Evolution of the Double Warm-Core Structure in the Eyewall Replacement Cycle of Typhoon Trami (2018)

The evolution of double warm-cores (DWC) during the secondary eyewall formation (SEF) and the subsequent eyewall replacement cycle (ERC) in Typhoon Trami (2018) was investigated in this study. Using a coupled atmosphere-ocean model, the impacts of the typhoon-induced sea surface cooling (SSC) on the DWC structure were also examined. In both the coupled and uncoupled simulations, the middle-level warm-core (MWC) decayed as the tangential wind expanded outward during the SEF, even in the absence of SSC. Under the thermal wind balance, the cooling of the MWC was an inherent result to ensure the dynamic consistency during the SEF. In contrast to the uncoupled simulation, the upper-level warm-core (UWC) decayed under the influence of SSC, significantly contributing to the rapid weakening of Trami. The negative effects of the SSC on the UWC were more detrimental to the intensity of Trami. Results from the potential temperature (PT) budget of improved accuracy indicated that the decrease in the warming due to the radial eddy advection was mainly responsible for the decay of the MWC. The flattened radial gradient of PT during the SEF accounted for the reduction in the dynamical eye warming associated with the asymmetric mixing. In addition to the convectively induced subsidence, the upper-level asymmetric structures could also be affected by the SSC. Consequently, the increase in the cooling due to the enhanced asymmetry collaborated with the decrease in the adiabatic warming, leading to the decay of the UWC in the coupled simulation.