Tropical cyclogenesis associated with extratropical precursors in the North Atlantic Basin

This thesis investigates genesis pathways of tropical cyclones (TCs) over the North Atlantic basin during 2004--2008 with a focus on TCs that form in conjunction with extratropical precursor disturbances. These genesis pathways are nonbaroclinic, low-level baroclinic, trough-induced, transient-trough interaction, weak tropical transition (TT), and strong TT. The transient-trough interaction, trough-induced, weak TT, and strong TT genesis pathways all occur in conjunction with extratropical precursor disturbances. This thesis will assess the physical and dynamical mechanisms relevant to TC genesis among these genesis pathways via composite and case study analyses.;An automated vortex-tracking algorithm identified 158 null cases and 84 TCs during the 2004--2008 North Atlantic TC seasons. The nondeveloping disturbances (incipient TCs) were categorized by their TC genesis pathway 12 h prior to their initial peak intensity (TC genesis). The developing and nondeveloping disturbances in the extratropical genesis pathways were stratified by the character of the synoptic-scale flow pattern as defined by the 300-hPa streamfunction. Three distinct composite categories emerged: tropospheric-deep cutoff low (CUTOFF), upstream trough (TROUGH), and foldover ridging (FOLDOVER).;CUTOFF composite precursor disturbances frequently follow the strong TT pathway and are typically associated with mature extratropical cyclones initially. Deep, moist convection (hereafter, convection) develops along an axis of strong low-level frontogenesis north and northwest of the disturbance. Diabatic processes are important in vertical potential vorticity redistribution, contributing to the transformation of a deep cold core extratropical cyclone to a warm core subtropical cyclone (and eventual TC), and vertical wind shear reduction. CUTOFF composite null cases feature the concentration of convection away from the incipient low-level disturbance and vertical wind shear remains strong over the disturbance throughout. TROUGH and FOLDOVER composite developing disturbances exhibit structures similar to subtropical cyclones with strong quasi-geostrophic forcing for ascent aloft and weak baroclinicity below. Diabatic processes, organized by frontogenesis for the FOLDOVER-type developments, are important in the growth of the incipient vortex and reduction of deep-layer wind shear. TROUGH and FOLDOVER nondeveloping disturbances are characterized by strong vertical wind shear and deformation, and a relative absence of DMC.;Selected case studies of developing and nondeveloping disturbances are used to highlight the spectrum of disturbances -- extratropical and subtropical -- that are capable of potentially producing a TC. These case studies illustrate how low-level baroclinicity, vertical wind shear, and diabatic processes interact to define TC genesis pathways and extratropical and subtropical cyclone characteristics.