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Thunderstorm Structure and Evolution Eric A. Pani The University of Louisiana at Monroe
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Background Thunderstorm Project (1949) Building block: Cell Cell identified by visual observation of cumuliform turrets during early stages of development before precip formed radar observations once precip formed Thunderstorms consist of more than one cell Each cell has distinct life cycle
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Visual appearance of cell Source: http://www.photolib.noaa.gov/historic/nws/wea00079.htm Cell 1Cell 2
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Radar appearance of cell Source: http://www.met.tamu.edu/ADRAD/tutor/RHI.gif) Cell 1 Cell 2 Cell 3 Cell 4 Cell 5
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Life Cycle of Thunderstorm Three stages (Towering) Cumulus Mature Dissipating Entire life cycle ~ 30-60 minutes but thunderstorms composed of many cells so storm life is longer Form where convergence exists and air conditionally unstable
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Towering Cumulus Stage (~15 minutes) Updraft strong enough so identifiable from ordinary convection Dominated by updrafts but may have downdrafts at cloud edge Air converges into cell from miles around, perhaps not uniformly w max ~ 10 ms -1 @ ~ 5 km with convergence ~ 2 x 10 -3 s -1 (10 -6 s -1 synoptic scale) May have adiabatic ascent in updraft core Precip begins in latter part of this stage with first radar echo detected around melting level Source: http://cjonline.com/images/050799/form1.jpg
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Towering Cumulus Stage (Source: http://www.photolib.noaa.gov/historic/nws/wea00088.htm)
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Mature Stage (~10 minutes) Often defined when precip reaches ground; presence of updraft/downdraft couplet may be better Downdraft results from precip drag initially, then evaporative cooling Downdraft air from middle troposphere -1°C buoyancy over 5 km produces downdraft of 15-20 ms -1 Gust front produced when downdraft reaches ground Downdraft dissipates cloud by evaporating droplets (d~20 μm) (Source: http://cjonline.com/images/050699/form2.jpg)
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Mature Stage
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Dissipating Stage (~10 minutes) Downdraft formation signals death of storm in non-severe cases Spreading downdraft below storm cuts off supply of high θ e air Precip aloft no longer supported by updraft, descends, and scavenges cloud drops Dissipates lower portion of cloud Source: http://cjonline.com/images/050699/form3.jpg)
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Dissipating Stage (Source: http://www.worldsurface.com/browse/entry.asp?entryid=5448)
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Classes of thunderstorms Single cell Non-severe: air mass thunderstorm Severe: supercell Multicell (all non-severe, all severe, or mixture) Individual: thunderstorm Cluster: MCC, tropical cloud cluster Line: squall line
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Single Cell Thunderstorms (Source: http://www.photolib.noaa.gov/historic/nws/wea00087.htm) (Source: http://www.inclouds.com/display.php?picID=WxClds06b_06) Air Mass Thunderstorm Supercell Thunderstorm
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Multicell Thunderstorms (Source: http://operations.ocs.ou.edu/~nebain/chase7/imagery.htm) (Source: http://orbit-net.nesdis.noaa.gov/arad/fpdt/pix/WDXSE19a.gif) Thunderstorm Squall Line
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Multicell Thunderstorms (Source: http://wind.mit.edu/~emanuel/geosys/fig6.8f.gif) (Source: http://www.ncdc.noaa.gov/img/satellite/satelliteseye/tornadoes/galva94/galvair1.gif) Mesoscale Convective Complex Tropical Cloud Cluster
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Propagation of Multicell Storms Cell motion Storm motion
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