Intensity and Structural Changes During the Eyewall Replacement Cycle of Typhoon Khanun (2023) Observed by Five POES Microwave Imagers

This study investigates characteristic variations of the eye, eyewall, and moat structures during the eyewall replacement cycle (ERC) of Typhoon Khanun in 2023 based on brightness temperature observations from microwave imagers onboard five sun‐synchronous meteorological satellites. Khanun was initially positioned south of the subtropical high and moved northwestward. Two distinctive aspects of Typhoon Khanun's ERC are (a) its ERC occurred during a 2‐day period (∼0600 UTC 1 to ∼0600 UTC 3 August) when its motion slowed down being sandwiched between the midlatitude and low‐latitude subtropical highs and (b) after replacing the inner eyewall, the radius of the outer eyewall expands instead of shrinking and the typhoon's intensity quickly decreased. As revealed in microwave imager brightness temperature observations, the phases of wavenumber‐2 disturbances on the outer side of the inner eyewall and the inner side of the outer eyewall during the ERC had opposite signs. After the ERC was completed, the absolute angular momentum decreased at the outer eyewall, causing expansion of the outer eyewall and a decrease in typhoon intensity. There were also wavenumber‐1 perturbations near both the primary and secondary eyewalls, with the negative phase being located on the left side of the vertical wind shear during most of the ERC period.