Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*

Abstract

The authors use a high-resolution supercell simulation to investigate the source of near-ground vertical vorticity by decomposing the vorticity vector into barotropic and nonbarotropic parts. This way, the roles of ambient and storm-generated vorticity can be isolated. A new Lagrangian technique is employed in which material fluid volume elements are tracked to analyze the rearrangement of ambient vortex-line segments. This contribution is interpreted as barotropic vorticity. The storm-generated vorticity is treated as the residual between the known total vorticity and the barotropic vorticity.In the simulation the development of near-ground vertical vorticity is an outflow phenomenon. There are distinct ''rivers'' of cyclonic shear vorticity originating from the base of downdrafts that feed into the developing near-ground vortex. The origin of these rivers of vertical vorticity is primarily horizontal baroclinic production, which is maximized in the lowest few hundred meters AGL. Subsequently, this horizontal vorticity is tilted upward while the parcels are still descending. The barotropic vorticity remains mostly streamwise along the analyzed trajectories and does not acquire a large vertical component as the parcels reach the ground. Thus, the ambient vorticity that is imported into the storm contributes only a small fraction of the total near-ground vertical vorticity.

VORTEX2 Observations of a Low-Level Mesocyclone with Multiple Internal Rear-Flank Downdraft Momentum Surges in the 18 May 2010 Dumas, Texas, Supercell*

“…A final observation on the evolution of surges B-D is that a near-surface vortex developed and persisted to the north and trailing the apex of each surge, reaching a maximum intensity as it interacted with the leading edge of surge D (Figs. 7, 8), which is similar to the evolution of horizontal momentum surges in prior dual-Doppler studies Wurman et al 2010;Marquis et al 2012;Kosiba et al 2013) and numerical simulations (Dahl et al 2014;Schenkman et al 2014).…”

Simulating the 20 May 2013 Moore, Oklahoma tornado with a 100‐metre grid‐length NWP model

“…Downdrafts (rather than updrafts) are considered important in generating near‐surface vertical vorticity by transporting vertical vorticity from mid‐levels to the surface (e.g. Rotunno and Klemp, ; Markowski and Richardson, ; Kosiba et al , ; Dahl et al , ; Markowski and Richardson, ; Naylor and Gilmore, ). Due to the deep‐layer wind shear and upper‐level winds, the bulk of the hydrometeors in a supercell is deposited on the forward flank of the main updraft.…”

Comment on "Eliminating the major tornado threat in Tornado Alley"

“…25,33,34 These surges transport vertical vorticity to the surface as described above, and into the region beneath the storm's updraft where the vertical vorticity may be amplified to tornadic strength via the conservation of angular momentum. 31,32 This process works best when the low-altitude vertical wind shear and relative humidity are particularly large.…”

“…However, during the past decades, observational, theoretical and modeling studies [18][19][20][21][22][23][24][25] have shown that this mechanism cannot explain the development of near-ground rotation because horizontal near-ground vortex lines are lifted away from the ground as they are tilted at the updraft edge. Instead, a downdraft is needed for the development of vertical vorticity next to the ground.…”

“…[24][25][26] Moreover, observational and numerical modeling research on the development of near-ground rotation indicates that the horizontal vorticity that is tilted in downdrafts owes its existence mainly to baroclinic generation (horizontal air density gradients associated with the storm's precipitation regions generate horizontal vorticity via the torque they exert). 12,15,16,19,[25][26][27][28][29][30][31] That is, the vorticity is generated within the storm. This is inconsistent with Tao's Fig.…”