四川盆地东北部山地夜间暴雨特征的合成分析

Composite Analysis of Nocturnal Rainstorm Characteristics in Northeastern Sichuan Basin

利用四川省4890个气象观测站逐时降水数据,筛选出2015—2019年四川盆地东北部发生的24例夜间暴雨过程,在根据影响系统对夜间暴雨进行分型并合成分析的基础上,运用绕流和爬流方程,将近地层流场分解为绕流和爬流运动,重点探究盆地东北部地形对气流的影响及其对夜间暴雨的作用。结果表明:四川盆地东北部山地夜间暴雨可分为西南涡型和偏南风型,两种雨型分别存在西南低涡和低槽在夜间加强的特征,导致夜间偏南气流明显加强,夜间山脉迎风坡出现强垂直上升运动,盆地形成垂直环流,有利于夜间暴雨的形成。两种类型在850 hPa有大量水汽从贵州、重庆上空进入盆地东北部,午夜时分形成强水汽辐合中心;西南涡型水汽的输送、辐合强于偏南风型,西南涡型在盆地东北部后半夜层结不稳定更强,偏南风型后半夜在大巴山迎风坡垂直运动更强。盆地东北部地形抬升区域爬流和绕流运动也具有夜间加强的特征,西南涡型爬流和绕流运动相比偏南风型更强。爬流强迫出的垂直运动能很好地反映偏南气流在地形抬升处形成的垂直上升运动,而地形阻挡产生的绕流运动则有利于局地涡旋的产生和加强,两种类型暴雨的空间分布与夜间正涡度大值区对应较好。

Using hourly precipitation observation data of 4890 meteorological observation stations in Sichuan,24 cases of nocturnal rainstorm processes occurring in northeastern Sichuan basin during the period from 2015 to 2019 were screened out.Based on the classification and synthesis analysis of nighttime rainstorms according to the influence system,the near-surface flow field was decomposed into circumfluence and creeping flow by using the equations of bypass flow and creeping flow,and the influence of topography on airflow and nocturnal rainstorms in the northeast part of the basin were explored.The study shows that the nocturnal rainstorm in northeastern Sichuan basin can be divided into southwest vortex type and southerly wind type.The southwest low vortex and low trough are strengthened at night in each types,causing a significant strengthening of the southerly flow and vertical motion at night.A large amount of water vapor enters northeastern basin from Guizhou and Chongqing at 850hPa and forms a strong water vapor convergence center at midnight.The transport and convergence of water vapor in the southwest vortex type are stronger than those in the southerly wind type.The southwest vortex type is more unstable in the late midnight in the northeast of the basin,and the vertical motion of the southerly wind type is stronger in the windward slope of Daba Mountain in the late night.The creeping and circumfluence movements in the topographic uplift area of northeastern basin also have the characteristics of nighttime strengthening.The creeping and circumfluence movements of southwest vortex type are stronger than that of the southerly wind type.The vertical movement forced by the creeping flow can well reflect the vertical upward movement formed by the southerly airflow at the terrain uplift,while the flow around the terrain is conducive to the generation and strengthening of the local vortex.The spatial distribution of the two types of rainstorms corresponds well to the large value area of positive vorticity at night.