Intensity Change of Binary Tropical Cyclones (TCs) in Idealized Numerical Simulations: Two Initially Identical Mature TCs

• Published in: Journal of the atmospheric sciences 2021-04, Vol.78 (4), p.1001-1020
• Main Authors: Liu, Hao-Yan, Wang, Yuqing, Gu, Jian-Feng
• Format: Article
• Language: English
• Subjects: Advection; Anticyclones; Asymmetric structures; Cyclones; Deformation; Distance; Evolution; Hurricanes; Numerical simulations; Potential temperature; Separation; Simulation; Tropical climate; Tropical cyclone intensities; Tropical cyclones; Vertical wind shear; Wind shear
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-20-0116.1

This study investigates the intensity change of binary tropical cyclones (TCs) in idealized cloud-resolving simulations. Four simulations of binary interaction between two initially identical mature TCs of about 70 m s −1 with initial separation distance varying from 480 to 840 km are conducted in a quiescent f -plane environment. Results show that two identical TCs finally merge if their initial separation distance is within 600 km. The binary TCs presents two weakening stages (stages 1 and 3) with a quasi-steady evolution (stage 2) in between. Such intensity change of one TC is correlated with the upper-layer vertical wind shear (VWS) associated with the upper-level anticyclone (ULA) of the other TC. The potential temperature budget shows that eddy radial advection of potential temperature induced by large upper-layer VWS contributes to the weakening of the upper-level warm core and thereby the weakening of binary TCs in stage 1. In stage 2, the upper-layer VWS first weakens and then restrengthens with relatively weak magnitude, leading to a quasi-steady intensity evolution. In stage 3, due to the increasing upper-layer VWS, the nonmerging binary TCs weaken again until their separation distance exceeds the local Rossby radius of deformation of the ULA (about 1600 km), which can serve as a dynamical critical distance within which direct interaction can occur between two TCs. In the merging cases, the binary TCs weaken prior to merging because highly asymmetric structure develops as a result of strong horizontal deformation of the inner core. However, the merged system intensifies shortly after merging.