Numerical Simulation of Supercell Tornadogenesis: The Tsukuba Tornado on 6 May 2012

On 6 May 2012, F3 supercell tornado, which is one of the most destructive tornadoes ever in Japan, hit Tsukuba City in eastern Japan and caused severe damage. The environment around the supercell storm was characterized by strong low-level veering sheer and a high CAPE of about 2000 J/kg. Backward trajectories and vorticity budget analysis revealed that the secondary rear-flank downdraft (RFD) surge played a key role in tornadogenesis by transporting the baroclinically generated streamwise vorticity and enhancing the horizontal convergence. The evolution of vortex lines within low-level mesocyclones is also examined. Some vortex lines emanating from the low-level mesocyclone form the arches straddling the hook-shaped hydrometers distribution. This result indicates that baroclinic vorticity generation within RFD region is effective for a low-level mesocyclone formation as in the recent observational studies of suprecells in midwestern United States. However, the change in circulation around the low-level mesocyclone revealed that not only storm-generated baroclinicity but also environmental vorticity contributed to the formation of a low-level mesocyclone. In the meanwhile, most of the circulation of material circuits that converge upon the tornado is acquired from baroclinicity associated with the so-called descending reflectivity core (DRC).