Structure and dynamics of axisymmetric tornado-like vortices simulated with a semi-slip lower boundary

No-slip boundary conditions are often employed in idealized axisymmetric models to study tornadoes. These boundary conditions result in a poor representation of near-ground winds, which are of crucial importance for tornadoes. In this study, the boundary layer of tornadoes is investigated using more realistic semi-slip conditions as the lower boundary of axisymmetric, idealized simulations. The drag law formulation of semi-slip conditions introduces the drag coefficient Cd as a control parameter, alongside the swirl ratio Sr (related to the system’s rotation) and a Reynolds number (describing diffusive effects) already employed in previous studies employing no-slip conditions. The exploration of a wide range of Cd values shows that the analytical two-tiered potential-vortex boundary layer is preserved under semi-slip conditions. The lower tier becomes shallower as Cd decreases, finally vanishing for Cd = 0.001. The drag coefficient plays a fundamental role in determining the structure of the vortex. A decrease in Cd causes the same one-celled to two-celled transition previously observed for an increase of Sr under no-slip conditions. For the range of swirl ratios and Reynolds number used in the present study, a decrease in Cd leads to the intensification of the surface inflow and a reduced dissipation of angular momentum of parcels advected towards the center of the vortex. This intensification for decreasing Cd occurs only for subcritical vortices, thus the effect appears dependent on vortex structure and corner flow swirl ratio.