Abstract This study investigates the mechanisms by which stratospheric intrusion contributed to a heavy snowfall event in China's Middle‐Lower Yangtze River (MLYR) region during 19–24 February 2024, which prompted a second‐highest‐level emergency response. Using station and ERA5 reanalysis data, we found that accompanying a Ural blocking‐transverse trough system and an intensified East Asian subpolar jet, a stratospheric intrusion of unprecedented depth (air with 1.5/1.0 ‐PVU potential vorticity (PV) penetrating to ∼600/700 hPa) occurred on 20 February, coinciding with the peak daily snow accumulation. Using HYSPLIT trajectories and piecewise PV inversion, we found that descending stratospheric air was not a direct cold air source in the lower troposphere. Instead, the key mechanism of stratospheric impact on snowfall is the dynamical forcing from upper‐level PV anomalies. From 18 February, a high‐PV band near the transverse trough guided the southeastward convergence of three lower‐level cold air streams from high latitudes, promoting frontogenesis and cold conditions. On 20 February, intrusion‐induced PV anomalies intensified the polar jet (accounting for ∼45% of intensification) and induced a Northeast China Cold Vortex anomaly. They drove a secondary circulation with subsidence over Northeast China at the northern side of the jet entrance, ascent over the MLYR, and northerlies in between. Concurrently, lower‐boundary PV anomalies strengthened a Mongolian High‐like anticyclone. The upper‐level PV‐induced vortex transported cold air from high latitudes, while the lower‐boundary PV‐induced anticyclone advected it further southward, forming a “cold air relay mechanism.” The mid‐tropospheric latent heating intensified moisture transport, creating a deep baroclinic zone when clashing with cold air.
Yu et al. (2026) studied this question.