Analyzing the Structural Variability of Eyewall Replacement Cycles in WRF Ensemble Simulations

Open Access
- Author:
- Stow, Justin
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 09, 2022
- Committee Members:
- David Stensrud, Program Head/Chair
Anthony Carl Didlake, Jr., Thesis Advisor/Co-Advisor
Steven J Greybush, Committee Member
Eugene Edmund Clothiaux, Committee Member - Keywords:
- tropical cyclone
secondary eyewall formation
ensemble modeling
numerical weather prediction
rainbands - Abstract:
- Secondary eyewall formation (SEF) and eyewall replacement cycles (ERCs) are commonly occurring events in tropical cyclones (TCs), but there remains a lack of complete understanding for many aspects of these phenomena. The structure and radial location of secondary eyewalls can vary widely, as well as the degree to which the inner eyewall weakens or decays. This study seeks to characterize this structural variability in a suite of idealized WRF ensemble simulations of TCs embedded in environments with deep-layer wind shear. Following the approach from Zhang et al. (2017), we develop quasi-objective methodologies applied to individual ensemble members to determine the timing of secondary eyewall formation (SEF) timing, to categorize the prevailing ERC type (FullERC vs. PartialERC), and to track the distance separating the two eyewalls during an ERC. To test the validity of this approach, kinematic and thermodynamic structures are investigated to compare the SEF structural behavior of the two ERC types. Whole-storm and shear-relative quadrant azimuthal average composites are calculated both temporally in two-hour segments relative to the time of SEF initiation and by ERC type. Our classification system identified a higher percentage of FullERC cases in the higher shear experiments while lower shear experiments contained no SEF/ERC events. The axisymmetric analysis revealed stronger secondary circulations as represented by deeper, stronger, and more upright updrafts and stronger radial outflow from the secondary eyewall. In the asymmetric analysis, the FullERC members better resolve the stationary band complex (SBC) than in the PartialERC members. More specifically, a more pronounced convective-to-stratiform precipitation transition is observed beginning in the downshear-right (DR) quadrant, rotating cyclonically downwind, and ending with a clear mesoscale descending inflow (MDI) jet and stratiform-induced cold pool in the upshear-left (UL) quadrant. The FullERC type contains stronger self-sustaining updrafts in the left-of-shear quadrants that help promote the axisymmetrization process of the developing secondary eyewall downwind. While the PartialERC case also exhibits an SBC, the eyewalls merge in the left-of-shear quadrants before SEF initiation and undergo a weaker ERC.