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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.

Flows in a cylindrical tank over a rotating bottom are investigated by laboratory experiments. Despite the axisymmetry of the experimental setup, various anisotropic phenomena are observed. The slow rotation of the bottom disk induces a circular flow according to the axisymmetric environment, but polygonal vortices form under faster rotation. Between these two vortex flow states, the flow undergoes a transition with clear hysteresis during which the elliptical shape assumed under faster rotation is retained when the rotation is subsequently slowed to rates that previously supported axisymmetric flow. Sloshing is also observed; here, a calm circular flow state alternates with an oscillation of the water surface along the sidewall of the container. A phase diagram showing the phenomena observed under different combinations of the initial water depth at rest and the rotation rate of the bottom disk is developed following thorough experimental testing over a wide range of parameter values. The features of the dependences of the range for each phenomenon to occur on these parameters are also elucidated.

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To improve short‐term rainfall forecasts through direct assimilation of radar reflectivity, atmospheric variables associated with rainfall should be modified based on their correlation with reflectivity. However, it is difficult to estimate such correlations. The ensemble Kalman filter can estimate the correlation by means of ensemble forecasts, although the estimation is limited to when rainfall is forecast by at least one member at analysis points. To assimilate reflectivity effectively even at points at which no rainfall is forecast, we suggest adding ensemble reflectivity perturbations, which are correlated with atmospheric variables, before ensemble Kalman filter assimilation. In the present study, this correlation is calculated in the whole computational domain including the rainfall regions. We apply this procedure to assimilation experiments with 1‐km horizontal grid interval for two tornadic supercells that occurred on 6 May 2012 and on 2 September 2013, and we succeed in improving short‐term rainfall forecasts by modifying wind, temperature, and water vapor.