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An Assessment of Low-Level Baroclinity and Vorticity Within a Simulated Supercell

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An Assessment of Low-Level Baroclinity and Vorticity Within a Simulated Supercell FEBRUARY 2013 BECK AND WEISS 649 An Assessment of Low-Level Baroclinity and Vorticity within a Simulated Supercell JEFFREY BECK Centre National de Recherches Me´te´orologiques, Me´te´o-France, Toulouse, France CHRISTOPHER WEISS Atmospheric Science Group, Texas Tech University, Lubbock, Texas (Manuscript received 7 May 2011, in final form 12 July 2012) ABSTRACT Idealized supercell modeling has provided a wealth of information regarding the evolution and dynamics within supercell thunderstorms. However, discrepancies in conceptual models exist, including uncertainty regarding the existence, placement, and forcing of low-level boundaries in these storms, as well as their importance in low-level vorticity development. This study offers analysis of the origins of low-level bound- aries and vertical vorticity within the low-level mesocyclone of a simulated supercell. Low-level boundary location shares similarities with previous modeling studies; however, the development and evolution of these boundaries differ from previous conceptual models. The rear-flank gust front develops first, whereas the formation of a boundary extending north of the mesocyclone undergoes numerous iterations caused by competing outflow and inflow before a steady-state boundary is produced. A third boundary extending northeast of the mesocyclone is produced through evaporative cooling of inflow air and develops last. Con- ceptual models for the simulation were created to demonstrate the evolution and structure of the low-level boundaries. Only the rear-flank gust front may be classified as a ‘‘gust front,’’ defined as having a strong wind shift, delineation between inflow and outflow air, and a strong pressure gradient across the boundary. Trajectory analyses show that parcels traversing the boundary north of the mesocyclone and the rear-flank gust front play a strong role in the development of vertical vorticity existing within the low-level mesocyclone. In addition, baroclinity near the rear-flank downdraft proves to be key in producing horizontal vorticity that is eventually tilted, providing a majority of the positive vertical vorticity within the low-level mesocyclone. 1. Introduction low-level dynamics to assess the development and im- pact of vorticity generated in different parts of the Conceptual models of supercell thunderstorm struc- storm. ture were developed in the late 1970s (Brandes 1978; One of the pioneering modeling studies documenting Lemon and Doswell 1979) and have remained relatively the dynamics of a supercell in detail (with 1-km hori- unchanged. While these conceptual models have gen- zontal grid spacing) was performed by Klemp et al. erally been accepted, verification of boundaries and air (1981). The simulation contained features and structure masses through in situ measurements has been limited that compared well with observations. The ability to because of the difficulty and potential hazards of re- replicate an observed supercell through model simula- cording direct measurements within the storm itself tion solely via a proximity sounding led Klemp et al. (Shabbott and Markowski 2006). Specifically, measure- (1981) to suggest that the larger-scale environment plays ments of low-level thermodynamics have been lacking, an important role in the structure and dynamics of these particularly within the downshear region of the storm. storms. Using similar modeling methods, Klemp and Therefore, the necessity exists for more research of Rotunno (1983) discovered a mechanism through which low-level vertical vorticity is strengthened and sub- sequently repositioned around the simulated low-level Corresponding author address: Jeffrey R. Beck, Centre National de Recherches Me´te´orologiques, Me´te´o-France, DT/AD/RH, 42 mesocyclone. Analysis revealed that low-level air north- Avenue G. Coriolis, 31057 Toulouse CEDEX 1, France. east of the mesocyclone flowed parallel to the ‘‘cold E-mail: [email protected] frontal boundary’’ as it moved toward the low-level DOI: 10.1175/MWR-D-11-00115.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/06/21 09:59 PM UTC 650 MONTHLY WEATHER REVIEW VOLUME 141 mesocyclone. This boundary was identified by the 218C many different environments in which supercells form perturbation isotherm; therefore, a quantifiable amount most certainly change the low-level evolution and of solenoidally produced horizontal vorticity was pres- boundaries within these storms. Moreover, recent obser- ent in this region of the simulation. A parcel traversing vational studies using Doppler-on-Wheels (DOW) and this boundary acquires significant (mesocyclonic) values Shared Mobile Atmospheric Research and Teaching Ra- of horizontal vorticity in a short period of time (e.g., dar (SMART-R) data (e.g., French et al. 2004; Beck et al. ;300 s). Klemp and Rotunno (1983) found that this 2006; Wurman et al. 2007a,b) have shown variability in horizontal vorticity, with a magnitude comparable to low-level boundary strength within the forward flank of that within the environmental inflow, is subsequently observed storms [many showing the absence of a forward- tilted within the gradient of vertical velocity near the flank gust front (FFGF) in the convergence field]. While mesocyclone, thus enhancing low-level vertical vortic- these studies lacked thermodynamic data, project Verifi- ity. Rotunno and Klemp (1985) found a buoyancy gra- cation of the Origins of Rotation in Tornadoes Experiment dient associated with this cold-air boundary, concluding (VORTEX) collected mobile mesonet measurements that it is important to the solenoidal generation of hor- within a number of supercells. Using the data collected izontal vorticity. during this project, Markowski et al. (2002) documented Wicker and Wilhelmson (1995) found two separate the virtual potential temperature perturbations and origins for parcels with trajectories terminating in the equivalent potential temperature values around hook mesocyclone: one northeast of the mesocyclone, with echoes and the near-mesocyclonic region of supercells. the other aloft, northwest of the mesocyclone, near the Both tornadic and nontornadic storms were sampled, with forward-flank gust front (defined by the authors as the evidence of a gradual east-to-west increase in the virtual 21-K potential temperature isotherm) extending north potential temperature deficit north of each mesocyclone. from the center of rotation. Parcels originating from the The conceptual model presented at the conclusion of the northeast were found to contribute most significantly to study showed a north–south boundary extending out of the vertical vorticity budget of the low-level mesocy- the low-level vertical vorticity maximum, separate from clone. These parcels then traverse a portion of the cold- the rear-flank gust front (RFGF), very similar to Brandes air boundary immediately north of the mesocyclone, (1978). These results mirror those found in Markowski acquiring baroclinically produced horizontal vorticity. et al. (2011), where again a north–south-oriented bound- An earlier paper by Davies-Jones and Brooks (1993) ary was found to exist separate from the RFGF in found that the method in which the baroclinity associ- a number of nontornadic supercell thunderstorms. Finally, ated along the cold-air boundary is tilted ultimately Shabbott and Markowski (2006) studied similar mobile proved fundamental in the production of positive ver- mesonet-collected thermodynamic properties from the tical vorticity near the ground. Specifically, in the pres- forward flank of supercells. A gradual east-to-west in- ence of a rear-flank downdraft, solenoidal generation of crease in density potential temperature deficit was found horizontal vorticity would allow the vorticity vector to exist in many of the storms, but no sharp thermody- within the downdraft to depart from trajectory paths, namic or kinematic boundary was found. Therefore, given resulting in a positive component of vertical vorticity as the discrepancies in forward-flank characteristics and im- the parcel reaches the surface within the downdraft. In plied low-level dynamics between many of these studies, it this sense, the authors’ findings, concerning the impor- is important to focus additional research to determine how tance of baroclinity along the boundary north of the this region of the storm impacts the development of the mesocyclone, mirror those found by Rotunno and Klemp low-level mesocyclone. (1985) and Wicker and Wilhelmson (1995). These results A common thread throughout these research papers is are corroborated by Adlerman et al. (1999), who found a lack of a mutual definition for a baroclinic boundary parcels entering the mesocyclone originate from regions within a supercell. Terms such as cold-frontal or cold-air northwest, north, and northeast of the mesocyclone and boundary, in addition to gust front, are used, sometimes experience positive vertical vorticity tendency upon de- defined using values of perturbation potential temper- scent in the rear-flank downdraft (RFD). ature, or in other cases, the convergence of the wind Established observational conceptual models, along field. However, general consensus is now to use the term with past numerical supercell simulations, have not al- ‘‘gust front’’ as the primary description of a baroclinic ways agreed on low-level baroclinity/boundary strength boundary within a supercell thunderstorm. Per the Glos- and position, specifically within the forward flank. In ad- sary of Meteorology (Glickman 2000), a gust front is de- dition, a spectrum of supercell
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