Outer Rainband Formation in a Sheared Tropical Cyclone

In this study, the formation of outer rainbands of a tropical cyclone (TC) in a vertically sheared environment is investigated with a high-resolution numerical model. The simulated TC shows a quasi-stationary structure of primary rainbands on the left side of downshear. The formation of outer rainbands (primary rainbands) is closely related to the activity of inner rainbands. Well-organized inner rainbands prevail downshear. As these inner rainbands are moving radially outward, active convective cells develop upstream and out of the rapid filamentation zone, finally leading to the formation of outer rainbands. Furthermore, there exist two types of inner rainbands in the sheared TC. One corresponds to sheared vortex Rossby waves, and the other originates from convection upshear forced in the rapid filamentation zone, which are quickly spiraled due to deformation and cyclonically advected by the rotating TC flow. It is indicated that three scenarios are associated with the formation of outer rainbands; they are formation from an inner rainband which was organized from a blend of deformation-induced inner rainbands, directly from a single deformation-induced inner rainband, and from a vortex Rossby wave-related inner rainband, respectively.