Idealized Numerical Experiments on Tornadogenesis in Horizontally Heterogeneous Environments

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. Tornadic supercells are often accompanied by preexisting convection, which makes outflow boundaries near the surface. In addition, tornadoes are frequently observed near such boundaries. The objective of this study is to investigate the influence atmospheric inhomogeneity on tornadogenesis associated with supercells by preforming idealized numerical experiments.

A restart technique is used. After conducting the control simulation of a supercell in a homogeneous base state, wind/temperature perturbations representing an outflow boundary of preexisting convective systems to the domain, and then the calculation is restated. 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.