Development and Evolution of Convective Bursts in WRF Simulations of 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 known as convective bursts (CBs) in the inner-core of TCs. Building off of previous observational studies, this study analyzes the evolution of CBs in numerical simulations of two hurricanes (Dean 2007 and Bill 2009) using the Weather Research and Forecasting (WRF) model.

The CBs are identified based on the 99th percentile of eyewall vertical velocity (calculated over the layer from z = 6-12 km) in each simulation (8.4 m/s for the Dean simulation and 5.4 m/s for the Bill simulation). The highest density of CBs is found in the downshear-left quadrant of the simulated TCs, consistent with prior studies of TC updrafts. 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-CB composites. The analysis of individual CBs also confirms the importance of the eye-eyewall exchange in the development of CBs, potentially by providing buoyancy, as parcel trajectories show that many parcels are flung outward from the eye and rapidly ascend in the CB cores, with as much as 500 J/kg of CAPE along the parcel path.

Initial analysis of intensity change in the simulations shows that there tend to be more inner-core CBs during times when the TCs are intensifying, while weakening/steady times appear to be associated with a higher density of CBs outside the radius of maximum winds (RMW). In addition, rapid intensification (RI) seems to be associated with an even higher density of CBs inside the RMW. Further work will make use of the high temporal resolution of the simulations to study the timescale of the intensity response relative to CB development, in order to determine whether CBs are a precursor of RI or a result of ongoing intensification.