DECEMBER 2014 W E I 1425 Surface Wind Nowcasting in the Penghu Islands Based on Classified Typhoon Tracks and the Effects of the Central Mountain Range of Taiwan CHIH-CHIANG WEI Department of Digital Content Designs and Management, Toko University, Pu-Tzu City, Chia-Yi County, Taiwan (Manuscript received 5 March 2014, in final form 21 September 2014) ABSTRACT The purposes of this study were to forecast the hourly typhoon wind velocity over the Penghu Islands, and to discuss the effects of the terrain of the Central Mountain Range (CMR) of Taiwan over the Penghu Islands based on typhoon tracks. On average, a destructive typhoon hits the Penghu Islands every 15–20 yr. As a typhoon approaches the Penghu Islands, its track and intensity are influenced by the CMR topography. Therefore, CMR complicates the wind forecast of the Penghu Islands. Six main typhoon tracks (classes I–VI) are classified based on typhoon directions, as follows: (I) the direction of direct westward movement across the CMR of Taiwan, (II) the direction of northward movement along the eastern coast of Taiwan, (III) the direction of northward movement traveling through Taiwan Strait, (IV) the direction of westward movement traveling through Luzon Strait, (V) the direction of westward movement traveling through the southern East China Sea (near northern Taiwan), and (VI) the irregular track direction. The adaptive network-based fuzzy inference system (ANFIS) and multilayer perceptron neural network (MLPNN) were used as the forecasting technique for predicting the wind velocity. A total of 49 typhoons from 2000 to 2012 were analyzed. Results showed that the ANFIS models provided high-reliability predictions for wind velocity, and the ANFIS achieved more favorable performance than did the MLPNN. In addition, a de- tailed discussion on the interaction of the CMR with the Penghu Islands based on various track directions is provided. For class I, the CMR is observed to have significantly influenced variations in wind speed when typhoons approached the Penghu Islands. In addition, the winds on the Penghu Islands were ob- served to have been influenced by the distance from the typhoon center to the Penghu Islands for all classes except class II. 1. Introduction a destructive typhoon hits the Penghu Islands every 15– 20 yr (Wu et al. 2013). For the Penghu Islands, the wind The Penghu Islands (Fig. 1), situated in the main path data were measured by a weather station at an eleva- of western North Pacific typhoons, form an archipelago tion of 10 m above sea level, indicating highly fluctu- off the western coast of Taiwan in the Taiwan Strait. The ating wind velocities during typhoons, as evidenced Penghu Islands comprise excellent land and sea areas for by historical records. Compared with weather stations wind power generation. Recently, the Penghu Islands at this altitude, a typical wind turbine, which is ap- were selected as low-carbon islands because numerous proximately 40–60 m in height, might be subjected to researchers have found the Penghu Islands to be an relatively stronger winds. Because fluctuations in wind excellent location for developing wind power (Lee and velocity influence wind power production, accurate Huang 2004; Lin 2012). Wind power meteorology can knowledge of wind is required for the planning, design, provide information to the wind power industry over and operation of wind turbines (Akhmatov 2007; the Penghu Islands, such as the siting of wind tur- Vincentetal.2010). bines, regional wind resource assessment, and the short- As a typhoon approaches the Penghu Islands, the ty- term prediction of wind resources (Petersen et al. 1998; phoon circulation with the Central Mountain Range Hsieh and Dai 2012). According to typhoon statistics, (CMR) of Taiwan, as indicated by the yellow dotted line in Fig. 1, produces considerable mesoscale variations in Corresponding author address: Chih-Chiang Wei, No. 51, Sec. 2, pressure, wind, and precipitation distribution over and University Rd., Pu-Tzu City, Chia-Yi County 61363, Taiwan. nearby Taiwan. The CMR is the principal range of E-mail: [email protected] mountains in Taiwan, and runs from the north to the DOI: 10.1175/WAF-D-14-00027.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/05/21 02:09 PM UTC 1426 WEATHER AND FORECASTING VOLUME 29 FIG. 1. Location of the Penghu Islands and schematic diagram of the typhoon track classifications. south of Taiwan. The tallest peak of the CMR is Jade confining the storm to a cyclonic track around the Mountain (3952 m). The CMR is 340 km long and 80 km northern end of the CMR. Bender et al. (1985) observed wide, with an average height of 2500 m. As a typhoon that the mountain range affects the decay rate through approaches Taiwan, the topography of the CMR in- reduction in the supply of latent and kinetic energy into fluences its track and intensity (Wu and Kuo 1999; Tsai the storm circulation both during and after the passage and Lee 2009). The topography increases the rainfall of the storm over the mountain. Moreover, Hsu et al. amount significantly by lifting moist air over the wind- (2013) examined the effect of topographically phase- ward side of the mountains. The topography modulates locked convection with the CMR on the westward the wind field, and the rainfall pattern adjusts with the movement of typhoons across Taiwan. They deter- wind pattern (Wu et al. 2002). mined that topographically phase-locked convection acts A number of review studies on typhoons affecting to slow the northern landfalling typhoons and accelerate Taiwan have been investigated and have indicated that, the southern landfalling typhoons. These studies have when a typhoon approaches Taiwan, the CMR signifi- shown the substantial influences of the CMR topogra- cantly affects its circulation and track (Chang et al. phy on typhoons. 1993; Tsay 1994; Wu et al. 2002; Wu and Kuo 1999). Based on historical records, the Central Weather Brand and Blelloch (1974) found that westbound ty- Bureau (CWB) of Taiwan defined 10 typical track phoons tend to move cyclonically around the northern types based on typhoon statistics. For simplicity, the side of the CMR, and experience an average intensity typhoon tracks that affected the Penghu Islands are decrease of over 40%, starting approximately 12 h be- classified into the following six main track directions fore the storm centers reach the CMR. Wang (1980) (classes I–VI) (Fig. 1): direct westward movement documented and analyzed the path, intensity, propa- across the CMR of Taiwan (class I, comprising types gation speed, and evolution of 53 westbound typhoons 2–4); northward movement along the eastern coast of that approached Taiwan, indicating that the typhoon Taiwan (class II, comprising types 6 and 8); northward center was deflected toward the north as it approaches movement traveling through Taiwan Strait (class III, Taiwan, and toward the south after its passage over comprising types 7 and 9); westward movement traveling the CMR, because of the deflection of the mean through the Luzon Strait, which is the strait between steering flow upstream of the CMR. Using a primitive Taiwan and Luzon Island of the Philippines (class IV, equation model, Chang (1982) showed that, because comprising type 5); the direction of westward movement of blocking effects, mountain-induced flow deflections traveling through the southern East China Sea (near are mainly confined to the lower levels, and the passage northern Taiwan) (class V, comprising type 1); and the of the westward-approaching typhoon induces a mean irregular track direction except classes I–V (class VI, cyclonic circulation pattern around the mountain, comprising type 10). Unauthenticated | Downloaded 10/05/21 02:09 PM UTC DECEMBER 2014 W E I 1427 TABLE 1. Typhoons affecting the Penghu Islands during 2000–12 and typhoon information. Track Period affected Max wind in Max wind in class the Penghu Magong typhoon center Intensity 2 2 (type) Typhoon Islands (LST) (m s 1) (m s 1) category Class I Bilis 22–23 Aug 2000 15.9 53 Category 3 (types 2–4) Toraji 29–31 Jul 2001 9.3 38 Category 1 Lekima 26–28 Sep 2001 12.7 35 Category 1 Morakot 3–4 Aug 2003 6.7 23 Tropical storm Haitang 17–19 Jul 2005 16.8 55 Category 3 Talim 31 Aug–1 Sep 2005 17.0 53 Category 3 Longwang 1–3 Oct 2005 10.8 51 Category 3 Bilis 13–15 Jul 2006 12.9 25 Tropical storm Kaemi 24–26 Jul 2006 13.8 38 Category 1 Bopha 8–9 Aug 2006 8.0 23 Tropical storm Pabuk 7–9 Aug 2007 8.4 28 Tropical storm Sepat 17–19 Aug 2007 13.7 53 Category 3 Krosa 6–7 Oct 2007 12.6 51 Category 3 Kalmaegi 17–18 Jul 2008 8.9 33 Category 1 Fung-Wong 27–29 Jul 2008 15.9 43 Category 2 Sinlaku 12–14 Sep 2008 10.1 51 Category 3 Jangmi 27–29 Sep 2008 13.4 53 Category 3 Morakot 7–9 Aug 2009 15.4 40 Category 1 Fanapi 19–20 Sep 2010 18.9 45 Category 2 Nanmadol 28–31 Aug 2011 10.6 53 Category 3 Saola 31 Jul–3 Aug 2012 12.3 38 Category 1 Class II Kai-Tak 8–9 Jul 2000 8.2 35 Category 1 (types 6 and 8) Prapiroon 27–30 Aug 2000 7.1 33 Category 1 Xangsane 30 Oct–1 Nov 2000 12.0 38 Category 1 Cimaron 11–13 May 2001 9.9 23 Tropical storm Melor 2–3 Nov 2003 9.0 25 Tropical storm Mindulle 30 Jun–2 Jul 2004 9.0 45 Category 2 Haima 12–13 Sep 2004 5.6 18 Tropical storm Nock-Ten 24–25 Oct 2004 8.9 43 Category 2 Class III Chebi 22–24 Jun 2001 25.8 35 Category 1 (types 7 and 9) Nanmadol 3–4 Dec 2004 9.9 38 Category 1 Nakri 9–10 Jul 2002 11.9 18 Tropical storm Chanchu 17–18 May 2006 14.6 45 Category 2 Linfa 20–22 Jun 2009 14.8 28 Tropical storm Lionrock 31 Aug–2 Sep 2010 8.5 23 Tropical storm Megi 21–23