Tornadogenesis in a Simulated Mesovortex within a Mesoscale Convective

33 34 The ARPS model is used to simulate a tornadic mesovortex with the aim of 35 understanding the associated tornadogenesis processes. The mesovortex was one of two tornadic 36 mesovortices spawned by a mesoscale convective system (MCS) that traversed southwest and 37 central Oklahoma on 8-9 May 2007. The simulation used 100-m horizontal grid spacing, and is 38 nested within two outer grids with 400-m and 2-km grid spacing, respectively. Both outer grids 39 assimilate radar, upper air, and surface observations via 5-min 3DVAR assimilation cycles. The 40 100 m grid covers a 50 km x 60 km domain with 20-m vertical grid spacing at the surface, and is 41 initialized from a 40-min forecast on the 400 m grid. 42 Results from the 100-m simulation provide a detailed picture of the development of a 43 mesovortex that produces a sub-mesovortex-scale tornado-like vortex (TLV). Closer 44 examination of the genesis of the TLV suggests that a strong low-level updraft is critical in 45 converging and amplifying vertical vorticity associated with the mesovortex. Vertical cross-46 sections and backward trajectory analyses from this low-level updraft reveal that the updraft is 47 the upward branch of a strong rotor that forms just northwest of the simulated TLV. The 48 horizontal vorticity in this rotor originates in the near surface inflow and is caused by surface 49 friction. An additional simulation with surface friction turned off does not produce a rotor, strong 50 low-level updraft, or TLV. Comparison with previous two-dimensional numerical studies of 51 rotors in the lee of mountains shows striking similarities to the rotor formation presented herein. 52 A four-stage conceptual model for tornadogenesis in this case is proposed. 53 54 2