Tilting of Horizontal Shear Vorticity and the Development of Updraft Rotation in Supercell Thunderstorms

Abstract

The question of how rotation arises in sheared updrafts is analyzed using the shear and curvature vorticity framework. Local rotation exists where the shear and curvature vorticity have a similar magnitude and the same sign, such that parcels are in near-solid-body rotation. It is shown that the tilting terms of the vertical vorticity equation cannot explain the development of local rotation in the canonical cases where the horizontal vorticity is either purely streamwise or purely crosswise. Rather, vertical shear vorticity develops if crosswise vorticity is tilted, and vertical curvature vorticity develops if streamwise vorticity is tilted. To analyze how local rotation develops, two simulations of updrafts in an environment with crosswise and mostly streamwise vorticity, respectively, are discussed. A trajectory analysis is performed and shear and curvature vorticity budgets are analyzed. It is found that much of the horizontal vorticity near the updraft becomes streamwise, which results from pressure gradient accelerations in the vicinity of the updraft. Consequently, in the analyzed scenarios, the tilting mechanism results primarily in vertical curvature vorticity. Local rotation is achieved via an interchange process that facilitates a partial conversion of vertical curvature vorticity to vertical shear vorticity. Updraft rotation in supercells thus does not result from tilting of horizontal vorticity alone, but partial conversion of curvature to shear vorticity is also required.

Processes Preventing the Development of a Significant Tornado in a Colorado Supercell on 26 May 2010

“…Second, vertical vorticity must develop near the surface. Previous studies have found that downdrafts play an important role in this step (e.g., Davies-Jones and Brooks 1993) and that the vorticity bound for the surface is often augmented by baroclinic generation (Klemp and Rotunno 1983;Rotunno and Klemp 1985;Dahl 2015). Third, vertical vorticity near the surface is amplified to tornado strength by stretching.…”

“…Upon reaching the updraft, vorticity becomes more streamwise and larger in magnitude, likely owing to the generation of streamwise vorticity by the ''riverbend'' effect and subsequent stretching of this streamwise vorticity (Dahl 2017). As the parcels ascend into the OW minimum, significant vertical vorticity develops (Figs.…”

The Influences of Effective Inflow Layer Streamwise Vorticity and Storm-Relative Flow on Supercell Updraft Properties

“…The relevancy of v s within a storm's EIL to tornado formation is well supported by dynamics studies. Though the vertical vorticity in tornadoes themselves is primarily baroclinically generated by storm outflow (e.g., Dahl et al 2014) and surface friction (e.g., Schenkman et al 2014;Roberts and Xue 2017), the tilting of ambient near-surface v s within a storm's inflow enhances low-level upward oriented dynamic pressure accelerations below the updraft, which facilitates the vertical stretching of near-surface vertical vorticity and consequently tornadogenesis (e.g., Coffer et al 2017). There are also possible connections between v s within a storm's inflow and the properties of supercell updrafts above the lowest few kilometers of the atmosphere such as vertical acceleration, vertical velocity w, vertical vorticity z, and updraft steadiness.…”

Modes of Storm-Scale Variability and Tornado Potential in VORTEX2 Near- and Far-Field Tornadic Environments

“…These same processes are also thought to be responsible for the formation of the low-level mesocyclone in a few observational (Markowski et al 2012a) and modeling studies (Markowski and Richardson 2014;Coffer and Parker 2015Coffer et al 2017). Additionally, baroclinic generation within the storm can augment the environmental vorticity along parcel trajectories entering the updraft (Klemp and Rotunno 1983;Rotunno and Klemp 1985;Markowski et al 2012b;Dahl et al 2014;Orf et al 2017).…”

“…Mechanisms influencing the formation of near-surface rotation have been the focus of many modeling and observational studies. A few processes may contribute, including baroclinic vorticity generation and tilting in storm-scale downdrafts (e.g., Davies-Jones and Brooks 1993;Wicker and Wilhelmson 1995;Markowski and Richardson 2009;Dahl et al 2014;Markowski and Richardson 2014;Parker and Dahl 2015) and the generation and modification of vorticity via surface friction (Schenkman et al 2014;Roberts et al 2016). Regardless of the processes at work, unless preexisting near-surface vertical vorticity is present, a downdraft is theorized to be required for the development of near-surface vertical vorticity (Davies-Jones 1982a,b).…”