Roles of Barotropic Instability across the Moat in Inner Eyewall Decay and Outer Eyewall Intensification: Three-Dimensional Numerical Experiments

Abstract

Radar imagery of some double-eyewall tropical cyclones shows that the inner eyewalls became elliptical prior to their dissipation during the eyewall replacement cycles, indicating that the barotropic instability (BI) across the moat (a.k.a. type-2 BI) may play a role. To further examine the physics of inner eyewall decay and outer eyewall intensification under the influence of the type-2 instability, three-dimensional numerical experiments are performed. In the moist full-physics run, the simulated vortex exhibits the type-2 instability and the associated azimuthal wavenumber-2 radial flow pattern. The absolute angular momentum (AAM) budget calculation indicates, after the excitation of the type-2 instability, a significant intensification in the negative radial advection of AAM at the inner eyewall. It is further shown that the changes in radial AAM advection largely result from the eddy processes associated with the type-2 instability, and contribute significantly to the inner eyewall decay. The budget calculation also suggests that the type-2 instability can accelerate the inner eyewall decay in concert with the boundary layer cut-off effect. 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 and also strengthen the outer eyewall with non-negligible effect.

Roles of Barotropic Instability across the Moat in Inner Eyewall Decay and Outer Eyewall Intensification: Essential Dynamics

“…In addition, radar imagery of some doubleeyewall TCs (McNoldy 2020) did show elliptical features (especially the inner eyewall) for a period of time prior to the inner eyewall dissipation [e.g., Super Typhoon Maria (2018) 1 ]. The observed elliptical features from the radar imagery may imply a possible role played by a type-2 BI in inner eyewall decay as demonstrated in Lai et al (2019). By performing a simulation study of Hurricane Wilma's (2005) first ERC, and using the vorticity profiles from Wilma, they showed the occurrence of type-2 BI (especially at the lower levels) by a 2D linear stability analysis and a series of 2D nondivergent barotropic vorticity experiments.…”

“…Since Kossin et al (2000) simplified the inner eyewall as a monopole, their results did not reveal the BI operating across the inner eyewall documented in Schubert et al (1999). By adopting a five-region framework (i.e., eye, inner eyewall, moat, outer eyewall, far field; Rozoff et al 2008;Lai et al 2019), the BI across the inner eyewall is recovered and is referred to as a type-3 BI. Yang et al (2013) speculated that BIs might play a role in determining the maintenance time of the multieyewall structures.…”

Inner‐Core Wind Field in a Concentric Eyewall Replacement of Typhoon Trami (2018): A Quantitative Analysis Based on the Himawari‐8 Satellite

“…As with Lai et al. (2021, 2019), in order to quantitatively assess this interpretation, we would need numerical simulations with three‐dimensional models with full‐physics with an adequate initialization to reproduce the actual TC structure because the barotropic model in Appendix does not have processes in surface friction and convection, unlike the observational results. This task is left for future works.…”

“…Lai et al. (2021) indicated a significant contribution of the asymmetric eddies to the inner‐eyewall decaying, based on an angular momentum budget with a full‐physics simulation.…”

“…The elliptic shape in the inner eyewall suggests the type-II instability across the moat in Kossin et al (2000). Lai et al (2021) indicated a significant contribution of the asymmetric eddies to the inner-eyewall decaying, based on an angular momentum budget with a full-physics simulation.…”