Tropical cyclone energy dispersion in a baroclinic model and its associated cyclogenesis

Abstract

A case study on typhoon Prapiroon (2000) is performed to examine the role of TCED in the following cyclogenesis in nature. Sensitivity experiments suggest that the previous TC would modulate the large-scale environmental circulations, thus affect the formation of its sequential storm. A remarkable asymmetry appears in the perturbation growth of the wave train in the presence of vertical wind shears. That is, an easterly (westerly) wind shear confines the wave to the lower (upper) level. It is suggested that the vertical shear may impact the Rossby wave train development through both the barotropic-baroclinic mode coupling and the modulation of the group velocity by the mean flow through a "Doppler shift effect". The destabilization of Rossby wave train by regional easterly vertical shears has important implications.The 3D Rossby wave energy dispersion of a TC is studied using a baroclinic primitive equation model. The numerical results demonstrate more complex 3D energy dispersion characteristics than 2D barotropic dynamics. A key feature associated with the formation of 3D wave train is a downward propagation of the relative vorticity and kinetic energy. The upper anticyclonic circulation rapidly induces an intense asymmetric outflow jet in the southeast quadrant, which influences the lower-level Rossby wave train formation. On one hand, the outflow jet exerts an indirect effect on the strength of lower-level wave train through changing in TC intensity and structure. On the other hand, it triggers downward energy propagation, which may further enhance the lower level Rossby wave train formation.The interaction between a tropical cyclone and idealized intertropical convergence zone (ITCZ) is investigated. Once a TC develops in the ITCZ region which satisfies barotropic and baroclinic instability, the southeastward energy dispersion from the TC may accelerate ITCZ breakdown, and the interaction between the convective heating and the perturbation circulations may lead to the generation of new tropical cyclone to the east. Through repeating of the above process, a synoptic-scale wave train oriented in the northwest-southeast direction can be generated and self-maintained.