A possible mechanism of tornadogenesis associated with the interaction between a supercell and an outflow boundary without horizontal shear

Using the insert-restart method developed in this study, tornadogenesis processes associated with the interaction between a supercell and an outflow boundary were investigated. A highly idealized outflow boundary promoted surface vortex intensification in a preconditioned supercell having a strong surface vortex. In particular, a tornado-like vortex was observed in an experiment with a moderate outflow boundary. This intensification was associated with the enhancement of the near-surface horizontal convergence by the outflow boundary. The optimal coldness of the outflow boundary was explained by the balance between the enhancement of the near-surface horizontal convergence and the buoyancy reduction by the outflow boundary. Sensitivity experiments demonstrated that, despite a less favorable environment, a storm with a boundary interaction might reach a similar maximum near-surface vorticity as a storm without a boundary in a more favorable environment.