Tornadogenesis and Tornado Dynamics as Revealed by Ultra-high-resolution Numerical Simulations of a Tornadic Thunderstorm

Due to their anomalous computational requirement, numerical simulations of tornadoes have so far been limited to idealized simulations without the parental thunderstorm, or the simulation of tornado vortex with parental storm but without resolving the internal structure of tornado itself. In the latter case, all past simulations used either nested grids or local grid refinement. Recent development in computational technology and improved parallel capability of numerical models has made it possible (though still challenging) to resolve parental storm and internal structure of tornado using uniform resolution grid.

A set of successful numerical simulations of tornadoes embedded within a supercell storm have has recently completed. The simulations used the ARPS (Advanced Regional Prediction System) and up to 2048 Alpha processors at the Pittsburgh Supercomputer Center. A tornado with surface ground-relative wind speeds of 120m/s and a pressure drop of over 80mb was obtained in these simulations. Uniform 25m horizontal and 20m near-surface vertical resolutions were used, with a grid of about 2000x2000x83 grid points. The use of uniform resolution grid large enough to contain the entire parental storm eliminates uncertainties associated with grid nesting or local refinement.

The structure and evolution of the simulated tornadoes are carefully examined. The sources of low-level rotation are studied via time-dependent trajectory analysis and diagnostic calculations along the trajectories. An improved conceptual model of supercell tornadogenesis is proposed. A very realistic 3D volume rendering of the supercell storm will be shown at the symposium.

The sensitivity of tornadogenesis to the microphysical processes and the assumed parameters in the drop size distributions will also be examined, and an understanding of the behaviors is sought through the understanding of tornadogenesis dynamics.