A numerical study of the genesis of concentric eyewalls in hurricanes

We use two axisymmetric numerical models to explore the dynamics of concentric eyewalls in hurricanes. The first is a simple two‐layer model using balanced dynamics and parametrized convection, while the second is a cloud‐resolving non‐hydrostatic model. In the case of the balanced model, infinitesimal disturbances amplify into secondary eyewalls provided the lower troposphere is sufficiently moist; otherwise, finite‐amplitude perturbations are necessary to initiate amplifying structures. But experiments with the full‐physics model show that finite‐amplitude disturbances are always necessary to initiate secondary eyewalls, regardless of the initial humidity of the lower troposphere. Experiments with both models, in which the surface wind is held constant in the surface flux formulations, fail to develop secondary eyewalls, demonstrating that in these models the disturbances, once initiated, grow through the wind‐induced surface heat exchange (WISHE) mechanism. Based on this work, we hypothesize that real secondary eyewalls result from a finite‐amplitude WISHE instability, triggered by external forcing, such as interaction of the tropical cyclone with baroclinic eddies, topography, or local perturbations in sea surface temperature.