Convection-permitting Ensemble Forecasts of the Rapid Intensification of Hurricane Edouard (2014): Predictability, Dynamics and Thermodynamic Structure

The dynamics and predictability of the rapid intensification (RI) of Hurricane Edouard (2014) are explored through a 60-member convection-permitting ensemble generated by the real-time Pennsylvania State University (PSU) WRF-based ensemble Kalman filter (WRF-EnKF) hurricane analysis and forecast system. Dropsonde data collected during the 2014 phase of NASA's Hurricane and Severe Storm Sentinel (HS3) are assimilated, in addition to other non-radiance observations. The 126-h forecasts are initialized at 1200 UTC 11 September 2014, which is the time at which Edouard was designated as a tropical depression, and includes the storm's RI from a tropical storm to a strong Category-2 hurricane. Although the deterministic forecast was very successful and many ensemble members correctly forecasted Edouard's RI, there was significant spread in the timing of RI amongst the members.

Utilizing composite groups created according to the RI onset times of the members, it is shown that for increasing magnitudes of deep-layer vertical wind shear, RI onset is increasingly delayed. In addition, a critical shear threshold appears to exist in which the TC will not intensify once it is exceeded. Although the timing of RI varies by as much as 48-h, a decrease in wind shear is observed across the intensifying composite groups approximately 12–24 h prior to RI. This decrease in wind shear is accompanied by a reduction in the magnitude of the tilt of the vortex, as the precession and subsequent alignment process begins approximately 24–48 h prior to RI. Sensitivity experiments reveal that some of the variation in RI time can be attributed to the initial intensity of the vortex, as the earliest developers intensify regardless of their environment. In addition, the non-developing members fail to undergo RI because of a less conducive environment, although significant sensitivity exists in which very small differences in dynamic and thermodynamic fields produce divergent forecasts. Using HS3 dropsondes, as well as the PSU WRF-EnKF ensemble simulations, the thermodynamic structure of Edouard is also being examined. Temperature profiles are calculated from both environmental and inner-core observations, with particular attention paid to the vertical structure and time evolution of the warm core leading up to RI onset, throughout RI and once peak intensity has been reached.