High-resolution Ensemble Experiments for the Tsukuba City Supercell Tornado in Japan on 6 May 2012

To develop a probabilistic forecast of tornadoes and clarify their genesis mechanisms, it is useful to perform statistical analyses using ensemble numerical experiments of observed tornadoes. In our recent paper (Yokota et al., 2016, MWR), we have made ensemble simulations on Tsukuba City (50-90 km northeast of Tokyo) F3 tornado on 6 May 2012, which is one of the strongest tornadoes in Japan. Horizontal resolution of the numerical model was 350 m, and the LETKF (local ensemble transform Kalman filter) analysis, which assimilated dense C-band polarimetric radar and surface data, was used as the initial conditions. The results of ensemble-based sensitivity analysis shows that low-level convergence in the front side of the storm and low-level water vapor in the rear side of the storm control the strength of the forecasted low-level mesocyclone (LMC).

In the present study, we carried out downscale ensemble experiments with 50-m horizontal resolution. The downscale experiments produce a wide range of tornadic vortices: some members spawn strong tornadoes, and some do not have a vortex of tornado strength. Thus the results of the downscale experiments can be used to clarify the relationship between the LMC and the tornado.

To investigate the relationship between the LMC and the tornado, we produced the composite fields with the coordinate system relative to the point and time (defined by t₀ min) of near-surface maximum vertical vorticity (ζ_max). This composite field shows that ζ_1km(t₀-3) correlates to ζ_max more strongly than ζ_2km(t₀-3) and ζ_30m(t₀-3), where ζ_zkm(t) denotes the maximum vertical vorticity at z-km height at t min. Furthermore, in the members where stronger ζ_max were generated, low-level water vapor was more humid, minimum surface pressure was lower, and horizontal distance between ζ_1km(t₀-3) and ζ_30m(t₀-3) was shorter at t₀-3 min. These results show that this tornado was strengthened several minutes after a strong LMC reached right above the near-surface maximum vorticity.