On the Geographic Asymmetry of Typhoon Translation Speed across the Mountainous Island of Taiwan

Abstract

This paper examines the effect of topographically phase-locked convection on the motion of typhoons across the island of Taiwan. Data for 84 typhoons that reached Taiwan’s eastern coast from 1960 to 2010 are analyzed, with motions compared to the long-term average overland translation speed. For 61 continuous-track typhoons among all cases, 77% of the slow-moving tropical cyclones (TCs) made landfall on the northern end of Taiwan’s eastern coast, while 60% of the fast storms had southeastern coastal landfalls. This geographic asymmetry with respect to typhoon translation speeds widened after landfall, as the slow movers typically decelerated during the overland period, whereas the faster TCs sped up. In particular, the average overland duration was 16 h for the slow class, compared to only 3 h for the fast-moving typhoons. The combination of slower translation with longer duration for the northern class of TCs led to large rainfall on the southwestern slope of the island’s Central Mountain Range. Weather Research and Forecasting model experiments are used to study the effect of convection on storm motion over a mountainous island resembling Taiwan. The authors find that the topographically phase-locked convection acts to slow down (speed up) the northern (southern) landfalling typhoons. The model results also suggest that a positive feedback mechanism exists for the slow storms, in which the convective heating pattern forced by topography acts to reduce the TC motion, leading to even more prolonged precipitation and heating, yielding further speed reductions.

Effects of Landfall Location and Approach Angle of an Idealized Tropical Cyclone over a Long Mountain Range

“…Note that in the present study, we found that the vorticity tendency (VT) approach is appropriate since the TC track is consistent with the VT, similar to that applied to TC motion, such as northwestward beta drift (e.g., Holland, 1983) and shear effects (e.g., Wu and Emanuel, 1993). The PV Tendency (PVT) diagnostic approach, as originally proposed by Wu and Wang (2000) and Chan et al (2002) and applied to real-case simulations (e.g., Hsu et al, 2013;Chan, 2013, 2014;Wang et al, 2013), appears to be an attractive method and will be considered in our future research. …”

“…Due to complicated interactions between orographic and thermal forcing, both idealized numerical simulations (e.g., Chang, 1982;Bender et al, 1987;Yeh and Elsberry, 1993a,b;Lin et al, 1999Lin et al, , 2005, denoted as L05 hereafter, Lin and Savage, 2011;Chan, 2013, 2014;Wu et al, 2015), real-case simulations (e.g., Wu, 2001;Lin et al, 2006;Jian and Wu, 2008;Huang et al, 2011;Hsu et al, 2013;Wang et al, 2013;Wu et al, 2015), and observations (Brand and Blelloch, 1974;Wang, 1980;Hsu et al, 2013) have shown that when a typhoon approaches Taiwan's CMR, its track may deflect either to the north or south upstream of the mountain. It has been proposed that the track deflection may be due to mean cyclonic circulation (e.g., Chang, 1982;Bender et al, 1987), channeling mechanism (e.g., Lin et al, 1999;Jian and Wu, 2008;Huang et al, 2011), blocking effect (e.g., Elsberry, 1993a,b, L05, Lin andSavage, 2011), latent heating (e.g., Hsu et al, 2013;Chan, 2013, 2014;Wang et al, 2013), terrain-induced gyes Chan, 2013, 2014), approaching angle and landing location (Lin and Savage, 2011;Tang and Chan, 2014), and midtropospheric northerly asymmetric flow (Wu et al, 2015).…”

“…It has been proposed that the track deflection may be due to mean cyclonic circulation (e.g., Chang, 1982;Bender et al, 1987), channeling mechanism (e.g., Lin et al, 1999;Jian and Wu, 2008;Huang et al, 2011), blocking effect (e.g., Elsberry, 1993a,b, L05, Lin andSavage, 2011), latent heating (e.g., Hsu et al, 2013;Chan, 2013, 2014;Wang et al, 2013), terrain-induced gyes Chan, 2013, 2014), approaching angle and landing location (Lin and Savage, 2011;Tang and Chan, 2014), and midtropospheric northerly asymmetric flow (Wu et al, 2015). Thus, the basic dynamics deserve further investigation.…”

“…In addition, unlike a dry cyclone vortex used in LS11 and L05, a more realistic bogus TC is initiated in a conditionally unstable atmosphere with land surface, moist and planetary boundary layer (PBL) processes included. A vorticity budget analysis is adopted to help understand the effects of combined forcing of orography and convection, instead of the potential vorticity tendency (PVT) diagnostic approach taken by some previous studies (e.g., Wu and Wang, 2000;Chan et al, 2002;Hsu et al, 2013;Chan, 2013, 2014;Wang et al, 2013). The direct diabatic heating effects will be investigated along with the rainfall distributions.…”

Revisiting the steering principal of tropical cyclone motion in a numerical experiment

“…An interesting finding of the study is that the contribution of diabatic heating becomes important for irregular tropical cyclone motion, suggesting that track oscillations as well as irregular track changes may be explained by changes in the convection pattern. The PVT approach has been used in understanding tropical cyclone motion in the presence of the effects of land surface friction (FR), river deltas, coastal lines, mountains, islands, cloud-radiative processes and sea surface pressure gradients (e.g., Wong and Chan, 2006;Yu et al, 2007;Fovell et al, 2010;Hsu et al, 2013;Wang et al, 2013;Choi et al, 2013).…”