The Impact of Tropical Remnants on Extratropical Cyclogenesis: Case Study of Hurricanes Danielle and Earl (1998)

Abstract

The importance of remnant tropical cyclone (TC) circulation and moisture structures is investigated for a simultaneous extratropical transition (ET) event involving ex-Hurricanes Danielle and Earl (September 1998). Although both storms undergo prolonged periods of reintensification following ET, the forcings involved in each of their redevelopment processes differ fundamentally. A review of the tropical and baroclinic ET modes in the North Atlantic stresses the importance of jet/front structures to the nature of the reintensification process. Ex-Hurricane Danielle begins to redevelop in the eastern half of the basin in the downstream, poleward sector of an intensifying polar jet. The system undergoes a tropical mode of reintensification, resulting in a troposphere-deep warm environment surrounding the storm, devoid of near-surface fronts and maintained by strong tropopause folds at its periphery. Ex-Hurricane Earl reintensifies near the eastern seaboard according to a baroclinic mode, under the influence of an upshear upper-level trough. A rapid cyclonic rollup of upper-level potential vorticity over the reintensifying low-level center results in a strong baroclinic system with well-defined frontal boundaries. The two elements of the remnant TCs considered here are circulation and moisture. Potential vorticity-based modifications are made to the initial atmospheric state of the Mesoscale Compressible Community model in order to remove either one or both of these possible cyclogenetic forcings. The resulting set of sensitivity tests is analyzed in terms of system intensity and structure. It is found that the tropical-mode reintensification (ex-Hurricane Danielle) process requires the presence of the remnant's circulation and moisture for rapid redevelopment. However, the baroclinic-mode transition studied (ex-Hurricane Earl) is remarkably insensitive to the removal of the ex-tropical vorticity and moisture structures of the TC remnant.