Towards a better understanding of and ability to forecast the wind field expansion during the extratropical transition process

The extratropical transition (ET) process, that by which an initially warm-core tropical cyclone transitions into a cold-core extratropical cyclone, is examined for Hurricane Bonnie (1998) utilizing an angular momentum and potential vorticity (PV) framework in the storm's environment. Radial cross-sections of absolute angular momentum and PV fields are created and analyzed to gain insight into the structural evolution of the near-vertical momentum surfaces and vertical PV tower during the ET process. Momentum, heat, and Eliassen-Palm (EP) fluxes are calculated within the storm vortex utilizing a storm-following cylindrical coordinate system to determine the physical processes impacting the alignment of the momentum surfaces.

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 200hPa (e.g. in 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 a tropical cyclone in its 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) are drawn in conjunction with the observed results.

The inner-core evolution of the momentum field highlights the transition of the vortex from that characteristic of a tropical cyclone – with wind and momentum magnitudes decreasing with increasing height, the strongest winds found near the center, and an outward tilt of the eye wall with increasing height – to that characteristic of an extratropical cyclone – with wind and momentum magnitudes increasing with increasing height and a larger radius of maximum winds. PV analysis clearly shows a destruction of the inner core upper level vortex structure as the storm became extratropical, with a gradual weakening process continuing from the top-down in the days following transition.

EP and momentum fluxes are calculated to confirm the hypothesis that the observed evolution of the momentum surfaces in both the inner and outer cores of the storm is directly related to eddy fluxes of angular momentum imparted by the storm's environment. Similarly, it is theorized that the observed evolution of the momentum fields within a tropical cyclone undergoing ET may result in the expansion of the cyclone's wind field during ET in the opposite manner of the momentum fields driving a contraction of the radius of maximum winds with an intensifying tropical cyclone. Implications towards this hypothesis are presented, highlighting the forecasting and societal impacts of an improved prediction of the wind field expansion with an extratropical transitioning tropical cyclone.