Evaluation of tornadic environments for Japan using multiple data sources and their potential responses under future climate change

Environmental conditions associated with severe thunderstorms in Japan/East Asia, particularly those producing tornadoes, is examined using a synthesis of data sources. For this study, the performance of four reanalysis datasets (ERA5, ERA-interim, JRA-55, and CFSR), and one regionally downscaled version of the JRA-55 (DSJRA-55), is compared with radiosonde sounding measurements throughout Japan to determine the overall effectiveness in reproducing thermodynamic (instability) and kinematic (vertical wind shear, helicity) parameters commonly employed when examining convective potential. In addition, we make use of the tornado database provided by the Japanese Meteorological Agency to determine appropriate discriminators between weak and significant tornadoes using indices such as the Significant Tornado Parameter (STP) and the Energy Helicity Index (EHI). As anticipated, our results support the use of STP and EHI over individual indices such as convective available potential energy (CAPE) or deep layer shear when discriminating between tornado intensities.

In addition, changes to severe weather environments in response to climate change is investigated using the database for Policy Decision making for Future climate change (d4PDF) large ensemble simulations. d4PDF provides over 3000 years of historical and future-scenario climate simulations from the 60km Meteorological Research Institute-Atmospheric General Circulation Model (MRI-AGCM) and the 20km dynamically downscaled regional climate simulations from the Non-Hydrostatic Regional Climate Model (NHRCM). For future-scenario experiments, a 2K (near-future) and 4K (end of century) warming simulations are produced under the RCP8.5 scenario. Results show notably higher instability (CAPE) in both 2K and 4K climates, with the greatest response seen along the southern and eastern coasts of the Japanese archipelago. While decreases in deep layer shear are pronounced throughout much of Japan, changes in STP and EHI, among other severe indices examined, all reveal projected increases in environments favorable for convective storm/tornado genesis, emphasizing the contribution of a more unstable environment in response to global warming.