Baroclinic and Barotropic Vorticity in Simulated Near-surface Mesocyclones

Tornadogenesis in supercells has been described as three-stage process, involving i) mid-level mesocyclogenesis, ii) development of near-surface rotation, and iii) concentration of the near-surface rotation into tornadic strength. This study is concerned with the second stage, one of the main questions being whether the source of vorticity in the near-surface mesocyclones is in-situ generated ("baroclinic") vorticity, or merely reoriented ambient ("barotropic") vorticity. The authors use 3D vorticity dynamics in a Lagrangian framework to analyze the origin of near-surface rotation in simulated supercells. The technique employed in this study allows for a separation of barotropic and baroclinic components. It is found that the development of surface vertical vorticity is rather unsteady and tied to surges of horizontal momentum emanating from downdrafts adjacent to the main updraft. The vorticity at the flanks of these surges is of baroclinic origin and horizontal initially, being reoriented into the vertical while the air is still descending. Ambient (barotropic) vorticity is also reoriented in the downdraft but contributes to anticyclonic rotation in the near-surface mesocyclone. However, the baroclinic vorticity dominates, leading to net cyclonic near-surface rotation.