Friday, 28 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
Exact solutions of the Navier-Stokes or Euler equations of motion and the continuity equation in cylindrical coordinates for three-dimensional axisymmetric flows are utilized to represent evolving vortices that roughly model tornado cyclones or misocyclones contracting to tornadoes. These solutions are unsteady versions of the diffusive Burgers-Rott vortex and the inviscid Rankine combined vortex. Different vortices are obtained by choosing different values of the constant eddy viscosity and uniform horizontal convergence while holding the circulation at infinity constant. A simulated WSR-88D radar is employed to generate time-varying Doppler velocity signatures of these analytical vortices at ranges of 25 and 50 km from the radar. Mean Doppler velocities are determined by computing 3D integrals over effective resolution volumes. Magnitudes of Doppler vortex signatures at different times in the evolution of the vortices are computed for effective beamwidths of 1.02º and 1.39º, which correspond to azimuthal sampling intervals of 0.5º and 1.0º, respectively. Four tornado predictors, rotational velocity, shear, excess rotational kinetic energy, and circulation, are examined.
Results of the simulations show that for smaller effective beamwidths, Doppler vortex signatures are stronger and exceed fixed threshold values of rotational velocity and shear earlier. With finer azimuthal resolution, tornado-cyclone, misocyclone, or tornado signatures switch to tornadic vortex signatures later. Circulations of the vortex signatures give good estimates of the circulations of the simulated tornadoes and tornado cyclones with relative insensitivity to range, effective beamwidth, and stage of evolution. All the other predictors increase significantly during tornadogenesis and so reveal the potential for tornadoes later than circulation does.
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