It is well known that the vast majority of tornadoes are accompanied by supercell storms. To prevent tornado disasters, it is important to elucidate the mechanism that governs tornadogenesis associated with supercells.
Most previous numerical studies do not include the influence of horizontal inhomogeneity of the atmosphere on tornadogenesis. In other words, they have investigated the behavior of a supercell embedded within horizontally homogeneous base states. However, there is no doubt that such a situation is not always the case in the real atmosphere. In particular, tornadic supercells are often accompanied by another preexisting convection, which makes outflow boundaries near the surface. In addition, tornadoes are frequently observed near such boundaries. The aim of this study is to elucidate the influence of atmospheric inhomogeneity on tornadogenesis associated with supercells by idealized numerical experiments.
We apply a restart technique. After conducting the control simulation of a supercell in a homogeneous base state, we add a perturbation field representing an outflow boundary to the domain, and then restart the calculation. As the outflow boundary arrives at the simulated supercell, one of surface vortices is significantly intensified. A vorticity-budget analysis reveals that the amplification of the surface vortex is caused by vortex stretching associated with the enhancement of horizontal convergence as the boundary arrived. Thus, preexisting outflow boundaries play an important role in tornadogenesis.