2011年1月九江地区暴雪过程的流场特征及强回波成因分析

Analyses on Air-Flow Characteristics and Abnormal High Reflectivity for a Winter Snowstorm in Jiujiang Region in January 2011

利用九江、南昌多普勒天气雷达资料和常规气象观测资料及0.5°×0.5°的NCEP再分析资料,对2011年1月19—20日九江地区的暴雪天气过程进行了分析。结果表明:南支槽前强盛的西南暖湿气流沿低层东北冷回流在长江流域形成的冷垫爬升是暴雪产生的动力机制;高低空急流的耦合作用形成次级环流,700 hPa以上正涡度平流和低层暖平流表明天气尺度上升运动较强;700 hPa等θ_(se)线密集,表明锋区稳定维持。低层暖平流,中下层风速加强,急流中心降低,预示降雪回波将发展;低层暖平流结构转为冷平流流场结构,预示降雪减弱。分析表明,本次过程没有强雷暴影响,回波异常增强不是由大水滴和霰雹等固态水凝物粒子造成,雪花下降经过0℃以上的暖层时表面融化,使雪花外裹了一薄层水膜,产生类似于大水滴的等同效果,从而出现了雷达图上反射率因子异常增强的现象;同时,中下层强的上升运动和凝结潜热释放的反馈机制可导致局部较强的反射率因子而产生强降雪。

One snowstorm process in January 19-20, 2011 was analyzed by using Doppler radar data from Jiujiang and Nanchang, observation data and NCEP reanalysis data （0.5°×0.5°）. The results showed that the ascending of strong and ample warm wet southwest airflow ahead of south trough and along the low- level return cold airflow over the Yangtze River Basin is the dynamic mechanism responsible for this heavy snow. The secondary circulation is produced by the coupling of low and upper level jets. The positive vorticity advection over 700 hPa and the low-level warm advection denote the strong upward motion. The sta- ble Ose isoline dense section at 700 hPa indicates that the frontal zone is maintained. The low-level warm ad- vection, the increase of wind velocity in the middle and lower level and the decrease of jet stream center are good indices for the enhancement of snow echoes. The low-level warm advection structure develops into cold advection structure, indicating the snow echoes get weakened. The abnormal high radar reflectivity is not caused by large water droplets and solid precipitation particles such as hails. Ice crystals melt as they fall through warm layer above 0℃ coalescing, aggregating and growing by themselves, which leads toabnormal high radar reflectivity similar to large water droplets, without convective weather such as thun- ders. The strong upward motion in the middle and lower troposphere and the feedback mechanism of latent heat create partial high radar reflectivity, so producing the snow storm.