How Does the Extratropical Transition of Tropical Cyclones Impact Tornadoes and Supercells?

Landfalling hurricanes produce multiple, concurrent hazards, including tornadoes, that can compound the impact of one another. These tornadoes in landfalling hurricanes can impact regions that infrequently experience tornadoes (i.e., Northeast U.S.), or have vulnerable infrastructure and marginalized communities with rapidly growing populations (i.e., Southeast U.S.). Nearly all those hurricanes that have produced the largest number of tornadoes in the modern historical record have transformed into high-latitude low-pressure systems either before, during, or after landfall as part of a process called extratropical transition. However, incomplete knowledge exists of how extratropical transition impacts tornadoes. Hence, this research seeks to understand how the extratropical transition of a hurricane impacts tornadoes, their associated thunderstorms, and near-thunderstorm environments. The knowledge gained from this work will serve as the foundation for improving tornado forecast skill at all lead times, which will be facilitated by the involvement of a Storm Prediction Center lead forecaster on the science team. This proposal will also train the next generation of scientists by funding the research of multiple undergraduate and graduate students. The science team will test whether extratropical transition impacts tornadoes using a climatological observational analysis paired with a two-part modeling study. First, we will investigate how extratropical transition impacts tornadoes and their attendant supercell (i.e., strongly rotating thunderstorms) attributes using long-term tornado and radar datasets. To identify the factors responsible for these changes in tornadoes and supercells, we will examine how tornado frequency varies with hurricane and its large-scale environmental traits during extratropical transition using an ensemble of high-resolution simulations of hurricanes that do and do not undergo transition. We will then characterize how convective-scale kinematic and thermodynamic environments respond to these changes in the hurricane and its large-scale environmental factors using a large sample of radiosondes and model-derived soundings sourced near supercells. Finally, these near-supercell soundings will be used as a base state for a series of idealized model experiments to assess key small-scale environmental traits that impact supercell characteristics associated with transition. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.