登陆福建台风外围环流中宁波地区强对流天气分析

AN ANALYSIS OF SEVERE CONVECTIVE WEATHER IN NINGBO RELATED TO PERIPHERIES OF TYPHOON MAKING LANDFALLS IN FUJIAN PROVINCE

利用中央气象台台风定位定强、常规气象观测、浙江省自动气象站、宁波及华东多普勒天气雷达、美国NCEP/FNL(1°×1°)再分析等多种资料,对2015—2016年5个福建中南部沿海登陆后西北行的台风进行对比分析。这5个台风路径相似,宁波地区仅受外围环流影响,但均出现了暴雨到大暴雨,其中4个出现强对流。分析表明:浙江沿海保持较强的高层辐散和低层辐合,为强对流天气发生提供了环流背景。强对流天气发生在台风中心位于闽赣交界处、强度迅速减弱阶段,浙北沿海中低层处于台风气旋性环流、副热带高压环流和中纬度西风环流之间,宁波地区上空低层(约1.5 km以下)风向随时间变化不大,并可能出现逆时针旋转,1.5 km往上则为明显的顺时针旋转,风向在垂直方向上表现为随高度顺时针旋转且切变增大,同时中上层风速往往同时增大,进一步增大了风垂直切变,有利于强对流天气的发生,强对流均发生在风垂直切变(有时仅表现为风向切变)增强阶段。强对流天气发生在台风外围螺旋雨带中,但强对流回波走向与螺旋雨带明显不同,多个个例表现出由东南-西北逐步转为西南-东北走向,与中上层引导气流的变化一致。出现强对流的台风个例,宁波地区低层存在较明显的温度梯度,其他热力不稳定因素表现不明显,倒槽、中尺度涡旋等为需要密切关注的动力触发因子。最后归纳出此类台风强对流天气典型的高、中、低层大气环流配置模型,为预报提供参考。

The article analyzes the characteristics of severe convections in Ningbo caused by five typhoons from 2015 to 2016 that landed at the middle to southern coast of Fujian province and all headed northwest later. Research data inclucle the typhoon location and intensity data from the Central Meteorological Observatory, a variety of conventional observation data, the automatic weather station data in Zhejiang Province, the Doppler radar data from Ningbo and East China, and NCEP/FNL (1 °×1 °) reanalysis data. These five typhoons had similar tracks and all resulted in heavy rainfalls in Ningbo. Although affected only by the outer rain bands of these typhoons, four of them led to severe convective weather. Results showed that strong divergences at high levels and convergences at low levels over Zhejiang coastal areas provided suitable circulation background for the severe convections. Convective weather occurred when the typhoon centers were located near the border of Fujian and Jiangxi provinces, while typhoon intensities were decreasing rapidly. At lower to middle levels, the coastal areas of northern Zhejiang province were encircled by typhoon cyclonic circulations, subtropical high circulations, and mid-latitude westerly circulations. Over Ningbo, wind directions showed few changes with time at lower levels (below 1.5 km), with some counter clockwise rotations though;at higher levels, they displayed more obvious clockwise rotations with time. The winds were veering with height and the wind shears were increasing. At the same time, the wind speeds at the middle and upper levels were strengthened, leading to more vertical wind shears. All these were favorable for the initiation of severe convections. Strong convections all took place during the enhancement periods of vertical wind shears within the outer spiral rain bands of these typhoons. On the other hand, the movement orientations of the strong convection echoes were quite different from those of the typhoon outer rain bands, and tended to change from southeast-to-northwest to southwest-to-northeast eventually, in consistent with the steering flows at middle and high levels. In the strong convection cases, noticeable low level temperature gradients were observed, but other thermal instabilities were not obvious. Inverted typhoon troughs and mesoscale vortices deserve close attention as triggering factors for convections. To help improve local operational forecast a conceptual model has been summarized to describe the typical circulation patterns at high, middle and low levels for convection initiations in Ningbo caused by typhoon with landfalls in Fujian province.