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[AAS04-01] 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
Keywords:tropical cyclone, eyewall replacement, concentric eyewalls, mesoscale descending inflow
Eyewall Replacement Cycle (ERC) is often seen in 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 not only scientifically but also socially. Several mechanisms of secondary eyewall formation have been proposed, including axisymmetrization of vorticity anomalies (Terwey and Montgomery 2008), frictional updraft due to local enhancement of vorticity gradient (Kepert 2013), and nonequilibrium dynamics in the boundary layer (Huang et al. 2012), but there is still no consensus. 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.
First, idealized numerical experiments were conducted using the nonhydrostatic 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. We conducted sensitivity experiments by increasing 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. The mechanism by which water vapor in the middle and upper troposphere prevents secondary eyewall formation is as follows. First, the increase in water vapor weakens the diabatic cooling due to evaporation and sublimation, weakening the downdraft. The weakening of the downdraft changes the pressure field, and the associated change in the pressure gradient causes the agradient wind component to weaken the inflow in both the middle and upper and lower troposphere. Angular momentum is less likely to be transported to the secondary eyewall formation region when the inflow is weakened. In this way, the mechanism of secondary eyewall formation by agradient force becomes less effective.
Next, a real numerical experiment was conducted using the nonhydrostatic model, asuca. The target case was Typhoon Haishen in 2020, confirmed by ground-based radar observations to have a secondary eyewall. Two experiments were conducted at different initial times. In the case of the later initial time, the secondary eyewall was formed. Dry air inflow was observed on the west side of the typhoon in the middle and upper troposphere. Comparing the radial wind and vapor fields in the middle and upper troposphere for the experiments started from different initial times, the experiment in which the secondary eyewall was clearer had stronger and drier inflow in the middle and upper troposphere. This dry inflow existed in the northwest of Amami and Okinawa at the initial time of the experiment, whose secondary eyewall was clearer. This area is downwind of the aerological observation station considering the wind direction around the typhoon. This result suggests that more detailed observation of the water vapor field in the middle and upper troposphere by observations may improve the prediction of the secondary eyewall of a TC.
First, idealized numerical experiments were conducted using the nonhydrostatic 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. We conducted sensitivity experiments by increasing 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. The mechanism by which water vapor in the middle and upper troposphere prevents secondary eyewall formation is as follows. First, the increase in water vapor weakens the diabatic cooling due to evaporation and sublimation, weakening the downdraft. The weakening of the downdraft changes the pressure field, and the associated change in the pressure gradient causes the agradient wind component to weaken the inflow in both the middle and upper and lower troposphere. Angular momentum is less likely to be transported to the secondary eyewall formation region when the inflow is weakened. In this way, the mechanism of secondary eyewall formation by agradient force becomes less effective.
Next, a real numerical experiment was conducted using the nonhydrostatic model, asuca. The target case was Typhoon Haishen in 2020, confirmed by ground-based radar observations to have a secondary eyewall. Two experiments were conducted at different initial times. In the case of the later initial time, the secondary eyewall was formed. Dry air inflow was observed on the west side of the typhoon in the middle and upper troposphere. Comparing the radial wind and vapor fields in the middle and upper troposphere for the experiments started from different initial times, the experiment in which the secondary eyewall was clearer had stronger and drier inflow in the middle and upper troposphere. This dry inflow existed in the northwest of Amami and Okinawa at the initial time of the experiment, whose secondary eyewall was clearer. This area is downwind of the aerological observation station considering the wind direction around the typhoon. This result suggests that more detailed observation of the water vapor field in the middle and upper troposphere by observations may improve the prediction of the secondary eyewall of a TC.