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四川南部一次局地暖区暴雨中尺度特征及成因

Mesoscale Characteristics and Causes of a Local Warm-sector Rainstorm Process in Southern Sichuan

  • 摘要: 本文利用地面自动气象站逐时降水资料、多源融合逐时气温资料、高空观测资料、FY-4A气象卫星资料、X波段双偏振雷达资料和ERA5再分析资料,对四川省南部攀西地区2022年10月4日20时-5日12时的局地暖区暴雨过程中尺度特征及成因进行分析,结果表明:(1)在卫星云图上,此次暴雨天气过程为β-γ中尺度对流系统,择木龙村一带中尺度对流系统三次经历了发展-加强-减弱的过程。暴雨中尺度对流系统自下而上向东倾斜,使得对流系统得以较长时间的维持和发展。(2)在雷达回波上,择木龙村一带回波呈带状分布,表现出明显的后向传播特征,“列车效应”显著,较强回波质心较低,多在4.5km以下,其单体质心减弱十分迅速,表明其具有较高的降水效率。(3)此次暴雨天气过程大气层结呈现典型的位势不稳定层结结构,对流凝结高度、抬升凝结高度和自由对流高度较低,使得气块不需要太强的抬升作用就能产生垂直上升运动和水汽凝结,使雷暴系统迅速发展。垂直风切变较弱,降低了未饱和干冷空气的卷入,减弱夹卷率,提高了降水效率。(4)攀枝花西北部边缘地形的阻挡作用形成的辐合流场和气旋式流场是该地不断有对流生成,形成“列车效应”,造成择木龙村一带出现极端降水的主要原因。而观音堂村一带,西南低空急流与其东北部山脉接近正交,地形能强迫气流抬升,在山前的低洼地带形成了较强降水。西南低空急流和由于地形作用形成的风速辐合,地形温度梯度造成的地形环流也对暴雨的形成起到促进作用。

     

    Abstract: Based on the hourly precipitation data from automatic weather stations, hourly temperature data from multi-source fusion, high-altitude observation data, meteorological satellite data of FY-4A, X-band dual polarization radar data and ERA5 reanalysis data , the mesoscale characteristics and causes of a warm-sector rainstorm process in Panxi area, southern Sichuan from 20:00 on October 4th, 2022 to 12:00 on October 5th, 2022 is analyzed.The results show that:(1)This process manifested as a β-γ mesoscale convective system on satellite imagery, which has gone through three stages of developing, strengthening and weakening in Zemulong Village area. The mesoscale convective system of rainstorm tilts east from bottom to top, allowing the convective system to maintain and develop for a longer period.(2)The radar echo in Zemulong Village area shows a strip-shaped distribution, the back-propagation and echo training effects is significant. The strong echo nuclei is relatively low, mostly below 4.5km and its individual nuclei weakens rapidly, thus it has a high precipitation efficiency.(3)In this process,the atmosphere is in an unstable state of dryness above and wetness below, with a low level of CCL, LCL, LFC, thus the gas mass does not need to be lifted too strongly to move upward vertically and cause water vapor condensation, then the thunderstorm system can develop rapidly. The weak vertical wind shear also reduces the involvement of unsaturated dry cold air, weakens the entrainment rate and improves the precipitation efficiency. (4)The convergence and cyclonic flow field formed by the blocking effect of the terrain on the northwest edge of Panzhihua are the main factors generating the continuous convection and forming the training effects, thus extreme precipitation occurs in Zemulong Village area. In Guanyintang Village, the southwest low-level jet stream is nearly orthogonal to the northeast mountains, which force the airflow to rise and forming strong precipitation in the low-lying areas in front of the mountain. Due to the southwest low-level jet, convergence of wind speeds caused by terrain, the terrain circulation caused by terrain temperature gradient also promotes the formation of heavy precipitation.

     

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