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

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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 boundaries 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 impact 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 subsequently repositioned around the simulated low-level Corresponding author address: Jeffrey R. Beck, Centre National de Recherches Me´teórologiques, Me´teó-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 types exists (Beatty et al. fined as having a strong wind shift, with delineation 2008), depending upon many variables, including vertical between inflow and outflow air, and a strong pressure wind shear, hodograph shape, and sounding profile. The gradient across the boundary. Furthermore, outflow air is

Unauthenticated | Downloaded 10/06/21 09:59 PM UTC FEBRUARY 2013 BECK AND WEISS 651 defined to specify an advancing cold pool with relatively higher pressure than the air it is displacing, and is therefore linked to the definition of a gust front within a supercell thunderstorm. In this study, a simulated supercell is generated to analyze the low-level evolution and dynamics as the storm evolves. A conceptual model of the simulated supercell is defined to describe each low-level feature or boundary found within the storm. This conceptual model shows three types of low-level boundaries associated with the simulated thunderstorm, with each boundary produced via different means. Trajectory analyses are conducted to assess the importance of horizontal vorticity along these boundaries and any impact on vertical vorticity development. Baroclinity along these boundaries is shown to be of importance to the development of the horizontal (and, ultimately, vertical) vorticity, so long as parcels parallel these boundaries for a sufficient amount of time, typically near the end of trajectories terminating in the low-level mesocyclone. Findings from this research show that a consensus is needed when defining baroclinic boundaries within the low levels of supercells. In addition, differences between the conceptual models of this simulated storm and previous research indicate a potential spectrum of boundary position, evolution, and dynamics.

2. Methodology a. Simulation description An idealized supercell simulation was created using the Weather Research and Forecasting (WRF) model. A base state sounding was developed (Fig. 1a) using analytical equations of potential temperature and relative humidity, as detailed by Weisman and Klemp (1982). An L-shaped hodograph was chosen (Fig. 1b) as the wind profile for this FIG. 1. (a) Thermodynamic profile and (b) hodograph used for study. Storm motion was calculated from Bunkers et al. the simulations, created using the method of Weisman and Klemp (2000) and was subtracted uniformly from the initial ho- (1982). In (a), the gray line represents the dewpoint profile (8C), dograph to obtain a stationary storm (Fig. 1b), making all whereas the black line represents the temperature profile (8C). The variables storm relative. dashed line indicates the ascending surface-based parcel trajectory. In the horizontal, 501 grid points exist in both the x and y Wind barbs are in knots. direction, with 250-m grid spacing, creating a horizontal domain of 125 km 3 125 km. In the vertical, 51 levels vertical, 20 vertical levels exist below 2 km, allowing for exist, beginning near the surface at 50 m AGL, extending high resolution in the boundary layer. An importance is to 17 km AGL. Because of the Arakawa C-grid employed placed on this region, specifically for sufficient depiction of by the WRF model, vertical velocity values a half grid cold pool dynamics, in relation to low-level boundary point below and above each vertical level are averaged, structure, development, and evolution. along with horizontal wind components at the sides of the The simulation is run for 3 h to include the full evo- grid to arrive at the values of u, y,andw at the center of the lution of the storm. Data are saved every 5 min for the grid. The vertical grid is explicitly defined, beginning at first hour, after which, data are archived every 60 s for 50-m spacing and terminating at 150-m spacing at the top improved tendency calculations and trajectories. The of the domain. Owing to stretching of grid points in the WRF single-moment 6-class (WSM6) scheme is chosen

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TABLE 1. WRF model simulation specifications. b. Calculation of variables, tendency equations, and trajectories Grid dimensions 501 3 501 3 51 3 3 (125 km 125 km 17 km) The u, y, and w components of the wind, temperature, Horizontal grid spacing 250 m mixing ratio of all hydrometeor species, evaporation Vertical grid spacing Stretched (min 5 50 m, max 5 1500 m) tendency, potential temperature, and total pressure are Initialization Single sounding all variables output by the WRF model. However, Terrain None postprocessing is necessary for other variables. Specifi- Microphysics WSM6 cally, horizontal gradients and vertical advection of Turbulence 1.5 turbulent kinetic energy (TKE) theta are calculated in a Cartesian framework based on parameterization closure (3D) Advection Fifth order for both vertical the calculated x, y, and z positions of the grid points equation discretization and horizontal within the domain. Furthermore, the density potential Short- and longwave None temperature is computed using contributions from the radiation mixing ratios of water vapor, rain, and graupel. How- Surface drag coefficient 0 21 ever, it should be noted that graupel only exists (and Coriolis parameter 0 s Boundary conditions Open therefore impacts the density potential temperature) at Initial convective 5 K thermal, levels above 2 km AGL in the analyzed forward-flank perturbation horizontal radius 5 10 km, region of the storm. The density potential temperature vertical radius 5 0.5 km, 0 perturbation ur is computed by subtracting the initial center 5 1.0 km AGL uniform density potential temperature field from the Time of simulation 3 h Time interval for data 5 min for first hour, 1 min thereafter time-specific density potential temperature. Equiva- output lent potential temperature is also calculated (Bolton Time step 1 s 1980) and is a psuedoadiabatically conserved quantity allowing for retrieval of the approximate origin height of air parcels (Markowski 2002). Vorticity and vorticity tendency (both vertical and horizontal) are computed for the microphysics parameterization, with five classes as well (Bluestein 1992). The equations for the x and of hydrometeors: rain, snow, cloud ice, cloud water, and y components of horizontal vorticity tendency are graupel. The varying impact of microphysical schemes (respectively): on the strength of the supercell cold pool has been well documented (e.g., Gilmore et al. 2004; Dawson et al. Dz ›u ›u ›u › ›p ›F 2010). Acknowledging that the use of different micro- x 52d z 1 f 1 z 1 z 2 2a 2 y D x x ›z ›z y›y ›z ›y ›z physics schemes could change the strength of the cold t pool sufficiently to alter the low-level dynamics discussed (1) in forthcoming sections, sensitivity to the WSM6 scheme was investigated in this study through alterations to the and drop size distribution for rain—the hydrometeor of in- Dz ›y ›y ›y › ›p ›F terest in the region studied within the storm. While these y 52d z 1 f 1 z 1 z 2 2a 2 x y y › › x› › › › , sensitivity simulations produced different cold-pool Dt z z x z x z strengths, the overall evolution of low-level features (2) remained very similar (further discussed in section 3a). The domain utilizes open lateral boundary conditions where zx and zy are horizontal vorticity in the x and y with rigid upper and lower boundaries. Rayleigh directions (respectively), z is the vertical vorticity, f is damping is employed near the top of the domain to help the Coriolis parameter, a is inverse density, and Fx and control vertical reflection; divergence damping is used to Fy are the forces of friction in the x and y directions control sound waves. For subgrid-scale turbulent mix- (respectively). However, surface friction is not included ing, a three-dimensional 1.5-order turbulent–kinetic in the analysis. Finally, to better compare simulated mixing closure scheme is used. No Coriolis forcing, storms to observational research, a simulated reflectivity surface fluxes, surface friction, or radiation are included field was generated, calculated using the formula out- in the simulation. The initial convective thermal per- lined by Smith et al. (1975). This formulation includes turbation of 5 K has a horizontal radius of 10 km, ver- the contribution from both rain and graupel. tical radius of 500 m, and is centered at 1 km AGL. A The thermodynamic tendency equation (e.g., Bluestein summary of the model characteristics can be found in 1993) is used as the basis for calculations of air mass or- Table 1. igin, comparison, and transport, and is defined as

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›u ›u u dQ reflectivity and remnants of a left-split storm moving off 52v $ u 2 v 1 , (3) ›t p ›p C T dt to the northwest (not shown). In addition, a broad area of p 2 2 vertical vorticity of mesocyclonic strength O(10 2 s 1) has formed at low levels just east of the hook echo. Later where v is vertical velocity, Cp is the specific heat of air at constant pressure, and dQ/dt is the rate of heat in the evolution, the low-level features become nearly transfer. Over small vertical and horizontal distances steady state in nature. Therefore, a time from this period (hundreds of meters), where pressure and height sur- (8700 s into the simulation) is chosen for analysis as it is faces are relatively parallel, it is possible to rewrite this representative of mature low-level supercell structure equation in terms of height in a finite difference (Fig. 2). framework: At this time, three main boundaries within the wind field can be observed. One boundary extends northeast Du Du Du Du u D from the mesocyclone, south of the reflectivity gradient 52 2 y 2 1 Q D u D D w D D . (4) of the forward flank. Another boundary extends due t x y z CpT t north from the mesocyclone into the core of the storm. Density potential temperature, rather than potential A final boundary, the RFGF, runs south from the meso- temperature, tendency is used in the context of this re- cyclone. The boundaries are associated with differing search. A scale analysis of the density potential tem- amounts of low-level convergence (Fig. 3). The strongest ; 3 22 21 perature tendency equation was conducted, indicating convergence is associated with the RFGF ( 2 10 s ), that terms involving absolute values and time tendencies followed by weaker convergence along the boundary of hydrometeor species and water vapor were negligible extending north out of the low-level mesocyclone ; 3 22 21 in comparison to that of potential temperature. There- ( 1.5 10 s ). The weakest convergence is associated with the boundary extending northeast out of the meso- fore, it can be shown that Dur/Dt ’ Du/Dt. ; 3 22 21 Trajectory analyses for parcels terminating in the cyclone, with values around 1 10 s or less. Only low-level mesocyclone are performed using the Read/ this latter boundary shows distinct spatial variation in both Interpolate/Plot (RIP) program developed by the Na- strength and breadth of convergence, with strength de- tional Center for Atmospheric Research (NCAR) and creasing eastward, away from the low-level mesocyclone. the University of Washington, which incorporates an On either side of the boundary extending northeast from Eulerian integration scheme. During the time of the the hook, the equivalent potential temperature values are trajectory analysis, data are input every 60 s, and velocity identical (Fig. 4a), indicating a common air mass origin, as data are linearly interpolated in time every 5 s through- equivalent potential temperature is conserved for both out the duration of each trajectory. Accuracy assessment dry and moist adiabatic processes. Backward trajectory of each trajectory was conducted to ensure the location analyses at 50 m AGL confirm this conclusion (Fig. 4b), and variable quantities along all trajectories were rea- with air parcels on either side originating from the inflow sonable. To accomplish this assessment, sample trajec- region. However, the westernmost parcels are influenced tories were selected with model data saved every second, by increasing amounts of evaporative cooling (calculated instead of every 60 s. Trajectories acquired from the 1-s from the microphysics scheme), as rain falls into this in- data were compared to those from the 60-s data and flow air, with density potential temperature decreasing were found to be nearly identical. In addition, summation from east to west (Fig. 4b). Specifically, a parcel traveling of vertical vorticity tendency quantities were conducted along a trajectory through this region for 15 min experi- along each trajectory, with total vertical vorticity values ences an average density potential temperature change of 2 matching well with instantaneous values along each tra- approximately 2 K due to evaporative cooling based on jectory and at the origin points. Eq. (4), comparing well with simulation results at the western termination points of the trajectories (Fig. 4b). Given the findings of a weak gradient in both per- 3. Analysis turbation pressure and density potential temperature across this portion of the forward flank and a lack of a. Established thermodynamic and kinematic strong convergence, the latter evident from horizon- low-level structure tal velocity vectors and trajectories in Figs. 4a,b, this Initial consideration is given to the evolution of the boundary may not be considered a gust front, using supercell and a representative time is chosen for analysis the traditional definition of the term. Instead, the term of the main features of the low-level flow. After 3600 s ‘‘forward-flank convergence boundary’’ (FFCB) is deemed into the simulation, the storm has formed supercellular more appropriate, and is used through the remainder of features at low levels, including a hook echo in simulated this paper.

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FIG. 2. Simulated reﬂectivity (shaded, dBZ) overlaid with horizontal velocity (every eighth 2 vector shown, m s 1, scale at bottom left) at 50 m AGL for 8700 s into the control simulation. Arrows indicate the location of the main boundaries associated with the supercell.

For the boundary extending north out of the low-level trajectory, the density potential temperature tendency rate 2 2 mesocyclone, a strong eastward-directed gradient in is 27.2 3 10 3 Ks 1, resulting in a change of approxi- equivalent potential temperature exists (Fig. 4a). This mately 24 K. Therefore, over the path of the trajectory, result is consistent with air parcels descending from aloft evaporation of rainwater results in ;7 K of density po- to the west of the boundary. Keeping in mind that an tential temperature cooling. The computed amount of exact parcel origin cannot be found because of diabatic cooling matches well with density potential temperature effects, an estimation can be made using the equivalent perturbations seen in the simulation west of the boundary potential temperature. Air descending west of this and north of the mesocyclone (Fig. 6). Yet, while tem- boundary has equivalent potential temperature values perature varies little north of the mesocyclone, a density of ;327–333 K (Fig. 4a). Comparing these values to the gradient does exist, owing to differences in water vapor initialization sounding (Fig. 1a), approximate origin content across the boundary, resulting in the convergence heights are around 2–3 km AGL. Indeed, backward that is seen (Fig. 3). trajectories across the boundary extending north out of While convergence is evident along this boundary, the the mesocyclone support the conclusion that air is de- change in wind vector orientation across it is minor, with scending west of the boundary, while to the east, air no strong outflow on either side (inferred from pressure parcels originate from just above the surface (Fig. 5), field). In addition, this boundary does not separate with equivalent potential temperatures of 343 K. outflow from environmental air, such that use of the To assess the impact of evaporative cooling on density term gust front is not accurate in this case. Therefore, potential temperature, backward trajectories are used. hereafter, this boundary will be referred to as the ‘‘left- Trajectories terminating west of the boundary at 6300 s flank convergence boundary’’ (LFCB). are utilized in this sense to compute the diabatic term in Evaluating the RFGF, equivalent potential tempera- Eq. (4) in a Lagrangian framework. Considering an average tures in the region just west of the boundary are ;333 K, 2 2 2 evaporation rate of approximately 22 3 10 6 kg kg 1 s 1 resulting in an approximate parcel height origin of ;3km for the first 20 min of the trajectories, the first part of the AGL. This result suggests that air is not descending from trajectory experiences a density potential temperature very high levels within the RFD (similar results were 2 2 tendency rate of 22.4 3 10 3 Ks 1, equating to a change found in a number of other studies, including Knupp 1987 of approximately 23 K. For the last 10 min of the and Adlerman et al. 1999, for example, and summarized

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21 21 FIG. 3. Convergence values (shaded, s ) with horizontal velocity (vectors, m s , scale at bottom left) and simulated reﬂectivity (contoured at 20, 30, 40, 50, and 60 dBZ) at 50 m AGL for 8700 s into the simulation. in Markowski 2002); however, the air west of the RFGF is (similar to the LFCB). Further trajectory analysis colder and moister than west of the LFCB. In this case, (not shown) indicates that parcels terminating within there is an area of enhanced evaporative cooling in the northeastern portions of the RFD arrive from the the hook echo, therefore it is likely that evaporative north, and traverse an area of strong evaporative cooling is the main forcing term acting in this case cooling west of the RFGF. However, values of liquid

21 FIG. 4. (a) Equivalent potential temperature (colored, K) with horizontal velocity (every tenth vector shown, m s , scale at bottom left) and simulated reﬂectivity (contoured at 20, 30, 40, 50, and 60 dBZ) at 50 m AGL for 8700 s into the simulation, and (b) density potential temperature perturbation (colored, K) overlaid with 15-min backward trajectories from either side of the northeastward-directed boundary at 8700 s and 50 m AGL. Green parcel paths indicate consistent heights of 50–100 m AGL. The white box in (a) indicates the domain of (b).

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FIG. 5. Perturbation density potential temperature (colored, K) with LFCB and FFCB boundaries outlined in black at 50 m AGL for 6300 s into the simulation, and 30-min backward trajectories terminating on either side of the LFCB. Colors along the trajectories depict height, with black, green, blue, red, yellow, light blue and magenta signifying higher heights, respectively, from the surface up to .400 m AGL (legend inset at bottom right). Refer to section 3b for a description of the dashed line.

2 2 water mixing ratio are nearly ;7 3 10 3 kg kg 1 in this cooling a parcel experiences traversing north of the region, indicating a high concentration of rainwater and boundary. The strength of the frontogenetical forcing therefore a strong contribution from water loading in analysis matches well with the previous descriptions of the addition to evaporative cooling. The RFGF is the only development of each boundary. boundary of the three presented to display a sharp Based on the analyzed low-level structure, a conceptual boundary in perturbation pressure (Fig. 7), a distinct model for this simulation (Fig. 8) is created to depict the wind shift, and a separation of environmental from position, orientation, and strength of the three low-level storm-altered, outflow air, therefore making it a true boundaries. Each boundary is shown as a solid line, with the gust front. thickness of each line representing the strength of conver- Having identified the origin of these surface bound- gence associated with each boundary. For the FFCB and aries, it is useful to quantify the strength of the fronto- the LFCB, dashed lines represent the portion of the genetical forcing that results (frontogenesis can be boundary that is less defined and becomes less distin- defined as a measure of the tendency of a horizontal gra- guishable from the general flow, farther away from the low- dient in density or temperature for a given boundary). Of level mesocyclone. Arrows represent the general low-level the three boundaries analyzed, the RFGF shows the flow near and around the boundaries and mesocyclone. 2 2 2 strongest frontogenetical forcing (;5 3 10 5 Km 1 s 1), A number of sensitivity studies were conducted re- as descending air is strongly impacted by evaporative garding changes to the low-level and cloud-level specific cooling and spreads out near the surface and meets inflow humidity within the initialized sounding, as well as changes air from the east. This frontogenetical forcing is uniform to the rain drop size distribution within the WSM6 mi- along the length of the RFGF. Frontogenesis for the crophysics package. Regarding the microphysical sensi- LFCB is slightly weaker than that for the RFGF (2–3 3 tivity tests, alterations were made to the intercept value 2 2 2 10 5 Km 1 s 1), but is also uniform in strength for a fair within the drop size distribution of rain. The default value 2 extent along the boundary toward the north. Finally, the (used for this study) was 8.0 3 106 m 4;therefore,two forcing for the FFCB is the weakest of all three boundaries sensitivity studies were conducted at intercept values of 2 2 2 2 2 (1–2 3 10 5 Km 1 s 1),andtrailsoffinstrengthtoward 8.0 3 105 m 4 and 8.0 3 107 m 4 (Fig. 9) to analyze the the northeast, in relation to the amount of evaporation impact this change might have on the conceptual model.

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FIG. 6. Perturbation density potential temperature (shaded, K) with horizontal wind (every 2 seventh vector shown, m s 1, scale at bottom left) and simulated reﬂectivity (contoured at 20, 30, 40, 50, and 60 dBZ) at 8700 s for 50 m AGL.

2 Alterations to the rain intercept value produce differences present. In the 8.0 3 107 m 4 simulation, an intense uni- in the low-level dynamics for each storm. For the 8.0 3 ﬁed cold pool develops that overtakes all three boundaries 2 105 m 4 rain intercept value simulation, convergence as- by 8700 s. Therefore, analysis of this simulation was con- sociated with the FFCB is weakened at 8700 s, but still ducted at 5700 s, showing that while convergence along

FIG. 7. Perturbation pressure (colored, Pa), with horizontal wind vectors (every seventh 2 vector shown, m s 1, scale at bottom left), and simulated reﬂectivity (contoured at 20, 30, 40, 50, and 60 dBZ) at 8700 s for 50 m AGL.

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of evaporative cooling are present (Fig. 11). These regions are not as strong or as large in aerial coverage as that seen later during the steadier evolution. Surface outflow is not as prominent, and any westerly outflow generated by downdrafts behind recurrent LFCBs is countered by the stronger easterly flow north of the low- level mesocyclone. This fact can be seen in the second hour of the simulation, as the easterly flow in the upshear region of the FFD is still stronger than the outflow caused by these discrete pockets of evaporative cooling. There- fore, as a downdraft nears the surface, an LFCB forms via convergence between these downdrafts (Fig. 12), but is then advected westward and dissipates. New LFCBs develop and follow a similar pattern. This process creates as many as four LFCBs simultaneously (Fig. 12) at 5700 s into the simulation. A backward trajectory analysis through these boundaries confirms that there is descent from differing heights occurring on either side of the LFCBs, where pockets of evaporative cooling exist (Fig. 13). It should be noted that each boundary starts as a weak convergence line farther east within the forward flank, FIG. 8. Steady-state conceptual model of the low-level bound- then strengthens as it moves west, peaks in magnitude of 2 aries and flow in the simulated supercell. The thick solid line rep- convergence (;0.015 s 1), and then gets progressively resents the radar reflectivity boundary, whereas the relatively weaker with time. This process continues until a larger, thinner lines represent the boundary locations and strength of convergence, with thicker lines representing stronger convergence. stronger region of evaporative cooling (and, therefore, Dashed lines represent the areas where the boundaries become near-surface divergence) develops, from which the weak and begin to dissipate. Arrows show typical streamlines. eastward outflow counters the prevailing mesocyclonic flow. At this point (7200 s into the simulation), the se- quence of LFCBs evolves into one relatively stationary the LFCB is stronger than in the control simulation, the LFCB. After the evolution of the LFCBs commences, FFCB, LFCB, and RFGF all coexist at this time. the FFCB begins to develop (Fig. 12), but is still very 2 2 2 In addition, forward-flank boundary orientation is al- weak (convergence from 10 3 to 10 4 s 1) at 5700 s into 2 2 tered in both the 8.0 3 105 m 4 and 8.0 3 107 m 4 sim- the simulation. The FFCB becomes more convergent ulations, compared to the control simulation. However, with time. The evolution of the low-level boundaries and all three boundaries—the FFCB, the LFCB, and the other features of the simulation at this time are shown in RFGF—are present in the sensitivity simulations and a conceptual model (Fig. 14). The portion of the simu- baroclinity remains similar to the initial simulation, lation conceptual model at 7200 s represents the steady- lending credence to the representativeness of the con- state structure of the supercell outlined in section 3a. ceptual model presented previously. However, this model c. Vorticity and trajectory analysis should not be necessarily considered to be applicable to all modeled or observed supercells. Persistent, but discrete midlevel mesocyclones (;5 km AGL) exist throughout the duration of the time b. Evolution of low-level structure 2 2 analyzed, with pulses of strong (.6 3 10 2 s 1) vertical During the early portion of the simulation, the super- vorticity (Fig. 15). Near the surface, however, appre- 2 2 cell undergoes large fluctuations in terms of downdraft ciable rotation (vertical vorticity .4 3 10 3 s 1) is ab- structure and strength and boundary development, posi- sent from the storm until approximately 8000 s. At this tion, and strength. Initially, around 3600 s, of the three point, a near-ground maximum in vertical vorticity is 2 boundaries discussed previously, only the RFGF is present with magnitude of about 0.05 s 1 (another oc- 2 present (Fig. 10). During the time analyzed in the pre- curs at 8700 s with vertical vorticity of about 0.06 s 1). vious section of the paper, one strong region of evapo- This time also marks the first connection of vertical rative cooling exists within the upshear portion of the vorticity between low levels and midlevels. The stron- forward-flank downdraft, around 400–1000 m AGL. gest near-ground vertical vorticity associated with the 2 However, earlier in the simulation, many discrete regions low-level mesocyclone (;0.07 s 1) occurs just prior to

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21 FIG. 9. Positive convergence (shaded, s ) with simulated reflectivity (contoured at 20, 30, 40, 2 50, and 60 dBZ) and horizontal wind (vectors, m s 1, scale at bottom left) at (a) 8700 and (b) 2 5700 s for 50 m AGL for rain intercept values (a) 8.0 3 105 and (b) 8.0 3 107 m 4. the interaction of the analyzed storm with outflow vertical vorticity maxima is next examined. Assessment originating from the initial left-split storm (Fig. 15). of the low-level horizontal vorticity field at 8700 s shows The impact of preexisting environmental and baro- that environmental horizontal vorticity within the inflow 2 2 clinically produced horizontal vorticity on low-level air has an average value of 8 3 10 3 s 1 (Fig. 16). The

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21 FIG. 10. Convergence (colored, s ) with simulated reﬂectivity (contoured at 20, 30, 40, 50, 2 and 60 dBZ) and horizontal wind (every eighth vector shown, m s 1, scale at bottom left) at 3900 s for 50 m AGL.

2 horizontal vorticity associated with the three persistent of 0.04 s 1 are intrinsic to both the FFCB and LFCB, with 2 boundaries identiﬁed within the supercell (RFGF, LFCB, values of 0.06 s 1 associated with the RFGF. The hori- and FFCB) is substantially larger than that in the envi- zontal vorticity vectors are directed parallel to each ronmental inﬂow (Fig. 16). Horizontal vorticity values boundary and in a direction that suggests that solenoidal

21 21 FIG. 11. Liquid water mixing ratio tendency/evaporation rate (shaded, kg kg s ) with simulated reﬂectivity (contoured at 20, 30, 40, 50, and 60 dBZ) and horizontal wind vectors 2 (every eighth vector shown, m s 1, scale at bottom left) at 5700 s for 1000 m AGL.

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21 FIG. 12. Convergence (colored, s ) with simulated reﬂectivity (contoured at 20, 30, 40, 50, 2 and 60 dBZ) and horizontal wind vectors (every eighth vector shown, m s 1, scale at bottom left) at 5700 s for 50 m AGL. Arrows point to the four LFCBs present at this time and to the developing FFCB.

generation of horizontal vorticity is impacting the orien- A final analysis at 9400 s shows a strong similarity to tation of the vector. At 8700 s, the RFGF has the stron- that at 8700 s. Two source regions of parcels, both west gest values of solenoidal horizontal vorticity tendency and east of the low-level rotation, converge as before 2 2 2 2 (;5 3 10 4 s 2), followed by the LFCB (;3 3 10 4 s 2) (not shown). However, at 9400 s, a few parcels originate 2 2 and then the FFCB (;2.5 3 10 4 s 2). The amount of east of the rotation, translate west of the rotation, and solenoidally generated horizontal vorticity that is tilted then are enveloped by the RFD. One specific parcel and stretched within the vertical vorticity maximum is traverses the FFCB for a brief period of time, close to related to the residence time of parcels that move along the low-level circulation, before curving back to the east. these boundaries. While these few parcel trajectories differ slightly in lo- Backward trajectories at 8700 s reveal that parcels cation from those seen at 8700 s, their horizontal and originate in separate groups to the east and west of the vertical vorticity tendencies and impact on the vertical, vertical, low-level rotation (Fig. 17a). Parcels that arrive low-level rotation at 9400 s are similar to trajectories at from the east, entering the mesocyclone, do not vary 8700 s (described later). Parcels passing through the much in elevation and traverse the low-level inflow di- RFD at 9400 s again have two origins. One group of rectly into the area of maximum vertical vorticity. How- trajectories originates near the surface, whereas a few ever, parcels arriving from the west follow a much more originate from aloft. Overall, the two analyses at 8700 complicated path. All of these parcels pass through the and 9400 s show remarkable resemblance. RFD; however, some arrive from aloft, and descend to Keeping in mind that trajectory analyses inherently the ground, whereas others originate near the surface, contain some error, it is informative to look at vorticity ascend first, and then descend in the RFD (Fig. 17a). tendency terms along specific trajectories to assess the Parcels that descend from aloft contain positive vertical forcing of each low-level vertical vorticity maxima. vorticity from the start of their descent, whereas parcels Assessing the amount of streamwise vorticity that exists beginning at the surface contain solely horizontal vor- along each trajectory is important, as streamwise vorticity. Only certain parcel trajectories briefly parallel the ticity directly impacts the amount of vertical vorticity LFCB/RFGF before entering the region of low-level that can be stretched within the low-level mesocyclone vorticity situated along the RFGF. (Davies-Jones 1984; Rotunno and Klemp 1985). Two

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FIG. 13. Perturbation density potential temperature (colored, K) with simulated reflectivity (contoured at 20, 30, 40, 50, and 60 dBZ) and 10-min backward trajectories ending at 50 m AGL at 4200 s on either side of transient LFCBs. The solid black line indicates the main LFCB position, whereas dashed lines indicate developing LFCBs. representative trajectories were chosen for the main vorticity associated with the LFCB aloft (which exists vertical vorticity maximum at 8700 s in an attempt to west of the LFCB at the surface). This effect increases as quantify vorticity generation. Trajectory 13 was chosen parcels accelerate, descending toward the low-level me- for the easterly inflow, while trajectory 8 was chosen for socyclone. Specifically, the strong increase in negative y the westerly flow (Fig. 17b). solenoidal vorticity tendency at 8300 and 8600 s is noted Looking at trajectory 8 first (Fig. 18), a general increase as the trajectory approaches the west side of the low-level exists in both the positive x and negative y components of LFCB (see termination point of trajectory 8 in Fig. 17b). horizontal vorticity owing to the proximity and impact Analysis of trajectory 13 reveals less horizontal vor- of stretching and production of solenoidal horizontal ticity fluctuation for the air parcel coming from the

FIG. 14. Conceptual model of the evolution of low-level boundaries within the simulated supercell. The thick solid line represents the radar reﬂectivity boundary, whereas the other lines represent the RFGF (thick line), the LFCBs (evolving boundaries associated with the counterclockwise arrow), and the FFCB (thinnest line). Dashed segments indicate weak, developing, or dissipating portions of the boundaries.

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21 FIG. 15. Averaged vertical vorticity (s , colored) time–height plot for the control simulation and for the area directly encompassing the low-level mesocyclone from the surface to 6 km AGL and from 3600 to 10 800 s. The black line indicates the time at which ancillary outflow affects the analyzed storm, after which vorticity analysis is stopped. inflow (Fig. 19). Horizontal vorticity forcing and parcel for long over the duration of the trajectory (Fig. 20), height change little over the first 600 s of the parcel path. indicating that the vorticity associated with these par- However, as parcels near the low-level mesocyclone, cels is not being tilted downward appreciably during they are exposed to the strong solenoidal generation of descent. horizontal vorticity (8500 s) along the RFGF. The pro- In interpreting the ability of the principal storm up- duction of horizontal vorticity along this boundary is in draft to stretch tilted horizontal vorticity, it is important the opposite sense of the horizontal vorticity already to assess the streamwise component of this vorticity for existing in the inflow environment. In turn, the horizontal vorticity of air parcels along the trajectory is altered drastically by the end of the trajectory (;8650 s), resulting in a strongly east–southeastward-directed horizontal vorticity vector about equal in magnitude, but opposite in direction to that found in the storm-relative inflow air. Whether or not these inflow parcels attain horizontal vorticity opposite to that prescribed by the background environment is clearly a function of residence time along the RFGF. The east–southeastward horizontal vorticity vector in this case is tilted downward by the RFD, generating positive vertical vorticity (Fig. 20). Analysis of the vertical vorticity along trajectory 8 (Fig. 20) shows a gradual decrease in positive vertical vorticity over time as parcels descend to the surface, owing to a negative value of stretching as the parcel decelerates (especially beginning at 7800 s). Eventually, the stretching term reverses sign (8500 s), and vertical vorticity increases sharply through convergence as parcels reach the low-level updraft and are stretched in the FIG. 16. Horizontal vorticity (vectors), absolute magnitude 2 vertical direction. Vertical vorticity is never negative as shaded (s 1, scale at bottom left), and simulated reflectivity (con- the parcel descends from aloft to the surface. In addi- toured at 20, 30, 40, 50, and 60 dBZ) for 8700 s at 150 m AGL tion, the vertical tilting term does not drop below zero within the region of the low-level vertical vorticity maximum.

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22 FIG. 17. (a) Solenoidal generation of horizontal vorticity (colored, s ) with simulated reflectivity contours (at 20, 30, 40, 50, and 2 60 dBZ), horizontal solenoidal tendency (vectors, s 2, scale at bottom left), and 15-min backward trajectories preceding 8700 s. Ter- mination points of the trajectories are bordered by two white boxes. Selected times are indicated with arrows for trajectories 8 and 13. (b) Zoomed in on region of the low-level mesocyclone. Trajectories 8 and 13 are displayed. both trajectories (Fig. 21). Though there are fluctuations trajectory. As a result, inflow trajectory parcels provide along the path, the horizontal vorticity along trajectory 8 about one-third the positive vertical vorticity of those ultimately becomes mostly streamwise by the time the descending in the RFD to the LLVVM. parcel reaches the low-level vertical vorticity maximum. In summary, a remarkably varied set of processes in- The vorticity of the parcel in trajectory 13 initially starts fluence the LLVVM along incoming trajectories. Tra- out streamwise to the storm-relative inflow, but is al- jectory paths are critical in identifying not only the tered by the strong solenoidal generation of horizontal origin of air parcels interacting with the low-level me- vorticity along the RFGF and becomes mostly anti- socyclone but also how these air parcels impart vertical streamwise as it is tilted by the RFD into the vertical as vorticity to the LLVVM. For example, the RFGF has the parcel reaches the interface between the RFGF and a dramatic influence on the horizontal vorticity of any the downdraft air. Therefore, it is clear that both parcels inflow air parcels that move through it. Downdraft tilting impart positive vertical vorticity to the low-level vertical of antistreamwise vorticity created from this interaction is vorticity maximum, even though one trajectory exhibits a prominent source of positive vertical vorticity, tempered streamwise horizontal vorticity and the other anti- by the direction and strength of the solenoidal production streamwise horizontal vorticity. of horizontal vorticity, a strong function of residence time Another trajectory analysis is conducted for a low- within baroclinic zones. All trajectories from the north and level vertical vorticity maximum (LLVVM) at 9400 s. northwest of the LLVVM move through the RFD and are However, for brevity, only a brief discussion will be in- associated with solenoidal generation of horizontal vor- cluded here. As before, two representative trajectories ticity along the LFCB as they descend, with vertical vor- were chosen for the two different air mass origins affecting ticity remaining positive, and at periods even increasing the LLVVM. For the selected trajectory originating from with time. The updraft/downdraft interface is the ultimate the storm-relative inflow, parcels experience a similar source for the tilting seen in these low-level vertical vor- reversal of streamwise to antistreamwise horizontal ticity maxima. vorticity as they reach the RFGF. However, in this case, their interaction with the boundary is limited in time, 4. Comparison of simulation to past findings resulting in extremely weak antistreamwise vorticity that is then tilted into positive vertical vorticity by the While the supercell simulation presented shows RFD. For the trajectory originating from aloft within the a varied distribution and evolution of boundaries and air RFD (;400 m AGL), parcels descend cyclonically and, mass origins, there are similarities to past observational on average, undergo an increase in the strength of ver- and numerical modeling work. Keeping in mind that the tical vorticity, with solenoidal generation of horizontal three-boundary conceptual model is only being directly vorticity remaining positive for the majority of the inferred from this simulated storm, it is in many ways

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21 FIG. 18. All for trajectory 8 ending at 8700 s: (top left) x component of horizontal vorticity (s ), (top right) y 2 2 component of horizontal vorticity (s 1), (bottom right) y components of horizontal vorticity tendency terms (s 2), 2 and (bottom left) x components of horizontal vorticity tendency terms (s 2). a combination of past work. The observational studies of This feature could have been an early LFCB that Lemon and Doswell (1979) and Brandes (1978) show quickly dissolved, as in the conceptual model of the the presence of what could possibly be a FFCB and three-boundary evolution in this research (Fig. 13). In LFCB, respectively, but both boundaries together are addition, boundaries similar to the LFCB, extending not present in either of these papers. The presence of northward out of the low-level mesocyclone, were iden- both an LFCB and FFCB has not been previously tified in both Markowski et al. (2002) and Markowski identified or discussed as such in the literature. In fact, et al. (2011). Shabbott and Markowski (2006) found an 0 confusion may exist as to whether a specific boundary east–west gradient in ur in a number of supercells com- identified in a previous paper could be either the LFCB parable to that found in the simulated supercell in this or the FFCB, as north–south boundaries have been de- study. fined previously only as FFGFs before. Past modeling studies have identified a boundary Some observational papers have found supcrcells extending north from the low-level mesocyclone (e.g., without a LFCB (e.g., Dowell and Bluestein 2002a,b; Klemp and Rotunno 1983; Wicker and Wilhelmson Beck et al. 2006), while Brandes (1984) and many of the 1995; Adlerman et al. 1999). These boundaries appear to recent mobile Doppler radar papers (e.g., Wurman et al. most likely correspond to LFCBs, both because of their 2007a,b) show no evidence for any boundary aside from location and the indication of descending air west of the RFGF (it should be noted that these recent mobile their position, as revealed by trajectory analyses. Wicker radar papers lack thermodynamic data). Dowell et al. and Wilhelmson (1995) and Adlerman et al. (1999) show (2002) found evidence of a transient north–south times where the boundaries could be identified as either boundary north of the investigated mesocyclone prior FFCBs or LFCBs, if analyzed solely on their orientation to tornadogenesis and full hook echo development. and position relative to the low-level mesocyclone. Both

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21 FIG. 19. All for trajectory 13 at time 8700 s: (top left) x component of horizontal vorticity (s ), (top right) y 2 2 component of horizontal vorticity (s 1), (bottom right) y components of horizontal vorticity tendency terms (s 2), 2 and (bottom left) x components of horizontal vorticity tendency terms (s 2). Note scale of x and y components of 2 2 horizontal vorticity tendency: 10 4 and 10 5, respectively. papers also show evidence of two boundaries coexisting between inflow and outflow air, and a strong pressure in the forward flank of their modeled storm. gradient across the boundary. Past observational re- Many numerical modeling studies of supercells have search (e.g., Beck et al. 2006; Wurman et al. 2007a,b) used a potential temperature perturbation contour to has documented mainly minor wind shifts when these mark the leading edge of outflow in supercell thunder- boundaries are present, and modeling studies have not storm simulations (e.g., Klemp and Rotunno 1983; found them to be coincident with strong gradients of Rotunno and Klemp 1985; Wicker and Wilhelmson 1995; temperature or vertical motion (e.g., Wicker and Wil- Adlerman et al. 1999). In addition, simulations have shown helmson 1995; Adlerman et al. 1999). that multiple kinematic and baroclinic gradients can be The simulation shows two distinct regions of parcel associated with boundaries north of the mesocyclone. origins for the low-level mesocyclone: east and west/ For example, a distinct wind shift (associated with northwest of the low-level vertical vorticity maxima, a strong gradient in potential temperature) is evident with the latter origin containing parcels descending north and southeast of the mesocyclone presented by from aloft. The disparate location of these parcel ori- Adlerman et al. (1999, their Fig. 8a), yet the 21-K po- gins is similar to findings from Klemp and Rotunno (1983), tential temperature perturbation contour is displaced Rotunno and Klemp (1985), Wicker and Wilhelmson farther to the east. In addition, terms such as cold front (1995), and Adlerman et al. (1999). However, the gen- and gust front have been used in previous research to eration, direction, and impact of parcel horizontal vorticity define forward-flank boundaries found using perturba- along trajectories inbound to the low-level mesocy- tion potential temperature contours. A gust front is clone from the inflow differs from that of Rotunno and defined as having a strong wind shift, with delineation Klemp (1985). The simulation in the current study

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21 22 FIG. 20. (top left) Vertical vorticity (s ) and (top right) vertical vorticity tendency terms (s ) for trajectory 2 2 8 ending at 8700 s, (bottom right) vertical vorticity (s 1) and (bottom left) vertical vorticity tendency (s 2) terms for 2 2 trajectory 13 ending at 8700 s. Note scales on images [(bottom left) 10 3; (bottom right) 10 5]. shows that environmental inflow horizontal vorticity is present along the LFCB and RFGF during the descent along trajectories is mostly eradicated upon reaching of parcels through the RFD. the LLVVM. The simulation shows that the RFD does not pro- 5. Conclusions duce significant tendencies in negative vertical vorticity within descending parcels at the time analyzed, and that The purpose of this study is to use a high-resolution parcels originating west of the LLVVM maintain their simulation of a supercell thunderstorm to identify and positive vertical vorticity throughout the path and de- investigate the development, evolution, and impact of scent of the trajectory. This maintenance of positive low-level boundaries, assign an appropriate naming vertical vorticity during the descent of an air parcel convention, and elucidate the role of these boundaries in within the RFD has been explained theoretically by the generation of both horizontal and vertical vorticity. Davies-Jones and Brooks (1993). They showed that if Each of these boundaries is identified and named ac- baroclinity is present during such descent, the genera- cordingly, based on their location relative to the primary tion of horizontal vorticity when tilted just before updraft, and the dynamics associated with the creation and reaching the surface will produce a positive component maintenance of each boundary. Conceptual models have of vertical vorticity. This finding has been corroborated been developed to define the low-level position (Fig. 8) in past modeling studies (e.g., Adlerman et al. 1999), and and evolution (Fig. 14) of the boundaries in this case. the theory applies very well to the results of the simula- The RFGF is created early in the evolution of the tion, where baroclinic generation of horizontal vorticity storm as precipitation drag and evaporative cooling

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21 21 FIG. 21. Streamwise horizontal vorticity (dashed, s ) and total horizontal vorticity (solid, s ) along both trajectories (left) 8 and (right) 13 ending at 8700 s. within the RFD develop quickly, generating a strong horizontal vorticity along the RFGF. Trajectories region of higher pressure relative to the inflow air. The traversing the RFD originate from aloft (.500 m LFCB is created later, as modified environmental inflow AGL), containing parcels which, during descent, can (low-level inflow impacted by precipitation) meets maintain positive vertical vorticity. These parcels ac- downdraft air to the west of the boundary, originating quire solenoidally generated streamwise horizontal from hundreds of meters aloft. A dominant LFCB de- vorticity along the LFCB and RFGF during descent, velops much more slowly than the RFGF, with an evo- which is converted into positive vertical vorticity upon lution including development and dissipation of many reaching the surface ahead of the LLVVMs. This pro- precursor boundaries, as cyclonic storm-relative flow cess has been described through theoretical study by dominates transient bursts of outflow in the forward Davies-Jones and Brooks (1993) and confirmed in other flank. Ultimately, enough outflow develops for an LFCB supercell modeling studies (Wicker and Wilhelmson to become relatively stationary (Fig. 14). Lastly, the 1995; Adlerman et al. 1999). However, when inflow FFCB develops as precipitation in the northeastward parcels parallel the RFGF for enough time, just as much extent of the forward flank cools inflow air and begins to antistreamwise vorticity can be tilted negatively by the produce a broad pressure gradient. This air in turn meets RFD, also producing positive vertical vorticity. The inflow air that has not been modified by the storm and residence time for parcels paralleling boundaries with creates the FFCB. solenoidally generated horizontal vorticity ultimately The term ‘‘gust front,’’ as it has been used in previous controls the amount of positive vertical vorticity gen- studies, is not applicable when used to describe anything erated as tilting and stretching take place within the but the RFGF. Therefore, the term ‘‘convergence updraft. boundary’’ has been used to replace gust front in the The ability to determine whether the conceptual conceptual models put forth in this research. In the case models put forth in this study can be confirmed using in of the LFCB, horizontal pressure gradients are weak situ measurements within supercells is an important and the boundary does not separate outflow from un- future goal for understanding the impact of baroclinity modified storm inflow. The FFCB is also not a gust front, within these regions of the storm and, potentially, their as it does not exist within a sharp horizontal pressure impact on mesocyclogenesis and tornadogenesis. De- gradient and does not contain descending air west of the tection of any temporal variation of these boundaries boundary. during the lifespan of the supercell, as described in the The baroclinity and solenoidally generated hori- conceptual models presented within this paper, is crucial. zontal vorticity associated with the FFCB contribute Findings from the recent field phase of the VORTEX2 weak low-level vertical vorticity to the LLVVMs, as project may be significant to this end. In addition, use of trajectory analyses show few parcels parallel this different microphysics schemes (such as double-moment boundary. Trajectories do reveal, however, that en- schemes) and environmental soundings/hodographs is vironmental horizontal vorticity within inflow parcels of particular interest to identify changes that occur reaching the updraft is reduced or totally reversed in within the development and evolution of the low-level sign owing to the production of solenoidally generated boundaries.

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Acknowledgments. Portions of this research were ——, Y. P. Richardson, and J. Wurman, 2002: Observations of the supported by National Science Foundation Grant AGS- formation of low-level rotation: The 5 June 2001 Sumner 0800542, the Texas Tech Wind Science and Engineering County, Kansas storm. Preprints, 21st Conf. on Severe Local Storms, San Antonio, TX, Amer. Meteor. Soc., 465–468. Research Center, the Visiting Graduate Student program [Available online at https://ams.confex.com/ams/SLS_WAF_ at the National Center for Atmospheric Research, and NWP/techprogram/paper_47335.htm.] from supercomputing allocations donated by the Na- French, M. M., H. B. Bluestein, D. C. Dowell, L. J. Wicker, M. R. tional Center for Supercomputing Applications. Thanks Kramer, and A. L. Pazmany, 2004: Mobile, dual-Doppler are due to David Dowell, Curtis Alexander, Morris analysis of tornadogenesis: The 15 May 2003 supercell in Skamarock,Texas.Preprints,22nd Conf. on Severe Local Weisman, George Bryan, and Sylvie Lorsolo for their Storms, Hyannis, MA, Amer. Meteor. Soc., P10.3. [Available constructive input during this research. David Schultz online at https://ams.confex.com/ams/11aram22sls/techprogram/ and the other anonymous reviewers of this manuscript paper_81119.htm.] are also due thanks for their comments during the pub- Gilmore, M. S., J. M. Straka, and E. N. Rasmussen, 2004: Pre- lication process. cipitation uncertainty due to variations in precipitation particle parameters within a simple microphysics scheme. Mon. Wea. Rev., 132, 2610–2627. REFERENCES Glickman, T. S., Ed., 2000: Glossary of Meteorology. 2nd ed. Amer. Meteor. Soc., 850 pp. Adlerman, E. J., K. K. Droegemeier, and R. Davies-Jones, 1999: A Klemp, J. B., and R. 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