超级单体风暴研究进展

Advances in research on Supercell storms

  • 摘要: 目的 超级单体是所有对流风暴中最为猛烈且持续时间较长的一种高度组织化的类型,亦是致灾性最强的对流风暴,引发极端天气概率较高,文章从超级单体风暴的近风暴环境特征、风暴自身的结构形态和中气旋等涡旋形成机制三方面对超级单体研究进展进行回顾,从而给出对超级单体的整体画像和最近进展。结果 结论 浮力不稳定是超级单体发生的必要条件,而深层垂直风切变和低层风暴相对螺旋度等动力因子是区分超级单体与非超级单体更为敏感的参数,基于多源观测资料构建的近风暴环境廓线有望进一步提升对超级单体风暴的短临精细预报能力。不同类型超级单体以及引发不同灾害性天气的超级单体在反射率因子形态、动力和云微物理结构特征上有所不同,如:龙卷超级单体具有强低层中气旋,强冰雹超级单体具有强而深厚的中气旋,强直线型对流大风超级单体的中气旋伴有明显的中层径向辐合,而引发强降水的超级单体中气旋多位于低层。中气旋本质上由强上升气流扭转环境水平涡度形成,其中层中气旋的环境水平涡度源自深层垂直风切变(风向风速随高度的变化),而低层中气旋的环境水平涡度存在低层垂直风切变和阵风锋区斜压性两种机制导致的水平涡度来源,哪一种机制更合理或占主导目前并不十分清楚。此外,中气旋被强降水包裹、风暴合并、以及边界层的中尺度边界(锋面、干线和阵风锋等及其伴随的辐合线)与超级单体相互作用等情况下的中气旋维持和增强机制复杂多样,有待进一步研究。近年来,基于高分辨率数值试验结果,一些学者给出了超级单体龙卷形成的更新的概念模型和物理图像,具有双偏振功能的多普勒天气雷达给出超级单体中一些新发现的动力和微物理特征,能够更加准确探测大雹尺寸,但精细的大冰雹增长物理图像有待完善,超级单体极端风雨形成机制的研究进展有限,尚待进一步研究。

     

    Abstract: Purpose Supercells are the most severe and long-lasting type of highly organized convective storms, representing the best organized type with the highest potential for causing disasters and a higher likelihood of producing extreme weather events. This article provides an overall portrait and recent highlights of supercells research, including the unique structure, environmental characteristics, and the formation and maintenance mechanisms of the mesocyclone. Results Conclusion Buoyancy instability is a necessary ingredient of supercells environment, while the dynamic factors such as vertical wind shear and low-level storm-relative helicity are more sensitive parameters for distinguishing supercells from non-supercells. The near-storm environmental profiles based on multi-source observation data are expected to further improve high resolution and highly skilled nowcasting of supercell storms. Supercells of different types and those causing different hazard weather exhibit distinct characteristics in reflectivity factor morphology, dynamic, and cloud microphysical structures. For instance, tornadic supercells have a strong low-level mesocyclone, severe hail supercells have a strong and deep mesocyclone, high wind mesocylones are accompanied by significant mid-level radial convergence, and supercells causing heavy precipitation often have a lower-level mesocyclone. The vertical vorticity of mesocyclone comes from tilting of environmental horizontal vorticity by intense updrafts related to storm. The horizontal vorticity of the mid-level mesocyclone originates from vertical wind shear (wind direction and speed vary with height), while the horizontal vorticity of the low-level mesocyclone has two different origins: one is the low-level environmental vertical wind shear,the other is horizontal vorticity producd by baroclinic near the gust fronts. It’s currently unclear which mechanism is more reasonable or dominant. Moreover, the maintenance and enhancement mechanisms of the mesocyclone are complex and diverse in situations where the mesocyclone being surrounded by heavy precipitation, storm mergers, and the interaction between supercell and mesoscale boundaries in Planet Boundary Layer (fronts, dry lines, gust fronts, etc., and their associated convergence lines). In recent years, based on superhigh-resolution numerical experiment results, the physical conceptual models of the supercell tornadogenes is have been updated. New microphysical characteristics and some dynamic characteristics have been revealed by the dual-polarization Doppler weather radar, enabling more accurate detection of hail sizes. However, the refined physical conceptual model of the growth of large hailstones remains to be improved, and the understanding on the formation mechanism of extreme wind and flash flood related to supercells is still limited.

     

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