An Analysis of the Low Moving Speed of Landfalling Typhoon In-Fa in 2021

The movement speed of Typhoon In -Fa (2021) was notably slow, at 10 km h-1 or less, for over 20 hours following its landfall in Zhejiang, China, in contrast to other typhoons that have made landfall. This study examines the factors contributing to the slow movement of Typhoon In -Fa, including the steering flow, diabatic heating, vertical wind shear (VWS), and surface synoptic situation, by comparing it with Typhoons Yagi (2018) and Rumbia (2018) which followed similar tracks. The findings reveal that the movement speed of Typhoons Yagi and Rumbia is most closely associated with their respective 500 hPa environmental winds, with a steering flow of 10-12 m s-1. In contrast, Typhoon InFa's movement speed is most strongly correlated with the 850 hPa environmental wind field, with a steering flow speed of only 2 m s-1. Furthermore, as Typhoon In -Fa moves northwest after landfall, its intensity is slightly greater than that of Typhoons Yagi and Rumbia, and the pressure gradient in front of Typhoon In -Fa is notably smaller, leading to its slow movement. Additionally, the precipitation distribution of Typhoon In -Fa differs from that of the other two typhoons, resulting in a weak asymmetry of wavenumber-1 diabatic heating, which indirectly affects its movement speed. Further analysis indicates that VWS can alter the typhoon's structure, weaken its intensity, and ultimately impact its movement.