Dynamic Processes and Multiscale Systems Interaction During an Extreme Heavy Rainfall Event in North China

Abstract The research reveals the origins and persistence of the extremely heavy rainstorm in North China in July 2023 (known as the “23.7” rainstorm). Unlike previous research, it applies scale separation and an advanced energy diagnostic equation to analyze energy transformations among multi-scale weather systems, highlighting their collective impact. Utilizing hourly rainfall data from over 70,000 national stations, along with daily global Final Analysis Data (FNL 1 º × 1 º) from NCEP/NCAR, the study employs Barnes bandpass filtering for atmospheric field separation and an energy equation to diagnose energy conversions. It finds that the “23.7” rainstorm is a complex, multi-scale phenomenon, with the synoptic to meso-α scales scales playing a pivotal role. The storm’s sustenance mechanism resembles the conditional instability of second kind (named R-CISK), focusing on upper tropospheric westerly jet divergence that triggers mid-tropospheric ascent, diverging from CISK’s emphasis on boundary layer friction. This initiates cumulus convection, followed by lower tropospheric jet convergence and typhoon-induced low-pressure trough interaction, uplifting lower layer air and coupling with mid-to-upper level convection. The energy diagnosis shows that large scale and synoptic scale systems are the primary energy sources. The kinetic energy interplay and conversion of buoyancy work and baroclinic energy are the main drivers of energy transformation, essential for storm development. These insights and methods enhance traditional rainstorm models and diagnostic techniques, offering valuable references for future regional heavy rainstorm forecasting and analysis.