A Numerical Study on Hurricane Patricia (2015). Part II: The Topographic Effect and the Secondary Eyewall Formation

Abstract Hurricane Patricia (2015), the most powerful tropical cyclone (TC) on record, formed its secondary eyewall when its center was about 113 km offshore before its landfall at the southwestern coast of Mexico at around 2300 UTC 23 October. The ARW-WRF Model reproduced well the main features, allowing for a detailed investigation of the secondary eyewall formation (SEF). Our results show that the secondary eyewall developed from a stationary banding complex (SBC), originating from the intersection of two outer rainbands (OR1 and OR2) on the western side of the TC. This process was largely regulated and enhanced by the coastal terrain through the orographic channel effect. The results from sensitivity experiments show that increasing terrain height amplified the channel effect, accelerating airflow between the TC vortex and the terrain, strengthening convergence into OR1, and promoting midlevel descending inflow conducive to convective enhancement downstream in the SBC. While the terrain weakened low-level moisture transport, it also positioned OR2 closer to OR1, facilitating the formation of the SBC and accelerating the moat development. Backward trajectory analysis revealed that the inflows below the upper-level outflow layers of both the primary and secondary eyewalls contributed to moat development. With increasing terrain height, dry air transported into the moat region by the upper-level inflows from the secondary eyewall significantly increased, further suppressing convection in the moat. These findings offer novel insights into the understanding of SEF processes and underscore the importance of the topographic effects in shaping outer rainband organization, contributing to the moat and SEF.