Structure and Environment of Tornado-Spawning Extratropical Cyclones around Japan

Structural and environmental characteristics of extratropical cyclones (ECs) that are associate with tornadoes (tornadic ECs; TECs) from 1961-2011 in Japan are examined through a reanalysis data set (JRA-55) and are compared with those of ECs that are not associated tornadoes (non-tornadic ECs; NTECs). A composite analysis indicates that differences in the structures and environment of ECs between TECs and NTECs vary with seasons. In spring (MAM), upper-level high potential vorticity (PV) and mid-level temperature for TECs are stronger and colder, respectively, than those for NTECs, resulting in larger CAPE for TECs. TECs in winter (DJF) and in the north region of 40 N in autumn (SON) are accompanied by CAPE larger than that for NTECs. The larger CAPE in the north region of 40N in SON is due to the mid-level cold temperature associated with an upper-level trough and that in DJF is due to low-level large moisture and warm temperature. The distributions of energy helicity index (EHI) also show significant differences between TECs and NTECs in NSON and DJF. On the other hand, the distributions of 0-1 km SREH show no notable differences between TECs and NTECs in most of seasons except for DJF. Additionally, other parameters such as K-index (KI) and K-helicity index (KHI) are not effective for distinguishing TECs from NTECs either except for DJF. The comparisons of TECs between Japan and the United States (US) show that SREH and CAPE are notably larger in the US. It is suggested that these differences occur because TECs in the US (Japan) are located over land (ocean) that exerts larger (less) surface friction and diurnal heating.