Eyewall Replacement Cycles in Hurricane Edouard (2014): Dynamics and Ensemble Variability

The dynamics, predictability, and variability of an eyewall replacement cycle (ERC) event that occurred in Hurricane Edouard (2014) is explored through a 60-member convection-permitting ensemble generated by the real-time Pennsylvania State University (PSU) WRF-based ensemble Kalman filter (WRF-EnKF) Atlantic hurricane forecast and analysis system. This simulation benefits from the assimilation of both dropsonde data collected during the 2014 campaign of NASA’s Hurricane and Severe Storm Sentinel (HS3), as well as other non-radiance observations. The 7-day forecasts are initialized at 1200 UTC 11 September 2014, coincident with Edouard’s designation as a tropical depression, and encompass both Edouard’s near-rapid intensification (RI) to a major hurricane, as well as an ERC event that occurs just after peak intensity (1200 UTC 18 September). Despite significant spread in RI onset timing in the ensemble, both the deterministic forecast and the majority of the ensemble members accurately capture Edouard’s intensification in terms of point metrics of intensity (minimum sea level pressure and maximum 10-m wind speeds). However, since similarities in these intensity metrics do not necessarily imply that the structural evolutions are comparable, a closer examination of TC structure is performed to assess the capability of the ensemble to accurately simulate Edouard’s ERC event.

Utilizing composite groups created based on RI onset timing that were employed in both Munsell et al. (2017a and b), approximately 75% of the developing members (23 out of 30) simulate at least a partial ERC event just after Edouard’s peak intensity. These structural changes are clearly illustrated in evolutions of the low-level tangential winds, vertical velocity, and the radius of maximum winds (RMW) despite a resolution of 3-km in the innermost domain. An investigation into the mechanisms that lead to the ERCs in the simulation is currently being conducted, with particular emphasis placed on the importance of boundary layer dynamics and interactions between the eyewall and the primary rainband evolution. In addition, there is some variability in the ensemble of the exact timing of the ERC, with a smaller percentage of members (7 out of 30) failing to produce any significant secondary eyewall (SEW) formation. The factors contributing to these structural changes, or lack thereof, are currently being explored, although the moderate vertical wind shear that the TCs are embedded in throughout Edouard’s intensification period may ultimately limit the predictability of the ERCs in this simulation.