Formation Mechanism of TC Secondary Eyewall by Numerical Experiments: Role of Dry Air Inflow from the Middle and Upper Troposphere and Cooling by Evaporation and Sublimation

Eyewall Replacement Cycle (ERC) is often seen in tropical cyclones (TCs). ERC occurs when secondary eyewall forms outside the inner eyewall, and the inner eyewall disappears. ERC significantly impacts TC intensity, so revealing the mechanism is an important issue both scientifically and socially.

Several mechanisms of secondary eyewall formation have been proposed. According to Huang et al. (2012), tangential wind enhancement associated with lower-level inflow causes secondary eyewall formation. The relationship between the mesoscale descending inflow (MDI) formed by diabatic cooling of stratiform precipitation areas and secondary eyewall formation has also been pointed out (Didlake et al. 2018). However, the detailed processes by which dry air inflow and diabatic cooling affect secondary eyewall formation through MDI are not yet well understood. Therefore, in this study, the role of dry air inflow from the middle and upper troposphere and diabatic cooling in secondary eyewall formation is investigated using numerical experiments.

Idealized numerical experiments were conducted using the plane version of the nonhydrostatic icosahedral atmospheric model, NICAM. Control experiments confirmed the existence of dry air inflow in the middle and upper troposphere and the formation of downdrafts due to diabatic cooling. It was also confirmed that the mechanism of secondary eyewall formation by agradient force was working, as pointed out in previous studies. In a sensitivity experiment, we conducted an experiment to increase water vapor in the middle and upper troposphere outside the TCs. The results showed that the secondary eyewall formation was hindered and slowed down as the water vapor increased. We further conducted a realistic case study for Typhoon Haishen in 2020 and confirmed the above mechanism works for the ERC of Haishen.