4220 MONTHLY WEATHER REVIEW VOLUME 136 The Effect of Mesoscale Heterogeneity on the Genesis and Structure of Mesovortices within Quasi-Linear Convective Systems DUSTAN M. WHEATLEY* AND ROBERT J. TRAPP Purdue University, West Lafayette, Indiana (Manuscript received 26 June 2007, in final form 2 January 2008) ABSTRACT This study examines the structure and evolution of quasi-linear convective systems (QLCSs) within complex mesoscale environments. Convective outflows and other mesoscale features appear to affect the rotational characteristics and associated dynamics of these systems. Thus, real-data numerical simulations of two QLCS events have been performed to (i) identify and characterize the various ambient mesoscale features that modify the structure and evolution of simulated QLCSs; and then to (ii) determine the nature of interaction of such features with the systems, with an emphasis on the genesis and evolution of low-level mesovortices. Significant low-level mesovortices develop in both simulated QLCSs as a consequence of mechanisms internal to the system—consistent with idealized numerical simulations of mesovortex-bearing QLCSs— and not as an effect of system interaction with external heterogeneity. However, meso-␥-scale (order of 10 km) heterogeneity in the form of a convective outflow boundary is sufficient to affect mesovortex strength, as air parcels populating the vortex region encounter enhanced convergence at the point of QLCS–boundary interaction. Moreover, meso-␤-scale (order of 100 km) heterogeneity in the form of interacting air masses provides for along-line variations in the distributions of low- to midlevel vertical wind shear and convective available potential energy. The subsequent impact on updraft strength/tilt has impli- cations on the vortex stretching experienced by leading-edge mesovortices. 1. Introduction In the idealized numerical simulations of QLCSs (e.g., Thorpe et al. 1982; Rotunno et al. 1988; Weisman An important unresolved issue in severe weather re- 1992, 1993; Skamarock et al. 1994; Weisman and Davis search is the relationship between quasi-linear convec- 1998; Weisman and Trapp 2003; Trapp and Weisman tive systems (QLCSs) such as squall lines and bow ech- 2003) and other convective storms, the initial condi- oes and the complex mesoscale environments within tions are supplied by a one-dimensional sounding that which they evolve. Some observational results (e.g., varies only with height; horizontal homogeneity along Klimowski et al. 2000; Przybylinski et al. 2000; model vertical levels is assumed. The resultant class of Schmocker et al. 2000) suggest that mesoscale variabil- solutions—while arising from an incomplete formula- ity in the wind and thermodynamic fields due, for ex- tion of deep moist convection (see Balaji and Clark ample, to convective outflows can significantly affect 1988)—agree with observations of highly organized, the rotational characteristics of severe QLCSs. Never- self-sustaining QLCSs (e.g., Burgess and Smull 1990; theless, idealized numerical simulations produce appar- Jorgensen and Smull 1993). Many hypotheses put forth ently severe QLCSs in the absence of environmental to explain the morphology, dynamics, and severe heterogeneity. weather potential of QLCSs are derived from idealized modeling, and some have been validated by subsequent * Current affiliation: NOAA/National Severe Storms Labora- studies. For example, a growing body of observational tory, Norman, Oklahoma. evidence (e.g., Atkins et al. 2005; Wheatley et al. 2006; Wakimoto et al. 2006) has confirmed the role of low- level mesovortices in the production of damaging sur- Corresponding author address: Dustan M. Wheatley, NOAA/ National Severe Storms Laboratory, National Weather Center, face winds within QLCSs, as described in Trapp and 120 David L. Boren Blvd., Norman, OK 73072. Weisman (2003). E-mail: [email protected] But idealized numerical simulations that develop me- DOI: 10.1175/2008MWR2294.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 09/24/21 01:00 PM UTC MWR2294 NOVEMBER 2008 WHEATLEYANDTRAPP 4221 soconvective systems are unable to fully document the An issue especially in the latter observational studies environmental conditions affecting the life cycles of is the difficulty in establishing the unambiguous effect these systems, as they cannot reproduce the inherent of environmental heterogeneity, due to the lack of a four-dimensionality of the environment. homogeneous control. Toward this end, Atkins et al. (1999) used a cloud-resolving model to quantify the a. Conceptual models of storm–boundary influence of preexisting boundaries on supercell evolu- interactions that produce tornadoes tion. In their control “no-boundary” run, the model was A long series of modeling and observational studies initialized in the standard way, using a single-sounding have focused on the life cycles of convective storms in representation of a tornadic environment. Initial con- the presence of environmental heterogeneity. Consider ditions for the second, “boundary” experiment were the evidence that a kinematic boundary is at times a chosen to resemble the meteorological conditions near necessary condition for the development of nonsuper- Garden City, Kansas, on 16 May 1995, which was char- cell tornadoes (e.g., Carbone 1983; Brady and Szoke acterized by a preexisting trough line. In both sets of 1989; Wakimoto and Wilson 1989; Mueller and Car- experiments, warm thermal bubbles were used to initi- bone 1987; Lee and Wilhelmson 1997a,b). In the con- ate convection. ceptual model of nonsupercell tornadogenesis, small- The inception and evolution of low-level rotation scale circulations form along a convergence or shear [i.e, significant vertical vorticity of O(10Ϫ2 sϪ1) within boundary through the release of a horizontal shearing the primary updraft] within the storms simulated in instability. The circulations strengthen to tornadic in- the boundary runs tended to be earlier, stronger, and tensity through stretching in the updraft of overhead more persistent, when compared to the no-boundary convection. runs. Moreover, vorticity analyses of the two storms 1 A preexisting thermal boundary has been suggested revealed that the origin of rotation in the boundary as a necessary condition for the development of signif- runs differed in part from the usual conceptualization icant tornadoes [i.e., F2 or greater on the Fujita damage of low-level mesocyclogenesis (e.g., Rotunno and intensity scale (Fujita 1981)]. Maddox et al. (1980) de- Klemp 1985; Davies-Jones and Brook 1993), in which veloped a physical model of boundary layer wind fields streamwise horizontal vorticity along the forward- to explain the seeming natural tendency of thunder- (rear) flank gust front is vertically tilted by the primary storms to become more severe and even tornadic, upon updraft (rear-flank downdraft). Now, the low levels interaction with a synoptic-scale front or external thun- on the cool-air side of the boundary were a source re- derstorm outflow boundary. In their conceptual model, gion for parcels rich in horizontal baroclinic vorticity, boundary layer vertical wind profiles owing to shallow which then flowed into the updraft in a streamwise baroclinic zones enhance moisture content, conver- manner. This behavior was consistent in those simu- gence, and vertical vorticity, providing for tornadic lated storms that propagated along the boundary, thunderstorms in an otherwise unsupportive environ- sometimes at small angles toward the warm-air side of ment. the boundary. Observational studies born of the Verification of By employing an otherwise idealized model with me- the Origins of Rotation in Tornadoes Experiment soscale variability in the environmental conditions, the (VORTEX; Rasmussen et al. 1994) have considered a number of cases where tornadogenesis via thunder- experimental methodology of Atkins et al. (1999) and storm interactions with shallow baroclinic zones ap- more recently Richardson et al. (2007) represent the peared likely. In particular, Markowski et al. (1998) most robust treatments of numerically simulated con- found that nearly 70% of the significant tornadoes that vective storms interacting with their heterogeneous en- occurred during VORTEX were associated in some vironment. It should be noted, though, that their initial way with external shallow baroclinic zones. Moreover, conditions are defined analytically (without enhance- of this subset, most of the tornadoes occurred within ment by observations) to closely resemble desired en- the baroclinic zones of the boundaries, and generally vironmental conditions, and significant uncertainties within 30 km of the boundaries’ leading edges. exist within this input, as stated by the authors. Fur- thermore, the interaction is also necessarily “one way,” hence, the convective scale is not permitted to modify 1 Here, the qualifiers “preexisting” and “external” refer to a the mesoscale, which then cannot feedback to the con- thermal and/or kinematic boundary generated by a source outside the convective cell/system affected. Synoptic-scale fronts (i.e., vective scale. For these reasons, it is uncertain whether warm fronts and stationary fronts) and old thunderstorm outflow their idealized results can be generalized to the real boundaries are both examples of preexisting thermal boundaries. atmosphere. Unauthenticated | Downloaded 09/24/21 01:00 PM UTC 4222 MONTHLY WEATHER REVIEW VOLUME 136 b. Conceptual models of a QLCS interacting with