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

Abstract

Intense tropical cyclones (TCs) often experience secondary eyewall formations and the ensuing eyewall replacement cycles. Better understanding of the underlying dynamics is crucial to make improvements to the TC intensity and structure forecasting. Radar imagery of some double-eyewall TCs and a real-case simulation study indicated that the barotropic instability (BI) across the moat (a.k.a. type-2 BI) may play a role in inner eyewall decay. A three-dimensional numerical study accompanying this paper pointed out that type-2 BI is able to withdraw the inner eyewall absolute angular momentum (AAM) and increase the outer eyewall AAM through the eddy radial transport of eddy AAM. This paper explores the reason why the eddy radial transport of eddy AAM is intrinsically non-zero. Linear and nonlinear shallow water experiments are performed and they produce expected evolutions under type-2 BI. It will be shown that only nonlinear experiments have changes in AAM over the inner and outer eyewalls, and the changes solely originate from the eddy radial transport of eddy AAM. This result highlights the importance of nonlinearity of type-2 BI. Based on the distribution of vorticity perturbations and the balanced-waves arguments, it will be demonstrated that the non-zero eddy radial transport of eddy AAM is an essential outcome from the intrinsic interaction between the mutually growing vortex Rossby waves across the moat under type-2 BI. The analyses of the most unstable mode support the findings and will further attribute the inner eyewall decay and outer eyewall intensification to the divergence and convergence of the eddy angular momentum flux, respectively.

Barotropic Instability during Eyewall Replacement

“…The present work focuses on a barotropic interpretation of the Hurricane Maria observations, thereby extending the analyses of Kossin et al [53], Slocum et al [68], Taft et al [69], Lai et al [34,[65][66][67], and Rostami and Zeitlin [70], and further exploring possible instabilities that may occur after the formation of a concentric eyewall structure, if the moat between the two eyewalls is sufficiently narrow. In our analysis we shall make use of a five-region, linearized barotropic model and then perform non-linear numerical integrations that illustrate how instability across the moat could have led to the disintegration of the inner vorticity ring of Hurricane Maria.…”

“…Their results show the importance of wavenumber-two barotropic instability across the moat in diluting the vorticity of the inner eyewall of Hurricane Wilma. To further explain the dynamics of instability across the moat, Lai et al [65][66][67] conducted numerical experiments with a hierarchy of models, including the full-physics WRF model. Two simulations with the three-dimensional, non-hydrostatic, 1 km resolution, doubly nested, WRF model were made.…”