On the Onset of Rapid Intensification of Typhoon Hato (2017) under Moderate-to-strong Vertical Wind Shear: Roles of Multi-timescale Interactions

Using observational and reanalysis datasets, this study explores the mechanisms by which the interactions among multi-timescale flows impacted the onset of rapid intensification (RI) of Typhoon Hato (2017). Hato formed within a northwest-southeast-oriented synoptic-scale (with periods < 10 days) wave train, concurring with a developing intraseasonal (10–90 days) oscillation and an elongated low-frequency (> 90 days) monsoon trough in the western North Pacific. Impacted by continuously-increasing vertical wind shear, Hato long maintained a highly asymmetric convective structure. Prior to RI onset, the synoptic-scale circulation and the inner-core asymmetric convection of Hato greatly strengthened, which are the key factors believed to trigger RI. A multi-timescale eddy kinetic energy budget indicates that the wind convergence associated with the intraseasonal circulation and monsoon trough led to barotropic energy conversion that largely enhanced the synoptic-scale cyclonic circulation. Besides, the pronounced increases in midlevel relative humidity (RH) and surface latent heat flux (LHF) were observed upshear before RI onset, which were primarily driven by the strong intraseasonal and synoptic-scale RH anomalies and the strengthened low-level wind speed, respectively. The increased LHF and midlevel RH, together with the enhanced downshear confluence between synoptic-scale and ISO/low-frequency winds, could have helped the intensification of asymmetric convection that supports RI onset. Overall, this study suggests that the interactions across multiple timescales may create favorable dynamic and thermodynamic conditions that promoted RI onset, offering new insights into RI processes for highly asymmetric tropical cyclones like Hato.