Tropical Cyclone-Trough Interactions

The interaction between tropical cyclones and upper-tropospheric troughs is a common occurrence in the North Atlantic, and presents complications in forecasting tropical cyclone intensity. Troughs can be both favorable and unfavorable for tropical cyclone intensification. Troughs that are favorable for intensification, versus troughs that are unfavorable, are typically weaker, shallower, and longitudinally-narrower, resulting in reduced vertical wind shear and ventilation. In these situations, convection is able to wrap upshear in the tropical cyclone and promote intensification. Tropical cyclone-trough interactions can also affect the size of a tropical cyclone. Idealized WRF experiments are used to study how a tropical cyclone evolves when interacting with a longitudinally narrow, medium, and wide trough. In these experiments, tropical cyclones interacting with wider troughs tend to grow larger in size. This growth is due to two primary processes. First, tilt-induced lift and moistening results in a more coherent precipitation shield downshear of the tropical cyclone. Second, sustained Q-vector convergence between the trough and tropical cyclone results in greater lift and moistening left of shear. Both of these processes result in greater coverage of convection and greater potential vorticity generation at large radii that results in an increase in size.