山东半岛连续两天高架对流降水相态差异的形成机制分析

Analysis of the formation mechanism of phase differences caused by elevated convection in Shandong for two consecutive days

  • 摘要: 2024年2月19—20日山东半岛连续2 d高架对流造成的降水存在显著的相态差异(19日为暴雪、20日为冻雨),文中利用常规观测、闪电定位仪、雷达探测及ERA5再分析等资料进行了对比分析,并给出了概念模型。结论如下:(1)19和20日降水过程是在相似环流背景下发生的:即低层存在气温低于0℃的深厚楔形冷垫,700 hPa南支槽前西南暖湿急流在冷垫上爬升,不仅为暴雪和冻雨的形成提供了充沛的水汽和动力条件,其强度的变化也是形成不同厚度融化层和导致降水相态变化的重要因子。(2)本次过程多种不稳定机制并存:低层,冷、暖空气交界面附近存在条件对称不稳定,暖湿气流在冷垫上倾斜上升;中层,对称不稳定区之上伴随着西南急流两次阶段性北推,500 hPa附近建立了条件不稳定区,低层弱倾斜上升进入该区域后触发高架对流。对流层中-下层局地动力锋生引起的次级环流上升支也是重要的影响机制。(3)19日暴雪是典型的锋生动力学过程造成的,暴雪带平行于锋生带;由于仅存在微弱且浅薄的暖层,绝大多数水凝物以雪花相态落到地面。20日中、低层暖平流造成对流层中层(700 hPa附近)锋消,而低层暖锋锋生,这一过程不仅造成了冷垫厚度变薄,并且形成了更厚更强的融化层,大部分冰相水凝物融化为雨滴和小冰粒,造成大范围冻雨;降水相态分布与低层暖锋锋生的位置关系密切:低层暖锋锋生区附近为冻雨带,其北侧为降雪带,二者之间为冰粒带。(4)降水相态差异是环境场差异和云微物理结构差异共同造成的,融化层及冻结层的厚度、强度和维持时间及云中降水粒子微物理特征,融化层液态水含量等对降水相态均有重要影响。19日对流发展高度高,30 dBz回波扩展到−20—−10℃高度之上,多个降雪云团持续作用且降雪效率高,短时降雪量大,期间的雷达偏振特征表现为:ZDR为−1—0.5 dB,CC>0.98,KDP 不超过1°/km,呈现出均一性强降雪的特征。20日冻雨期间,雷达反射率上存在显著的融化层亮带,该区域对应的CC小于0.9(0.7—0.9)且梯度较大,而该高度以下ZDR显著增大到1—3 dB,对应为粒径较大的冻雨滴。

     

    Abstract: From 19 to 20 February 2024, precipitation caused by elevated convection occurred in Shandong peninsula for two consecutive days, but there were significant differences in phases (heavy snowfall on the 19 February 2024 and freezing rain on the 20 February 2024). In this paper, utilizing multisource meteorological data from conventional meteorological observation, three-dimensional lightning locator, dual polarization radar and ERA5 reanalysis, the differences of their thermodynamic mechanism were compared and their conceptual models were proposed. The results shows: (1) The precipitation on the 19 and 20 February 2024 occurred under the background of similar circulation: That is, there was a deeply sphenoid cold air pad above the ground with a temperature below 0℃, forming a low-level strong inversion layer. The climb of the southwest jet along the cold air pad in front of the southern trough over 700 hPa not only provides abundant water and dynamic conditions, but also its intensity change is an important factor of formatting melt layer. (2) There are a variety of instability mechanisms in this process: In the lower layer, there is a conditional symmetrical instability zone near the interface between cold and warm air, and the warm and humid airflow rises obliquely on the cold pad, and in the middle layer, above the symmetrical instability zone, with the southwestern jet pushing northward twice, the conditional instability zone is established near 500 hPa, and the tilted rising flow into this area triggers elevation convection. The vertical circulation caused by local dynamic frontogenesis is also an important influencing mechanism of the extreme disaster weather by elevated convection. (3) The blizzard on 19 February 2024 was caused by frontogenesis dynamic process, and the blizzard belt was parallel with the frontogenesis belt. On the 20 February 2024, the warm advection caused the front dissipation near 700 hPa, and the frontogenesis of the warm front in the lower layer, which not only caused the cold pad to gradually become thinner, but also formed a thicker melting layer, so most of the hydrometeors melted into raindrops, forming freezing rain. The distribution is closely related to the low-level warm front: The freezing rain is near the warm frontogenesis, the snowfall zone distributes more northern, and the ice particle belt is between them. (4) The differences in precipitation phases are caused by both differences in environmental fields and differences in cloud microphysical structures. The microphysical characteristics of precipitation particles in clouds, the liquid water content in the melting layer, and the thickness, strength, and duration of the melting and freezing layers all have significant impacts on the precipitation phase. On the 19 February 2024, the convection developed higher, and the 30 dBz extended to the height above −20—−10°C, and multiple snowfall clouds continuously act with high snowfall efficiency. During heavy snowfall, the polarization features of radar observation indicate that ZDR is −1—0.5 dB, CC>0.98, and KDP does not exceed 1°/km, showing the characteristics of uniform heavy snowfall. There was a significant melting layer bright band in radar reflectivity during freezing rain on the 20 February 2024, and the CC was less than 0.9 (0.7—0.9) and the gradient was larger, while the ZDR below this height increased to 1—3 dB, corresponding to water droplets or ice particles with larger particle size.

     

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