Secondary Eyewall Formation in an Idealized Axisymmetric Model

Hurricanes are among the costliest natural disasters, but they remain a challenge for forecasters, especially for predicting a hurricane’s intensity. One source of uncertainty in intensity prediction lies in the formation of secondary eyewalls, which are rings of enhanced wind and precipitation that encircle a hurricane’s primary eyewall. Secondary eyewall formation (SEF) occurs in most major hurricanes and is commonly a precursor to an eyewall replacement cycle (ERC), in which the primary eyewall dissipates while the secondary eyewall intensifies and becomes the new primary eyewall of the storm. ERCs are associated with an expansion of the hurricane’s winds and rainfall, as well as fluctuations in the hurricane’s intensity. Predicting their onset is an urgent problem for forecasters and motivates this study.

Although secondary eyewalls have formed in simulations of hurricanes using three-dimensional, full-physics models such as HWRF and HMON, they form less commonly in simulations using idealized models. We evaluate the hypothesis that this discrepancy arises because SEF is the response of a hurricane to an external stimulus. Such a stimulus would be represented in realistic models that include changing environmental conditions, but may not be included in idealized models. To test this hypothesis, we introduce perturbations to the surface physics of the axisymmetric Cloud Model 1 (CM1) and evaluate the role that the size of the hurricane and wind-surface flux feedbacks play in the incipient formation of a secondary eyewall.