The term “double eyewall structure” is sometimes mentioned in the Tropical Cyclone Reports of the Hong Kong Observatory. What exactly is a double eyewall structure?

To understand what a double eyewall structure is, one must first understand the basic structure of a mature tropical cyclone. A mature tropical cyclone can be divided into three regions, namely the eye, the eyewall, and the spiral rainbands^[1]. According to some research^[2][3], tropical cyclones of typhoon intensity^[4] or above may undergo an eyewall replacement cycle^[5][6]. During this process, a double eyewall structure is developed, that is, a new eyewall develops outside the original eyewall. Subsequently, the inner eyewall gradually dissipates, and the outer eyewall takes its place. The resultant eye becomes larger than before, and the wind speed near the centre of the tropical cyclone may temporarily decrease. Then, the new eyewall contracts inwards. If the eyewall replacement process comes to a successful completion, the structure of the tropical cyclone will become compact again, and its intensity may recover or even increase (Figure 1).

The double eyewall structure of a tropical cyclone can be analysed through remote sensing observations such as satellite and radar imagery^[7]. Taking Super Typhoon Ragasa^[8], which necessitated the issuance of the Hurricane Signal No. 10 by the Observatory in September 2025, as an example, Ragasa experienced its first eyewall replacement cycle before and during its passage over the Luzon Strait (Table 1). Infra-red satellite images showed that Ragasa developed a compact eye over the seas east of Luzon, and then the eye expanded as it moved across the Luzon Strait. Microwave satellite images clearly showed that Ragasa developed a double eyewall structure during the eyewall replacement process.

After completing the first eyewall replacement, Ragasa entered the South China Sea and immediately began its second eyewall replacement process. Hong Kong's weather radar^[11][12] showed that as Ragasa approached the coast of Guangdong, both its inner and outer eyewalls contracted slightly. While the wind speed in the inner eyewall gradually weakened, the wind speed in the outer eyewall remained high (Figure 2). Therefore, when Ragasa skirted past about 120 km south of Hong Kong on the morning of 24 September, the hurricane-force winds in its outer eyewall persistently battered the southern part of Hong Kong^[8]. Finally, upon its landfall near Yangjiang of Guangdong on the afternoon of 24 September, Ragasa had not yet completed the second eyewall replacement process.

The example of Ragasa demonstrates that remote sensing tools such as satellites and radars play a crucial role in monitoring changes in the wind structure of tropical cyclones, helping to assess their impact on coastal areas. The Observatory will continue to strengthen meteorological observations, and work closely with meteorological agencies in neighbouring regions to promote regional meteorological information sharing.