Transcript chapter14PowerPointLecture

Chapter 14
Wall Cloud associated with
a super cell thunderstorm
Thunderstorms
A storm containing lightening and
thunder; convective storms may have
heavy rain hail
 Ordinary Cell Thunderstorms

 Air-mass thunderstorms: limited wind sheer
 Stages: cumulus, mature, dissipating
 Entrainment, downdraft, gust front
A dissipating thunderstorm
Thunderstorms

Multi-cell Thunderstorms
 Thunderstorms that contain a number of
convection cells, each in a different stage of
development, moderate to strong wind
shear; tilt, over shooting top
Multicell storm
Thunderstorms

Multi-cell Thunderstorms
 Micro-bursts: localized downdraft that hits
the ground and spreads horizontally in a
radial burst of wind; wind shear, virga
Dust clouds from microbursts
Thunderstorms

Multi-cell Thunderstorms
 Gust Front: leading edge of the cold air out-
flowing air; shelf cloud, roll cloud, outflow
boundary
Thunderstorms

Severe thunderstorms: one of large hail,
wind gusts greater than or equal to
50kts, or tornado, tilted
updraft/downdraft
Shelf cloud
Thunderstorms

Multi-cell Thunderstorms
 Squall-line thunderstorms; line of multi-cell
thunderstorms, pre-frontal squall-line,
derecho
Pre frontal
Squall
line
Pre frontal
Squall
line
The thunderstorms are producing strong straight-line winds called a derecho
Thunderstorms

Multi-cell Thunderstorms
 Meso-scale Convective Complex: a number
of individual multi-cell thunderstorms grow in
size and organize into a large circular
convective weather system; summer,
10,000km2
Thunderstorms

Supercell thunderstorms
 Large, long-lasting thunderstorm with a
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single rotating updraft
Strong vertical wind shear
Outflow never undercuts updraft
Classic, high precipitation and low
precipitation supercells
Rain free base
A supercell thunderstormwith a tornado sweeps over Texas
Thunderstorms

Supercell thunderstorms
 Strong vertical wind shear
 Surface, 850mb, 700mb, 500mb, 300mb
conditions
 low-level jet
Thunderstorms

Supercell thunderstorms
 Cap and convective instability
Thunderstorms

Thunderstorms and the Dryline
 Sharp, horizontal change in moisture
 Thunderstorms form just east of dryline
 cP, mT, cT
Fig. 14-23, p. 384
Thunderstorms

Floods and Flash Floods
 Large floods can be created by training of
storm systems, Great Flood of 1993
 Flash floods rise rapidly with little or no
advance warning; many times caused by
stalled or slow thunderstorm
downtown Des Moines, Iowa, during July, 1993
Fig. 14-25, p. 387
Thunderstorms

Topic: Big Thompson Canyon
 July 31, 1976, 12 inches of rain in 4 hours
created a flood associated with $35.5million
in damage and 135 deaths
Fig. 1, p. 386
Flash Floods
Slow moving or Stalled thunder Storm, especially in
canyon areas
135 deaths in 1976 flood
12 inches of rain in 4 hours
(normal ~16 inches /year)
Thunderstorms

Distribution of Thunderstorms
 Most frequent Florida, Gulf Coast, Central
Plains
 Fewest Pacific coast and Interior valleys
 Most frequent hail Central Plains
Thunderstorms

Lightening and Thunder
 Lightening: discharge of electricity in mature
storms (within cloud, cloud to cloud, cloud to
ground)
 Thunder: explosive expansion of air due to
heat from lightening
 Electrification of Clouds: graupel and
hailstones fall through supercooled water,
ice crystals become negatively charged
 Upper cloud positive, bottom cloud negative
Thunderstorms

Observations: Elves
 Blue jets, red sprite, ELVES
Fig. 2, p. 390
Thunderstorms

The Lightening Stroke
 Positive charge typically on ground, cloud to
ground lightening
 Stepped leader, ground stroke, forked
lightening, ribbon lightening, bead lightening,
corona discharge
Thunderstorms
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Observation: Apple tree
 DO NOT seek shelter during a thunderstorm
under an isolated tree.

Lightening Detection and Suppression
 Lightening direction finder detects
radiowaves produced by lightening, spherics
 National Lightening Detection Network
 Suppression: seed clouds with aluminum
Fig. 3, p. 395
Fig. 14-32, p. 392
Fig. 14-33, p. 393
fulgurite
Fig. 14-34, p. 393
Fig. 14-35, p. 394
sferics
Fig. 14-36, p. 394