Thunderstorm Structure and Evolution

Eric A. Pani

The University of Louisiana at Monroe

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

Visual appearance of cell

Source: http://www.photolib.noaa.gov/historic/nws/wea00079.htm

Cell 1 Cell 2

Radar appearance of cell

Source: http://www.met.tamu.edu/ADRAD/tutor/RHI.gif)

Cell 1

Cell 2

Cell 3Cell 4

Cell 5

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

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 wmax ~ 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

Towering Cumulus Stage

(Source: http://www.photolib.noaa.gov/historic/nws/wea00088.htm)

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)

Mature Stage

Dissipating Stage (~10 minutes) Downdraft formation signals

death of storm in non-severe cases

Spreading downdraft below storm cuts off supply of high θe 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)

Dissipating Stage

(Source: http://www.worldsurface.com/browse/entry.asp?entryid=5448)

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

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

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

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

Propagation of Multicell Storms

Cell motion

Storm motion