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Ensemble forecast experiments on tornadoes that occurred in Tsukuba on May 6, 2012

 Experiments were conducted by changing the initial value of a cloud resolution numerical model to forecast tornadoes. For the experiments, a two-way nesting system using a Local Ensemble Transform Kalman Filter (LETKF), a leading-edge data assimilation technique, was developed. Figure 1 shows a tornado developmental distribution obtained from the ensemble forecast by LETKF at the time the strongest tornado ever recorded in Japan landed around Tsukuba and caused enormous damage on May 6, 2012. Using a 350m resolution model, 10 out of 12 members showed cyclonic circulation with very strong vertical vorticity of 0.1 (1/s) at 3 locations corresponding to the actual observations. Even strong winds exceeding 50m/s were represented in the high resolution experiments with a 50-m grid using the K computer (Figure 2). These important results will contribute to probabilistic forecasts specifying locations tornadoes might occur.