致洪暴雨过程中尺度涡旋的涡散作用及准平衡流诊断分析

A diagnostic analysis of vorticity divergence effects and the quasi-balanced flow in a mesoscale vortex during the process of flash-flood-producing rainstorm.

中文关键词:  致洪暴雨，中尺度涡旋，深厚湿对流，涡散共存，PV-ω反演方法，准平衡流

英文关键词:Flash-flood-producing rainstorm, Mesoscale vortex, Deep and moist convection, Coexistence of vorticity and divergence, PV-ω inversion method, Quasi-balanced flow

中文摘要:

      利用较高分辨率的观测资料与模拟结果进行分析表明，影响2008年6月中旬广西致洪暴雨的主要中尺度天气系统是准静止的中尺度涡旋，它是中纬度天气尺度西风带波动与低纬度暖湿气流带在地形效应下相互作用的结果，其生成、发展和移动对此次致洪暴雨过程的降水强度和持续有重要作用，该中尺度涡旋具有强涡散共存、涡散度达同量级并伴随有较长生命史组织化深厚湿对流的特征。分析了由低空急流中大风速中心的非地转效应以及地形效应所产生的重力波与中尺度涡旋相互作用激发出大振幅组织化的β中尺度深厚湿对流，根据准平衡动力学的理论，准平衡流适用于描述这种同时包含辐散风和旋转风效应的中尺度运动。因此，本文引进基于准平衡流分析的 PV ω 方法，将其应用于反演诊断分析暴雨中具有涡散共存特征的长生命史的组织化深厚湿对流。结果表明，准平衡流能真实反映涡散共存的大振幅垂直运动特征，暴雨区的垂直环流中有50%—70%归于准平衡流。因此，准平衡流可以描述广西致洪暴雨过程中具有较长生命史组织化过程的深厚湿对流系统，准平衡流场具有涡散运动共存的特征。

英文摘要:

      The analysis in this paper based on the atmospheric observation data and the model data of higher resolution shows that one of the major synoptic systems that caused the flash flood producing rainstorm in mid June 2008 in Guangxi is a quasi-stationary mesoscale vortex. The vortex was resulted from the interaction of synoptic westerly waves in the mid-latitude with the warm moist flow in the low-latitude under the terrain effect. The genesis, development and movement of the mesoscale vortex have important influence on the precipitation intensity and continuance in the process of the flash-flood-producing rainstorm. The vortex has the characteristics of strong vorticity and divergence coexisting, and the two have the same order of magnitude as each other. A long-life organized deep and moist convection is accompanied with the mesoscale vortex. Gravity waves, which are generated by terrain effects and ageostrophic effects of large wind speed center in a low-level jet, interact with the mesoscale vortex, and as a result, they stimulate a β-mesoscale organized deep and moist convection of large amplitude. According to the theory of quasi-balanced dynamics, quasi-balanced flow is suitable to describe such mesoscale motions containing both divergent wind and rotational wind. Thus, the PV-ω method, which is based on the analysis of quasi balanced flow, is introduced in this rainstorm process, and is applied in the inversion and diagnostic analysis of the organized deep and moist convection of long life with vorticity and divergence coexising. The results show that the quasi-balanced flow can be a true reflection of the characteristics of large-amplitude vertical motion in which vorticity and divergence coexist. 50%—70% of the vertical circulation in the rainstorm area belong to the quasi-balanced flow. Therefore, the quasi-balanced flow is able to describe a deep and moist convection with a long life organized course. The quasi-balanced flow field has the characteristics of coexistence of vorticity and divergence motion.