Important Factors for Tornadogenesis as Revealed by High-Resolution Ensemble Forecasts of the Tsukuba Supercell Tornado of 6 May 2012 in Japan

Abstract

To identify important factors for supercell tornadogenesis, 33-member ensemble forecasts of the supercell tornado that struck the city of Tsukuba, Japan, on 6 May 2012 were conducted using a mesoscale numerical model with a 50-m horizontal grid. Based on the ensemble forecasts, the sources of the rotation of simulated tornadoes and the relationship between tornadogenesis and mesoscale environmental processes near the tornado were analyzed. Circulation analyses of near-surface, tornadolike vortices simulated in several ensemble members showed that the rotation of the tornadoes could be frictionally generated near the surface. However, the mechanisms responsible for generating circulation were only weakly related to the strength of the tornadoes. To identify the mesoscale processes required for tornadogenesis, mesoscale atmospheric conditions and their correlations with the strength of tornadoes were examined. The results showed that two near-tornado mesoscale factors were important for tornadogenesis: strong low-level mesocyclones (LMCs) at about 1 km above ground level and humid air near the surface. Strong LMCs and large water vapor near the surface strengthened the nonlinear dynamic vertical perturbation pressure gradient force and buoyancy, respectively. These upward forces made contributions essential for tornadogenesis via tilting and stretching of vorticity near the surface.

Tornado-Resolving Ensemble and Probabilistic Predictions of the 20 May 2013 Newcastle–Moore EF5 Tornado

“…Finally, we note that the 50-m ensemble dataset produced in this study could be used for diagnostic analyses aimed at better understanding tornadogenesis dynamics, as was done in the most recent study of Yokota et al (2018) for their ensemble forecasts at 50-m grid spacing of tornadoes occurring within a supercell storm over Japan. In future work, we plan to perform diagnostic analyses on the ensemble simulations for the understanding of tornadogenesis dynamics, following the approaches of Yokota et al (2018) and other recent dynamical studies of tornadoes and tornadogenesis processes (e.g., Schenkman et al 2014;Dawson et al 2016;Roberts et al 2016;Roberts and Xue 2017). Other possible future work using this dataset will include using the vortex wind analysis method of Q.…”

“…We also note that, because the 500-m run cannot explicitly resolve tornadoes, there appears not to be a direct relation between the simulated tornado on the 50-m grid and the simulated supercell characteristics on the 500-m grid. Investigating which aspects of the parent storm determine the characteristics of embedded tornado is a separate research question related to both tornado dynamics and predictability; for example, Coffer et al (2017) and Yokota et al (2018) found that the vertical perturbed pressure gradient force resulting from the mesocyclone to be vital for tornadogenesis, and Roberts and Xue (2017) found that the ingestion of frictionally generated vorticity into the low-level mesocyclone led to lowering of the mesocyclone circulation and the creation of a strong upward pressure gradient force, ultimately contributing to tornadogenesis. Investigating these topics will require a separate study.…”

“…More recently, Mashiko (2016a, b) obtained reasonable simulations for a supercell tornado of F3 intensity that occurred in Tsukuba, Japan, using 50-m grid spacing, again starting from operational mesoscale analysis. Most recently, Yokota et al (2018) produced ensemble forecasts at 50-m grid spacing for tornadoes occurring within a supercell storm over Japan, focusing on tornadogenesis dynamics.…”

“…The tornado-resolving simulation studies discussed above have, with the exception of Yokota et al (2018), all been based on deterministic prediction of tornadoes or tornado-like vortices, and have primarily focused on investigation of processes responsible for tornadogenesis within individual tornado forecasts. While deterministic forecasts can provide useful insights into dynamical and thermodynamic processes occurring within the model storm, Warn-on-Forecast predictions for tornadoes will require an ensemble-based probabilistic approach to forecasting in order to properly characterize the uncertainty that tends to grow very fast in forecasts at the subkilometer scale (Lorenz 1969).…”

Horizontal Vortex Tubes near a Simulated Tornado: Three-Dimensional Structure and Kinematics

“…An analysis of the videos presented in Figure 5 also shows that the observed vortices in the Tuscaloosa tornado (as well as other tornado events, e.g., [7][8][9]) consistently display the same behavior. This suggests that HVs near the surface have horizontal vorticity vectors that point to the left of the horizontal wind vector at large angles, implying considerable crosswise horizontal vorticity [11,13,14,52]. This can be seen in Figure 8a, which shows the horizontal vorticity field at 1 m and 158 m AGL.…”