Multi-scale Energy Transfer during the Offshore Intensification of Typhoon Hato

Wave-flow interactions within tropical cyclones are closely linked to their structural evolution and intensity variability. Using ERA5 reanalysis data and Tropical Regional Assimilation Model for the South China Sea (TRAMS) numerical simulation data, this study adopts wave-flow separation and Fourier decomposition techniques to investigate the interaction between the basic-state vortex  and asymmetric disturbance of Typhoon Hato. Cross-scale energy transfer processes are analyzed via energy spectra and energy budget analyses. The results are shown as follows.

Typhoon Hato intensified primarily due to strong southwesterly and southeasterly moisture convergence, which advected water vapor toward the inner core and facilitated upward vertical transport, sustaining condensation latent heat release and the subsequent intensifying. Vigorous updrafts created favorable dynamic conditions for latent heat release during the rapid intensification (RI) stage, whereas downdrafts dominated the stagnation stage, markedly suppressing latent heat release processes.

Kinetic energy transfer within Typhoon Hato is closely coupled to its intensity evolution and exhibits distinct radial heterogeneity. During the RI stage, a forward (positive) energy cascade from the symmetric basic-state vortex to asymmetric disturbances dominates in the lower troposphere (~1.5 km), while an inverse (negative) energy cascade from asymmetric scales back to the symmetric vortex prevails in the middle troposphere (~5 km). At the peak of RI, a complete energy transfer chain forms in the upper troposphere (~8 km), with energy sequentially conveyed from the symmetric vortex to outer asymmetric small-scale disturbances and then to spiral rainbands. In the lower stratosphere (~13 km), continuous conversion of asymmetric-scale available potential energy to kinetic energy serves as the dominant process.

The direction of kinetic energy cascade reverses periodically with fluctuations in typhoon intensity, especially at intensity extrema. Forward cascades dominate the RI stage, while inverse cascades prevail during the re-intensification stage. Compared with the inner core region, kinetic energy transfer in the outer region is relatively simpler, characterized by the predominance of asymmetric-scale kinetic energy.This study elucidates the coupled mechanism between abrupt typhoon intensity changes and multi-scale kinetic energy redistribution, in which nonlinear energy transfer at the 8-km level plays a critical regulatory role in overall energy repartitioning.

The Heat redistribution induced by hydrometeor phase transitions and the drag effect associated with hydrometeor sedimentation enhance multi-scale energy transport within the typhoon at the same time, thereby modulating tropical cyclone intensity. These results may provide a sound theoretical basis for understanding the physical mechanism by which hydrometeor phase changes in spiral rainbands trigger asymmetric disturbances and further regulate the overall intensity of tropical cyclones.