272 MONTHLY WEATHER REVIEW VOLUME 143 Supercell Low-Level Mesocyclones in Simulations with a Sheared Convective Boundary Layer CHRISTOPHER J. NOWOTARSKI,* PAUL M. MARKOWSKI, AND YVETTE P. RICHARDSON Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania GEORGE H. BRYAN 1 National Center for Atmospheric Research, Boulder, Colorado (Manuscript received 2 May 2014, in final form 5 September 2014) ABSTRACT Simulations of supercell thunderstorms in a sheared convective boundary layer (CBL), characterized by quasi-two-dimensional rolls, are compared with simulations having horizontally homogeneous environments. The effects of boundary layer convection on the general characteristics and the low-level mesocyclones of the simulated supercells are investigated for rolls oriented either perpendicular or parallel to storm motion, as well as with and without the effects of cloud shading. Bulk measures of storm strength are not greatly affected by the presence of rolls in the near-storm en- vironment. Though boundary layer convection diminishes with time under the anvil shadow of the supercells when cloud shading is allowed, simulations without cloud shading suggest that rolls affect the morphology and evolution of supercell low-level mesocyclones. Initially, CBL vertical vorticity perturba- tions are enhanced along the supercell outflow boundary, resulting in nonnegligible near-ground vertical vorticity regardless of roll orientation. At later times, supercells that move perpendicular to the axes of rolls in their environment have low-level mesocyclones with weaker, less persistent circulation compared to those in a similar horizontally homogeneous environment. For storms moving parallel to rolls, the opposite result is found: that is, low-level mesocyclone circulation is often enhanced relative to that in the corre- sponding horizontally homogeneous environment. 1. Introduction Klemp and Rotunno 1983; Rotunno and Klemp 1982, 1985; Wicker and Wilhelmson 1995; McCaul and Most numerical investigations of supercell thunder- Weisman 2001; Naylor et al. 2012)thisapproachdoes storms have used idealized, horizontally homogeneous not account for the inherent inhomogeneity of the environments without surface fluxes of heat, moisture, turbulent convective boundary layer (CBL) found in and momentum. Though fruitful in exposing basic many severe-storm environments. Increased comput- supercell dynamics and the relationship between storm ing power now allows for inclusion of more physical type and the characteristics of the environment (e.g., processes (such as radiation and land surface parame- Schlesinger 1975; Klemp and Wilhelmson 1978a,b; terizations) and better resolution of boundary layer turbulence than in earlier idealized simulations. With this expanded capability, we investigate how meso- * Current affiliation: Department of Atmospheric Sciences, g-scale horizontal variability in a CBL interacts with Texas A&M University, College Station, Texas. 1 The National Center for Atmospheric Research is sponsored supercell thunderstorms. by the National Science Foundation. Because the vast majority of significant tornadoes are spawned by supercell thunderstorms (Trapp et al. 2005), much of our research is concerned with their Corresponding author address: Christopher J. Nowotarski, Department of Atmospheric Sciences, Texas A&M University, low-level mesocyclones (LLMs). Numerous studies 3150 TAMU, College Station, TX 77843-3150. have demonstrated the importance of environmental E-mail: [email protected] properties such as low-level wind shear and moisture DOI: 10.1175/MWR-D-14-00151.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 09/29/21 04:52 AM UTC JANUARY 2015 N O W O T A R S K I E T A L . 273 (e.g., Kerr and Darkow 1996; Rasmussen and Blanchard that it might draw inflow from both roll updraft and 1998; Craven and Brooks 2004) in discriminating be- downdraft simultaneously, resulting in equal ingestion tween tornadic and nontornadic supercells, but it re- of air parcels with favorable and unfavorable thermo- mains unclear how the horizontal variability of these dynamic quantities. Yet, Crook and Weisman showed quantities in a CBL may affect the potential of a su- that inhomogeneities in the storm environment may percell to become tornadic. Moreover, inviscid dy- disrupt aspects of supercell evolution, including the namical arguments regarding tornadoes that form in gust front occlusion process. Other studies have noted supercell LLMs generally assume there is no preexist- the appearance of ‘‘feeder clouds’’ in the inflow envi- ing vorticity (z ) near the surface in supercell environ- ronment of supercells and have noted that these clouds ments; thus, the development of rotation next to the may be related to rolls and could signal storm in- surface (a prerequisite for tornadogenesis) requires tensification (Weaver et al. 1994; Weaver and Lindsey a downdraft to reorient and redistribute ambient hor- 2004; Mazur et al. 2009). izontal vorticity and/or horizontal vorticity generated Building upon these previous studies, this article pres- baroclinically within a supercell (e.g., Davies-Jones ents the results of simulations of supercells in a CBL, 1982; Davies-Jones and Brooks 1993; Markowski et al. focusing primarily on characteristics and evolution of 2008). As Nowotarski et al. (2014, hereafter NMRB14) LLMs. We compare these simulations against simulations and others (Arnottetal.2006; Markowski and Hannon that have horizontally homogeneous environments hav- 2006; Marquis et al. 2007) have shown, CBLs typically ing similar overall conditions (e.g., CAPE and shear). do contain patches of significant vertical vorticity that Given the quasi-linear organization of a CBL composed would precede storm development if it occurs. The extent of rolls, it is likely that the orientation of rolls relative to which this vorticity might affect the development to storm motion may, in part, dictate their effects on and evolution of LLMs in supercells has been an un- supercell LLMs. As such, we use two different hodo- answered question. graphs that result in sets of simulations with storm motion Previous studies have found sensitivity of low-level perpendicular to the boundary layer vertical shear and rotation to horizontal variations in the storm environ- convective rolls (hereafter ‘‘perpendicular-shear’’ simu- ment on the meso-b scale (;100 km). Richardson lations) or parallel to the boundary layer vertical shear (1999) found that isolated supercells in areas of in- and convective rolls (hereafter ‘‘parallel shear’’ sim- creased low-level moisture exhibited both stronger ulations). The following section describes our methods, updrafts and low-level rotation. Observations (Maddox including the experiment design and the model configu- et al. 1980; Weaver and Purdom 1995; Markowski et al. ration. Sections 3 and 4 present results and analysis from 1998; Rasmussen et al. 2000) have shown that many the perpendicular-shear and parallel-shear simulations tornadoes develop within supercells interacting with (respectively). Sensitivity to the initiation location of the mesoscale thermal boundaries. Atkins et al. (1999) supercells relative to the rolls is discussed in section 5.We found that stronger low-level rotation develops when offer concluding remarks in section 6. a simulated supercell moves along a mesoscale boundary, and that the mechanism for the development 2. Methods of the LLM is altered when a boundary is present. Wheatley and Trapp (2008) showed in real-data simu- NMRB14 describe the methodology used herein for lations that preexisting boundaries could also affect the simulating a realistic CBL composed of boundary layer strength and development mechanism of mesovortices rolls. The modeled rolls have aspect ratios (i.e., the in quasi-linear convective systems (QLCSs). horizontal distance between rolls divided by the Studies examining the effects of small-scale (order boundary layer depth) of ;3 and they are associated 1 km) horizontal variability on deep moist convection with quasi-periodic horizontal fluctuations in variables are limited (Carpenter et al. 1998; Crook and Weisman that are relevant to severe convective storms, such as 1998), and none have examined the specific case of temperature, moisture, convective available potential CBL rolls (often referred to as ‘‘horizontal convective energy (CAPE), convective inhibition (CIN), vertical rolls’’) interacting with supercells. Considering that the velocity, vertical wind shear, and storm-relative helicity horizontal scale of the perturbations associated with (SRH) throughout the boundary layer. The boundary rolls (,5 km) is smaller than that of a supercell thun- layer development and magnitude of the environmen- derstorm updraft (.10 km), bulk measures of storm tal variations are generally consistent with observa- strength (e.g., maximum updraft speed, maximum z) tions of rolls (e.g., LeMone and Pennell 1976; Reinking may be relatively unaffected by the CBL. The hori- et al. 1981; Atlas et al. 1986; Weckwerth et al. 1996, zontal extent of a supercell updraft is large enough such Markowski and Richardson 2007). Conditions in the Unauthenticated | Downloaded 09/29/21 04:52 AM UTC 274 MONTHLY WEATHER REVIEW VOLUME 143 TABLE 1. Summary of differences in configuration of supercell simulations in each experiment. Simulation name CBL FRAD CBL INVRAD CONTROL Radiation Full radiation No interaction of radiation with No radiation liquid water or ice Initial state