Mesoscale aspects of tropical cyclogenesis from extratropical precursors over the North Atlantic during 2004–2008

Previous research on North Atlantic tropical cyclone (TC) genesis for 1948–2004 has identified six TC genesis pathways. These genesis pathways, and the climatological percentage of all North Atlantic TCs in each genesis pathway, are nonbaroclinic (40%), low-level baroclinic (13%), transient-trough interaction (16%), trough induced (3%), weak tropical transition (13%), and strong tropical transition (16%). In this presentation we will focus on the nearly 50% of all TC genesis events that form from extratropical precursors along the transient-trough interaction, trough induced, and tropical transition pathways. The motivation for focusing on TCs in these genesis pathways is that they can be difficult to predict as they can form over relatively short timescales (< 24 h) and in close proximity (< 1000 km) to U.S. coastal zones, and they provide the opportunity to study the physical processes – quasigeostrophic (QG) forcing for ascent and deep-layer wind shear reduction – that contribute to the spectrum of TC genesis events.

The aim of this presentation is twofold. First, composites of genesis events and null cases associated with extratropical precursors for the 2004–2008 TC seasons will be examined and compared to assess the role of QG forcing for ascent in facilitating the development of deep moist convection, which can contribute to deep-layer wind shear reduction and column moistening in some cases. Second, the generation and evolution of mesoscale low-level relative vorticity and potential vorticity centers for selected genesis events and null cases will be compared using available observations and numerical simulations.