Maintenance Mechanisms of the Concentric Eyewall Structure of Typhoon Lekima (2019): Axisymmetric Perspective

This study examines the concentric eyewall structure of Typhoon Lekima (2019) from an axisymmetric perspective. Possible maintenance mechanisms for the concentric eyewalls are investigated using a high-resolution WRF simulation (nested down to 1-km horizontal grid size). The secondary-circulation responses to the latent heating in the inner eyewall, moat and outer eyewall are diagnosed by solving the Sawyer-Eliassen (SE) equation individually to examine the corresponding contribution to the moat downdraft. By calculating the dynamic efficiency factor (DEF), the conversion of latent heating to kinetic energy is evaluated in the moisture-restricted inner eyewall. The diagnoses from the SE equation show that the moat downdraft was contributed mainly by latent heating in the inner and outer eyewall, with a secondary contribution from latent cooling in the moat after concentric eyewall formation. DEF diagnoses show that the conversion of latent heating to kinetic energy in the inner eyewall was more efficient than that in the outer eyewall. Although tangential wind within the boundary layer was weakened by friction, the compensative tangential wind in the inner eyewall was larger than that in the outer eyewall. The compensative tangential wind indirectly accumulated moisture from the sea surface in the moat region, aiding the moisture supply to the inner eyewall and enhancing the amount of kinetic energy converted from latent heating. Although the inner eyewall of Typhoon Lekima eventually weakened due to the moisture cut off from the outer eyewall, the inner eyewall could be maintained for tens of hours by the high DEF from latent heating.