No abstract

Typhoons with long-lived concentric eyewalls (CEs) are more intense than those with short-lived CEs. It is important for more accurate prediction of typhoon intensity to understand the maintenance mechanism of the long-lived CEs. To study the mechanism of the long-term maintenance of CEs, a numerical experiment of Typhoon Bolaven (2012) is performed using a nonhydrostatic model with full physics. Two aspects of the maintenance of simulated CEs are investigated: the maintenance of the inner eyewall and the contraction of the outer eyewall. To examine the maintenance of the inner eyewall, the equivalent potential temperature budget and air parcel trajectories of the simulated inner eyewall are calculated. The results show that the entropy supply to the inner eyewall is sufficient to maintain the inner eyewall after secondary eyewall formation (SEF). During the early period after SEF, entropy is supplied by an axisymmetric inflow, and later it is supplied by nonaxisymmetric flows of the outer eyewall. To examine the contraction of the outer eyewall, the potential vorticity (PV) budget of the outer eyewall is diagnosed. The result reveals that the negative contribution to the contraction of the outer PV peak (i.e., the outer eyewall) in the early period is the negative PV generation due to axisymmetric advection and diabatic heating just inside of the outer PV peak. In the later period, the negative PV generation due to nonaxisymmetric structure is important for the prevention of contraction. The present study reveals that the structure of the outer eyewall plays important roles in the maintenance of long-lived CEs.

Typhoon Megi (2010), a very intense tropical cyclone with a minimum central pressure of 885 hPa, was characterized by especially rapid intensification. We investigated this intensification process by a simulation experiment using a high‐resolution (0.02° × 0.02°) three‐dimensional atmosphere‐ocean coupled regional model. We also performed a sensitivity experiment with a time‐fixed sea surface temperature (SST). The coupled model successfully simulated the minimum central pressure of Typhoon Megi, whereas the fixed SST experiment simulated an excessively low minimum central pressure of 839 hPa. The simulation results also showed a close relationship between the radial SST profiles and the rapid intensification process. Because the warm sea increased near‐surface water vapor and hence the convective available potential energy, the high SST in the eye region facilitated tall and intense updrafts inside the radius of maximum wind speed and led to the start of rapid intensification. In contrast, high SST outside this radius induced local secondary updrafts that inhibited rapid intensification even if the mean SST in the core region exceeded 29.0°C. These secondary updrafts moved inward and eventually merged with the primary eyewall updrafts. Then the storm intensified rapidly when the high SST appeared in the eye region. Thus, the changes in the local SST pattern around the storm center strongly affected the rapid intensification process by modulating the radial structure of core convection. Our results also show that the use of a high‐resolution three‐dimensional atmosphere‐ocean coupled model offers promise for improving intensity forecasts of tropical cyclones.

Concentric eyewalls (CEs) in tropical cyclones (TCs) in different basins were identified based on satellite imagery during 1997–2014. Their duration and structural parameters, including inner eyewall size, moat width, and outer eyewall width, were calculated. Differences in these parameters can best be distinguished by short‐lived and long‐lived CEs (i.e., CEs with durations shorter or longer than 20 h). A long‐lived CE tends to have a larger size, mainly contributed by a larger moat and a larger outer eyewall width. The inner eyewall size shows no significant difference between short‐lived and long‐lived CEs in the western North Pacific (WNP) but increases slightly and steadily with increasing CE durations in the Atlantic (ATL). Furthermore, the WNP has far more CEs than in ATL and in the eastern Pacific (EPAC) for all duration categories. Long‐lived CEs cover about 20% of all CEs and are associated with higher sea surface temperature and weaker vertical wind shear. In the WNP, the TC tracks associated with long‐lived CEs were with the less northward motion component. Furthermore, seven TCs with triple eyewalls in the WNP are identified during 1997–2014, with only one case in the EPAC and no ATL case. It is found that five cases of these triple eyewalls TC in the WNP evolved to long‐lived CEs. The results suggest that the CE internal dynamical process, the TC track during the CE period, and the environmental conditions are all critical to the CE size and duration.