Radar and Thermodynamic Analysis of the 6 April 2018 Monroe, LA Tornadic Supercell

Abstract

This case study analyzes a tornadic supercell observed in northeast Louisiana as part of the Verification of the Origins of Rotation in Tornadoes Experiment Southeast (VORTEX-SE) on April 6—7 2018. Two mobile research radars (SR2 and SR3), one WSR-88D equivalent (KULM) and two airborne radars (TAFT and TFOR) sampled the storm at close proximity for ~70 minutes through its mature phase, tornadogenesis at 2340 UTC, and dissipation and subsequent ingestion into a developing MCS segment. The 4-D wind field and reflectivity from up to five-Doppler analyses every five minutes, combined with 4-D Diabatic Lagrangian Analysis (DLA, Ziegler 2013a,b) retrievals, enabled kinematic and thermodynamic analysis of storm-scale boundaries leading up to, during, and after the dissipation of the 13 minute-long EF 0 tornado. Additional near-storm thermodynamic measurements from the Compact Ramen Lidar (CRL), a P-3 aircraft-mounted downward-pointing lidar which profiles boundary layer water vapor mixing ratio and temperature, were compared to far-field proximity soundings to provide an accurate representation of the storm inflow environment. Trajectory analysis using the DLA reveals that ambient environmental low-level vertical vorticity was present in the inflow region, and additional low-level vertical vorticity appeared to be generated by the shearing zone between the Rear-Flank Gust Front (RFGF) and inflow at the location of tornadogenesis. Baroclinically-generated horizontal vorticity which was tilted into vertical by downdrafts did not appear to be a significant source of vorticity for the tornado. The kinematic and thermodynamic analysis also reveal a transient current of baroclinically-generated low-level streamwise vorticity leading into the low-level supercell updraft, appearing similar to the Streamwise Vorticity Current (SVC) that has been identified in supercell simulations and observed only kinematically previously. Although the SVC did not directly feed streamwise vorticity to the tornado-cyclone, its development coincided with tornadogenesis. The evolution of the supercell’s updraft and its induced surface boundaries were investigated in the context of its unique vertical thermodynamic profile and hodograph compared to most previous observations and simulations based on Central Plains supercells. Although the mesoscale environment was not high-shear/low-CAPE, the Monroe supercell shared many similarities to such storms due to meager temperature lapse rates aloft which are commonplace in southeast severe convection events.