Observed Downdrafts and Ventilation During the Rapid Intensity Changes of Hurricane Delta (2020)

This study examines downdrafts and downdraft ventilation, and their effects on Hurricane Delta (2020). Delta experienced rapid intensification (RI) before abruptly weakening before making landfall. Two sampling periods by reconnaissance aircraft provided observations during RI and the subsequent rapid weakening, giving an opportunity to study the downdraft evolution and compare these two periods. Across both sampling periods, the vertical wind shear increased, causing the vortex to tilt and exposing Delta's inner core to the surrounding environment. The increased shear provided a pathway for downdraft ventilation, the transport of low-equivalent potential temperature $\left({\theta }_e\right)$ air aloft into the boundary layer via downdrafts. Aircraft Doppler radar measured intense, deep downdrafts toward the end of the RI period; however, minimal downdraft ventilation was observed. After peak intensity, during the weakening period, downdraft ventilation was more prevalent with moderate to intense downdrafts and low-${\theta }_e$ air concentrated on the left-of-tilt side of the storm. The observations show that as Delta weakened, the inner core became diluted with low-${\theta }_e$ air from downdraft ventilation and the convection was limited to the downtilt and left-of-tilt regions of the storm. These findings are consistent with previous idealized and real-case modeling studies that have shown downdraft ventilation weakens tropical cyclones and degrades their structure. Additionally, the location of the downdraft ventilation observations relative to the vortex tilt and shear directions agree with previous modeling studies. More broadly, observations of the downdraft structure and diagnostics of downdraft ventilation may provide cues of subsequent intensity change in sheared storms.