Generation and intensification of misocyclone related to a tornado formation in moist environment was confirmed by analyzing a data set of dense operational Doppler radar observation network in the central part of Japan. A tornado classified as F0 or F1 on Fujita scale struck Inabe city, central part of Japan, on the evening of 18 September 2012 within the observation range of three X-band polarimetric radars of Ministry of Land, Infrastructure, Transportation and Tourism, Japan. The mesocyclone was detected manually by finding a pair of maximum and minimum Doppler velocity in the PPI scan of every minute. Mesocyclone with the diameter around 4 km and vortex larger than 1.0 ◊ 10 2 s 1 was tracked for 65 minutes in succession of the cyclone lasted for 32 minutes with the diameter around 4 km and vorticity less than 1.0 ◊ 10 2 s 1. Misocyclone with the diameter around 1 km and vortex larger than 1.0 ◊ 10 1 s 1 was identified several minutes before the tornado occurence.
By dual Doppler radar analysis choosing a pair of appropriate Doppler radar among the three, we revealed the three-dimensional airflow structure of the parent storm and the formation process of the misocyclone relating to the tornado, named as Inabe tornado. The updraft and downdraft of the parent storm of the Inabe tornado was weaker than typical supercells in the Great Plains, though hook echo and overhanging of echo were clearly analyzed several minutes before the tornado occurrence. The environment of the storm had large low-altitude vertical wind shear, large storm-relative helicity (> 150 m2 s-2) and medium CAPE (> 1000 J kg-1), which are favorite for the generation and development of supercell storm. The misocyclone was observed adjacent to the mesocyclone a few minutes before the occurrence time of the Inabe tornado. It is noteworthy that a pair of positive and negative vortices with the diameter about 1 km was detected in the rear-flank of the parent storm before the appearance of the misocyclone in the mesocyclone. This vortex pair suggests a vortex tube that was formed by the tilting of the horizontal vorticity associated with a rear-flank downdraft and cold outflow. The cyclonic vortex of the vortex pair was advected into the mesocyclone just before the deep development of it. As the location of the intensified misocyclone of the parent storm corresponded to the damage track of the Inabe tornado, we propose that the regional intensification of an updraft by the misocyclone was a contributory factor to form the Inabe tornado which occurred in the humid environment in Japan. This indicates that the Inabe tornado was a supercell tornado but not non-supercell tornado though its duration time was short.
Based on the successful identification of mesocyclone and misocyclone related to a tornado, we will discuss a possible usage of dense network of X-band polarimetric radar for identification and nowcasting of extreme weather phenomena.