During Typhoon Haikui’s landfall in September 2023, Fuzhou received extreme rainfall exceeding 500 mm, even though the typhoon did not make landfall in this city, with two distinct nocturnal rainfall phases identified. This study examines the characteristics of the first phase (1600–2100 UTC 4 September) and its associated synoptic-scale patterns. A near-surface cold pool along the Fujian coast was observed to strengthen and spread several hours before rainfall began. This cold pool promoted strong warm frontogenesis, mainly driven by boundary-layer convergence, with scalar frontogenesis outweighing deformation effects. The convergence resulted from the interaction between two low-level jets—one over the Taiwan Strait and another to its north—whose confluence was located directly over southern Fuzhou. Frontogenetic circulation lifted moist air from the boundary layer, initiating deep convection. Following the initial release of conditional instability and conditional symmetric instability, lower-tropospheric conditional instability became the dominant factor sustaining convection and producing extreme rainfall.
Hu et al. (Wed,) studied this question.