An Observational Examination of Supercell and Squall Line Thunderstorm Interactions Kevin M. Wagner & Adam J. French South Dakota School of Mines and Technology.

An Observational Examination of Supercell and Squall Line Thunderstorm Interactions

An Observational Examination of Supercell and Squall Line Thunderstorm InteractionsKevin M. Wagner &Adam J. French South Dakota School of Mines and Technology Atmospheric and Environmental Sciences Program

NSF Grant #AGS-1339469Motivation:Widespread severe weather outbreaks often contain multiple storm organizationsSupercells and squall lines generally producing most of the severe weather hazardsIt is unclear, however what effect the presence of neighboring convective storms may have on convective storm intensity and severity.

Goal:This project seeks to address the question of how does a squall line affect the structure and intensity of a nearby supercell thunderstorm?Use radar data to identify common changes to the structure and intensity of the supercell as a squall line approaches.

Previous Research:

French and Parker (2012) observed increases in low-level storm rotation, and changes in the type and frequency of severe weather reportsPrzybylinski (1995) noted increasing echo size and radar reflectivity factor

Previous Research:Recent observations by Bryan and Parker (2010) documented the environmental evolution ahead of an approaching squall line, Changes to vertical wind shear, CAPE, CIN, and precipitable waterParameters that are well known to affect supercell structure and organization.This past work suggests that the presence of a nearby squall line may be sufficient to alter supercell structure and intensity

Hypothesis:Squall line-generated perturbations to the local environment will lead to changes in supercell structureincreases low-level rotationincreases in metrics related to updraft strength and storm intensityVertically integrated liquid (VIL), maximum estimated hail size (MESH), and echo top (ET) height

Methodology:Obtained Level-II WSR-88D data from NCDC archivesInput radar data in Warning Decision Support System Integrated Information (WDSS-II) software and converted to netcdf format, dealiased data, and performed quality control.Automated algorithms, tools, diagnosis and forecasts of severe weather phenomena Grid Analysis & Display System(GrADS) used for display and analysisOutput displayed on a constant height grid(1-3km) Isolated each supercell to analyzed individual storm characteristicsCases:Map with dates

05/23/200805/24/201504/07/201503/18/201304/25/201405/29/2013Strongly forcedFavorable upper-level dynamics (jet, trough/SW, dcva, WAA, shear)Favorable lower level sfc features ( sfc low, fronts, dryline, etc)8Radar Reflectivity:

9Azimuthal Shear:

Rotation evolution is variable All cases have some form of weakening prior to mergerCall Azshear rotationNo specific trendCommon characteristicsSome form of Weakening before merger

10Estimated Echo Tops:Short-lived supercells have a general increasing trendLong-lived supercells have a general decreasing trend

KJGX SC_B 18 March 2013KRAX SC_A 25 April 2014KDDC SC_C 18 May 2008KENX SC_A 29 May 2013KDDC SC_B 24 May 2008KDDC SC_A 24 May 2008KJGX SC_A 18 March 2013* 2015 cases are not includedET indicates life cycle of the storm SL developing to matureLL Mature to dissipating*other influences to the one increasing (LLJ or SC to SC interactions)11 VIL/MESH:

Short-Lived SupercellRapid intensification of the supercells (~2 hours before merger)MESH values taper off as the squall line approaches before the mergerLong-Lived SupercellInitial quasi-steady intensityDeclines as the squall line approachesKDDC SC_B 24 May 2008KDDC SC_A 25 May 2015KDDC SC_A 24 May 2008KJGX SC_A 18 March 2013KJGX SC_B 18 March 2013KJKL SC_A 07 April 2015KRAX SC_A 25 April 2014KDDC SC_C 18 May 2008KENX SC_A 29 May 2013SL developing LL Mature stage to dissipating 12Rotation Tracks:

Rotation tracks from a majority of the supercells share a unique wobble characteristicThe wobble may be indicative of the squall line modifying the shear in the supercells environment

Describe wobbleRight turn then gradual return to mean wind vector LL SC another right turn just before the mergerWobble baby Wobble baby Wobble baby Wobble yeahGet in there, yeah, yeahEy big girl make em' back it up, make em' back it up13How does a squall line affect a nearby supercell thunderstorm?Trends in Azimuthal shear seems to be case dependent, but generally, the supercells exhibit a pronounced weakening within two hours of the mergerEcho top time series for short-lived supercells shows a general strengthening trend, while long-lived supercells shows a weakening trendVIL/MESH trends capture the intensification of the developing short-lived supercells, but VIL/MESH decrease around the merger for both short and long-lived supercellsRotation tracks exhibit a wobble which may be indicative of change in shear profile caused by the approaching squall line

Some of these features (Wobble) are seen in the model simulations of eventsShameless plug for Dr. Frenchs poster on the modeling side of the project14Other Considerations:Each supercell is observed in a different stage in its lifecycle when the squall line develops and upon mergerVarious other meteorological influence may be in play:Lifecycle of storms, supercells interactions with other supercells, diurnal effects , LLJ, cold pool interactions, anvil shading, etc.Errors with Radar dataMore cases are needed to examine the squall lines roleFuture Work:Continue to use this analysis framework on other cases to identify any common features in the structural evolution of supercellsCategorize the different types of events based on type of squall lineUtilizing VAD wind profiles to analyze wind variability

KUEX 06 May 2015Conclusions:Azimuthal shear seems to be time and case dependent, but generally, the supercells exhibit a pronounced weakening within two hours of the mergerEcho top time series for short-lived supercells shows a general strengthening trend, while long-lived supercells shows a weakening trendVIL/MESH trends capture the intensification of the developing short-lived supercells, but VIL/MESH decrease around the merger for both short and long-lived supercellsRotation tracks exhibit a wobble which may be indicative of change in shear profile caused by the approaching squall line

Thank You!

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Acknowledgments:NSF Grant # AGS-1339469Warning Decision Support System Integrated Information (WDSS-II)Grid Analysis and Display System Software (GrADS)National Climate Data Center (NCDC) Radar Data Archive