Tropical Cyclogenesis due to ITCZ Breakdown: Idealized Numerical Experiments and a Case Study of the Event in July 1988

Abstract

Abstract The mechanism of tropical cyclogenesis due to the breakdown of the intertropical convergence zone (ITCZ breakdown) and the structure of associated vortices are studied by numerical experiments using a nonhydrostatic mesoscale model. First, an idealized numerical experiment, in which a simple initial state without external disturbances is assumed, is performed without cumulus parameterization. A zonally uniform forcing of high sea surface temperature (SST) is imposed to generate an ITCZ-like structure. This “ITCZ” starts to undulate and eventually breaks down to form several tropical cyclones (TCs). These TCs merge and end up with a single TC. The energy budget analysis shows that barotropic instability of the low-level flow associated with the ITCZ is responsible for the genesis of vortices, and TC-scale buoyancy production soon takes over to contribute to the intensification of TCs. Conversion from the cumulus-scale kinetic energy into the TC-scale kinetic energy is found to be insignificant during ITCZ breakdown. Additional experiments show that the presence of the warm SST belt and an inclusion of the β effect are not essential for the occurrence of ITCZ breakdown. A numerical simulation of ITCZ breakdown over the Pacific Ocean in July 1988 shows that the mechanism of the tropical cyclogenesis is similar to that in the idealized numerical experiments from the viewpoint of the energy budget. Therefore, horizontal shear instability of the low-level flow and TC-scale buoyancy production are generally more essential than mergers of cumulus-scale vortices for the tropical cyclogenesis due to ITCZ breakdown.