Abstract:

Typhoon Chaba, the fourth typhoon of 2022, exhibited an abnormal northward turn after making landfall in Guangxi on the night of July 2, resulting in significant deviations in wind and rainfall forecasts and posing major challenges to typhoon defense efforts. This study employs a comprehensive approach utilizing multi-source meteorological observation data from the upper-air, surface, and satellite platforms, along with ERA5 (the fifth generation of European Centre for Medium-range Weather Forecasts) reanalysis data of the global climate. Through synoptic diagnostic analysis and quantitative diagnosis using the potential vorticity tendency equation, we investigate the causes of the typhoon's abnormal northward turn. The results indicate that: (1) The northward turn of typhoon Chaba resulted from the combined effects of changes in the deep-layer steering flow due to large-scale circulation pattern shifts and changes in the typhoon's internal asymmetric structure; (2) The deep-layer steering flow played a dominant role in the track deflection. Key factors driving the steering flow changes included the westward extension and intensification of the Western Pacific Subtropical High, the interaction between the southwest flow ahead of the upper-level westerly trough and the northwest side of the South Asian High, and the typhoon's northward outflow. Meanwhile, changes in positive vorticity advection served as important indicators of the typhoon's northward turn; (3) Typhoon Chaba displayed distinct asymmetric structural characteristics, with cumulonimbus convection triggered by its internal vertical motion significantly influencing the northward turn. Changes in the typhoon's cloud pattern also provided valuable indications of its directional shift; (4) Quantitative diagnosis using the potential vorticity tendency equation further revealed that during its movement over the South China Sea, the typhoon was primarily controlled by the steering flow formed by the external large-scale circulation. The abrupt northward turn after landfall resulted from the combined effects of this steering flow and vertical motion induced by the typhoon's asymmetric structure. Additionally, typhoon Chaba exhibited a consistent movement tendency towards the center of positive potential vorticity tendency.

Key words: northward turning of typhoon path, steering flow, asymmetric structure of typhoon, potential vorticity tendency equation