On Typhoon Track Deflections near the East Coast of Taiwan

Abstract

Typhoons with “deflection tracks” (DTs) within a 200-km distance of the mountainous island of Taiwan are examined. We analyze 84 landfalling typhoons that compose 49 DT cases turning to the left-hand side, including 18 with very large deflection angles (DA > 20°) and another 7 having looped tracks (LTs). Most of the large DA and LT cases are “northern landfall” type, reaching Taiwan’s east coast poleward of 24°N and originally possessing relatively slow translation speeds (~4 m s−1). Their average translation speeds, however, increase by 50% in the 3 h prior to landfall. The WRF Model is used to simulate DT cases, and potential vorticity (PV) tendency diagnosis is used to interpret the contributions of the horizontal advection (HA), vertical advection (VA), and diabatic heating (DH) terms. The northern landfall tropical cyclones (TCs) possess significant cross-mountain flow to the south of the storm near the coast, resulting in vorticity stretching (the VA effect) and subsidence warming. The subsidence suppresses storm convection and produces heating asymmetries (the DH effect) that can induce significant southwestward deflections. The cross-mountain VA and DH effects are weaker for the “southern landfall” storms. The results explain well the observed increase of translation speed prior to landfall in DT cases and show that the HA effect, in general, does not contribute to the track deflection. Our results highlight the impact of topography on TC track by the vorticity stretching effect and by asymmetric diabatic heating.

Track Deflection of Typhoon Nesat (2017) as Realized by Multiresolution Simulations of a Global Model

“…For the CMR, the typical distance of track sensitivity is thus about 100-300 km as shown by a variety of idealized as well as realistic case simulations. Such large variations of upstream influence may be related to the size, intensity, translational speed, and direction of an approaching typhoon (e.g., Wu et al 2015;Tang and Chan 2016;Hsu et al 2018). These observational and modeling results show large variations in track deflection associated with different approaching typhoons.…”

“…A cyclonic path of the track around the northern corner of the mountain ridge was first noticed when a westbound tropical cyclone (TC) approached the northern sector of the mountain ridge (e.g., Wang 1980;Chang 1982). Many numerical studies have been devoted to investigating the intriguing track evolution for idealized TCs over a bell-shaped mountain or realistic mountains like the Central Mountain Range (CMR) in Taiwan that is elongated mainly northsouthward with a maximum height over 3500 m (e.g., Chang 1982;Bender et al 1987;Yeh and Elsberry 1993a,b;Lin et al 1999Lin et al , 2005Kuo et al 2001;Huang and Lin 2008;Jian and Wu 2008;Y.-H. Huang et al 2011;Lin and Savage 2011;Hsu et al 2013Hsu et al , 2018Tang and Chan, 2014, 2016Wu et al 2015;Huang et al 2016a).…”

“…It has been generally recognized that influences upstream of the mountain range may be induced on the track within a critical distance from the CMR for westbound typhoons (e.g., Yeh and Elsberry 1993a,b;Y.-H. Huang et al 2011;Wu et al 2015;Tang and Chan 2016;Hsu et al 2018). Consequently, westbound tropical cyclones tend to take a northward or southward track deflection as they approach the mountains at a distance of about the half-width scale of the mountain elongation (e.g., Wu et al 2015;Huang et al 2016a;Tang and Chan 2016).…”

“…To directly identify the contribution of diabatic heating to vortex motion, the potential vorticity (PV) tendency budget has been diagnosed. A dynamical framework of PV tendency diagnostics presented by Wu and Wang (2000) has been widely applied to investigate TC translation, and it is found that the translation is essentially dictated by PV advection and latent heating (e.g., Chan et al 2002;Yu et al 2007;Huang et al 2016b;Tang and Chan 2016;Hsu et al 2013Hsu et al , 2018. Positive PV tendencies induced by asymmetric diabatic heating allow for a TC to shift to the region of upward heating gradient over an open ocean (Wu and Wang 2001).…”

An observational study of the inner core structure of Typhoon Meranti (2016) near landfall

“…Besides, the channeling effects between the topography and the storm, along with downslope wind from Taiwan, may also help to maintain the intensity of Meranti and the smaller RMW. Moreover, the distribution of diabatic heating associated with rainfall (large reflectivity) was also in favor of northward deflection (Hsu et al ., ).…”