Environmental ingredients that lead to tornado outbreak and tornado failure: A comparison between two similar recurving tropical cyclones

Abstract

Recurving tropical cyclones (TCs) can sometimes produce tornado outbreaks, while some TCs with similar tracks and intensities may produce none of tornado, which makes it challenging to assess tornado risk within recurving TCs. This study investigates two recurving TCs, Typhoon Yagi (2018) and Typhoon In-Fa (2021), that made landfall in eastern China. Despite the similar recurving tracks and intensities, Yagi produced 11 tornadoes while In-Fa produced none. Results show that both TCs were characterized by similar large-scale conditions that were dynamically favourable for tornadoes during the recurvature process. The non-tornadic In-Fa even featured a higher shear and helicity environment in its northeast sector than did the tornado-productive Yagi. The greatest difference between Yagi and In-Fa is the thermodynamic instability owing to the different lower–middle-tropospheric lapse rates that are attributable to the differences in air trajectories at low levels. In-Fa featured marginal instability due to the cooler air at low levels because almost all of the air parcels came from the Pacific Ocean while most air parcels for Yagi came from the warm land. The cooler low-level air tends to create higher relative humidity in In-Fa's interior and thus leads to widespread precipitation which in turn also contributes to the low-level cooling. The different air trajectories are demonstrated related to the TC's translation speed, size and synoptic characteristics days before TC's landfall. Numerical simulations suggest that the upward motions within the widespread precipitation regions of In-Fa are overall weaker than those of Yagi due to the limited instability in the former. These findings suggest that even though two TCs were characterized by similar tracks, intensities and large-scale forcings, their different low-level air pathways may have significant influence on priming the mesoscale environment for supercell or tornado formation.