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A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary

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A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Dissertations & Theses in Earth and Earth and Atmospheric Sciences, Department Atmospheric Sciences of 5-2010 A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary Jennifer M. Laflin University of Nebraska at Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/geoscidiss Part of the Earth Sciences Commons Laflin, Jennifer M., "A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary" (2010). Dissertations & Theses in Earth and Atmospheric Sciences. 6. https://digitalcommons.unl.edu/geoscidiss/6 This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Dissertations & Theses in Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. A TARGETED MODELING STUDY OF THE INTERACTION BETWEEN A SUPERCELL AND A PREEXISTING AIRMASS BOUNDARY by Jennifer M. Laflin A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree Master of Science Major: Geosciences Under the Supervision of Professor Adam L. Houston Lincoln, Nebraska May, 2010 A TARGETED MODELING STUDY OF THE INTERACTION BETWEEN A SUPERCELL AND A PREEXISTING AIRMASS BOUNDARY Jennifer Meghan Laflin, M. S. University of Nebraska, 2010 Adviser: Adam L. Houston It is theorized that supercell thunderstorms account for the majority of significantly severe convective weather which occurs in the United States, and as a result, it is necessary that the mechanisms which tend to produce supercells are recognized and investigated. Airmass boundaries have been identified as a preferred location for supercell development due to the enhanced horizontal vorticity and forced ascent that are found along the boundary. This study examines the specific influence of a preexisting airmass boundary on supercell development through a set of idealized simulations. These simulations are based on a supercell which formed along an outflow boundary in the panhandle of Texas on 25 May 1999, and involve both homogeneous environments from the warm and cool sides of the boundary, as well as a representation of the actual environment with a boundary present. Detailed analyses of these simulations are then performed to determine the specific influence of the preexisting airmass boundary on supercell formation and morphology. It was found that the airmass boundary has three main impacts on the simulated storm: 1) enhancement of the updraft by forced ascent along the boundary, which allows a stronger right-splitting storm to develop, 2) production of a gust front through a combination of storm outflow and the cool airmass, which allows the storm to transition away from precipitation and continually draw in warm air, and 3) provision of enhanced horizontal vorticity that supports the development and maintenance of a low-level mesocyclone. i Chapter 1. Introduction ......................................................................................................................... 1 Chapter 2. Methodology ........................................................................................................................ 9 Chapter 3. Results .................................................................................................................................. 24 Section 3.1. Time Series Analysis ..................................................................................... 28 Section 3.2. HP Analysis ....................................................................................................... 36 Section 3.3. Storm Split Analysis ...................................................................................... 38 Section 3.4. Trajectory Analysis ........................................................................................ 41 Chapter 4. Conclusions and Summary .......................................................................................... 73 Acknowledgements ............................................................................................................................... 77 References ................................................................................................................................................ 78 1 Chapter 1 Introduction A specific class of thunderstorms known as a supercell (Browning 1964) is defined by having a deep (approximately one third of the depth of the storm), s-1; persistent (lasting ~30 min or greater) mesocyclone (vertical vorticity ≥ 0.01 aMoller majority et al. of 1994; the significantly Doswell 2001). severe Supercell weather thunderstorms associated with have deep been convection correlated such to - -1 and significant toasrnadoes hail with (EF diameter-3 and greater) ≥ 5 cm, non and tornadicaccount forwind a large gusts portion ≥ 33 m ofs damage which ofresults supercell from thunderstorms thunderstorms is (Moller necessa etry al. to 1994, protect Doswell life and 2001). property Advanced and for warning this reason, it is important to recognize the mechanisms and environments that tend to produceA supercellnumber of thunderstorms. studies have focused on environments that favor the development of supercell thunderstorms or, rather, the formation of a supercell from existing d convective availableeep convection. potential energy For example, (CAPE), it high has beenshear, found and low that convective moderate inhibition (CIN) are often found where supercells develop (Rasmussen and ters have been developed that Blanchard 1998; Thompson et al. 2003). Parame 2 combine shear and CAPE and can discriminate between convective modes, such as storm- -Jones 1984), the energy- relative helicity (SRH; Davies helicity indexeisman (Hart and and Korotky 1991; Davies 1993), the bulk Richardson number (BRN; W Klemp 1982), the vorticity generation parameter (Rasmussen and Blanchard 1998), and the supercell composite parameterhe use (SCP;of parameters Thompson in etoperational al. 2003), forecasting,among others. haveStudies lead such to theas these, creation as wellof thresholds as t or benchmark values to distinguish between convective modes, and by calculating the values of these parameters, an environment can be diagnosednherently as supportive flawed inor that unsupportive the soundings of supercells. from which they However, these values are i were calculated may not be representative of the storm’s environment (Houston et al. 2008). In addition, it is also important to recognize mechanisms that create environmentalenvironments supportive shear can be of createdsupercells. by aWhile favorable optimal synoptic values pattern, of CAPE mesoscale and features such as an airmass boundary can enhance an environment that is marginally supportive, or even unsupportive, of supercells on a larger sc the introduction of a mesoscale feature may create more favorable valuesale. for While parameters such as shear, it also enhances the environment in methods that are not captured through parcel theory, such as locally enhanced vertical motion or additiona Anl airmassenvironmental boundary vorticity. is most simply defined as a demarcation between two airmasses with different densities, usually characterized by a temperature difference in the range of five to ten kelvins, and can be readily observed through 3 boundariessatellite, radar, are andthose surface that are observations not formed (Maddox by the storm et al. in 1980). consideration, Preexisting and airmass thus exclude include anthe outflow storm’s boundary own gust createdfront. Examples by another of astorm preexisting or by convection airmass boundary that has dissipatedAlthough or exited a boundary the area, comprises or a synoptic the frontarea betweensuch as a two cold relatively or warm front. homogeneous environments, the local boundary environment is quite different than either of the surrounding airmasses (Maddox et al. 1980). This local environment is anquite airmass complex, boundary and has help profound to support implications both storm for lconvection.ongevity and Several rotation properties in of thunderstorms, thus enhancing an environment to become more supportive of forcedsupercells. upward As denser (Figur air wedges underneath the strengthens less dense theair above,storm’s air updraft parcels and are e 1.1), creating lift which largeassists area with of storm CIN near maintenance the surface, and the longevity. forced ascent In environments along an airmass with lowboundary CAPE or a could promote updraft maintenance in an environment that may otherwise be detrimen tal to the storm. An area of moisture convergencethe lifted condensation is also present level along (LCL ) andboundaries level of (Maddoxfree convection et al. 1980), (LFC) whichand can lowers extend into the mid-levels, decreasing the chance for parce and moisture convergence,l dilution (Houston boundaries and also Niyogi generate 2007). vertical In addition vorticity to forced via the ascent four components of the vertical vorticity tendency equation: 4 = ( ) ( ) + − ∙ ∇ − − � − � � − � (1.1) which are, from left to right, advection of vertical vorticity, stretching of vertical viavorticity, the terms tilting in ofthe horizontal horizontal vorticity, vorticity and equations:
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