Multiple-vortex tornado

Abstract: Three characteristic features of tornadoes are simulated in a laboratory system and the associated flow is observed and discussed. These are (i) a characteristic surface pressure profile, (ii) a bulging deformation on the vortex core, and (iii) multiple vortices in a single convergence system. Vortex motion is very sensitive to the geometrical features of the larger flow in which it is imbedded. Only when the diameter of the updraft column exceeds the depth of the inflow layer can features (i) and (iii) be produced in the present model. When the updraft diameter is large compared to the depth of inflow, inertial effects associated with large changes in radial momentum produce significant convergent forces. When the updraft diameter is small compared to depth of inflow layer, the inflow speed is relatively small and related inertial effects are small. It is concluded that radial momentum flux is an important factor in the production of atmospheric vortices.

Abstract: This paper describes the Verification of the Origins of Rotation in Tornadoes Experiment planned for 1994 and 1995 to evaluate a set of hypotheses pertaining to tornadogenesis and tornado dynamics. Observations of state variables will be obtained from five mobile mesonet vehicles, four mobile ballooning laboratories, three movie photography teams, portable Doppler radar teams, two in situ tornado instruments deployment teams, and the T-28 and National Atmospheric and Oceanic Administration P-3 aircraft. In addition, extensive use will be made of the new generation of observing systems, including the WSR-88D Doppler radars, demonstration wind profiler network, and National Weather Service rawinsondes.

Abstract: The mature and dissipating stages of a strong tornado were observed from close range by the prototype Doppler On Wheels mobile radar. Volumetric observations repeated eight times over an 840-s period with resolution volumes at the center of the tornado as low as 61 m × 61 m × 75 m = 2.8 × 105 m3 revealed new details about three-dimensional tornado vortex structure and evolution. Observed structures included a conical debris envelope, a low-reflectivity eye, multiple windfield maxima, and multiple semiconcentric bands of reflectivity surrounding the eye. The three-dimensional structure of the debris and single-Doppler wind field were well characterized, as well as more rapid dissipation of the tornado aloft compared to near the ground. Volumetric measures of tornado strength are introduced. A downdraft exhibiting w ∼ −30 m s−1, indicative of a partial two-cell vortex, was observed only during the earliest radar scans when the tornado was near maximum intensity. Comparisons with simple conceptual m...

Abstract: High-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado vortex with the surface were performed in an attempt to answer questions about the character of turbulent transport in this unique flow. The authors demonstrate that sufficient resolution was achieved for the particular physical conditions of their example that the time-averaged velocity and pressure distributions showed little sensitivity in the region of maximum velocities to either finer resolution or modified subgrid turbulent model. The time-averaged velocity distributions show the maximum velocity values occurring within 50 m of the surface. The instantaneous velocity distributions show the turbulence dominated by a relatively small number of strong secondary vortices spiralling around the main vortex with the maximum instantaneous velocities typically one-third larger than the maximum time-averaged velocity. These eddies are centered a little inside of the cone of maximum mean swirl velocity and spiral around the mean vortex at velocities less than the average maximum velocity. Statistical analysis of the velocity fluctuations induced by the secondary vortices shows that the turbulent transport of angular momentum is predominantly inward at low levels, allowing the inner recirculating flow to acquire values of angular momentum of up to 30% of that provided by the inflow boundary conditions, thus enhancing the surface intensification of the velocities.