Contemporary, high-resolution numerical simulations replicating the fluid flow within the atmospheric tornadoes employ the axisymmetric Navier Stokes equations and user-defined domain and boundary conditions. The model of the tornado vortex can be differentiated into four regions: (1) the core and outer flow, which comprises the convective region in the tornado above a few hundred metres from the ground surface; (2) the boundary layer, which makes up the flow regime at low levels in the vicinity of the vortex core, and is strongly influenced by the ground surface; (3) the corner flow region that consists of the flow in the vortex core at low levels, where the air in the boundary layer converges and is directed upwards as a vertical jet; and (4) the upper flow, which represents the conditions with which the tornado is connected to the parent mesocyclone.
The maximum windspeeds in the most intense tornadoes have been estimated to be in the range of 110-125ms-1, and found close to the ground in the vicinity of the axis of rotation. In addition, the cyclostrophic nature of the tornado core results in steep pressure gradients, with the maximum pressure deficit possibly exceeding 100mb in the strongest tornadoes.
The structure and size of the tornado vortex are strongly influenced by the proportion of flow though the boundary layer flow to that above it, quantitatively determine by the swirl ratio S, which in the context of numerical and laboratory experiments is physically determined by the tangential wind velocity of the inflow divided by the vertical velocity of the exhausting air at the domain ceiling. As the swirl ratio is increased from zero, the tornado vortex becomes wider and more defined. At a critical value of S, vortex breakdown occurs along the axis of rotation at a certain altitude above the ground, where the flow transitions from laminar (upstream) to turbulent (downstream), appearing as a pronounced bludge in the vortex core. A larger value of S lowers this vortex breakdown to the surface, where it is called a Drowned Vortex Jump. Still higher values results in the formation of a vortex pair, comprising two secondary vortices orbiting on opposite sides of the parent tornado. Still larger values of S results in the formation of a third vortex, and so on.