On the dynamic structure of a tornado vortex near the ground

Abstract

The dynamic structure of a tornado vortex near the ground has been investigated by numerically solving the anelastic system of hydrodynamic equations with appropriate boundary conditions. The vertical and tangential velocity fields as observed for the lower 850 ft of the Dallas tornado of 2 April 1957 were used as input data into the equation system. Numerical solutions have been obtained for the radial velocity field, the velocity acceleration fields, and the deviations of pressure and potential temperature from basic state values. The results indicate that the airflow in the Dallas tornado was characterized by a very shallow, but strong, region of inflow close to the ground capped by a deeper layer of outflow above. The pressure field showed an intense radial gradient between 100 and 200 ft from the axis and a strong vertical gradient near the ground along the axis. In addition, a small region near the ground and close to the axis of the tornado was found to be the center of maximum velocity shear and accelerations (decelerations) for all three components of velocity. An accurate solution for the distribution of potential temperature could not be found for the entire region due to the accumulation of truncation error in the finite-difference approximations to the derivatives used in the solution procedure.