2024年3月25日浙江强飑线演变特征

Evolution and Mechanism of Strong Squall Line Occurred in Zhejiang on 25 March 2024

  • 摘要: 利用多源资料分析2024年3月25日浙江强飑线过程的演变特征,结果表明:此次过程发生在高空槽前、中低层切变和地面强冷锋附近,冷锋前地面辐合线上不断触发多个对流单体后,在低空强西南急流、强垂直风切变等有利环流条件下,组织化成飑线。飑线发展维持的主要原因是其自组织结构,其上有2个超级单体发展,且西南段与东北段发展速度不一致。飑线伴随的雷暴高压和冷池较强,地面冷出流叠加强辐合抬升,促使单体发展为超级单体,而超级单体内部水凝物相变导致大气剧烈降温形成强下沉气流,又加强了地面冷池和后侧入流急流,使飑线得以发展和维持。12级以上极端大风主要出现在飑线成熟阶段,其中超级单体A在并入飑线前及减弱过程中均出现12级以上极端大风,但形成机制有所不同。

     

    Abstract: Based on conventional upper-air and surface observations, S-band dual-polarization radar data, cloud ground lightning positioning data,ERA5 reanalysis data (0.25°×0.25°), environmental conditions, evolution characteristics, structure and mechanism of the strong squall line process in Zhejiang on 25 March 2024 are analyzed. Main conclusions are as follows: With a trough at 500 hPa, V-shaped shear at 850-925 hPa level and strong cold front on the ground, the initial convection is triggered within the strong convergence zone on the ground convergence line before cold front. Under favorable conditions such as strong upper-level westerly jet stream, strong low-level southwest jet stream, moderate unstable layering conditions and strong vertical wind shear of 0-3 km and 0-6 km, multiple isolated convective cells develop strongly and organize into the squall line. The primary cause for the development and maintenance of this squall line is its self-organizing structure, characterized by strong thunderstorm-induced high pressure and a cold pool. In the mature stage, the β mesoscale squall line merges with supercell A, resulting in the formation of the α mesoscale squall line. The surface cold outflow formed by the squall line confront the southwest wind ahead, creates a frontogenesis which continuously lifts the warm and humid airflow in front of it, stacks the convergence and uplift effect, and leads to the development of cells into supercells. At the same time, there is a significant angle between low-level vertical wind shear and the squall line, which facilitates the continuous uplift of low-level vertical airflow and supports the long-term development and maintenance of supercells. During the phase transition of water vapor within supercells, significant heat absorption leads to a sharp cooling of the atmosphere and the formation of a strong downdraft. Combined with the descending airflow generated by the inflow from the rear, the surface temperature decreases significantly, thereby intensifying the cold pool at the ground. The strengthening of cold pool facilitates the formation and development of the inflow jet stream from the rear, thereby promoting the emergence and strengthening of squall lines. Extreme winds exceeding level 12 primarily occur during the mature stage of squall lines. Supercell A experiences extreme winds of level 12 or above both before merging into the squall line and during its weakening phase. The former is mainly caused by strong downdrafts with features such as mid-level echo overhang and bounded weak echo area, while the latter is induced by combined effects of strong downdrafts and momentum transfer mechanisms with features disappearing.

     

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