A numerical study on the evolution of tornadoes

Typescript (photocopy).The evolution of the thermal and dynamic structure of tornadoes is studied via numerical simulation. The model, which is axisymmetric, includes prognostic equations governing motion, a diagnostic pressure equation, and prognostic equations for potential temperature, water vapor, and liquid water. Condensation processes are included so as to provide a physically realistic driving mechanism for the tornado. Subgrid turbulence is parameterized by a deformation-dependent eddy viscosity. The numerical domain is 1 km x 1 km with a grid resolution of 12.5 m. The lateral and top boundaries are open to mass flow. The equations are solved with the aid of quadratic-conservative differences in space and Adams-Bashforth differences in time. Seven major experiments are performed of which the first six simulate tornado development in the presence of constant ambient circulations with differing surface roughness and vertical stability near the ground. In the last experiment the early stages of tornado demise are simulated by suddenly reducing the ambient circulation. The numerical simulations show that frictional convergence, induced by an initial field of rotation, forces air upwards through the conditionally-unstable model atmosphere. This leads to condensation and the release of buoyancy, which enhances the upward motion and maintains the frictional convergence in a manner similar to the CISK mechanism in large-scale flows of the tropics. Angular momentum transported inwards by the frictionally-driven inflow increases the tangential velocity. Correspondingly, the central pressure lowers, and a condensation funnel extends downward from the base of the parent cloud. Coupling between the tangential and vertical velocities, through the pressure gradient, gives rise to strong vertical accelerations near the ground. The resulting updraft is a maximum at the axis over a rough surface, while over a ground of moderate roughness the maximum is annular, with weak velocities near the axis. The study revealed other structural features, such as vortex breakdown, reversed axial flow, Bodewadt oscillations, inertial and gravity waves, toroidal vortices, shear eddies and downburst