Cold Pool Forcing of the Streamwise Vorticity Current and Low-Level Mesocyclone in the 9 June 2009 Supercell during VORTEX2

This case study analyzes a nontornadic supercell observed on 9–10 June 2009 in southwest Kansas during the Verification of the Origins of Rotation in Tornadoes Experiment 2 (VORTEX2). Time-series multi-Doppler radar analyses and diabatic Lagrangian analysis retrievals document the kinematic and thermal–microphysical evolution of the storm’s strengthening low-level mesocyclone during the period 2342–2351 UTC, an apparent “tornadogenesis failure” event around 2351 UTC, and subsequent storm decay through 0024 UTC. An analyzed current of low- and midlevel streamwise vorticity enters the supercell updraft, appearing similar to the streamwise vorticity current (SVC) identified in previous supercell simulations. However, the present SVC primarily feeds into the midlevel updraft and mesocyclone, with relatively limited inflow to the low-level occlusion updraft and mesocyclone. Lagrangian vector vorticity dynamical calculations demonstrate that baroclinity, differential hydrometeor loading, and horizontal stretching all play significant roles in the generation and amplification of streamwise vorticity associated with this SVC. The origins of concentrated vertical vorticity in this mature low-level mesocyclone are consistent with off-trajectory separation of streamwise vorticity in downdraft previously identified in supercell simulations. However, the off-trajectory vorticity vector displacement in the present case is forced by differential hydrometeor loading instead of thermal gradients, since the cold pool is penetrating the low-level mesocyclone core. Another unique feature of this study is the detailed validation of surface radar-derived airflow and retrieved thermal fields with observations from a dense array of surface in situ measurement platforms in the storm.