北京"23.7"持续性暴雨的天气事件极端性及强降水不同阶段非平衡流场的分析OA

Under ongoing global warming,the climate system has exhibited increasingly frequent and unevenly distributed extreme weather and climate events worldwide.In particular,extreme precipitation events over the North China Plain(NCP)have shown a pronounced increase in sub-daily and hourly extremes since 2000.Ex-traordinary persistent heavy rainfall events(PHREs),often accompanied by record-breaking flash heavy rainfall(FHR),have repeatedly affected the Beijing-Tianjin-Hebei urban agglomeration,resulting in severe casualties,environmental damage,economic losses,and public concern,partly due to limitations in accurately predicting FHRs embedded within PHREs.Originating from complex interactions among multiscale atmospheric systems and the coupling between mesoscale or storm-scale structures and intense weather phenomena,FHRs remain a major challenge in meteorology.Compared with isolated FHRs occurring during individual rainstorms,those embedded within PHREs—characterized by varying intensities,durations,and spatial distributions—are generally more de-structive.Previous studies have emphasized the dominant roles of meridional and zonal synoptic patterns in PHREs.However,the characteristics of FHRs during PHREs have often been obscured by based on daily-mean datasets,with primary focus on planetary-and synoptic-scale systems.In this study,the record-breaking extreme rainfall event in late July 2023(hereafter referred to as the"23.7"event),which produced an average accumula-ted precipitation exceeding 300 mm over Beijing and a maximum hourly rainfall exceeding 111 mm,is compre-hensively examined. Using multi-source precipitation observations,reanalysis dataset,and historical PHREs over the NCP during the past 60 years,we investigate the synoptic characteristics and unbalanced flow features of the"23.7"event across different FHR stages.Diagnostic analyses include calculations of characteristic thermal and moisture varia-bles,statistical comparisons,and dynamic approximations to separate ageostrophic and unbalanced flows.Compar-ative studies indicate that the"23.7"event is historically rare,persisting for approximately 83 h,with several rain gauge stations recording cumulative precipitation exceeding 800 mm.Quantitative analyses reveal that anomalies in 850 hPa moisture flux,lower-level warm and humid centers,vertical velocity,and convergence between southeast-erly flows and topography were approximately 3-4,4-5,1.5,and 3 times larger,respectively,than those in his-torical PHREs.Further comparisons with two other extreme PHREs occurring after 2010 show that the"23.7"e-vent was characterized by exceptionally persistent southeasterly moisture transport and convergence.Analyses of ageostrophic and unbalanced winds derived from geostrophic and nonlinear balance approximations demonstrate that unbalanced flows in the lower and middle troposphere better correspond to the spatial locations and intensity evolution of rainfall centers during the two major FHR stages over the central and northern Taihang Mountains.These findings are further supported by S-band and X-band radar observations,which reveal the evolution of me-soscale convective systems responsible for the two FHR episodes during the"23.7"event.This study provides a quantitative and comparative assessment of FHR processes embedded within an extreme PHRE and highlights the importance of unbalanced flows in modulating rainfall intensity and distribution.Future work should incorporate higher-resolution,multi-source observations to further elucidate the physical mechanisms governing FHRs across multiple temporal and spatial scales,with the goal of improving early-warning and forecasting of extreme rainfall during PHREs.