台风“海棠”特大暴雨数值模拟研究

在福建中北部登陆的台风,往往会严重影响浙江,尤其值得注意的是台风引起特大暴雨经常会发生在浙江东南沿海的南雁荡山区和北雁荡山区,2005年在福建省连江黄歧登陆的台风"海棠"(0505)对浙江东南沿海造成严重影响,是这类台风比较典型个例。文中利用非静力模式MM5模拟"海棠"台风在浙东南沿海造成的特大暴雨,模拟结果与实况对比分析表明,模式较好地模拟了台风降水强度和分布,特别是成功模拟出南雁荡山区特大暴雨中心(南部暴雨区)和雁荡山区特大暴雨中心(北部暴雨区);运用高时空分辨率模拟资料对特大暴雨成因进行诊断分析表明,南部暴雨区涡度低层到高层向西倾斜结构和北部暴雨区高低空强辐散辐合的耦合结构有利于形成暴雨区强烈上升运动,环境风场垂直切变产生次级环流进一步加强暴雨区上升运动;暴雨区持续不稳定层结和特殊水汽输送通道为特大暴雨提供热力条件和水汽条件。最后对浙南闽北地形对台风特大暴雨影响进行数值敏感性试验表明,温州南、北雁荡山脉地形等高线与台风水汽输送路径正交是造成特大暴雨的重要原因,地形使暴雨增幅明显,地形越高对暴雨增幅越明显,降水分布更加不均匀。比较台风造成南、北特大暴雨条件,发现两者既有环境风场垂直切变产生次级环流进一步加强暴雨区上升运动、持续不稳定层结以及地形对暴雨增幅作用等相同之处,又有动力结构、维持持续不稳定层结条件以及水汽输送等不同之处。

A NUMERICAL STUDY OF THE SEVERE HEAVY RAINFALL ASSOCIATED WITH THE TYPHOON HAITANG (0505)

Typhoons landing in the middle and north of Fujian province,often seriously impact Zhejiang province.More attention has been given to exceptionally torrential rain in the South/North Yandang mountainous regions in the southeast of Zhejiang province associated with typhoon-landing.Typhoon Haitang(2005) is a typical case of such a category,which landed in lianjiang Huangqi town in Fujian province,and meanwhile greatly impacted southeast Zhejiang.A numerical simulation has been performed with the PSU/NCAR non-hydrostatic model MM5V3 to study the torrential rain associated with typhoon Haitang.The comparison of simulated and observed rainfalls show that the MM5V3 was able to well simulate not only the intensity but also location of the severe heavy rain of typhoon Haitang,especially the locations of the south /north heavy rain center areas in the South/North Yandang mountainous regions.Meanwhile,the diagnostic analysis has been also carried out to gain a better understanding of the severe heavy rain mechanism by using the model output data of high resolution.The diagnostic analysis indicates that the westward tilt of the axis of vorticity form lower layer to upper layer over the south heavy rain center area and the coupled structure of convergence in the lower layer and divergence in the upper level over the north heavy rain center area,were both propitious to stronger upward motion in the layers between the middle and upper atmosphere,and the secondary circulation induced by the vertical shear of the ambient winds further strengthened the upward motion in the heavy rain areas.After Haitang passed through the island of Taiwan into the Taiwan strait,the water vapor east of Taiwan island was continuously transferred by typhoon circulation towards south Wenzhou,leading to the torrential rainfall in the South Yandang mountainous region south of Wenzhou.Subsequently,Haitang moved northwards,the water vapor belt east of Taiwan island slowly advanced northwards,the precipitation rate obviously enhanced in the North Yandang mountainous region north of Wenzhou.After landing in Fujian province,Haitang moved northwestwards,the water vapor belt coming from the Taiwan strait merged into the strong water vapor belt from the ocean surface east of the Taiwan island, and passed through the north of Wenzhou,resulting in the torrential rainfall in the north of Wenzhou.The specific moisture transport passage offered the abundant water vapor condition for torrential rainfalls associated with typhoon Haitang.The unstable stratification at the middle and lower layers over rainstorm areas also provided favorable thermal condition for torrential rainfall.The sensitivity simulation experiments of terrain effect on Haitang's heavy rain were also conducted.The moisture flow derived by Haitang's southeast flow,which was perpendicular to the south and north Yandang mountain ranges,played an important role in the torrential rainfall associated with Haitang.The experimental results show that orographic lifting contributed greatly to the enhancement of precipitation,and made the distribution of precipitation more uneven.The some causes for typhoon heavy rainfalls in the south and north of Wenzhou are common,but some causes different.The common aspects are strong ascending motion forced by vertical wind shear,sustained unstable stratification,and amplification effect of orography on precipitation,while different aspects are dynamic structure,conditions for maintenance of unstable stratification,and moisture transport condition.