2021 Volume 99 Issue 6 Pages 1453-1482
In 2018, Typhoon Trami made landfall in Japan and maintained its intensity for a few days, then rapidly weakened after its recurvature. Subsequently, Typhoon Kong-Rey passed through the waters cooled by Trami while rapidly weakening. The region where both typhoons rapidly weakened is a region rich in oceanic mesoscale eddies overlying the Subtropical Countercurrent. To understand the role of a cold-core eddy, in the intensity change of these two typhoons, we examined the similarity and differences between the two typhoons, utilizing numerical simulations with a 2-km-mesh nonhydrostatic atmosphere model and an atmospheric-wave-ocean coupled model. Sensitivity experiments were performed by assuming a significant magnitude on the weakening of Trami during the mature phase; for example, we embedded an artificial cold-core eddy with a magnitude not based on in situ observations to gauge initial oceanic conditions. In contrast for Kong-Rey, nine ensemble simulations for initial atmospheric conditions were conducted instead of different-day initial oceanic conditions. The simulated rapid weakening of two typhoons was related to the low upper-ocean heat content caused by typhoon-induced sea surface cooling (SSC). Most simulations for Trami and Kong-Rey show a tendency of overdevelopment during the mature or weakening phase; the overdevelopment of Trami is caused by insufficiently simulated SSC and the embedded artificial cold eddy, which promoted the SSC; whereas, the overdevelopment of Kong-Rey is related to the failure of track simulation. A reasonable simulated track of Kong-Rey required greater time traveling over the Trami-induced SSC area to enhance weakening by reduction in inner-core moisture transport toward the center near the surface and in the inflow boundary layer on the upshear side. The reductions in downward motion in the center and the associated adiabatic heating were closely related to weakening in both typhoons.
