Near-Ground Rotation in Simulated Supercells: On the Robustness of the Baroclinic Mechanism*

Abstract

This study addresses the robustness of the baroclinic mechanism that facilitates the onset of surface rotation in supercells by using two idealized simulations with different microphysics parameterizations and by considering previous results. In particular, the importance of ambient crosswise vorticity relative to baroclinically generated vorticity in the development of near-ground cyclonic vorticity is analyzed. The storms were simulated using the CM1 model in a kinematic base state characterized by a straight-line hodograph. A trajectory analysis spanning about 30 min was performed for a large number of parcels that contribute to near-surface vertical-vorticity maxima. The vorticity along these trajectories was decomposed into barotropic and nonbarotropic parts, where the barotropic vorticity represents the effects of the preexisting, substantially crosswise horizontal storm-relative vorticity. The nonbarotropic part represents the vorticity produced baroclinically within the storm. It was found that the imported barotropic vorticity attains a downward component near the surface, while the baroclinic vorticity points upward and dominates. This dominance of the baroclinic vorticity is independent of whether a single-moment or double-moment microphysics parameterization is used. A scaling argument is offered as explanation, predicting that the baroclinic vertical vorticity becomes increasingly dominant as downdraft strength increases.

Transition of Near-Ground Vorticity Dynamics during Tornadogenesis

“…These studies are summarized in Table 1. The downdraft mechanism was identified in numerical simulations of supercells by Davies-Jones and Brooks (1993), Markowski and Richardson (2014), Wicker and Wilhelmson (1995), Adlerman et al (1999), Dahl et al (2014), Schenkman et al (2014), Dahl (2015), Parker and Dahl (2015), and Fischer and Dahl (2020), and a similar argument exists for quasi-linear convective system (QLCS) tornadoes (Trapp and Weisman 2003;Schenkman and Xue 2016;Flournoy and Coniglio 2019;Boyer and Dahl 2020). In most of these studies, the mechanism relies heavily on baroclinic production of horizontal vorticity at the periphery of a downdraft.…”

“…A large subset of these parcels (Fig. 1) originated between 2 and 4 km above ground level (AGL), a relatively high altitude compared to other studies (e.g., Markowski et al 2014;Dahl 2015). In these areas, the horizontal vorticity was artificially enhanced around the forced updraft (Figs.…”

“…(iii) The ambient vorticity does not seem to contribute meaningfully to vertical vorticity production in this scenario (Dahl et al 2014;Dahl 2015).…”

Additional Evaluation of the Spatiotemporal Evolution of Rotation during Tornadogenesis Using Rapid-Scan Mobile Radar Observations

“…With the advent of high-resolution computer models (grid spacing , ∼100 m) (e.g., Wicker and Wilhelmson 1995;Bryan and Fritsch 2002;Xue et al 2003;Mashiko et al 2009) and rapid-scan radar (t , 30 s between consecutive volume scans) (Wurman and Randall 2001;Bluestein et al 2010;Pazmany et al 2013;Isom et al 2013), the details of tornadogenesis have been revisited with improved spatiotemporal resolution over the past decade. Results from such studies have not found evidence of a traditional [i.e., O(5-10) min] DPE top-down evolution and have instead supported a non-descending 1 evolutionary process in supercells (e.g., Mashiko 2016;Mashiko et al 2009;French et al 2013;Kosiba et al 2013;Dahl 2015;Dahl et al 2014;Markowski and Richardson 2014;Naylor and Gilmore 2014;Xue et al 2014;Houser et al 2015;Parker and Dahl 2015;Schenkman et al 2014;Markowski et al 2018;Yokota et al 2018;Bluestein et al 2019;Wienhoff et al 2020;Noda and Niino 2010). Moreover, French et al (2013) determined that observations of tornadogenesis with insufficient temporal resolution (e.g., that of traditional WSR-88D observations) could be misidentified as having top-down evolution when they truly have non-descending evolution owing to the common presence of transient tornadic vortex signatures (TVSs) (Burgess et al 1975;Brown et al 1978;Brown and Wood 2012) at midlevels.…”

Distinguishing Characteristics of Tornadic and Nontornadic Supercell Storms from Composite Mean Analyses of Radar Observations

“…One potentially important reason for this lack of progress in tornado warning performance is our incomplete knowledge of tornadogenesis. While a variety of processes have been found that generate broadscale near-ground vorticity in supercells and/or tilt and stretch this vorticity to tornadic magnitude (Rotunno and Klemp 1985;Davies-Jones and Brooks 1993;Straka et al 2007;Marquis et al 2012;Kosiba et al 2013;Schenkman et al 2014;Markowski and Richardson 2014;Dahl 2015;Davies-Jones 2015;Rotunno et al 2017;Roberts et al 2020), much remains unknown about the relative importance of these tornadogenesis processes, how they are modulated by dynamically adjacent processes (e.g., inhibition of vertical vorticity stretching by negatively buoyant outflow), how the importance of all of these processes varies with the near-storm environment, and so forth. These knowledge gaps imply the existence of yet-unknown relationships between tornado potential, storm characteristics, and nearstorm environment characteristics that could be exploited to improve tornado forecasting.…”