Diagnosis of banded precipitation patterns associated with extratropically transitioning tropical cyclones using CSI theory

The synoptic and mesoscale environments associated with two North Atlantic tropical cyclones interacting with the midlatitudes, Bonnie (1998) and Charley (2004), are analyzed to determine the potential role of conditional symmetric instability (CSI) in the formation of convective bands associated with each storm. This topic is an outgrowth of work highlighting the presence of moist symmetric instability associated with the extratropical transition of Hurricane Floyd in 1999 and other work highlighting the potential for CSI to exist within the warm frontogenetical sectors of a tropical cyclone undergoing extratropical transition.

Charley provides a case of a tropical cyclone interacting with the midlatitudes as it approached landfall along the southwest Florida coast. Outer convective bands responsible for heavy rainfall rates and strong winds propagated outward ahead of the storm to a larger degree than those associated with a tropical cyclone typically do, suggesting that another mechanism may be responsible for their formation and maintenance away from the core of the cyclone. Analysis of model data shows that the environment ahead of the system is conducive to the presence of CSI. Quasi-geostrophic (QG) forcing for vertical motion through differential vorticity advection associated with the translation of the cyclone provided the dominant lifting mechanism for the convection. Moderate values of convective available potential energy (CAPE) ahead of the system and a saturated atmospheric column associated with the advection of moist air into the region ahead of the mid-tropospheric trough provide sufficient conditions for the formation of the observed convection.

Bonnie provides a classical case of a tropical cyclone undergoing extratropical transition in the North Atlantic Ocean, highlighted by the development of a frontal structure to the cyclone. It is shown that there is strong forcing for rising motion in the warm frontogenetical sector of the surface cyclone induced primarily by strong QG forcing and frontogenetical forcing resulting from deformation and convergence processes in the lower troposphere. However, despite a sufficiently moist column as a result of moist tropical air being transported along shear vectors into the midlatitudes and favorable conditions for rising motion, surface-based convective instability likely dominates over the low-level potential for CSI; elevated convection may be present within such a region. This convective instability, not present in association with Charley, arises as a result of the decoupling of the boundary layer circulation of the rapidly-translating tropical cyclone from that in the mid-troposphere as it encounters the increasingly statically stable midlatitude environment.

Implications from and physical reasons behind the findings, the utility of this approach to operational (e.g. real-time) forecasting, and suggestions for future research are discussed in conjunction with the results. Additional cases, both of tropical cyclones impacting land that undergo extratropical transition as well as of those that do not, will be presented to further highlight the role of CSI within the observed convective bands associated with these cyclones.