thunderstorms: ‘ordinary’ or ‘single cell’ storms, multicell storms, supercell storms

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Thunderstorms: ‘ordinary’ or ‘single cell’ storms, multicell storms, supercell storms. Typical cumulonimbus – single cell thunderstorm – produces heavy shower, possibly with hail and lightning. Reading. Ahrens, Chapter 14: Thunderstorms and Tornadoes - PowerPoint PPT Presentation

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  • Thunderstorms: ordinary or single cell storms, multicell storms, supercell stormsTypical cumulonimbus single cell thunderstorm produces heavy shower, possibly with hail and lightning

  • ReadingAhrens, Chapter 14: Thunderstorms and TornadoesThis lecture + next (Lightning, tornadoes) will cover the topic.

  • What meteorological conditions precede a thunderstorm?A conditionally unstable atmosphereSubstantial boundary layer moistureA trigger to release the instabilityOn a skew T-log p plot: CAPE: Convective Available Potential Energy = energy that can be releasedCIN: Convective INhibition: = energy barrier that has to be overcome

  • Real example tephigram large amount of CAPE thunderstorm v.likelyCAPECAPE is given by the area between SALRand environmental lapse rate

  • An important forecaster tool for predicting thunderstorms: Maps of CAPE (contours) and vertical velocity (+)Fri Nov 7 12Z 2008http://expert.woeurope.eu/cape_frame.htm

  • Sunday 1200 (8 Nov 2009)

  • Monday 31 Oct 2011 (03z)

  • Ordinary or single cell thunderstorms Relatively smallIsolatedTypically just produce a single heavy shower, then dissipate.Very little vertical wind shear (come back to this later)

  • Stage 1: Cumulus

  • Cumulus stage (continued)Buoyant updraughtVertical velocity increases with height, to ~10 ms-1 at topSurrounding air mixed in (entrainment)Inside cloud, raindrops and supercooled drops grow, releasing latent heatAt edges, drops evaporate into entrained air moistens the surrounding air.As the environment moistens, successive updraughts sustain clouds to higher and higher levelsNo rainfall at this stage

  • Stage 2: Mature

  • Mature stage (continued)Top of cloud extends to near tropopause levels (>10 km), well above 100% freezing levelGrowth of drops & ice continues until updraught can no longer support them start to fallEntrainment of surrounding drier air tends to evaporate drops, cooling airBoth these processes lead to development of a downdraughtUpdraught+downdraught=cell single cell thunderstormMost intense stage heavy rain, thunder, lightningAnvil starts to form at top-40C0C5 km10 km

  • Stage 3: Dissipating

  • Dissipating stage (continued)Downdraught grows until it cuts off flow of air to the updraught the storm has its fuel supply stoppedRainfall declines and the lower part of the cloud evaporatesRainfall stops; all that is left is the anvilAll 3 stages pass in typically about 1 hour - a rapid, heavy shower

  • Summary: single cell stormCumulusMatureDissipating

  • Vertical wind shearWhy might this be important?

  • Approaching mature stageDissipating stage

  • Multi-cell thunderstormsThis type of thunderstorm is where once one cell subsides, another grows in its place, adjacent to the last cellThe downdraught causes a gust front when it meets the surface. This may push up surrounding moist air and trigger a new cell to develop.The presence of vertical wind shear can help thunderstorm development and persistence by separating the updraught from the downdraught

  • Vertical Wind ShearShear tilts the storm, helping it propagate, increases its lifetime and severityPromotes formation of new cells i.e. a multicell storm

  • Shear and rotation

  • Horizontal shear combined with an updraught can lead to a storm acquiring vorticity about a vertical axisVorticity associated with horizontal shear

  • Generating a supercell storm

  • Supercell, Kansas, rotating updraught

  • Supercell thunderstormsRotating updraughtRotation causes the storm to be more robust longer-lived, and therefore more dangerousForms an area of low pressure at centre of rotation, called a mesolowUpdraught centred on the low pressureCirculation around the low is in cyclostrophic balance

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