Comparative analysis of thermodynamic and dynamic structures of two heavy rainfall events triggered by the Huanghuai low vortex over southern Shandong

The Huang-Huai vortex is a key influencing system for heavy rainfall during Shandong's main flood season, yet numerical models exhibit significant biases in forecasting rainfall intensity associated with the vortex. On July 17, 2024 (referred to as the "24·7" event) and September 12, 2025 (referred to as the "25·9" event), regional torrential rains occurred in southern Shandong under the influence of the Huang-Huai vortex, with notable differences in rainfall intensity and spatial distribution relative to the vortex center. Based on dense ground observations, upper-air soundings, precipitation weather phenomenon instruments, and reanalysis data, this study quantitatively compares the physical mechanisms behind the differences in rainfall intensity between these two events. Results show that the "24·7" event featured warm-sector convective rain on the southeast side of the vortex, characterized by high hourly rainfall rates; whereas the "25·9" event had intense rainfall along the northern edge of the warm shear line within the vortex, with weaker hourly rainfall intensity. During the "24·7" event, deep vertical updrafts extending to 200 hPa, with peak vertical velocities of −7.5 Pa·s⁻¹, developed due to dynamic coupling between low- and high-level jet streams and release of conditional instability energy. In contrast, during the "25·9" event, a low-level warm and moist jet ascended along the northward-tilting warm front, forming a moist baroclinic symmetrically unstable stratification, resulting in weaker and shallower updrafts extending only to 400 hPa with a maximum vertical velocity of −3.7 Pa·s⁻¹. The vertical motion during the "24·7" event enhanced both horizontal and vertical moisture transport, with condensation latent heat centers located between 400 and 600 hPa, peaking at 4.6–4.9 W·kg⁻¹, enhancing rainfall intensity through positive feedback. In contrast, moisture transport was weak during the "25·9" event, with latent heat release concentrated between 650 and 700 hPa, peaking at 1–1.5 W·kg⁻¹. The thermodynamic tendency remained dominated by warm advection throughout, while diabatic heating from latent heat release was relatively weak, resulting in limited positive feedback. Raindrop size distributions were broader during the "24·7" event, including large raindrops exceeding 6 mm, with intense coalescence growth among large particles, while the "25·9" event consisted mainly of small raindrops below 2 mm, exhibiting weak particle collision and coalescence processes, resulting in overall lower rainfall intensity. These findings provide valuable insights for improving the fine-scale forecasting of rainfall intensity and quantifying key influencing factors in critical regions affected by the eastern side of the Huang-Huai vortex.