两种类型短时强降水形成机理对比分析——以甘肃两次短时强降水过程为例

Case Study on the Formation Mechanism of Two Types of Short-term Strong Rainfall Occurred in Gansu Province on 18 June 2013 and 19 June 2014

利用"2014·06·18"和"2013·06·19"两次短时强降水过程的实况资料及NCEP 1°×1°再分析资料,对比分析了发生在甘肃省中南部地区相同季节、相似气候背景下的不同类型短时强降水过程实况特征、天气形势配置、动力热力特征、云图及雷达特征。结果表明:两次过程雨强均较大,但"2014·06·18"降水过程分散性强、持续时间短,且伴随冰雹、雷暴大风等多种强对流天气,而"2013·06·19"降水过程区域性强、持续时间长。前者是发生在中低层冷暖空气强烈交汇,并伴有明显温度锋区和锋生,地面有冷锋活动形势下,是斜压锋生类短时强降水。后者是发生在低层强烈发展的暖湿平流中,暖湿平流对建立热力不稳定起了主导作用,是暖平流强迫类短时强降水。不稳定指数显示前者不稳定能量大于后者,且存在一定的对流抑制能量,有利于强对流的发展。暖平流强迫类短时强降水湿层厚度高于斜压锋生类,而斜压锋生类短时强降水高层垂直风切变表现得更强。"2013·06·19"暖平流强迫类短时强降水云图特征为沿暖湿气流迅速发展北上的带状云系。"2014·06·18"斜压锋生类短时强降水则表现为与低空"人"字形切变相对应的逗点云系,云系的发展变化与形势场变化密切相关,是降水落区及其发展变化的重要原因。雷达反射率因子显示"2013·06·19"是积状云为主的混合性降水回波,回波梯度小,质心低。"2014·06·18"是层积云中分散着块状对流单体回波,回波梯度大,回波质心发展较高,回波强度可发展到很强。当50 d Bz强反射率因子核心区接近8 km高度,达到-20℃层高度,回波顶高也达到12 km时,有冰雹产生。

By using ＂2014· 06· 18＂ and ＂2013 ·06 · 19＂ short-term strong precipitation observation and NCEP l^xl^reanalysis data, the characteristics of the situation, the configuration of the weather situation, the dynamic thermal characteristics, cloud and radar features for different kinds of strong short-term rainfall were compared and analyzed, which happened in same season and similar climate background at the middle and south parts of Gansu Province. The results show that the formation mechanism have both similarities and significant differences. The rain intensity of the two processes is large, and the former occurred in the case of intense con- vergence of cold and warm air in the middle and lower level, which accompanied with obvious temperature fron- tal zone, frontogenesis and the ground cold front activity. It is a kind of short-term strong precipitation of ba- roclinic frontogenesis. The latter occurred in the strongly developed warm and moist advection in the lower level, the warm and wet advection played a leading role in establishing thermodynamic instability, it is a kind of short time strong precipitation of the warm advection force. The instability index showed that the unstable energy of the former was much larger than that of the latter, and there was convective inhibition, which was conducive to the development of strong convection. The thickness of the wet layer of warm advection forced short-term heavy rain- fall was higher than that of oblique pressure front short-term heavy rainfall, and high-level vertical wind shear performance of the latter short-term heavy rainfall was much stronger. The image features of warm advection force was a banded cloud which was quickly move northward along with warm air on 19 June 2013. The image features of oblique pressure front was a comma cloud system which corresponding to the low-level herringbone shaped shear on 18 June 2014. The change and development of cloud system is closely related to the changes of situation field, which is an important reason for the development and changes of precipitation area. Radar reflec- tivity factor on 19 June 2013 showed that it was a mixed cloud-based mixed precipitation echo, and the echo gra- dient was small with low center of mass. Radar reflectivity factor on 18 June 2014 was a stratified cloud scattered block convection single echo, and the echo gradient was large with high center of mass, besides, the echo inten- sity can be developed to very strong. When the core area of the strong reflectivity of 50 dBz is close to 8 kin, reached the height of -20℃ layer, and the echo top height reached 12 km at same time, there would be hail there.