How does HAIKUI remnant produce heavy precipitation in 2023

In 2023, HAIKUI's prolonged remnant caused exceptional destruction, exposing critical gaps in understanding tropical cyclone (TC) remnant dynamics. While existing studies have documented decay processes of TC remnant, systematic analyses of stage-dependent moisture transport remain lacking. This study combines multi-source data to reveal HAIKUI remnant's structural evolution, moisture trajectories, and precipitation drivers. During the persistence of the HAIKUI remnant, the mountain ranges together with environmental airflow played a pivotal role in isolating the low-pressure system, maintaining its coverage largely unchanged. Based on temporal variations, the lifecycle of HAIKUI remnant can be segmented into three distinct stages: the moving stage, the charging stage, and the separating stage. In the moving stage, the center of HAIKUI remnant and the corresponding rain band gradually shifted westward, exhibiting both barotropic and baroclinic characteristics. The charging stage marked a period where HAIKUI remnant's center hovered over Guangxi Province. The dominance of barotropic structure enhanced moisture convergence, generating an arc-shaped heavy rainfall belt. In the separating stage, the mid- and low-level centers of HAIKUI remnant moved toward opposing directions, resulting in a primarily baroclinic circulation structure. The convergence of cold and warm air led to precipitation in the Pearl River Delta region. The findings highlighted the impact of topography and moisture transport on the evolution and precipitation of TC remnant, offering valuable insights for future predictions of precipitation- and flood-related disasters caused by such remnants.