Understanding the thermodynamic evolution of the tropical cyclone warm core during the extratropical transition process

It is well known that as a tropical cyclone (TC) undergoes extratropical transition (ET), it loses its warm core structure and becomes, at least at the completion of ET, a cold core extratropical cyclone. Previous research has suggested that the upper tropospheric warm core gets lost in the highly vertically sheared environmental flow associated with the ET event, while little research has been performed toward understanding the evolution of the lower tropospheric warm core. As such, a comprehensive understanding of the full tropospheric thermal evolution of a TC undergoing ET is lacking. Such an understanding can not only close one of the remaining gaps in our understanding of the ET process but potentially lead to improved forecasts of the timing of the completion of an ET event as well as specific mesoscale evolutions, such as the expansion of the radius of maximum winds, observed to occur during ET.

Here, we use previous results aimed at understanding the wind field evolution during ET as motivation toward understanding the thermal core evolution during ET. Analyses of thermodynamic and momentum budgets obtained directly from very high resolution (dx = 4 km, 70+ vertical levels) MM5 simulations of North Atlantic TC Bonnie (1998) are presented, highlighting the primary factors at work within the inner core of the cyclone to bring about its thermal evolution during ET. Results from additional simulations of other cyclones are presented to provide a means of generalizing the results to all ET events. Connections are drawn between the thermal evolution and other previously observed mesoscale structural evolutions that occur during the ET process. A conceptual model of the entire ET process using the results from this study is proposed, specifically highlighting the contributions of the warm to cold core evolution to the process.