Concentric Eyewall Formation in Typhoon Sinlaku (2008) – Part I: Axisymmetric Processes

Based on a dataset assimilated with special flight-observation data from T-PARC (THORPEX – Pacific Asian Regional Campaign), this study investigates the evolution of the concentric eyewall structure in Typhoon Sinlaku (2008). The dataset is particularly helpful to identify key dynamical processes in secondary eyewall formation (SEF). The identified precursors to SEF suggest a possible application of a newly proposed spin-up paradigm to the SEF problem. Based on our diagnoses, we present a new model for SEF from an axisymmetric view of the problem. The model contains precursor characteristics, the associated evolution of the boundary layer flow and a dynamical interpretation.

The findings point to a sequence of vortex structure changes that occur in the outer-core region of a mature tropical cyclone and which culminate in SEF provided that the environmental conditions remain favorable. This diagnosis features an unbalanced response in the boundary layer to radially expanding swirling flow, suggesting that the combined growth in storm size and boundary layer response serves as an important controlling mechanism for initiating and sustaining an approximate ring of deep convection in a narrow supergradient-wind zone in the vortex's outer-core region. The results also imply that the boundary layer scheme and its coupling to the interior flow need to be adequately represented in numerical models to improve the understanding of SEF, as well as the accuracy of SEF forecasts, including the timing and preferred radial intervals.

Further investigation of the new paradigm for SEF is presented in a companion paper (part II).