**Acknowledgements**

**5. Summary and conclusions**

186 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

basins, respectively.

for a long time.

and NRC cases.

tively.

An objective method is developed to categorize concentric eyewall structures in the western North Pacific (WNP) TCs using the NRL SSM/I and TMI satellite imagery database. For the WNP (ATL) basin, there are 91 (33) CE TCs and 113 (50) CE cases identified from 29,785 (19,001) satellite images between 1997 and 2014. Excluding the cases that are 200 km close to landfall and cases with temporal resolution higher than12 h, 83 (34) CE cases were studied for the

**1.** Three CE types are categorized: CE with an eyewall replacement cycle (ERC; 51 and 56% in the WNP and ATL), CE with no replacement cycle (NRC; 27 and 29% in the WNP and ATL), and CE that is maintained for an extended period of time (CEM; 23 and 15% in the WNP and ATL). The mean duration of CEM type is 32 and 27 h in the WNP and ATL

**2.** The CEM cases have relatively high intensity, large widths of both the moat and outer eyewall that last for a long duration time. The large widths of the moat and outer eyewall may reduce the barotropic instabilities of CE storms and thus maintain the CE structures

**3.** Most CEM cases in the WNP basin are located to the west of 140°E with the smallest northward translation speed of 2.2 m s−1. The average location of CEM cases in the ATL basin is farther west than that of ERC and NRC cases with the smallest northward translation speed of 1.4 m s−1. The NRC cases often have fast northward translation speeds and are located in higher latitudes of relative strong vertical shear and cold SST zones. The unfavorable environmental conditions thus act to weaken the convective activity for these cases. On the other hand, the CEM cases occur in favorable environment and tend to form in the warm episode of ENSO in the WNP basin. There are 12% (3%) of CE cases

**4.** The variabilities of intensity and structural changes in the WNP basin are larger than that in the ATL basin. For example, the moat size and outer eyewall width in the WNP (ATL) basin are approximately 20–50% (15–25%) larger in the CEM cases than that in the ERC

**5.** In the WNP basin, a very good relationship of 0.77 was found between the annual CE TC numbers and the NCEP Oceanic Niño Index (ONI). The probability of CE TCs formation is strongly influenced by the ENSO environmental factors. For example, there are 29 out of 69 (42%) TCs that possess CE structures in the warm episode. In contrast, there are only 16 out of 65 (25%) TCs that possess CE structures in the cold episode. The averaged CE formation frequencies are 0.8 and 0.3 per month in the warm and cold episodes, respec-

**6.** In the ATL basin, the correlation between ONI and CE formation frequency suggests that the ENSO may not influence the CE formation. It may be due to the average location of CE formation during warm episode farther south over the ATL in the relative warm SST

structural and intensity changes. The primary findings are as follows:

with more than 30 h long duration in the WNP (ATL) basin.

This work was supported by the Ministry of Science and Technology in Taiwan under grants MOST 104-2111-M-002-002-MY3 and MOST 104-2625-M-002-006, and the Naval Research Laboratory under grant N62909-15-1-2008.
