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.
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 .