“Coupling development”: An efficient mechanism for the development of extratropical cyclones

Abstract

"Coupling development", or the rapid cyclogenesis with a coupling process of the upper and the lower tropospheric vortices is studied numerically using a β-plane channel, dry primitive equation model. The "coupling development" occurred when a lower vortex is initially located at the latitude of the jet axis, and an upper vortex to the northwest of the lower vortex. We displayed the following three stages of the "coupling development". 1) Deformation of the upper vortex associated with the "upper-level frontogenesis" (from day 0-day 2). 2) The northward penetration of the lower vortex through advection of surface high-PV (from day 2-day 3). 3) Continuous rapid growth of the vortex near the surface triggered by the "coupling" of the upper and the lower vortices (day 2.5-day 3.5). We have also made a brief data analysis of the 25-28 April 1986 cyclone developed around the Japan Islands. We observed the following two characteristics in that cyclone. 1) The northward penetration of the lower vortex before the "coupling". 2) The downward intrusion of the upper vortex along the sloping isentropic surface, and the "coupling" with the lower vortex at the beginning of the rapid development. These observed features are consistent with our numerical results, which indicate the importance of the "coupling development" mechanism in the mid-latitudinal cyclone development.