Identification of downdraft generated near-surface vertical vorticity using TTUKa radar data and idealized numerical simulations

Prior research breaks tornadogenesis into a three-step process. A mesocyclone forms aloft, near-surface rotation develops, and concentration/stretching of near-surface rotation can lead to intense surface vortex development. Stage two, the development of near-surface rotation remains an area of research. As shown in Parker and Dahl (2015), sole downdrafts within a background flow field will produce near-surface vertical vorticity via the Davies-Jones and Brooks mechanism (DJB93). Prior research tends to be focused on supercells while investigating near-surface vertical vorticity production mechanisms. However, the results of Parker and Dahl (2015) would suggest that the DJB93 mechanism of near-surface vorticity generation be similar across various storm modes. Using CM1 version 19, both an idealized supercell and an observed sounding simulation was conducted. Near-surface vertical vorticity was examined for each simulation. Additionally, an observational TTU Ka-radar data set from June 15th, 2016 was analyzed. Horizontal wind retrievals from the radar data was done with SingleDop, a radar software package. Across both simulations and the observational data, non-zero near-surface vertical vorticity was found. Various near-surface vortices and vorticity river like features were seen emanating from the periphery of low-level downdrafts. These vorticity features were concentrated along internal convergence boundaries, propagated downstream, and merged into tornado like vortices (TVS) or individual vortex features. Similar results are seen across both supercell and multi-cell/linear storm environments. Simulation and observational data seem consistent with the DJB93 mechanism for near-surface vertical vorticity production regardless of storm mode.