Projected Changes in Atmospheric Stability for Japan Using Large Ensemble Simulations

In this study, we use large ensemble simulations (50 members) produced as part of the “database for Policy Decision making for Future climate change (d4PDF)” to evaluate how a warming climate may enhance the potential for convective storm activity in Japan. Model results utilize the Non-Hydrostatic Regional Climate Model (NHRCM), with a horizontal resolution of 20km. For contemporary climate simulations, ensemble members are produced through slight perturbations using COBE-SST2, while future-scenario simulations use six sea surface temperature (SST) patterns from the Coupled Model Intercomparison Project – Phase 5 (CMIP5). Preliminary analysis, and results in this abstract, focus on end of century projections, where global-mean air surface temperatures are projected to be 4˚C warmer than pre-industrial climate. Additionally, results from near future projections, with global-mean air surface temperatures 2˚C warmer than pre-industrial climate, will also be presented. Developed under the Social Implementation Program on Climate Change Adaptation Technology (SI-CAT), this dataset provides climate projection results for the immediate future, which is more likely to be relevant for decision makers and stakeholders who are concerned about changes over the next 20-30 years, and less so for changes beyond the middle of the 21^stcentury.

While several indices have been developed for measuring atmospheric stability, we analyze Convective Available Potential Energy (CAPE), Convective Inhibition (CIN), K index, and the Severe Weather Threat Index (SWEAT) during the summer months (MJJA). For climatological CAPE, large increases are seen predominantly near coastal areas, with some areas such as the southern island of Kyushu and the Kanto region (near Tokyo) showing increases excess of 400-500 J kg^-1 in August. For both the K index and SWEAT, greatest increases are seen in July/August, and compared to CAPE, are more evenly distributed throughout Japan (with a few exceptions). In addition, extremely unstable environments, defined here as CAPE > 3500 J kg^-1 (with CIN > -100 J kg^-1), K index > 40˚C, or SWEAT > 400, also increase in frequency, some areas rather substantially. These results are in line with previous studies, where a warming climate will likely produce conditions more favorable for severe thunderstorm activity.