It is recognized in the fluid mechanics of swirling flows that distinctly different vortex structures can occur beneath a single rotating updraft. Such structures are commonly observed in tornadic thunderstorm events as well as in laboratory simulations of tornado-like vortex flows. Multiple vortex (MV) structures have been simulated in the laboratory with great success. However, laboratory simulations did not simulate different MV scales simultaneously, as observed in thunderstorm and tornado systems.

This study proposes that MV structures in thunderstorms and tornadoes occur on three clearly distinct and different spatial and temporal scales. The largest scale of MV phenomena is the mini-tornado cyclone, defined as two or more local vorticity maxima embedded within a mesocyclone that revolve about a common central axis. This scale can be identified by doppler velocity fields and sometimes by two or more visible rotating wall clouds within a mesocyclone. If tornadoes should form from each vortex center this results in the parallel mode tornado family. The intermediate scale of MV phenomena is multiple vortex columns that extend beneath a single wall cloud, collectively composing a single tornado event (defined as a multiple vortex tornado, T_m). For clarification, a single vortex tornado, denoted as T_s, is a tornado composed of a single vortex column. The smallest scale of MV phenomena is suction debris vortices. These intense vortices form in the surface boundary layer and revolve around the base of a given tornadic column, whether it be the vortex column of a T_s or any given vortex column of a T_m event. A classification tree is presented for all conceivable MV events and observational examples for each scale are discussed.