Intensity Change of Binary Tropical Cyclones in an Idealized Three-Dimensional Model

This study investigates the intensity change of binary tropical cyclones (TCs) under the influence of their mutual interaction in an idealized three-dimensional full-physics numerical model with a finest horizontal resolution of 3 km. The two identical TCs merge within the initial separation distance of 600 km.

Due to the interaction between binary TCs, the intensity evolution presents two weakening stages and an unchanged stage between them. Such intensity change of each one in binary TCs is correlated to the upper-layer vertical wind shear (VWS) caused by the other TC. During the first stage, the upper-layer anticyclone (ULA) of one TC results in the upper-layer VWS and ventilates the warm core of the other TC above the outflow layer, which causes the intensity of the binary TCs decreasing. During the second stage, as the ULA stretches downward and outward, the upper-layer VWS changes to the opposite direction, along with the intensity decreasing first and then increasing. Meanwhile, the intensity of the binary TCs stays unchanged. In the last stage, the binary TCs weaken again as the upper-layer VWS increases to some extent except the merging cases. When the two TCs approach each other before merging, the upper-layer VWS in one TC is almost caused by the upper-layer cyclone and outflow of the other, which induces highly asymmetric structure and weakens the vortex. In addition, the horizontal size of the ULA quantified by the Rossby radius of deformation seems to be a critical separation distance of binary TCs, exceeding which the VWS is small enough to influence the intensity.