A Numerical Simulation of Multiple Vortices

It often occurs that tornadoes contain smaller subsidiary vortices which revolve about the tornado, which in turn, rotate in the same sense as the tornado (see FUJITA, 1970; FORBES, 1978). This phenomenon goes by various names (“suction vortices”, “satellite vortices”, and “secondary vortices”), however, for this work we follow CHURCH et al. (1979) and refer to the multiple vortex phenomenon (MVP). So, when we speak of the MVP we refer to that type of “suction vortex” which FUJITA (1976) has termed the “orbiting vortex”. That the MVP may be more than a minor detail of the tornadic flow is suggested by damage surveys which indicate the most intense destruction of life and property is associated with cycloidal paths which FUJITA has termed “suction swaths”. As with the tornado, very little is known about the internal circulation of the MVP. Photogrametric and ground survey data are inconclusive on such important questions as to i) what maximum wind speed is achieved by the tornado and ii) the relation between this and the MVP. MVs are clearly visible in photographs but it is extremely difficult to infer actual flow patterns.