XXIII General Assembly of the International Union of Geodesy and Geophysics (IUGG2003)

Simulation Experiment of Tornado Associated with Supercell using the Cloud Resolving Storm Simulator

Kazuhisa TSUBOKI and Atsushi SAKAKIBARA


We have developed a cloud-resolving numerical model which was named theCloud Resolving Storm Simulator (CReSS). In this paper, characteristicsof CReSS will be introduced and results of simulation experiment oftornado which occurred within a supercell will be presented.

CReSS is formulated based on the non-hydrostatic and compressibleequation system with terrain-following coordinates. Prognostic variablesare three-dimensional velocity components, perturbations of pressure andpotential temperature, subgrid-scale turbulent kinetic energy (TKE), andcloud physical variables. A finite difference method is used for thespatial discretization and the mode-splitting technique is adapted fortime integration. Cloud physics is formulated by a bulk method of coldrain with variables of mixing ratios and number densities of watersubstance. The turbulence is parameterized by the 1.5 order closer withTKE. Parallel processing is performed by the Massage Passing Interface (MPI).

Three severe tornadoes occurred in the Tokai District, the central part ofJapan, on 24 September 1999. Observation showedthat their width was several hundred meters and their rotation wascyclonic. The Doppler radar of Nagoya University observed the parentmesoscale convective systems (MCSs) of the tornadoes and found characteristics of supercell in the MCSs: a hook-shaped echo and a bounded weak-echo region. Vorticity of themeso-cyclones within the MCSs estimated from the Doppler velocity was anorder of 0.01/s.

The simulation was aiming at resolving the vortex of the tornado withinthe supercell. The computation domain was about 45 km in horizontal with agrid size of 75 m and 12.5 km in vertical with the smallest gird size of 25 m. The basic field was give by a sounding at 00 UTC, 24 September 1999. After about one hour from the initial time, aquasi-stationary supercell was simulated. An intense updraft occurredalong the surface flanking line. At the central part of the updraft, atornadic vortex was formed after 90 minutes from the initial time. Thesimulated tornado was successively developed and maintained at thecentral part of the strong updraft. The diameter of the vortex was about 500 m and the maximum of vorticitywas about 0.5 /s. These characteristics correspond to the observedtornado. The central pressure perturbation of -12 hPa was in the cyclostrophicbalance with the rotating flow of the tornado.

In this simulation experiment, both the supercell and the tornado weresimulated in the common uniform grids. The tornado was produced purelyby the dynamic process formulated in CReSS. Detailed analyses of thesimulated data showed the mechanism of the tornado genesis within thesupercell.


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