利用NCEP再分析资料，对安徽省夏季高空槽前形势下两类强对流天气各5次个例的环流特征、热力和动力条件进行了对比分析。结果表明：以大风、短时强降水天气为主的非龙卷类表现为高空的低槽比较深厚，而龙卷的产生多是由于较浅的短波槽引起的，并且低层有西南急流存在，导致较强的垂直风切变。通过比较热力和动力物理量平均场的分布特征发现：在槽前形势下水汽条件都比较好，夏季整层大气可降水量平均在55mm以上，但出现龙卷时中低层的垂直风切变非常强，龙卷类0-1km垂直风切变大约是非龙卷类的3倍。由于存在较强的垂直风切变，龙卷类低层的风暴相对螺旋度也强于非龙卷类。从动力和热力条件综合来看，出现龙卷时的对流有效位能并不是很大，但能量螺旋度很大，即风暴相对螺旋度上差异更加明显。因此在预报槽前类龙卷天气时，应重点关注环境风场的垂直切变和风暴相对螺旋度。%Based on NCEP (1°× 1°) reanalysis data, the large-sale circulation features as well as thermaland dynamic conditions for two types (5 cases for each type) of severe convective weathers under ahead of trough pattern during summer in Anhui Province are analyzed. The results show that. the nontornadic type mainly causes strong wind and short-time heavy rainfall which is characterized by deep upper trough. However, the tornado is usually triggered by short wave trough with the low-level jet which causes strong vertical wind shear. By comparing the composite thermal and dynamic parameters, it is demonstrated that the moisture condition is good for both types of severe convective weathers and the average precipitable wa- ter during summer is above 55 mm. However, the vertical wind shear at middle-to-low levels is very strong whe'n tornado occurs and the 0--1 km vertical wind shear is about 3 times higher than that of non- tornadie type. Because of larger vertical wind shear, the storm relative helicity (SRH) of tornadie type isStronger than that of nontornadic type. According to the thermal and dynamic conditions,the CAPE of tor- nadic type is not very large while the energy helicity is large and significant differences are found in storm relative helicity. Therefore, more attention should be paid to the vertical wind shear and SRH in forecas- ting the tornado under ahead of trough pattern.
展开▼
