“罗莎”台风造成浙江特大暴雨的过程分析

Process analysis of the extraordinarily torrential rainfall from Typhoon Krosa in Zhejiang

0716号台风"罗莎"于2007年10月7日下午15:30在浙江省苍南县霞关镇附近的浙闽交界处登陆。受"罗莎"台风的影响,浙江省出现暴雨到大暴雨,过程平均降雨量全省达163.0mm。利用NCEP/NCAR1°×1°再分析格点资料、浙江省自动站降水数据和MICAPS的Ki与Ky指数资料(浙江省部分),对由0716号台风"罗莎"造成的浙江大暴雨到特大暴雨过程的大环流天气形势演变、动力条件、水汽输送及物理量特征等进行诊断研究。结果表明,在10月6～9日"罗莎"台风影响浙江期间,欧亚500hPa中高纬度为平直环流,乌拉尔山以西为高压脊,从河西走廊至中原地区有一移动性冷槽,西太平洋副热带高压逐渐加强,呈东西向带状,位置偏北强度偏强,江淮以南地区为副热带高压控制,脊线位于30°N附近;在对流层中、低层(气压为700hPa、850hPa)有明显的冷温槽,途经河西走廊、河套地区、江淮和长江中下游地区,7日14时至8日14时地面冷锋与"罗莎"台风倒槽在浙江相互作用,形成了台风外围受冷空气侵入的特定环境场,为浙江大暴雨到特大暴雨的形成创造了条件。"罗莎"台风螺旋云带中含有大量水汽从东海向浙江大陆输送,10月6日20时至8日14时,从东海到浙江大部出现强的水汽通量大值区,最大水汽通量出现在10月7日02时低层(气压为850hPa),中心为40g/(s.hPa.cm)以上的水汽通量大值区出现在东海,低层高含水量从东海向浙江陆地持续输送,时间长达42h,为本次大暴雨到特大暴雨提供了有利的水汽条件,诊断得出,大气柱可降水量达65kg/m2时可出现大到暴雨,大气柱可降水量达70kg/m2时可出现特大暴雨。从流场、散度场和垂直速度场发现,台风环流区域内低层的强上升运动为本次大暴雨到特大暴雨提供了上升运动的促发机制,高层辐散区与中低层台风北侧倒槽强烈辐合区叠加,使得中低层的辐合更为强烈,这都有利于台风北侧暴雨的维持和加强;台风环流域内的垂直上升运动区有一个"增厚"过程。分析温度平流发现,6～8日在浙江上空(气压为550hPa以下),有明显的冷暖平流交汇,冷空气进入台风外围与台风携带的暖湿空气相遇,台风环流呈西冷东暖状态,加上低层的辐合机制,促使斜压对流不稳定能量的加大,并释放出最大有效的位能,使得大于35℃的Ki指数和4～6个单位的Ky指数高值区覆盖浙江全省,造成大气强烈的上升运动,导致台风倒槽降雨量的加强。

No.0716 Typhoon Krosa landed near Xiaguan town, Cangnan County, Zhejiang adjacent to Fujian province at 15：30 on October 7, 2007, which brought torrential rain in Zhejiang province with the average rainfall of 163.0 mm during the typhoon. Extraordinarily torrential rain occurred in Wenzhou, Taizhou and Ningbo areas and the maximum rainfall occurred at Shibalong Town, Yongjia County, Wenzhou, where the rainfall was up to 545.8 mm.Based on NCEP/NCAR 1°×1° reanalyzed grid data, data from the automatic observation stations in Zhejiang, and Ki and Ky indices from MICAPS analysis system general circulation evolution, dynamic conditions, water vapor transportation and the characteristics of physical variables that brought torrential rain by Typhoon Krosa （0716） in Zhejiang are analyzed. The results show that when Krosa hit Zhejiang in October 6 to 9, the circulation was straight in middle and higher latitudes at 500 hPa; high-pressure ridge was to the west of Urals and there was a moving cold trough from Hexi Corridor to the Central Plains; the gradually intensified western Pacific subtropical high was in the shape of east-west belt, its northern part was stranger than normal; the area to the south of Jianghuai was covered by the subtropical high, the ridge was located near 30°N; in the middle and low levels of 700 hPa and 850 hPa of the troposphere, there was an apparent cold temperature tough passing though the Hexi Corridor, Hetao area, Jianghuai and the middle and lower reaches of the Changjiang River. The interaction between the surface cold front and the inverted trough of typhoon Krosa from 14pm, October 7 to 14pm, October 8 formed special environmental conditions of cold air invasion at the surroundings of the typhoon which favors the torrential rain in Zhejiang. Huge amount of vapor content in the spiral cloud band of Krosa was transported from the East China Sea to Zhejiang continent, which resulted in an area of great water vapor flux value from the East China Sea to the most part of Zhejiang with the maximum flux greater than 40 g/（s·hPa·cm） at the low level of 850 hPa at 02am, October 7 in East China Sea. The water vapor transportation at the low level from the East China Sea to the continent of Zhejiang lasted for 42 h, which provided water vapor for this torrential rain. Diagnosis shows that there could be torrential rain when the precipitable water in the atmospheric column is up to 65 kg/m2 and extraordinarily torrential rain when it is 70 kg/m2. The stream, divergence and vertical velocity fields show that not only the vigorous rising motion at the low level in the typhoon region provided a trigger of the rising motion of the torrential rain, the overlap of the upper and middle level divergence area and the strong convergence area at the north of the typhoon＇s inverted trough enhanced the convergence of the middle and lower levels, which favored the maintenance and strengthening of the torrential rain at the north of the typhoon. Besides, the area of vertically rising motion in the circulation area of the typhoon experienced a thickening process. The analysis of temperature advection shows that the warm air and cold air interacted below 550 hPa from October 6 to October 8 over Zhejiang, which is resulted from the meeting of the cold air inserted into the typhoon and the warm and wet air brought by typhoon. The typhoon circulation exhibited a western-eastern seesaw mode cold in the west and warm in the east. Together with the affection of the convergence mechanism at the low level, the baroclinic instability energy was further increased and maximum effective potential energy released, which made the 35℃ Ki index and 4~6 high value Ky index control the whole Zhejiang. These lead to vigorous ascend motion and more rainfall in typhoon inverted trough.