Production of Near-Surface Vertical Vorticity by Idealized Downdrafts

Abstract

This study uses an idealized heat sink to examine the possible roles of the wind profile in modulating the production of surface vertical vorticity by a downdraft. The basic vorticity evolution in these idealized simulations is consistent with previous work: the process is primarily baroclinic and produces near-ground vertical vorticity within the outflow. Sensitivity experiments affirm that the only fundamental requirement for downdrafts to produce surface vertical vorticity is the existence of ambient downdraft-relative flow. Vertical vorticity production increases monotonically as the low-level downdraft-relative flow increases from zero up through intermediate values (in these experiments, 10–15 m s−1), followed by a monotonic decrease for greater values. This sensitivity has to do with the degree of cooling acquired by parcels as they pass through the idealized heat sink as well as the degree to which horizontal vorticity vectors subsequently attain an orientation that is normal to isosurfaces of vertical velocity. Although the addition of vertical wind shear is not directly helpful to surface vertical vorticity production in these simulations, increased realism of outflow structure is attained in hodographs with ambient streamwise vorticity. Furthermore, the necessary condition of flow through a region of downdraft forcing would in nature probably require the existence of ambient vertical shear. Therefore, shear in the lower troposphere has a possibly important indirect role in modulating the initial production of near-ground rotation.

Origins of Vorticity in a Simulated Tornadic Mesovortex Observed during PECAN on 6 July 2015

“…1). This lowlevel vertical vorticity is typically generated by tilting of horizontal vorticity in association with a downdraft (e.g., Atkins and St. Laurent 2009b;Parker and Dahl 2015) along an internal boundary (i.e., the system gust front) or an external boundary with preexisting vertical vorticity (Przybylinski et al 2000;Schenkman et al 2011b) prior to encountering the low-level updraft. These mechanisms and the resulting collocation of the updraft and cyclonic vorticity resemble the processes responsible for the formation of low-and midlevel mesocyclones and tornadoes in supercells (e.g., Markowski and Richardson 2009).…”

Horizontal Vortex Tubes near a Simulated Tornado: Three-Dimensional Structure and Kinematics

“…The increase in the degree of organization of the LFCB results in the formation of meridionally-oriented "vertical vorticity rivers" [35,36], which first appear at the base of downdrafts northwest of the low-level circulation (Figure 2d). These features, along with enhanced vertical vorticity in an RFD internal boundary, are thought to be the primary storm-scale sources of vertical vorticity to the developing low-level rotation in other simulations in the literature [35][36][37][38]. At 11,888.775 s, the first tornado forms at the tip of the hook as seen in the vertical vorticity field (Figure 2d, Figure 3a, and Figure 4a).…”

“…Immediately outside the tornado core, in predominantly rotational flow, near-surface horizontal vorticity vectors, consistent with the nearsurface HVs, point to the left of the prevailing horizontal wind in which they are embedded, thus having a large crosswise component. Above the surface (i.e., farther from the lower boundary; Figure 8b) the vorticity vectors become more streamwise, suggesting that HVs that arise from the near- Another possibility for the horizontal vorticity of near-surface HVs is generation by baroclinic torques [15,24,[34][35][36][37]43,45]. However, if the horizontal vorticity in the HVs were mainly created baroclinically at the leading edge of a negatively buoyant RFD, the generated horizontal vorticity would point to the right of the downdraft and cold pool flows, giving an opposite sense of rotation than observed [5,43].…”

Near-Surface Vortex Formation in Supercells from the Perspective of Vortex Patch Dynamics

“…The vortex patches result mainly from baroclinically produced, horizontal vorticity that is tilted into the vertical, as demonstrated by Klemp and Rotunno (1983) and Rotunno and Klemp (1985). This mechanism was further elucidated by Davies-Jones and Brooks (1993, hereafter DJB93), who showed that horizontal baroclinic vorticity production, and subsequent tilting into the vertical, occurs in descending air leading to a vertical vorticity component at the surface (DJB93; Dahl et al 2014;Markowski and Richardson 2014;Parker and Dahl 2015). This vertical vorticity tends to be accumulated along convergence boundaries within and at the edge of the cold pool, whereby elongated vortex patches result (Markowski et al 2014).…”

“…7a for an example of such an elongated vortex patch.) It is well established that the initial appearance of weak near-surface vertical vorticity extrema, or vortex patches, occurs within the cold pool of the thunderstorm (Rotunno and Klemp 1985;Davies-Jones and Brooks 1993;Markowski and Richardson 2014;Dahl et al 2014;Parker and Dahl 2015;. The vortex patches result mainly from baroclinically produced, horizontal vorticity that is tilted into the vertical, as demonstrated by Klemp and Rotunno (1983) and Rotunno and Klemp (1985).…”