台风“暹芭”引发鄂北暴雨不同阶段中尺度特征及成因分析

Analysis on Mesoscale Characteristics and Causes of Different Stages of Rainstorm in North Hubei Caused by Typhoon Chaba

利用常规气象观测资料、ERA5再分析资料和数值模式资料,对2022年7月4—5日台风“暹芭”深入内陆引发的鄂北暴雨不同阶段降水中尺度特征及环流背景、物理量环境场、可预报性等进行分析。结果表明:(1)此次过程是发生在台风北上与西风槽结合的环流背景下,中低层700~925 hPa“暹芭”东侧强盛的偏南急流配合高空台风低压环流为强降水提供了动力抬升条件和水汽供应条件。按照系统影响可将此次过程分为螺旋云带和低压主体两个降水阶段,两个阶段都有明显的对流性暖云降水特征,小时雨强大,但螺旋云带降水持续时间短,范围分散,而低压主体降水持续时间长,范围广,累计雨量大。(2)螺旋云带阶段雨团出现前向传播特征,导致雨团加速西移;低压主体阶段MCS(Mesoscale Convective System)快速高度组织化,东移速度缓慢,并出现“列车效应”。导致两个阶段雨团演变差异的主要原因是螺旋云带阶段具有强对流性不稳定和潜势能量,但中低层的垂直运动和水汽辐合弱,对流触发相对分散,维持时间短,主要以环境风场配合降水冷区形成的冷出流触发为主;而低压主体阶段也有较强的对流不稳定,低层辐合高层辐散形成的上升运动、中低层水汽输送及辐合均较螺旋云带阶段显著增强,同时北方冷空气侵入形成能量锋区,为大范围较长时间的强对流提供了有利的热力、动力和水汽条件。地形阻挡作用使MCS出现后向传播,与天气尺度系统共同影响形成“列车效应”。(3)此次过程暴雨的短期可预报性高,但降水极值及强中心落区可预报性低。相较而言,全球模式对于低压主体降水预报更有优势,其中ECMWF效果最优,而中尺度模式对于螺旋云带阶段降水预报更有优势。

Based on conventional meteorological observation data,ERA5 reanalysis data and numerical model data,the mesoscale characteristics,circulation background,physical quantity environment and predictability in different stages of typhoon Chaba rainstorm at northern Hubei are analyzed.The results indicate that:(1)This process occurred under the circulation background of the typhoon moving northward and the combination of the westerly trough.The 700-925 hPa strong southerly jet on the east side of typhoon Chaba,combined with the upper-level typhoon circulation,provided the dynamic lifting conditions and water vapour supply conditions for the heavy precipitation.According to the influence of the system,this process could be divided into two precipitation stages:spiral cloud belt and low pressure main body.Both stages had obvious characteristics of convective warm cloud precipitation,and the hourly rain was strong.However,the precipitation in the spiral cloud belt was short in duration and dispersed in scope,while the precipitation in the low pressure main body was long in duration,wide in scope and large in cumulative rainfall.(2)The primary factor contributing to the disparity in rain mass evolution between the two stages lay in the fact that the spiral cloud belt stage exhibited robust convective instability and potential energy,while vertical motion and water vapour convergence at mid-to-low levels were relatively weak.Consequently,convective triggering was more dispersed and short-lived,primarily driven by cold outflows generated by ambient wind fields and cold precipitation areas.Conversely,during the main stage of the low pressure main body,there was also strong convective instability.The ascending movement,water vapour transport and convergence formed by the convergence of the lower level and the divergence of the upper level were significantly enhanced compared with the spiral cloud belt stage.At the same time,the cold air from the north invaded into the energy front,providing favourable thermal,dynamic and water vapour conditions for a large range of strong convection for a long time.Additionally,topographic blocking effects induced backward propagation of mesoscale convective systems(MCS),which together with synoptic-scale systems gave rise to a“train effect”.(3)In this process,the short-term predictability of heavy rain was high,but the predictability of precipitation extremes and strong central falling areas was low.In comparison,the global model had more advantages for the precipitation forecast of the low pressure main body stage,among which ECMWF yielded the best results,while the mesoscale model had more advantages for the precipitation forecast of the spiral cloud belt stage.