Rapid Intensification of Typhoon Goni (2015) After Eyewall Replacement Cycle

After the completion of an eyewall replacement cycle (ERC), a tropical cyclone (TC) often continues to intensify and in some cases rapid intensification (RI) occurs (Sitkowski et al. 2011). However, RI processes after an ERC has not yet been investigated. An operational ground-based Doppler radar observed the RI of Typhoon Goni (2015) for 24 h immediately after it completed an ERC. We examined Goni’s RI processes by using radar reflectivity and wind fields retrieved by the ground-based velocity track display (GBVTD) technique. The maximum wind at 2-km altitude increased by 30 m s^–1 during the first 6 h of RI, and it further increased by 20 m s^–1 during the subsequent 12 h. Around the onset of RI, relatively strong outflow (>2 m s^–1) was present both inside and outside the radius of maximum wind (RMW) above the boundary layer (BL), suggesting the existence of supergradient flow in and just above the BL. Despite this outflow, angular momentum increased inside the RMW. In contrast, a budget analysis of absolute angular momentum (AAM) showed that the outflow contributed to the decrease in AAM just outside the RMW below 5-km altitude. As a result, the low-level RMW contracted rapidly from 50 to 33 km, causing the RMW to slope greatly outward with height. The radial profile of tangential wind became more peaked with time. The wind profile is consistent with that of other intensifying TCs (Rogers et al. 2015; Martinez et al. 2017). During RI, the low-level outflow changed into inflow immediately outside the RMW. Thus, the secondary circulation became well established. Then the tangential wind field and high inertial stability region expanded radially outward, followed by the formation of an outer reflectivity maximum at twice the RMW. The contraction speed of the low-level RMW slowed down.