2004. During the landfall of Mindulle's main circulation, a secondary low formed over the Taiwan Strait. However, the secondary low, after it developed significantly (vorticity exceeded 5 X 10(-4) s(-1) over a 30-km radius), did not replace the original center as was observed in many other storms. Instead, it moved inland and dissipated after the original center redeveloped near the north coast of Taiwan. In this study, the evolution of the secondary low, the redevelopment of the primary center, and the processes leading to the severe rainfall were examined. Results showed that the processes leading to the formation and the development of the secondary low were similar to those described in previous studies. These processes include the leeside subsidence warming, the horizontal transport of vorticity around the northern tip of the Central Mountain Range (CMR), and the overmountain upper-level vorticity remnant. However, because of the northward track, Mindulle preserved some strong vorticity on the eastern slope of the CMR. This strong vorticity remnant was steered northward over the ocean offshore from the north coast where the redevelopment of the primary center occurred. This quasi-continuous track of Mindulle has not been documented in previous studies. The vortex interaction between the redeveloped primary center and the secondary low resulted in the northeastward movement of the secondary low, which then dissipated after making landfall. Analyses also showed that even though heavy rainfall would occur over the mountain area when only the southwesterly flow prevailed, as on 3 July 2004, Typhoon Mindulle and the secondary low provided extra convergence that resulted in the west-east-oriented convective bands. These convective bands and the orographic lifting of the circulation associated with the secondary low resulted in the heavy rainfall over the central-western plains area.