An extreme storm over the Nanling Mountains during Typhoon Bilis and the roles of terrain

When the center of Typhoon Bilis was about 500 km away, an extreme flash-flooding storm hit Lechang City on July 14, 2006, which is located at Nanling Mountains in northern Guangdong Province, China. This study demonstrates that the extreme flash-flooding storm originated from the Dongjiang Reservoir basin in Hunan Province during the accelerated movement of the typhoon, with a life cycle of more than 4 h and travel distances of more than 100 km beyond the reservoir shoreline. Based on high-resolution weather radar data and terrain data, this study employs geographical information system techniques to reveal complex roles of Nanling Mountains in aggravating the extreme precipitation in Lechang by intensifying the storm and changing its propagation direction. Our investigations indicate an extreme storm changed direction two times as it passed Nanling Mountain and stayed over Lechang for a long time. The valley on the northwest of Tiantoushui River basin played a primary role in controlling the convective cells to hit Lechang in an unusual direction under the influence of typhoon. Detailed analysis shows meso-scale and miso-scale terrain, including narrow valley, low mountain and small reservoir, may promote convection as same as high mountain under the influence of typhoon, which are poorly resolved in numerical models. Besides, this research shows the shape of meso-scale mountain also has a significant impact on distribution of severe convection during this extreme storm. More importantly, we found atmospheric inversion over Lechang City before the major storm based on the latest AIRS product. Our findings suggest that the construction of large reservoir in typhoon-prone areas requires comprehensive analysis of the complex interactions between typhoons and terrain in order to avoid the threat of extreme storms to the surrounding areas of the reservoir.