Nat Hazards DOI 10.1007/s11069-013-0696-9 ORIGINAL PAPER Analysis of a major storm over the Dongjiang reservoir basin associated with Typhoon Bilis (2006) Yi Zou • Shifan Qiu • Yaoqiu Kuang • Ningsheng Huang Received: 12 January 2013 / Accepted: 16 April 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract An unexpected major storm on July 14, 2006, resulted in great loss to the Dongjiang reservoir basin in Zixing City, Hunan Province, China, during the dominance of Typhoon Bilis (2006). The rainfall characteristics and temporal evolution of this major storm were studied with rain gauge data and high-resolution radar reflectivity data to investigate the connections between typhoon, reservoir and convective storm. Our inves- tigations found that the intense convective storm, which was characterized by a banded structure, brought heavy rainfall concentrated in the Dongjiang reservoir basin while the center of Typhoon Bilis was nearly 450 km away from the basin. By applying geographical information system techniques, analyses of radar reflectivity demonstrated that the topography of the Dongjiang reservoir has big influence on the development of convective storm. Furthermore, intense convective cells with strong radar reflectivity ([50 dBZ) arose more frequently over the edges of the reservoir, especially over the southern mountain valley in the basin. More importantly, our investigations indicate that the occurrence of this convective storm is closely related to a strong atmospheric inversion by examining the Atmospheric Infrared Sounder data. Keywords Convective storm Á Reservoir basin Á Tropical cyclone Á Bilis Á Radar reflectivity Á AIRS Y. Zou Á Y. Kuang (&) Á N. Huang Sustainable Development Research Center, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China e-mail: [email protected] Y. Zou Á S. Qiu Á Y. Kuang Key Laboratory of Marginal Sea Geology, Chinese Academy of Sciences, Guangdong 510640, China Y. Zou Á S. Qiu University of the Chinese Academy of Sciences, Beijing 10049, China S. Qiu Department of Municipal Engineering, Guangdong Technical College of Water Resources and Electric Engineering, Guangdong 510635, China 123 Nat Hazards 1 Introduction Tropical cyclones (TCs), also known as hurricanes, typhoons or cyclones, are one of the most devastating weather phenomena in the world, but tropical cyclones are still poorly understood (Roy and Kovordanyi 2012). Tropical cyclone is an important trigger of many hazards, including floods, landslides and debris flows, and thus has drawn much attention from hydrologists, geomorphologists and engineering geologists (Catane et al. 2012; Chen et al. 2008; Huang et al. 2012; Liu and Wu 2011; Yu et al. 2006). As one of huge man-made projects, reservoir plays a key role in flood control. The understanding of precipitation characteristics over a reservoir basin is helpful in flood forecasting and warning systems. Although many investigations have explored the impacts of tropical cyclones on hydrological processes in reservoir basins (Huang and Hsieh 2010; Lin G-F et al. 2010, Lin G-W et al. 2011), few studies have paid attention to the char- acteristics of major storms triggered by tropical cyclones over a reservoir basin. Lack of observations with adequate spatial–temporal resolution largely hampered the study on convective storms associated with tropical cyclones over a reservoir basin. Being situated in a mountainous area with numerous towns and villages, the Dongjiang reservoir basin in Zixing City, China, is vulnerable to heavy rainfalls during tropical cyclone seasons. On July 14, 2006, Zixing City experienced a devastating major storm that caused 246 deaths, 95 missing and over 300 million dollars in damage during the domi- nance of Typhoon Bilis (2006). Previous investigations have shown that the surrounding areas of the Dongjiang reservoir in Zixing were the hardest hit with heavy loss of property and human lives caused by flash floods and landslides (Xu et al. 2011; Zhu and Zhuang 2009). However, the temporal and spatial scales of previous investigations on the Typhoon Bilis (Gao et al. 2009; Teng et al. 2008; Yang et al. 2009; Zhu and Zhuang 2009) were too rough to show the details of the extreme rainfall event in Zixing. Based on rain gauge data, radar reflectivity data and Atmospheric Infrared Sounder (AIRS) data, this research aims to study the characteristics of the major storm associated with Typhoon Bilis over the Dongjiang reservoir basin and explore the mechanisms behind the extreme storm event. Thus, we try to identify the rainfall characteristics of the major storm by analyzing rain gauge data and radar reflectivity data, study the local topographic influence on the development of convective systems by introducing geographical infor- mation system (GIS) techniques, and explore the connections between the local atmo- sphere and the major storm by examining the AIRS data. 2 Study area and data Zixing is a city in the southeast of Hunan Province, China, with Dongjiang reservoir located at the upstream of Leishui River (Fig. 1). The city is located in the north of the Nanling Mountains with the Luoxiao Mountains to the east and the landform declining gradually from the southeast to the northwest. The study area consists of three distinct geomorphic parts: 1. the hilly plain, 106–300 m a.s.l.; 2. the hilly valley, * 300–700 m a.s.l.; 3. the arc-shaped mountains of the Luoxiao Mountains near the eastern and southern boundaries, [700 m a.s.l.. 123 Nat Hazards Fig. 1 Study area The Dongjiang reservoir basin is the most important landform of Zixing, and the Dongjiang reservoir within it is the largest reservoir in the south of Hunan, with a water area of 160 km2 and a capacity of 8.12 9 109 m3. Topographic properties of Zixing were obtained from the 30-m ASTER Global Digital Elevation Model (version 2) of METI and NASA (Fig. 1). Radar reflectivity data of high temporal and spatial resolution were obtained by S band (10 cm) Doppler radar from Shaoguan weather radar station. The altitude of the radar antenna is 301.9 m a.s.l. Moreover, rainfall data from records of 21 rain gauges in and around Zixing were used to analyze the precipitation characteristic of the major storm in Zixing (Fig. 2). Of the 21 rain gauges, Fangshi (FS), Huangcao (HC), Lianping (LP), Longxi (LX), Qingjiang (QJ), Xingning (XN) and Zixing (ZX) are located in Zixing City. Recent developments in observation techniques have led to improvements in high- quality remote sensing data for better understanding the atmospheric and surface charac- teristics. Atmospheric Infrared Sounder (AIRS) is an advanced atmospheric remote sensing instrument on board the EOS-2/Aqua satellite. AIRS along with its accompanying microwave instrument, Advanced Microwave Sounding Unit (AMSU-A), represents the most advanced atmospheric sounding system at present. The AIRS temperature retrievals have nominal horizontal resolution of 45 km and vertical resolution at 28 pressure levels 123 Nat Hazards Fig. 2 Location of the weather Doppler radar and rain gauges. CZ Chenzhou, FS Fanshi, GD Guidong, HC Huangcao, HK Hankou, LHD Longhudong, LP Lianping, LX Longxi, ND Nandong, QJ Qingjiang, RC Rucheng, SD Sidu, ST Shatian, XF Xinfang, XLS Xiulishan, XN Xingning, YS Yanshou, YX Yongxing, YZ Yizhang, ZHX Zhangxi, ZX Zixing between 1,100 and 0.1 hPa. Generally, temperature profiles have an accuracy of 1 K per 1-km-thick layer in the troposphere (Li 2008). The surface pressure combined with AIRS standard temperature products is interpolated from the NCEP GFS 3-, 6- and 9-h forecast and the local DEM topography (Olsen 2011). The local atmospheric conditions in the study area were examined on the basis of AIRS standard temperature products and air surface pressure before and after the major storm. 3 Case overview Typhoon Bilis (2006) developed in the western North Pacific Ocean and landed on the coast of Xiapu County, Fujian Province, China, at 0450 UTC on July 14, 2006, with a maximum wind speed of 108 km/h. After landing, it shortly weakened into a tropical storm in Fujian Province and kept moving westward at a speed of 10–15 km/h. On July 14, Bilis weakened to a tropical depression at 0700 UTC on July 15 in Jiangxi Province, but it still kept moving westward. Finally, it disappeared in Hunan Province at 1800 UTC on July 16, 2006. The best track of Bilis from 0000 UTC on July 11 to 1800 UTC on July 16, 2006 (Fig. 3), was plotted using the online dataset (http://agora.ex.nii.ac.jp/digital-typhoon/) 123 Nat Hazards Fig. 3 The best track of Bilis provided by the Japan National Institute of Informatics. As shown in Fig. 3, the center of Bilis was about 450 km away from Zixing at 1200 UTC on July 14, 2006, and then heavy rain occurred and appeared to be persistent over Zixing. Rainfall observations are available for these date from rain gauge networks in Zixing from 0000 UTC on 14 July to 2400 UTC on 15 July as shown in Fig. 4. As shown in the 3-h rainfall curves of Zixing (Fig. 4), the first peak of rainfall intensity appeared at 1500–1800 UTC on July 14, which is a prominent characteristic of the major storm event in Zixing during the dominance of Bilis. The unexpected intense precipitation resulted in devastating floods in Zixing, especially in the Dongjiang reservoir basin. Figure 5 shows the inflow, outflow and water level of Dongjiang reservoir from 1200 UTC on 14 July to 0900 UTC on 19 July. As shown in Fig. 5, there are two peak values along the inflow curve during this period. The inflow of Dongjiang reservoir increased sharply from 590 to 9,298 m3/s during the 9-h period from 1200 UTC to 2100 UTC on 14 July and then decreased rapidly before reaching the second peak at 2100 UTC on 15 July.