The Role of Radiation in Accelerating the Genesis of Atlantic Hurricane Matthew (2016)

Despite active study, there is still much left to discover about the transformation from a tropical disturbance into a tropical depression, a process known as tropical cyclogenesis. Several recent studies have found that the presence of radiation in numerical models has a very pronounced effect on accelerating tropical cyclogenesis in both idealized and case study simulations. In idealized simulations this appears to be primarily due to a weak nocturnal transverse circulation brought about from differential radiative heating between the cloudy disturbance and the surrounding cloud-free region. This causes a slow lifting motion (vertical velocity on the order of millimeters per second) in the core of the system and weak compensating subsidence in the surroundings. This transverse circulation slowly moistens the core of the disturbance and provides a favorable environment for additional convection, thus contributing to a faster rate of tropical cyclogenesis as compared to simulations without radiation. Hurricane Matthew was a Category 5 Atlantic storm that made devastating landfall in Haiti, Cuba, and the southeast coast of the United States in October 2016. Matthew underwent tropical cyclogenesis near the Lesser Antilles on September 28, 2016 and reached a maximum sustained wind speed of 145 knots on October 1 at 0Z. The minimum pressure of 934mb was observed on October 4 at 0Z, as measured by a dropwindsonde. Matthew was responsible for 585 direct deaths and the evacuation of roughly 4 million people from coastal areas. Total damages were estimated at $15 billion US dollars, including about $10 billion along the coasts of Florida, Georgia, and the Carolinas. Results will be presented from a WRF case study of Hurricane Matthew’s genesis near the Lesser Antilles (beginning 2-3 days before genesis) and its early intensification. Sensitivity to choices of radiation scheme will be explored, including cases without radiation and shortwave and longwave only, compared to a control simulation with both longwave and shortwave radiation which closely matches Matthew’s observed track and intensity. A comparison of these simulations will illustrate that radiation plays a critical role in the development of the tropical cyclone. The distribution of vertical relative vorticity and convection (Vortical Hot Towers) will be examined to illustrate the role of the transverse circulation in moistening the core and accelerating tropical cyclogenesis in the presence of radiation.