Exploring the Causes of Difference in Moat Width in Concentric Eyewalls—Ensemble Simulation of Typhoon Haiyan (2013)

Abstract Ensemble simulations are conducted in this study to examine the cause of the narrow-moat concentric eyewall formation in Typhoon Haiyan (2013). Forty ensemble members out of 150 members exhibit a complete eyewall replacement cycle (ERC). A positive correlation is found between the initial moat width and ERC duration. The top 25% of members with the widest initial moats are selected as the wide group (WG), and the bottom 25% of members with the narrowest moats are selected as the narrow group (NG) for a comparative analysis. Results show that there is no major difference in the mechanism of secondary eyewall formation (SEF) between the two groups. However, before the SEF, NG experiences weaker vertical wind shear, leading to more active outer rainband convection and stronger diabatic heating, enhancing the low-level inflow and allowing air parcels to penetrate deeper into the inner-core region of tropical cyclones (TCs) and to generate agradient force closer to the center, resulting in the formation of an outer eyewall nearer to the original eyewall, and thus a narrower initial moat. In contrast, WG experiences stronger vertical wind shear, which suppresses the outer rainband activity. The weaker diabatic heating leads to weaker low-level inflow, preventing air parcels from reaching the inner core and limiting the agradient force to regions farther from the center. As a result, the outer eyewall forms at a greater distance from the center, producing a wider moat. This study highlights that the differences in environmental wind shear can significantly impact the inner-core structure and subsequent evolution of TCs.