Development and Evolution of Convective Bursts in WRF Simulations of TCs Dean (2007) and Bill (2009)

Understanding and predicting the inner-core structure and intensity of tropical cyclones (TCs) remains one of the biggest challenges in tropical meteorology. This study addresses this challenge by investigating the formation, structure, and intensity changes resulting from localized strong updrafts in TCs known as convective bursts (CBs). This study analyzes the evolution of CBs in high-resolution simulations of two hurricanes (Dean 2007 and Bill 2009) using the Weather Research and Forecasting (WRF) model. The simulations are able to capture the observed track and peak intensity of the TCs. With Dean, there is a slight lag between the simulated intensification and actual intensification, and the extreme rate of RI is not perfectly captured. However, the cycle of intensification, weakening, and re-intensification observed in both TCs is captured in the simulations, due to a combination of internal dynamics and the surrounding environmental conditions.

CBs are identified based on the 99th percentile of eyewall vertical velocity (over the layer from z = 6-12 km) in each simulation (8.4 m/s for Dean, 5.4 m/s for Bill). The highest density of CBs is found in the downshear-left quadrant, consistent with prior studies. The structure of the CBs is analyzed by comparing r-z composites of azimuths with CBs and azimuths without CBs. The CB composites show stronger radial inflow in the lowest 0-2 km, and stronger radial outflow from the eye to the eyewall in the 2-4 km layer. The CB composites also have stronger low-level vorticity than the non-CBs, potentially due to eyewall mesovortices. The analysis of individual CBs also confirms the importance of the eye-eyewall exchange in CB development, potentially by providing buoyancy, as parcel trajectories show that many parcels are flung outward from the eye and rapidly ascend in the CBs, with as much as 500 J/kg of CAPE along the parcel path. In addition, the location of radial convergence seems to play a key role in governing the radial location of CBs. Inner-core CBs seem to be associated with local convergence maxima in the eyewall, while CBs outside the radius of maximum winds (RMW) are associated with convergence maxima due to bands and/or secondary eyewalls.

Analysis of intensity change in the simulations shows that there are more inner-core CBs during times when the TCs are intensifying, while weakening/steady times appear to be associated with more CBs outside the inner radius of maximum wind (RMW), consistent with observational studies and theoretical work. However, times when the TC has already been intensifying and continues to do so have more CBs than times when the TC has been weakening but then intensifies. This suggest that CB development may not always be predictive, but rather may sometimes occur as a result of ongoing intensification. On the other hand, rapid intensification (RI) in the simulations is found to be associated with an even higher density of CBs inside the RMW than slower intensification. Lag correlations between CBs and intensity are calculated to investigate the time of the intensity response to CB development. These calculations reveal a broad peak in correlation, with the CBs tending to lead pressure falls by 0-3 hours. These results confirm the notion that convective heating inside the RMW is favorable for intensification, and suggests that the development of extreme updrafts does indeed tend to be associated with intensification, sometimes in a predictive sense, and other times in ongoing intensification.