Tropical cyclogenesis associated with extratropical precursors in the North Atlantic

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 on 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 that contribute to the spectrum of TC genesis events.

The aim of this presentation is to assess differences in pre-genesis ambient environments associated with TC genesis events and null cases that occurred during the 2004–2008 North Atlantic TC seasons. The TC genesis events and null cases associated with extratropical precursors were stratified into categories defined by the structure of the 200 hPa streamfunction pattern during initial intensification of the incipient low-level disturbance at 12 h prior to the time of genesis (initial peak intensity) for the TC (null) cases. The composite categories defined by the 200 hPa streamfunction pattern include (i) cutoff low, (ii) positively-tilted potential vorticity streamer, and (iii) upstream trough. Initial findings indicate that TC genesis cases, compared to null cases, have (i) a better-defined region of warm air advection on the south and east flank of the low-level disturbance, (ii) a more prominent anticyclonic thermal vorticity maximum in the 600-300 hPa layer, (iii) deeper moisture, and (iv) larger 1000-700 hPa layer-averaged convergence implying more low- and mid-level cyclonic vorticity growth.