Investigating the transition from elevated multicellular convection to surface-based supercells during the tornado outbreak of 24 august 2016 using a WRF model simulation

Kevin Gray, Jeffrey Frame

Research output: Contribution to journalArticle

Abstract

On 24 August 2016, a tornado outbreak impacted Indiana, Ohio, and Ontario with 26 confirmed tornadoes. Elevated multicellular convection developed into surface-based supercells that produced several tornadoes, particularly near a differential heating boundary. This convective mode transition is of particular interest owing to its relatively rare occurrence. A WRF Model simulation accurately captures the environment and storm evolution during this outbreak. Trajectory analyses indicate that the multicellular updrafts were ini-tially elevated. Since nearly all of the vertical wind shear was confined to the lowest 1 km, significant rotation did not develop via tilting of horizontal vorticity until the storms began ingesting near-surface air. Near-surface vertical wind shear decreased outside of cloud cover owing to vertical mixing, while it was preserved under the anvil, allowing for large values of 0–1-km storm-relative helicity to persist north of a differential heating boundary. Analysis of the perturbation pressure field from the WRF Model output indicates that the development of relatively large nonlinear vertical perturbation pressure gradients coincided with when near-surface air began to enter the updrafts, resulting in upward accelerations in the lowest 2 km, below the level of maximum rotation. In strengthening updrafts, upward-directed buoyancy perturbation pressure accelerations may have offset the downward-directed nonlinear perturbation pressure accelerations above the level of maximum rotation, allowing the updrafts to intensify further.

Original languageEnglish (US)
Pages (from-to)1051-1079
Number of pages29
JournalWeather and Forecasting
Volume34
Issue number4
DOIs
StatePublished - Aug 2019

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supercell
tornado
updraft
convection
perturbation
wind shear
simulation
heating
pressure field
air
vertical mixing
pressure gradient
vorticity
cloud cover
buoyancy
trajectory

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

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title = "Investigating the transition from elevated multicellular convection to surface-based supercells during the tornado outbreak of 24 august 2016 using a WRF model simulation",
abstract = "On 24 August 2016, a tornado outbreak impacted Indiana, Ohio, and Ontario with 26 confirmed tornadoes. Elevated multicellular convection developed into surface-based supercells that produced several tornadoes, particularly near a differential heating boundary. This convective mode transition is of particular interest owing to its relatively rare occurrence. A WRF Model simulation accurately captures the environment and storm evolution during this outbreak. Trajectory analyses indicate that the multicellular updrafts were ini-tially elevated. Since nearly all of the vertical wind shear was confined to the lowest 1 km, significant rotation did not develop via tilting of horizontal vorticity until the storms began ingesting near-surface air. Near-surface vertical wind shear decreased outside of cloud cover owing to vertical mixing, while it was preserved under the anvil, allowing for large values of 0–1-km storm-relative helicity to persist north of a differential heating boundary. Analysis of the perturbation pressure field from the WRF Model output indicates that the development of relatively large nonlinear vertical perturbation pressure gradients coincided with when near-surface air began to enter the updrafts, resulting in upward accelerations in the lowest 2 km, below the level of maximum rotation. In strengthening updrafts, upward-directed buoyancy perturbation pressure accelerations may have offset the downward-directed nonlinear perturbation pressure accelerations above the level of maximum rotation, allowing the updrafts to intensify further.",
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