地形作用下低空急流的演变与强降水对流风暴系统的相互作用

The interaction between low-level jet evolution and severe convective rainstorms under topographic effect

中文关键词:  强降水对流风暴  后向传播  低空急流  冷池  地形

英文关键词:Severe rain convective storm  Back-propagating  Low-level jet  Cold pool  Topography

中文摘要:

      利用雷达、卫星、风廓线雷达和地面加密区域自动气象站等观测资料，分析了2016年入梅后发生在鄂东地区一次极端强降水事件的中尺度对流系统发生发展过程、结构演变及其传播特征，旨在揭示造成强降水过程中的3个中尺度对流系统（MCS）的触发、发展、维持机理以及它们之间内在的中尺度动力学关系，尤其是地形作用下的低空急流的演变与强降水对流风暴系统相互作用过程。研究表明：（1）与大多数梅雨锋上的强降水带与低空切变线平行分布不同，此次极端强降水雨带呈倾斜的“n”字形，其中两条主雨带近乎与低空切变线垂直；此次极端强降水分别由大别山迎风坡上西北—东南向MCS、湖北中东部平原地区西北—东南向MCS和桐柏—大洪山东侧东北—西南向MCS造成。3个MCS移动缓慢，都具有后向传播的特征。（2）大别山迎风坡上MCS初始雷暴是低空急流下边界不断向下扩展过程中在地形抬升作用下触发的，而湖北中东部平原地区的MCS和桐柏—大洪山东侧MCS的触发、发展、加强都与大别山迎风坡上MCS形成的冷池加速推进形成的出流边界与环境气流形成的强烈辐合抬升作用有关。（3）垂直于大别山的边界层西南急流对山坡上的对流冷池产生的顶托作用不仅平衡了冷池密度流产生的向下作用力，而且进一步强化了山区的辐合抬升强度，使得大别山迎风坡上强降水风暴系统得以长时间维持和发展；当山坡上的对流冷池堆积到足够厚度，或者由于低空急流的下边界迅速抬升时，这种平衡被打破，大范围的冷池俯冲下山并在平原地区快速推进，造成了湖北中东部平原地区大范围的雷暴大风和MCS发展加强，并沿冷池前沿逐步组织化，形成平原地区东南—西北向的强降水带。

英文摘要:

      Observations of radars, satellite and wind profile radars are combined with data collected at regional automatic weather stations to analyze the initiation, development, structure evolution and propagation characteristics of three mesoscale convective systems (MCSs) occurred in 2016. These systems induced an extremely severe precipitation in eastern Hubei during the Meiyu period. This paper focuses on the trigger, development and maintenance mechanism of the three MCSs and the mesoscale dynamic relationship between them. In particular, the interaction between the low-level jet and the severe convective rainstorms under the topographic effect is discussed. The results are as follows：(1) Different to most of heavy rain bands caused by the Meiyu front, which are distributed along the lower-shear line, the extreme rain bands presented in this study show a slanted n-shape, and the two main rain bands are nearly perpendicular to the lower-shear line. The extremely severe rain is caused by the northwest—southeast oriented MCS on the windward slope of Dabie Mountain, the northwest—southeast oriented MCS in the central and eastern plain area of Hubei province, and the northeast—southwest oriented MCS on the east side of Tongbai—Dahong Mountains. These three MCSs move slowly and have the characteristic of back-propagating. (2) The convective rainstorm associated with the MCS on the windward slope of Dabie Mountain is triggered by topographic forcing during the continuous downward expansion of the lower boundary of the low-level jet. And the cold pool produced by the MCS in Dabie Mountain accelerates down the slope, forming the outflow boundary. The strong convergence produced by the outflow boundary and the environmental airflow leads to the initiation, development and strengthening of the MCSs in the central and eastern plain of Hubei province and the eastern side of Tongbai—Dahong Mountains. (3) The low-level southwesterly jet perpendicular to Dabie Mountain has a strong supporting effect on the convective cold pool, which not only balances the downward force generated by the density airflow of the cold pool, but also further strengthens the intensity of convergence in the mountain area. As a consequence, the heavy rainstorm system on the windward slope of Dabie Mountain maintains and develops over a long time. However, once the convective cold pool on the hillside accumulates to sufficient thickness, or the lower boundary of the low-level jet rises rapidly, the balance is broken. The cold pool dives downhill and advances rapidly in the plain area, resulting in a large-scale thunderstorm gale in central and eastern plain of Hubei province. Meanwhile, the MCS strengthens and organizes gradually along the cold pool frontier, generating the southeast—northwest oriented severe rain bands.