Ensemble Kalman Filter Analyses of Internal Rear-Flank Downdraft Momentum Surges within the 18 May 2010 Dumas, Texas Supercell

Several platforms participating in the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) observed a series of four internal rear-flank downdraft (RFD) momentum surges coincident with the development and decay of a low-level mesocyclone in a supercell near Dumas, Texas on 18 May 2010. Analysis of fine-scale dual-Doppler data from two Ka-band mobile Doppler radars operated by Texas Tech University (TTUKa) coupled with volumetric single-Doppler data collected by the X-band, phased array MWR-05XP mobile Doppler radar reveals that the second through fourth internal RFD surges develop in a region of inferred downward-directed vertical perturbation pressure gradient forcing, suggesting that they may be the surface manifestation of an occlusion downdraft.

The observational analyses are extended through data assimilation of Doppler radar data at X-band (DOW), C-band (SMART-R) and S-band (WSR-88D) with an ensemble Kalman filter (EnKF). A suite of three ensemble analyses have been produced, using varying microphysical parameterizations to address uncertainty in the thermodynamic environment within the Dumas supercell. Each of the analyses produces a representative three-dimensional wind field capturing the development and decay of the low-level mesocyclone and internal RFD surges, though with varying cold pool intensity. As the assimilation of radial velocity values does not update the perturbation pressure field in the numerical simulation, three-dimensional perturbation pressures have been dynamically retrieved from the simulated wind and buoyancy fields, allowing the relative contributions of dynamic and buoyancy forcing to the development of the internal RFD surges to be assessed.