Numerical Simulation of the 6 May 2012 Tsukuba City Supercell Tornado: Generation Mechanisms of a Tornado

On 6 May 2012, a F3 supercell tornado, which is one of the most destructive tornadoes ever in Japan, hit Tsukuba City in eastern Japan and caused severe damage. To clarify the structure of the tornado-producing storm and the mechanism of tornadogenesis, triply-nested numerical simulations were performed using a nonhydrostatic model with an innermost horizontal grid spacing of 50 m. The innermost simulation successfully reproduced a tornado spawned by a supercell storm. The tornado was generated on the rear-flank gust front when a secondary rear-flank downdraft (RFD) surge overtook the rear-flank gust front. Backward trajectories and vortex line analysis revealed that the secondary RFD surge played a key role in tornadogenesis by transporting streamwise vorticity. To identify the vorticity source of the tornado, the evolution of circulation was investigated for a material circuit integrated in time from the tornado region. The circuit acquired most of its circulation baroclinically at the tip of hook-shaped precipitation when the precipitation core was descending. The sensitivity experiments revealed that cooling due to the melting of hail/graupel was most effective for tornadogenesis. Meanwhile, evaporative cooling from rain drops weakened the tornado because strong cold pools and excessive negative buoyancy are detrimental to tornadogenesis and tornado maintenance.