Analyzing Asymmetric TC Structures during Secondary Eyewall Formation Using Flight-Level Observations

Most hypotheses for secondary eyewall formation (SEF) begin with axisymmetric processes that occur once a sufficient amount of rainband convection projects strongly onto the azimuthal mean. The asymmetric features that precede SEF initiation require further investigation in order to fully understand eyewall replacement cycles (ERCs). This project uses flight-level reconnaissance data from the FLIGHT+ dataset to examine the evolution of asymmetric thermodynamic and kinematic structures prior to and during SEF. This dataset includes observations from the USAF C-130 and NOAA P-3 aircraft from 1999-2016, during which a total of 32 ERCs in the Atlantic basin were identified. The axisymmetric tangential wind profiles are divided into groups that characterize ERC evolution. Then, the profiles are partitioned into quadrants relative to the deep-layer environmental wind shear. Axisymmetric and asymmetric characteristics of these storms undergoing ERCs are first compared to similarly intense TCs that did not undergo an ERC. Next, quadrant composites highlight the asymmetric evolution of tangential wind, temperature, equivalent potential temperature (θ_e), and vertical velocity during ERCs. Specifically, increases in temperature and θ_e are found in the DL quadrant prior to SEF, suggesting the presence of locally enhanced convection. Radar and satellite data supplement the analyses for individual case studies in ideal ERC events. Understanding the observed structures in flight-level data prior to SEF will improve our knowledge of ERCs and the resultant changes in TC intensity and structure.