Topographic Influence on the Motion of Tropical Cyclones Landfalling on the Coast of China
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OCTOBER 2016 C H E N A N D W U 1615 Topographic Influence on the Motion of Tropical Cyclones Landfalling on the Coast of China XIAOYU CHEN Key Laboratory of Meteorological Disaster, Ministry of Education/Joint International Research Laboratory of Climate and Environment Change/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters/Pacific Typhoon Research Center, Nanjing University of Information Science and Technology, Nanjing, China LIGUANG WU Key Laboratory of Meteorological Disaster, Ministry of Education/Joint International Research Laboratory of Climate and Environment Change/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters/Pacific Typhoon Research Center, Nanjing University of Information Science and Technology, Nanjing, and State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China (Manuscript received 17 March 2016, in final form 10 August 2016) ABSTRACT A few recent numerical studies investigated the influence of a continent, which is much larger in size than islands, on the track of a landfalling tropical cyclone. It has been found that land surface roughness and temperature tend to make a tropical cyclone move toward the continent. In this study, the effect of the elevated terrain on the western part of mainland China on landfall tracks is examined through idealized numerical experiments, in which an initially axisymmetric vortex is embedded into a monsoon trough and takes a landfall track across China’s mainland. It is found that the effect of the elevated terrain on the western part of mainland China can enhance the southerly environmental flows in the low and midtroposphere, leading to an additional landward component of tropical cyclone motion, suggesting that forecasts for tropical cyclone tracks over land should take into consideration the change in thermal condition over the continent. The study suggests that the increased duration of tropical cyclone overland tracks observed in China may be associated with the reduction of the thermal forcing over the Tibetan Plateau. 1. Introduction roughness and temperature can affect landfalling TC motion (Wong and Chan 2006, 2007; Au-Yeung and Economic damage and casualties associated with Chan 2010; Szeto and Chan 2010; Li et al. 2014; Xu et al. tropical cyclones (TCs) occur mostly during and after 2013). So far, few studies have examined the influence of landfall (Zhang et al. 2009, 2010) and thus it is very im- continental terrain on landfalling TC motion. portant to predict the timing and location of landfall in Early studies paid much attention to TC track changes operational TC forecasts. While many studies have fo- associated with topography. Idealized numerical and cused on the influence of islands on TC activity (e.g., real case studies suggest that island topography like Chang 1982; Bender et al. 1987; Yeh and Elsberry 1993a, that of Taiwan can significantly affect environmental b; Lin et al. 2005; Jian and Wu 2008; Yang et al. 2008; flows and TC tracks during landfall, leading to track Huang et al. 2011; Wang et al. 2012; Xie and Zhang 2012), deflection or looping tracks (Chang 1982; Yeh and attention has been paid recently to the effects of conti- Elsberry 1993a,b; Wu 2001; Jian and Wu 2008; Wang nents that are much larger than the TC circulation on the et al. 2012; Xie and Zhang 2012). For instance, it is horizontal scale. It has been found that land surface suggested that the unusual tracks of Typhoons Nari (2001) and Krosa (2007) were closely related to the terrain height of the island of Taiwan (Yang et al. 2008; Corresponding author address: Prof. Liguang Wu, Pacific Ty- phoon Research Center, Nanjing University of Information Sci- Huang et al. 2011). Additionally, the slowdown of Ty- ence and Technology, Nanjing 210044, China. phoon Morakot (2009) when crossing Taiwan was mainly E-mail: [email protected] attributed to the asymmetric latent heating induced by the DOI: 10.1175/WAF-D-16-0053.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 09/28/21 07:44 PM UTC 1616 WEATHER AND FORECASTING VOLUME 31 topography and monsoonal flows (Liang et al. 2011; Wu et al. 2011; Wang et al. 2012). It is indicated that the island topographic effect can affect lower-level flows and induce significant asymmetric diabatic heating that will result in TC track changes (Yeh and Elsberry 1993a,b; Wang et al. 2012). A few numerical studies found that TC asymmetric flows induced by land surface roughness can lead to a landward component of TC motion during landfall in a resting environment (Wong and Chan 2006, 2007; Au- Yeung and Chan 2010; Szeto and Chan 2010; Li et al. 2014). Wong and Chan (2006) for the first time performed idealized numerical experiments that investigated the effects of land surface roughness and moisture flux on TC motion. They found that the TC can drift toward the 2 continent at a speed of about 1 m s 1 in a resting envi- ronment on an f plane as a result of the induced TC asymmetric flows at lower levels; this process is now called land drift. Land drift was further demonstrated in the idealized numerical experiments with a triangle- FIG. 1. Domains for the numerical experiments and the initial 2 shaped coastal delta on the f plane and the numerical background flow (m s 1) at 850 hPa. The terrain height (m) is experiments on the b plane (Au-Yeung and Chan 2010; shaded and the TC track in OCN is plotted. Szeto and Chan 2010). Uniform environmental flows were added in numerical experiments and the land drift work differs from the previous numerical studies in the was also found by Li et al. (2014).However,wenotice following aspects. First, we use a surface roughness that the surface roughness length was set to be 0.5 m in length that is much smaller than the land drift experi- these land drift experiments, which is much larger than ments; this approach is more realistic for eastern main- those for common land surface conditions of eastern land China. Second, we use a relatively realistic China when using the Advanced Research version of the environmental flow including the typical monsoon Weather Research and Forecasting (WRF) Model trough and subtropical high over the western North (ARW). However, the influences of environmental flows Pacific for the embedded TC, which was derived from and continental topography have not been fully in- the low-frequency fields of Typhoon Matsa (2005). Fi- vestigated in previous studies. nally, the influence of the continental-scale terrain is Xu et al. (2013) conducted a 60-yr (1950–2009) sur- included. vey on TCs that made landfall on China’s mainland and suggested that supertyphoons (maximum sustained 2 2. Experiment design 1-min surface winds above 65 m s 1) tend to take a track toward the warmer part of the mainland. Based on The numerical experiments conducted in this study their observational analysis, Xu et al. (2013) further include five two-way interactive domains using the WRF performed a set of numerical experiments to show the Model (version 2.2). The horizontal resolutions are 27, 9, influence of land surface temperature on landfalling TC 3, 1, and 1/3 km, respectively, with 40 levels in the vertical tracks. In their experiments, they modified the land and a top of 50 hPa. The fifth domain, with subkilometer surface temperature and found that the simulated ty- resolution, is used for better simulation of the inner-core phoon gravitates to the region with higher land surface structure during TC landfall (Moon and Nolan 2015). In temperature. They proposed that the warmer land sur- fact, a sensitivity experiment was conducted without the face, with intense moisture supply from the summer subkilometer domain. While the simulated tracks are Asian monsoon, is conducive to deep convection that very similar, the intensity evolution is different as a result attracts supertyphoons. of the different horizontal resolutions. Two eyewall re- In this study, we also examine the influence of the placement processes occur in the subkilometer run. To- continent on the landfalling TC motion through ideal- pography data with a resolution of 30 s (about 1 km) are ized numerical experiments. The investigation of used in the study. The center of the outermost domain is changes in the motion of TCs making landfall is closely located at 30.08N, 132.58E, and the three innermost do- associated with TC track and damage prediction. Our mains move with the TC (Fig. 1). Unauthenticated | Downloaded 09/28/21 07:44 PM UTC OCTOBER 2016 C H E N A N D W U 1617 TABLE 1. Summary of the numerical experiments. Expt Terrain Land use Roughness length (m) TC vortex inserting OCN No Water Smooth Yes OCNB No Water Smooth No FLT Flat terrain Irrigated cropland and pasture 0.07 Yes FLTB Flat terrain Irrigated cropland and pasture 0.07 No TOP Topography Irrigated cropland and pasture 0.07 Yes TOPB Topography Irrigated cropland and pasture 0.07 No The physics parameterization schemes are adopted as mainland China (Fig. 1). The TC tracks in our experi- follows. The WRF single-moment 3-class scheme and the ments are very similar to that of Typhoon Matsa. Kain–Fritsch scheme (Kain and Fritsch 1993) are used in Two sets of numerical experiments are discussed in the outmost domain for microphysics and cumulus pa- this study and each one consists of three runs (Table 1). rameterization schemes, respectively, while the WRF Both the sea surface temperature and the land surface single-moment 6-class microphysics scheme (Hong and temperature are set to 298C to eliminate the possible Lim 2006) and no cumulus parameterization scheme are influences of the initial temperature contrast between used in the four inner domains.