Structure and Maintenance of Concentric Eyewalls in Simulated Typhoon Bolaven (2012)

An intense TC occasionally has multiple eyewalls which are called as concentric eyewalls. Striking concentric eyewalls of hurricanes are studied by radar observations and numerical simulations. These previous studies indicated that eyewall replacement often occurs after concentric eyewalls are formed. In contrast to the replacement of eyewall, Typhoon Bolaven, which passed over the main Okinawa Island in 2012, had steady concentric eyewalls for very long time. It is the case that the eyewall replacement does not occur even if concentric eyewalls are formed. Structure of concentric eyewalls and process of eyewall replacement are not fully known.

In this study, we investigate the structure of the concentric eyewalls and mechanism of maintenance in Typhoon Bolaven, using the Cloud Resolving Storm Simulator (CReSS) which is a three dimensional, nonhydrostatic model. To simulate the concentric eyewalls of Bolaven, we conducted numerical experiments with horizontal resolution of about 1 km. Simulated multiple eyewalls were located within about 150 km radius from Bolaven's center and they were steady for over one day. The simulated eyewalls has the moat regions, which is very dry with descending motion. These results almost agree with Doppler radar observations by the Japan Meteorological Agency (JMA). According to some studies, an inner eyewall of TC gradually decays when, in planetary boundary layer (PBL), supply of vapor to the inner eyewall is decreased by an outer eyewall. To investigate tendency of the supply of water vapor and condensations to the inner eyewall, we analyzed water budget for the inner eyewall of simulated Bolaven. The result showed that the tendency of the supply was not changed, after the concentric eyewall formed clearly.

The simulated outer eyewall had a structure tilted outward, relative to the inner eyewall. It suggests that upward flow in the outer eyewall is weakened due to the tilted structure. Thus, we suppose that the supply of water vapor to the outer eyewall is very weak and the supply to the inner eyewall is not changed because intensity of upward flow in an eyewall is related to a supplied amount of water vapor to the eyewall in PBL.