The Impact of Outflow Environment on Tropical Cyclone Intensification and Structure

Abstract

In this study, the impacts of regions of weak inertial stability on tropical cyclone intensification and peak strength are examined. It is demonstrated that weak inertial stability in the outflow layer minimizes an energy sink of the tropical cyclone secondary circulation and leads to more rapid intensification to the maximum potential intensity. Using a full-physics, three-dimensional numerical weather prediction model, a symmetric distribution of environmental inertial stability is generated using a variable Coriolis parameter. It is found that the lower the value of the Coriolis parameter, the more rapid the strengthening. The lower-latitude simulation is shown to have a significantly stronger secondary circulation with intense divergent outflow against a comparatively weak environmental resistance. However, the impacts of differences in the gradient wind balance between the different latitudes on the core structure cannot be neglected. A second study is then conducted using an asymmetric inertial stability distribution generated by the presence of a jet stream to the north of the tropical cyclone. The initial intensification is similar, or even perhaps slower, in the presence of the jet as a result of increased vertical wind shear. As the system evolves, convective outflow from the tropical cyclone modifies the jet resulting in weaker shear and more rapid intensification of the tropical cyclone?jet couplet. It is argued that the generation of an outflow channel as the tropical cyclone outflow expands into the region of weak inertial stability on the anticyclonic shear side of the jet stream minimizes the energy expenditure of forced subsidence by ventilating all outflow in one long narrow path, allowing radiational cooling to lessen the work of subsidence. Furthermore, it is hypothesized that evolving conditions in the outflow layer modulate the tropical cyclone core structure in such a way that tropical cyclone outflow can access weak inertial stability in the environment.