Tropical cyclones, often called hurricanes or tropical storms, can cause thousands of deaths and billions of dollars in damages to homes and businesses. Research on the formation of tropical cyclones, while spanning decades, has been limited compared to research on the dynamics and prediction of mature tropical cyclones. The relative de-emphasis on tropical cyclone formation research is linked with the tendency of these systems to form several days away form land. Sometimes, however, tropical cyclones form close to land, giving forecasters as little as a few hours to attempt to prepare the public for their potentially devastating impacts. With soaring coastal populations, this small amount of time is insufficient, necessitating better prediction of the formation of these tropical cyclones.

Hurricane Gabrielle (2001) formed a few hundred miles from the Florida coast only three days before making landfall. This project simulated the formation of Gabrielle using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research mesoscale model (MM5). The simulation results, along with available observations, were examined for the large-scale and mesoscale factors that influenced Gabrielle’s transition from a cold-core non-tropical disturbance to a warm-core tropical cyclone. This project found that asymmetric rainfall created circulation anomalies that, when axisymmetrized, provided the energy to fuel the transition of the disturbance to the warm-core Gabrielle. This asymmetric rainfall was organized by vertical wind shear interacting with the disturbance. This system’s reliance on vertical wind shear for development significantly differs from previous research on tropical cyclone formation, which stresses low vertical wind shear.