Dynamics and predictability of secondary eyewall formation in sheared tropical cyclones

This study examines the predictability and dynamics of tropical cyclone (TC) secondary eyewall formation (SEF) and intensity changes under moderate environmental vertical wind shear through a series of cloud-resolving ensemble simulations. It is found that intensity changes of sheared TCs can be extremely sensitive to small, unobservable, random initial condition errors due to random and chaotic moist convection during all formation, rapid intensification and SEF stages. Moreover, the same level of intensity divergence can also be found between experiments with exactly the same initial conditions under the same environmental but are performed on different computer clusters, which is reminiscent of the “butterfly effect” that led to the development of the chaos theory half a century ago. Through composite analysis from 5 ensembles with similar SEF, sensitivity experiments with and without ice microphysics, and diagnostics with a nonlinear boundary layer (BL) model, we identify several key factors in the SEF process: (1) expansion of outer wind fields and changing inertial stability through peripheral convection outside of the primary eyewall, (2) downward building and axis-symmetrization of the primary (outer) rainband due to stratiform processes and enhanced static stability, and (3) moat formation facilitated by the ice microphysical processes between the primary and secondary eyewalls. Diagnosis with the nonlinear BL model suggests that the SEF can be mostly described by an Ekman-like nonlinear force balance among the pressure gradient force, the centrifugal and Corilois forces, and the turbulence friction in the hurricane BL.