一次弱弓形飑线后方入流特征的观测分析

后方入流是中尺度对流系统内中尺度环流的一部分,表现为一支从风暴后部穿过层状回波区进入风暴系统的相对气流,对增强中尺度下沉气流和地面冷池具有重要作用。利用多普勒雷达探测资料、地面加密自动站和NCEP再分析资料,结合雷达径向剖面内反演的系统相对水平速度,对2012年5月16日江苏省一次弱弓形飑线的后方入流演变特征进行了分析。结果表明,此次飑线是在东北冷涡影响下,受高、低空温度平流差动、低空急流和低层温度暖脊的共同作用生成。飑线发展阶段,后方入流最早出现在对流层中层的层状回波区中,并向前伸展到对流回波区后缘;成熟阶段,后方入流逐渐下沉并与对流区前低层辐散外流合并,形成一条从飑线后部中层延伸到对流区前缘的持续性后方入流通道;消散阶段,后方入流中心下沉到地面附近,与冷池外流共同增强,与其前侧西南入流的局地辐合,可能是触发对流单体后向新生并促使双带状回波出现的有利条件。后方入流把中层干冷空气持续输送到对流区中下方,通过加剧降水粒子的蒸发冷却作用,增强地面冷池及其出流,导致成熟阶段地面大风生成,这与以往的研究结论一致。受后方入流中心下沉到地面以及新生带状回波系统的影响,地面冷池持续增强,可能是消散阶段地面大风形成的原因。此外,后方入流与飑前地面中尺度辐合线具有很好的对应关系。 

The observational analysis of the characteristics of rear inflow in a weak-bow squall line

The mesoscale system-relative rear inflow is a component of mesoscale convective systems. It is characterized by a current of air penetrating through the stratiform precipitation region from the rear side of the storm and plays an important role in the formation of a strong cold pool and mesoscale downdrafts. In this study, we have exploited Doppler radar observations,intensive surface observations and NCEP(National Centers for Environmental Prediction) reanalysis to investigate characteristics of the rear inflow in a weak bow echo squall line occurred on 16 May, 2012 in Jiangsu Province. The squall line developed under the combined effects of a northeast cold vortex, a large differential temperature advection, a low-level jet and a thermal ridge. Analysis of the system-relative horizontal winds retrieved from radar radial velocity shows that the rear inflow firstly appeared in the stratiform region in the middle troposphere, and then extended to the back edge of the convective region during the development stage of the squall line. At the mature stage, the rear inflow occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels at the leading convective region. During the dissipation stage,the rear inflow maximum gradually descended to the ground, where it enhanced the local convergence caused by southwesterly inflows and cold pool outflows. This convergence might trigger the formation of new back-building convective cells, thereby causing the occasional dual-band echo. The rear inflow transported the cold and dry air in the middle level to the middle lower convective region, strengthening evaporative cooling and promoting the generation of intensive cold pool outflows and high surface winds. This result is consistent with that of theoretical studies. Under the influence of the maximum rear inflow near the surface and the newly generated band echoes, the cold pool continued to intensify and led to strong surface winds during the dissipation stage. Note that the rear inflow well corresponded to the surface mesoscale convergence line.