Essential Dynamics of Secondary Eyewall Formation

The authors conduct an analysis of the dynamics of secondary eyewall formation in two modeling frameworks to obtain a more complete understanding of the phenomenon. The first is a full-physics, three-dimensional mesoscale model in which the authors examine an idealized hurricane simulation that undergoes a canonical eyewall replacement cycle. Analysis of the mesoscale simulation shows that secondary eyewall formation occurs in a conditionally unstable environment, questioning the applicability of moist-neutral viewpoints and related mathematical formulations thereto for studying this process of tropical cyclone intensity change. The analysis offers also new evidence in support of a recent hypothesis that secondary eyewalls form via a progressive boundary layer control of the vortex dynamics in response to a radial broadening of the tangential wind field.The second analysis framework is an axisymmetric, nonlinear, time-dependent, slab boundary layer model with radial diffusion. When this boundary layer model is forced with the aforementioned mesoscale model's radial profile of pressure at the top of the boundary layer, it generates a secondary tangential wind maximum consistent with that from the full-physics, mesoscale simulation. These findings demonstrate that the boundary layer dynamics alone are capable of developing secondary wind maxima without prescribed secondary heat sources and/or invocation of special inertial stability properties of the swirling flow either within or above the boundary layer. Finally, the time-dependent slab model reveals that the simulated secondary wind maximum contracts inward, as secondary eyewalls do in mesoscale models and in nature, pointing to a hitherto unrecognized role of unbalanced dynamics in the eyewall replacement cycle.