一次甘肃天水强冰雹的雷达回波特征及成因分析

Analysis on the Characteristic of Radar Echo and the Causes of a Strong Hail in Tianshui City of Gansu Province

利用常规气象观测资料、天气雷达以及NCEP FNL再分析资料,对2017年5月18日下午天水一次强冰雹天气的雷达回波结构演变特征和成因进行了详细分析。结果表明:(1)降雹对流单体低层反射率因子呈现出明显的"V"型缺口,最大回波强度出现在低层,为63 d Bz。反射率因子垂直剖面呈对流单体有界弱回波区和其上的回波悬垂,相应的径向速度垂直剖面呈中低层径向风有明显的辐合特征,高层转为辐散,尤其风暴顶附近。(2)对流单体发生在对流层中层河套地区至甘肃河东东部低涡及其附近冷区和河西中部高压脊及其东北部冷池和低层甘肃与宁夏交界处冷性低涡分别为干冷空气入侵和暖湿气流辐合抬升提供有利条件的环境背景下;较高CAPE值和低CIN值有利于强对流天气发生;对流层中低层深厚的上升气流,中层下沉气流和0℃层以上强气流上升有利于对流单体水汽输送以及生成、发展和维持;距地高度2600～2900 m的0℃层为大冰雹落地提供了环境条件。(3)冰雹临近预警的雷达参数化指标为最大反射率因子达55 d Bz,VIL最大值和VIL密度分别达25 kg·m-2和2. 3 g·m-3。

Using normal surface meteorological data、weather radar data and NCEP FNL reanalysis datasets on 18 May 2017,the radar echo structure evolution characteristics and the causes of a strong hail in Tianshui city of Gansu province was analyzed.The results showed that:(1)The low-level reflectivity factor of the convective cells showed obvious"V"gap,the strongest echo appears in the low-level and the value got to 63 dBz.The vertical cross-sections of reflectivity factors showed typical BWER,overhanging echo.The corresponding the vertical cross-sections of radial velocity showed low-level radial wind with obvious characteristic of convergence and high-level to divergence,especially at storm near the top.(2)The background conditions of the convective cells occuring was low vortex and its adjacent cold area from Hetao region to Hedong east and high pressure ridges and cold pool of northeastern in middle of Hexi in the centre of the troposphere,a cold vortex at the border of Gansu and Ningxia provide favorable condition for high-level dry and cold air coming and low-level updrafts pushed by the warm moist southerly flow in the lower troposphere,respectively.High values of the CAPE and low values of the CIN are conducive to strong convection.At the same time the configuration of the deep updrafts airflow in the center and lower troposphere and sinking airflow in the centre of the troposphere,strong convective updrafts above 0℃layer in favor of water vapor transport,generating,develop and maintaining of the convective cells.0℃layer height apart from above ground is about 2600~2900 meters,which provides environmental conditions for large hail floor.(3)Maximum reflectivity reaches 55 dBz,VIL value over 25 kg·m^-2and VIL density over 2.3 kg·m^-2 can be taken as early warning radar parameterized indicators of hail.