Internal Momentum Surges in a High-Resolution Simulation of the 8 May 2003 Oklahoma City Tornadic Supercell

A high-resolution simulation of the 8 May 2003 Oklahoma City tornadic supercell is examined to determine the origin of multiple internal momentum surges behind the primary rear-flank gust front. The simulation has 50-m grid spacing and is nested within three coarser domains. The Advanced Regional Prediction System (ARPS) 3DVAR and complex cloud analysis is used to assimilate conventional and radar data on the two coarsest grids. Examination of the simulation reveals that one of the internal momentum surges appears to trigger tornadogenesis. A decomposition of the vertical momentum forcing along backward trajectories that are initialized within this internal surge shows that the downdraft responsible for the surge is predominantly dynamically forced. A closer look at model fields reveals a persistent (but unsteady in magnitude) area of high perturbation pressure on the west side of the simulated low-level mesocyclone. This area of high-pressure is the result of stagnation between the mesocyclone flow and the environmental flow impinging from the west. Air parcels are forced downward as they encounter this stagnation zone. Unsteadiness in the mesocyclone strength and flow configuration leads to downdraft pulses which are responsible for the internal momentum surges.