Barotropic Instability across the Moat and Inner Eyewall Decay of Tropical Cyclone
Student Seminar Series
Department of Atmospheric & Oceanic Sciences
presents
a talk by
Tsz Kin (Eric) Lai
PhD student
Barotropic Instability across the Moat and Inner Eyewall Decay of Tropical Cyclone
The eyewall of a tropical cyclone (TC) is the region having the most intense winds and rainfall within the TC. It is frequently observed that the strongest TCs develop a secondary eyewall outside the primary eyewall with an annular moat region separating the two eyewalls. Generally, a double-eyewall TC undergoes an eyewall replacement cycle such that the inner eyewall gradually dissipates while the outer eyewall strengthens and contracts.
Radar imagery of some double-eyewall TCs shows that the inner eyewalls become elliptical prior to their dissipation. According to previous 2D idealised studies, this feature indicates that a barotropic instability across the moat (a.k.a. type-2 barotropic instability) may play a role. To further investigate the mechanism for dissipation, a WRF simulation of Hurricane Wilma (2005) is performed. The analyses reveal the occurrence of a type-2 instability, which led to the elliptical elongation of the inner eyewall, and the associated wavenumber-2 radial flow. A time series analysis of the inner core circulation indicates the weakening of the inner eyewall, largely due to the wavenumber-2 radial flow pattern.
To further examine the physics of inner eyewall decay, idealised 3D numerical experiments are performed. In the moist full physics run, the simulated vortex reproduces the type-2 instability and the wavenumber-2 radial flow pattern as in the Wilma simulation. The evolution of the absolute angular momentum (AAM) demonstrates that the region of negative radial transport of AAM located at and near the inner eyewall starts to significantly enlarge after the ellipticity of the whole vortex becomes prominent. The AAM budget calculation after the excitation of the type-2 instability indicates a significant intensification in the outward eddy radial advection of AAM resulting in the total radial AAM advection becoming negative in the later stage of the instability. The budget calculation also shows that the negative total radial advection of AAM contributes the most to the inner eyewall decay except in the boundary layer where frictional effects dominate. Another dry no-physics idealised experiment is conducted and the result shows that the type-2 instability alone is able to weaken the inner eyewall with non-negligible effect. Taking together, these two idealised experiments suggest that the type-2 instability can accelerate the decay of the inner eyewall in concert with the cut-off effect of the boundary layer.