东北冷涡背景下两类龙卷形成机制的对比分析

Comparison analysis of formation mechanisms of two tornado cases under the background of Northeast Cold Vortex

利用多种常规和非常规高时空分辨率的观测资料,对2015年通榆县两次龙卷过程(5月31日和6月8日,分别简称为"531"龙卷和"608"龙卷)形成机制进行详细对比分析。结果表明:直接影响系统均为东北冷涡前部的次天气尺度短波槽或切变线,强对流层中层偏西急流使700—500 hPa温差大值区东移,低空西南急流使低层湿区显著北伸,叠加在温差大值区之下,龙卷发生在湿舌边缘多尺度系统叠加区附近;但"531"龙卷急流风速、700—500 hPa温差及925 hPa露点温度均明显高于"608"龙卷,且850 hPa切变线和负变压区的存在导致辐合上升运动更强,龙卷强度更强。两次龙卷过程发生前对流有效位能均超过1 500 J·kg^(-1),低层存在逆温,抬升凝结高度较低,但湿层较薄,辐合切变线或冷锋是龙卷直接触发机制,且"531"龙卷辐合线两侧有明显风速辐合,为对流风暴发展提供了强入流。两次过程中"531"龙卷过程为强水平风垂直切变下的超级单体龙卷,"608"龙卷过程为弱水平风垂直切变下的非超级单体龙卷。

Using conventional and unconventional observational data with high temporal and spatial resolutions,the formation mechanisms of two tornado cases that occurred on May31and June8,2015in Tongyu County(referred to as"513"Tornado and"608"Tornado)were analyzed in detail.The results showed that the two tornados are both affected by the sub scale short wave trough or shear line in front of the Northeast Cold Vortex.The area of the large temperature difference between700hPa and500hPa moves eastwards due to the strong westerly jet at middle levels of the troposphere,and the area of high moisture obviously stretches northwards due to the low level southwest jet.Tornados occur at the edge of a moist tongue and the vicinity of overlaying area of the multi scale systems.The"531"Tornado has larger jet velocity,the temperature difference between700hPa and500hPa,and the925hPa dew point value and stronger intensity than those of the"608"Tornado,because the convergence and ascending motion are stronger due to the shear line and negative variable pressure at850hPa.Before the formation of the two tornado events,the atmosphere is characterized by the convective available potential energy(CAPE)larger than1500J·kg1,a temperature inversion layer at lower levels,the low lifting condensation level(LCL),and a shallow moist layer.The convergence shear line or cold front is the direct trigger mechanism of the two tornados,and the significant convergence of wind speed at both sides of the convenience line during the"531"Tornado event provides strong inflows to enhance a convective storm.Few clear physical characteristics occurred before the formation of the Tornado,such as the large pressure zone with the higher CAPE value,the lower height of LCL,but the thickness of the wet layer is very thin,the shear lines moving slowly to the east which are the direct facts triggering the formation of the tornados.The both sides of the shear line of"531"tonado have wind speed convergence which provides the strong inflow for the development of the convective storm.The"531"Tornado belongs to a supercell tornado under conditions of strong vertical shear of horizontal winds,while It has a clear hook structure,a weak echo area at lower levels and the draping characteristics of echos in the middle and high levels with the echo top height of higher than11km.The supper cell crosses the boundary of a shear line and locates at the side of cold air.The horizontal vorticity reached a maximum due to the effect of atmospheric baroclinicity.A strong meso cyclone occurs at low levels,with its bottom extending to the height of less than1km,and exhibits obvious characteristics of tornado vortexes,which is favorable for the formation of tornados.the"608"Tornado is a non supercell tornado and it is under the conditions of weak vertical wind shear.