Important Factors for Tornadogenesis as Revealed by High-Resolution Ensemble Forecasts of the Tsukuba F3 Tornado on 6 May 2012

Most of strong tornadoes are generated in supercell storms with mid- and low-level mesocyclones. However, the presence of the mesocyclones does not necessarily lead to tornadogenesis, implying that some other processes inside the storms play important roles in tornadogenesis. To clarify important factors for tornadogenesis, statistical analyses based on ensemble experiments on an observed tornado seem to be a promising approach. Using Japan Meteorological Agency Non-Hydrostatic Model (JMANHM, Saito et al. 2006) of 350-m horizontal grid spacing, Yokota et al. (2016) conducted 33-member ensemble forecasts of a supercell that spawned the Tsukuba City F3 tornado on 6 May 2012 in Japan and clarified the importance of low-level convergence and water vapor for the low-level mesocyclogenesis using ensemble-based sensitivity analysis. Since its resolution was not fine enough to resolve tornadoes, however, we further conducted 33-member downscaling ensemble forecasts with 50-m horizontal grid spacing to focus on the tornado and performed ensemble-based analyses to clarify important factors for tornadogenesis (Yokota et al. 2018).

In the ensemble forecasts, seven of 33 members produced tornadoes, where tornadoes were defined as vortices having 5-min moving average of maximum vorticity at 30 m AGL (above ground level) ζ^max_z*=30m^(5min) > 1.0 s^-1. Circulation analyses for several vortices at 30 m AGL showed that the change of the circulation was mainly contributed by the frictional term, but generally little by the baroclinic term. Correlation between these terms and ζ^max_z*=30m^(5min), however, was weak, suggesting that the source of the circulation is not essential for tornadogenesis and stretching of vorticity due to the storm-scale low-level updraft may be more important. The examination of correlations between ζ^max_z*=30m^(5min) and several near-tornado mesoscale factors affecting the stretching of vorticity shows that vertical vorticity of the mesocyclone at about 1 km AGL and water vapor near the surface are particularly important: The former factor strengthens the nonlinear dynamic vertical perturbation pressure gradient force below 1 km AGL, and the latter the buoyancy force through lowering of the lifted condensation level. These two upward forces intensify the storm-scale updraft, which in turn strengthen stretching of vorticity and contribute to supercell tornadogenesis.