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A Climatology Model for Forecasting Typhoon Rainfall in Taiwan

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A Climatology Model for Forecasting Typhoon Rainfall in Taiwan Natural Hazards (2006) 37:87–105 Ó Springer 2006 DOI 10.1007/s11069-005-4658-8 A Climatology Model for Forecasting Typhoon Rainfall in Taiwan CHENG-SHANG LEE1,2,w, LI-RUNG HUANG2, HORNG-SYI SHEN2 and SHI-TING WANG3 1Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan, ROC; 2Meteorology Division, National S&T Center for Disaster Reduction, Taipei, Taiwan, ROC; 3Central Weather Bureau, Taiwan, ROC Abstract. The continuous torrential rain associated with a typhoon often caused flood, landslide or debris flow, leading to serious damages to Taiwan. Thus, a usable scheme to forecast rainfall amount during a typhoon period is highly desired. An analysis using hourly rainfall amounts taken at 371 stations during 1989–2001 showed that the topographical lifting of typhoon circulation played an important role in producing heavier rainfall. A climatology model for typhoon rainfall, which considered the topographical lifting and the variations of rain rate with radius was then developed. The model could provide hourly rainfall at any station or any river basin for a given typhoon center. The cumulative rainfall along the forecasted typhoon track was also available. The results showed that the R2 value between the model estimated and the observed cumulative rainfall during the typhoon period for the Dan- Shui (DSH) and Kao-Ping (KPS) River Basins reached 0.70 and 0.81, respectively. The R2 values decreased slightly to 0.69 and 0.73 if individual stations were considered. However, the values decreased significantly to 0.40 and 0.51 for 3-hourly rainfalls, indicating the strong influence of the transient features in producing the heavier rainfall. In addition, the clima- tology model can only provide the average conditions. The characteristics in individual ty- phoons should be considered when applying the model in real-time operation. For example, the model could give reasonable cumulative rainfall amount at DSH before Nakri (2002) made landfall on Taiwan, but overestimated the rainfall after Nakri made landfall and weakened with significant reduction in convection. Key words: typhoon, typhoon rainfall, rainfall climatology, quantitative precipitation fore- cast, typhoon landfall, topographical effect 1. Introduction On average, about 80 tropical cyclones formed around the whole globe each year and about 30 of them formed in the western North Pacific (Gray, 1981). Taiwan is located on the main path of western North Pacific tropical cyclones and was affected by at least one system each year accord- ing to the official record of the Central Weather Bureau (CWB), Taiwan. w Author for correspondence: Phone: +886-2-2392-8260, Fax: +886-2-2363-3642, E-mail: [email protected] gg gy 88 CHENG-SHANG LEE ET AL. Table I. The occurrence of different numbers of typhoons per year that affected Taiwan and made landfall on Taiwan in 1961–2002 Number of year Number of typhoons 012 3 45 6789 Affected Taiwan 0 1 2 11 7 13 2 4 1 1 Made landfall 9 9 11 10 2 0 1 0 0 0 In this paper, we use the term ‘‘typhoon’’ in representing tropical cyclone generally, including tropical storm and typhoon. Table I shows the distri- bution of occurrence of typhoons that affected Taiwan and made landfall on Taiwan each year during 1961–2002. Here a typhoon was considered to affect Taiwan if CWB issued the sea warning (or the storm force wind would affect the area as far as 100 km from the coast in the next 24 h). Taiwan was usually affected by three to five typhoons every year (maxi- mum at nine), and the annual variation was large. The average number of typhoon affecting Taiwan in a year is 4.5, with 1.8 of them making land- fall. Table I also indicates that there were nine years during 1961–2002 in which no typhoon made landfall. Although typhoons could form through- out the whole year in the western North Pacific, Taiwan was only affected by typhoons from April to November, with 74% of them occurred in July- September. Taiwan is a small island 140 km wide and 394 km long, and the average elevation of the Central Mountain Range (CMR) is above 2000 m (Fig- ure 1). A typhoon often experienced tremendous changes in motion and structure as it was approaching or moving across the island (Wang, 1992, Chang et al., 1993, Lee, 1993, Lin et al., 2001). On the other hand, the ter- rain slope lifting of the warm and moist air associated with a typhoon often caused continuous torrential rainfall on the windward side leading to seri- ous flooding, landslide or debris flow. Official reports indicated that during the 10-year period of 1985–1994, typhoon-related disaster caused an eco- nomic loss of 403 million US dollars each year, which accounted for about 76% of the average total yearly loss or 534 million US dollars caused by all weather events. In the period 1996–2001, the average yearly property loss due to typhoon increased to 530 million US dollars (number adopted from the reports of Ministry of the Interior). Table II lists the top five typhoons that caused most serious damage to Taiwan in the past several years. In 1996, Typhoon Herb hit Taiwan and brought a total of 1,984 mm rainfall at Ah-Li Mountain located at the central mountain area, resulting in severe debris flows and flooding. The National Science & Technology (S&T) Pro- gram for Hazard Mitigation (the former program office of the National S&T Center for Disaster Reduction) was established after the event. In MODEL FOR FORECASTING TYPHOON RAINFALL 89 Table II. A list of the top five typhoons that caused the most damages to Taiwan in the past eight years Typhoon name (year/month) Brief description of damage and rainfall Life loss Life injured Property loss (million US dollars) Herb (1996/7) 73 463 978 Severe disaster around whole island. 1984 mm (1095 mm daily) rainfall at Ah-Li Mountain (central) Toraji (2001/7) 214 188 531 Landfall period at 10 h. 147 mm hourly (390 mm 3-hourly) and 141 mm (333 mm 3-hourly) rainfall at Hua-Lien (east) and Nan-Tou (central) Nari (2001/9) 104 265 454 Landfall period at 51 h. Severe disaster around western Taiwan. 1051 mm (425 mm daily) at Taipei and 1142 mm at Chia-Yi (south) Zeb (1998/10) 38 27 297 Severe flood at Taipei Metropolitan Area. 921 mm rainfall at Yang-Ming Mountain (north), 506 mm at Taipei and 805 mm at Hua-Lien (east) Billis (2000/8) 15 110 251 Maximum wind speed 78 m s)1. 912 and 824 mm rainfall at Hua-Lien (east) and I-Lan (northeast) 2001, Taiwan was affected by nine typhoons with some really damaging ones. For example, Typhoon Nari stayed on land for 51 h and brought con- tinuous torrential rain to many areas around the whole island. The daily rainfall reached 425 mm (1051 mm in total for the entire impact period) in Taipei city, leading to severe inundation at many places including the main station of the Taipei Metropolitan Rapid Transit System. Since most of the disasters caused by typhoons in Taiwan were due to the torrential rain that caused floods, landslides or debris flows, a useful scheme for quantitative precipitation forecast (QPF) during typhoon peri- od is highly desired. The purpose of this study is to investigate the features of precipitation and to develop a usable scheme for forecasting rainfall amounts during the typhoon period. In Section 2, the rainfall characteris- tics in Taiwan during a typhoon period are analyzed. The development and the evaluation of the typhoon rainfall climatology model are discussed in Sections 3 and 4, respectively. Finally, Section 5 gives the discussions and conclusions. 90 CHENG-SHANG LEE ET AL. 2. The Characteristics of Typhoon Rainfall in Taiwan To study the rainfall characteristics associated with typhoons affecting Tai- wan, we collected all hourly rainfall data taken at 371 stations (Figure 1a) during 1989–2001 (the data time periods for some stations were shorter). These stations include 21 conventional surface stations and 319 automatic rain gauges of CWB, 8 rainfall stations of Taiwan Water Resource Bureau and 23 rainfall stations of different Reservoir Administration Bureaus. Note that the spatial distribution of these stations is not uniform with 80% of them located on the western side of CMR. The elevation of the stations ranges from 1 to 3845 m. The 6-hourly fixes of the Joint Typhoon Warning Center (JTWC) best tracks were interpolated to obtain hourly center positions. However, when typhoon was around Taiwan, mesoscale analysis (as that of Wang, 1992) was performed to provide hourly center positions. In 1989–2001, there were 58 typhoons affecting Taiwan. Fig- ure 1b shows the hourly center positions of these typhoons around Taiwan. The average maximum intensity of these typhoons when their cen- ters were located within the domain (shown in Figure 1b) was 38.8 m s)1. But the average of all 6-hourly intensities (when the typhoons were within the domain) was only 31.8 m s)1 because typhoons often weakened signifi- cantly when moving across Taiwan. Figure 2 shows that the rain rate associated with typhoon generally de- creases with radius except near the center. The standard deviations are Figure 1. Locations of 371 rainfall stations in Taiwan (a) and hourly center positions of 58 typhoons that affected Taiwan in 1989–2001 (b). The contours in (a) show the 1000 and 2000 m height. The star in (b) shows the location of Taipei station and the rectangles show two grid boxes used in the climatology model. MODEL FOR FORECASTING TYPHOON RAINFALL Table III.
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