Analyzing tropical cyclone structures during secondary eyewall formation using aircraft in-situ observations

Open Access
- Author:
- Wunsch, Katharine Ed
- Graduate Program:
- Meteorology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 23, 2018
- Committee Members:
- Anthony Didlake, Thesis Advisor/Co-Advisor
Fuqing Zhang, Committee Member
George Young, Committee Member - Keywords:
- tropical cyclones
secondary eyewall formation
aircraft in-situ observations
eyewall replacement cycles - Abstract:
- In the evolution of mature tropical cyclones (TCs), intensity and structural changes can occur due to a process called an eyewall replacement cycle (ERC). Secondary eyewall formation (SEF) is the initial phase of an ERC, in which a ring of convection forms outside of the preexisting primary eyewall of the TC. The dynamical mechanisms for SEF remain unclear, but most hypotheses rely on the early presence of persistent and widespread rainband convection outside of the primary eyewall. The evolving rainband convection has both axisymmetric and asymmetric structures that play a role in SEF processes. This project uses aircraft reconnaissance observations from the FLIGHT+ dataset to examine the evolution of these structures. We create composites from this dataset which includes USAF C-130 and NOAA P-3 aircraft observations of Atlantic basin TCs from 1999-2015. The axisymmetric structures of TCs undergoing SEF are first compared to intensifying TCs that did not experience an ERC. Tangential wind and angular momentum profiles show a broadening of the outer wind field prior to SEF, while thermodynamic observations indicate features consistent with strengthening eyewall convection. Next, the ERC TCs are analyzed in quadrants relative to the deep-layer environmental wind shear to examine the evolution of asymmetric kinematic and thermodynamic structures. We utilize a new normalization technique based on the radii of both eyewalls to isolate structures that surround the secondary eyewall before and during SEF. We found that the kinematic structures of the developing secondary eyewall were most prominent in the storm half left of the wind shear vector. The thermodynamic structures of the secondary eyewall became more axisymmetric over time during SEF, but those of the primary eyewall became more asymmetric as it began to weaken prior to being fully replaced. Analyzing observations from Hurricane Earl as a case study illustrates variations in convective coverage that are captured in the composite study. Understanding the structures observed by aircraft reconnaissance and their relation to mechanisms that lead to SEF will improve our ability to predict the resultant changes in TC intensity and structure.