• A new formation mechanism of a mountain-induced secondary center inside Typhoon Morakot (2009)

    When a tropical cyclone (TC) passes over Taiwan Island, several vorticity centers or secondary lows form on the west side (leeside) of the Central Mountain Range, one of which may develop into a well-defined secondary center (SC) with closed isobars, closed circulation and a warm core. Several SCs can meanwhile appear around the island during a TC’s passage, due to their similar characteristics with the inner core of the typhoon, one of which may develop and even displace the primary center of the typhoon to affect the position of the TC’s track and the estimation of its intensity. 

    Fig. 1. Schematic diagram of the (a) thermodynamic and (b) dynamic mechanism of the SC in the initial formation stage.

     

    Based on observation and numerical simulation, Dr. Xuwei Bao from the Shanghai Typhoon Institute/China Meteorological Administration and coauthors investigated the formation mechanism of a mountain-induced SC during Typhoon Morakot (2009) crossing Taiwan. The SC inside Morakot was initially generated by the interaction among the TC’s cyclonic wind, southwesterly wind and orographic effects over southwestern Taiwan, which is a dynamic mechanism (Fig. 1b). This differs from previous studies in that the SC develops from a secondary low initially caused by the downslope adiabatic warming effect over northwestern Taiwan (a thermodynamic mechanism, Fig. 1a).

     

    Despite this difference, Dr. Xuwei Bao identified that the initial formation mechanism of the SC inside Typhoon Morakot described in his paper does not in fact contradict previous studies; rather, it serves as a complement, especially when a typhoon is embedded within a strong southwesterly monsoon during its passage across Taiwan.

     

    Reference

    Bao, X. W., L. M. Ma, J. Y. Liu, J. Tang, and J. Xu, 2018: Formation and development of a mountain-induced secondary center inside Typhoon Morakot (2009). Adv. Atmos. Sci., 35(9), https://doi.org/10.1007/s00376-018-7199-2 .

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