Dynamics and predictability of the rapid intensification of Hurricane Edouard (2014) using convection-permitting ensemble forecasting

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.

Preliminary analysis shows 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. However, it is important to note that variations amongst additional environmental variables are also present, particularly in the members that do not develop. These storms track further west than the developing members into an environment of not only increased vertical wind shear, but also somewhat less favorable sea surface temperatures. However, the moisture content in the non-developing members is more conducive for development. Using both the WRF-EnKF ensemble simulations and the HS3 sounding observations, the pathways to intensification (or lack thereof) are currently being examined in greater detail to assess the interplay of the various environmental variables on the structural evolution of the TC vortex.