Analyzing Simulated Convective Bursts in Two Atlantic Hurricanes, Part I: Burst Formation and Development

Abstract

nderstanding the structure and evolution of the tropical cyclone (TC) inner core remains an elusive challenge in tropical meteorology, especially the role of transient asymmetric features such as localized strong updrafts known as convective bursts (CBs). This study investigates the formation of CBs and their role in TC structure and evolution using high-resolution simulations of two Atlantic hurricanes (Dean 2007 and Bill 2009) with the Weather Research and Forecasting (WRF) model.Several different aspects of the dynamics and thermodynamics of the TC inner-core region are investigated with respect to their influence on TC convective burst development. Composites with CBs show stronger radial inflow in the lowest 2 km, and stronger radial outflow from the eye to the eyewall around z=2-4 km, than composites without CBs. Asymmetric vorticity associated with eyewall mesovortices appears to be a major factor in leading to some of the radial flow anomalies that lead to CB development. The anomalous outflow from these mesovortices, along with outflow from supergradient parcels above the boundary layer, favors low-level convergence and also appears to mix high-?e air from the eye into the eyewall. Analysis of individual CBs and parcel trajectories show that parcels are pulled into the eye, and briefly mix with the eye air. The parcels then rapidly move outward into the eyewall, and quickly ascend in CBs, in some cases with vertical velocity over 20 ms-1. These results support the importance of horizontal asymmetries in forcing extreme asymmetric vertical velocity in tropical cyclones.