Warm-seclusion extratropical cyclone development: Sensitivity to the nature of the incipient vortex

As tropical cyclones translate poleward, they encounter lower SSTs and upper-level vertical shear consistent with the baroclinic midlatitude circulation. During this extratropical transition, the TC evolves from an axisymmetric warm-core system into an asymmetric cold-core extratropical cyclone. Interaction with these midlatitude features may cause continued weakening, merger with existing frontal systems, and/or reintensification into a mature extratropical cyclone. One possible outcome of ET is the warm-core seclusion (Shapiro and Keyser 1990), and intense system with destructive winds often in excess of hurricane force. However, this outcome is hardly unique to transitioning tropical cyclones. Indeed, many intense extratropical cyclones without tropical origins develop warm core structure (Gyakum 1982a,b).

Observational studies (i.e., Maue 2004; Maue and Bourassa 2005; Hart 2003) have raised many fundamental and intriguing questions about the detailed nature of the warm seclusion development. QuikSCAT wind measurements illustrate the relatively high frequency of these events in all oceanic basins. In particular, the development of an intense low-level jet of winds in the SW flank of the cyclone core remains poorly understood and requires model simulation (Browning 2004). Although a warm-seclusion can develop with or without a tropical cyclone remnant, it remains unclear whether the presence of a TC remnant vortex will trigger or accelerate the frontal fracture and tropopause fold necessary for this development. Likewise, the rapid intensification of low-level diabatic potential vorticity (PV) maxima is often concomitant with the initialization of frontal fracture (Moore and Montgomery 2004).

This study will use the results of Browning (2004) as a foundation and simulate the evolution of various warm-seclusion events using the PSU/NCAR MM5 model. Cases examined include Hurricanes Jose (1999), Irene (1999), and extratropical cyclones in the same week, including the secondary development after Jose's demise. Detailed analysis of these simulations will examine the questions posed above.