Secondary Eyewall Formation in Idealized Axisymmetric Models

Abstract:

Eyewall replacement cycles (ERCs) are a major source of hurricane variability, but the specific physical processes that lead to their formation have not yet been confirmed. Even without being able to simulate spiral rainbands and other features commonly associated with secondary eyewall formation (SEF), axisymmetric models such as Cloud Model 1 (CM1) sometimes exhibit eyewall replacement cycle (ERC)-like behavior, in which a secondary wind maximum forms, intensifies, and replaces the primary eyewall over a period of 1-2 days. In the experiments presented in this study, these ERC-like features form relatively early in the simulations and show some qualitative similarities to ERCs in three-dimensional simulations, notably exhibiting an expansion of hurricane winds prior to SEF. The aim of this study is to examine the causes of these features and the scope of their applicability to understanding ERCs in real life.

The experiments in this study are run on a 6000-km axisymmetric domain with a fixed sea surface temperature and interactive radiation. We demonstrate that the ERC-like features are a means by which a hurricane’s wind field adjusts to a theoretical model for a hurricane’s outer wind field developed by Chavas et al. (2015, JAS), which solves for a radial distribution of angular momentum by assuming that radiative subsidence in the free troposphere is matched by Ekman suction in the boundary layer. Since these ERC-like features only form in experiments with interactive radiation, we also examine the roles of radiative cooling and feedbacks on their development.