Monday, 5 November 2012: 11:45 AM
Symphony I and II (Loews Vanderbilt Hotel)
A high-resolution simulation of the 8 May 2003 OKC tornadic supercell is examined in order to reveal the dynamical processes behind tornadogenesis. The high-resolution simulation, which has 50 m horizontal grid spacing and a stretched vertical coordinate (dzmin = 20 min) , is nested within three lower-resolution simulations. The two outermost grids have horizontal grid spacing of 9 km and 1 km, respectively, and are initialized with high-frequency (5 min) data assimilation cycles in which radar and other conventional observations are assimilated via the Advanced Regional Prediction System (ARPS) 3DVAR and complex cloud analysis. The 1-km simulation provides boundary and initial conditions to a 100-m simulation. This 100-m simulation provides boundary and initial conditions to the 50-m simulation. Comparison between the 100-m and 50-m simulations show that both produce a supercell that spawns vortices of tornadic strength very near the observed tornado damage path. In this presentation, we focus on the 50-m simulation because it captures the evolution of these tornadoes with greater detail. Examination of the 50-m simulation reveals the development of two tornadoes associated with the simulated supercell. The first tornado forms as an internal outflow surge merges with a small circulation near the rear-flank gust front (RFGF) occlusion point. The tornado intensifies rapidly and produces maximum wind speeds exceeding 70 m s-1. About 4 min after its formation, the tornado weakens dramatically before re-organizing about 1 km further to the north. The tornado then persists for ~ 6 min. The second tornado forms along the RFGF and persists for less than 3 min. Preliminary results suggest that the two tornadoes were formed in substantially different ways. Internal outflow surges and a low-level updraft/mesocyclone appear important in the genesis first tornado, while the second tornado appears to form initially in the supercell's inflow region. More detailed and most recent results of our analysis will be presented.
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