On the Distinct Nature of Tropical Cyclone Convergence Zones That Induce Core Reformation

Abstract Relatively weak tropical cyclones exposed to appreciable environmental vertical wind shear commonly possess localized off-center regions of prominent convective activity coinciding with enhanced low-level convergence. Under special circumstances, a subvortex can rapidly strengthen within this convergence zone (CZ) and become the kernel of a new inner core of the tropical cyclone. A previously proposed condition for such core reformation is revised to be less restrictive. With minimal ancillary assumptions, the revised condition is simply that τ e should exceed τ cr . Here, τ cr is the time scale of core reformation neglecting the disruptive impact of vorticity expulsion from the CZ by the nondivergent background flow, and τ e is the time scale for fluid parcels to horizontally escape the CZ. Semianalytical and analytical formulas are provided for estimating τ cr and τ e , respectively. The revised condition is tested against several cloud-resolving simulations of strongly misaligned tropical cyclones. Essentially consistent with theoretical expectations, downtilt CZs are found to induce core reformation only after maintaining τ e near or well above τ cr for a time scale comparable to the latter (hours). In a detailed illustration of the dynamics associated with the opposite condition of τ e falling well below τ cr , locally amplified vorticity anomalies readily leave the CZ, and the negative eddy flux of vorticity is large enough to prevent major CZ spinup.