Dynamics of the wind field expansion with extratropically transitioning tropical cyclones

The extratropical transition (ET) process, that by which an initially warm-core tropical cyclone transitions into a cold-core extratropical cyclone, is examined for North Atlantic Tropical Cyclone (TC) Bonnie (1998) from an observational and modeling standpoint utilizing an angular momentum and potential vorticity (PV) framework in the storm's environment. Momentum, heat, and Eliassen-Palm (EP) fluxes are calculated within the storm vortex utilizing a storm-following, cylindrical, isentropic coordinate system to determine the physical processes impacting the alignment of the momentum surfaces. Nested runs of the nonhydrostatic Mesoscale Model version 5 (MM5) are performed through the ET lifecycle at high resolutions (2-36km) to detail the evolution of the storm's wind field through the transition process.

Over the mean storm vortex, as the system begins to undergo ET, an upper-tropospheric trough imparts a source of angular momentum at large (~1000km) radii near the 340K isentropic surface (e.g. below the outflow layer). High values of angular momentum are transported downward at these large radii, leading to an increase in angular momentum at lower levels throughout the outer core of the vortex. This is akin to the response of the TC's secondary circulation to a source of momentum at outer radii in the outflow layer of the storm. Contrasts between the ET process and that of midlatitude-forced rapid deepening of a tropical cyclone (e.g. Hurricane Elena of 1985; Molinari et al. 1995) are drawn in conjunction with the observed results.

Within the inner core of the storm, high resolution trajectories and flux budgets are computed to detail the processes at work to bring about a weakening of the tropical cyclone vortex. It is theorized that the observed expansion of the TC's wind field during ET may be a combination of the weakening of the inner core of the system as the wind-induced surface heat exchange development and maintenance mechanism is terminated (WISHE; Emanuel 1986) and an intensification of the outer core of the system in response to midlatitude forcing through the processes observed within the EP flux formulation. Implications toward this hypothesis are presented in conjunction with the observed results.