Evaluation of tornadic environments and their trends and projected changes in Japan

Abstract

Tornadoes are responsible for several high-impact weather disasters in Japan. However, little is known about how these events have changed over the last several decades or how they may change in future climates. This study examines environmental conditions associated with tornados in Japan using pseudo-soundings from the high-resolution fifth-generation ECMWF reanalysis. We first determine appropriate discriminators of F2+ tornadoes using thermodynamic (convective available potential energy, convective inhibition, lifting condensation level, and the K-index), kinematic (bulk wind difference and storm-relative helicity), and multivariate tornado parameters (energy helicity index, K-helicity index, and the significant tornado parameter), and confirm that F2+ tornadoes occur in environments with higher instability and helicity, but are better distinguished using multivariate parameters. Recent trends indicate that F2+ environments have increased significantly in some regions over the last four decades. We also examined future changes for each parameter using a large ensemble 2-K warming experiment. Robust increases in strong tornado environments are depicted in many regions in Japan, particularly on the Sea of Japan side and the Kanto region. This indicates that despite projected decreases in bulk wind difference and higher convective inhibition, significant increases in atmospheric instability compensate, leading to more days with F2+ tornado potential.