Structure and Environment of Tornado-Spawning Extratropical Cyclones around Japan

This study used the JRA-55 reanalysis dataset for analyzing the structure and environment of extratropical cyclones (ECs) that spawned tornadoes (tornadic ECs: TECs) between 1961 and 2011 in Japan. Composite analysis findings indicated that the differences between the structure and environment of TECs, and those of ECs that did not spawn tornadoes (non-tornadic ECs: NTECs), vary with the seasons. In spring (March-May), TECs are associated with stronger upper-level potential vorticity and colder mid-level temperature than NTECs. The colder air at the mid level contributes to the increase in convective available potential energy (CAPE) of TECs. TECs in winter (December-February: DJF) and those northward of 40 degrees N in autumn (September-November: SON) are accompanied with larger CAPE than are NTECs. The larger CAPE for TECs in DJF is caused by larger moisture and warmer temperature at low levels and that for TECs northward of 40 degrees N in SON (NSON) is caused by the colder mid-level temperature associated with an upper-level trough. The distribution of the energy helicity index also shows significant differences between TECs and NTECs for DJF and NSON. On the contrary, the distribution of the 0-1 km storm-relative environmental helicity (SREH) showed no significant differences between TECs and NTECs in most seasons except DJF. A comparison of TECs between Japan and the United States (US) shows that SREH and CAPE are noticeably larger in the US. These differences possibly occur because TECs in the US (Japan) develop over land (ocean), which exerts more (less) surface friction and diurnal heating.