chapter 11: severe weather!!
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Chapter 11: severe weather!!. thunderstorms (classification) tornadoes lightning, and hail. Thunderstorm classification a brief review. Thunderstorms often cluster into systems 100 miles or more in size. These are called mesoscale convective systems. MCSs come in many shapes, - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 11: severe weather!!
(a) thunderstorms (classification)(b) tornadoes(c) lightning, and (d) hail
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Thunderstorm classificationa brief review
th e re a re 3 ba s ic type s o f thu n de rs to rm s
a irm a ss ts(s in g le -ce ll, o rd in a ry)
m u lt i-ce ll ts su p e rce ll ts
in d iv id u a l th u nd e rsto rm s
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Thunderstorms often cluster into systems 100 miles or more in size.
These are called mesoscale convective systems.
MCSs come in many shapes, including squall lines, bow echoes and MCC (complexes).
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Fig. 11.2a
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Fig. 11.2b
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Airmass Thunderstorms
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Photo by NSSL
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Airmass thunderstorms have a life cycle … with three stages
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Mature airmass thunderstorms over the Pacific seen by the Space Shuttle
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Airmass Thunderstorms
• These are scattered small thunderstorms that form in mainly summer.
• They typically develop in warm, moist air masses away far from any fronts.
• They occur when the winds aloft are weak (little wind shear)
• They are also typically short-lived and rarely produce extreme winds and/or hail.
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The reason why an airmass thunderstorms is so shortlived is that there is little wind shear, therefore the rainy downdraft
quickly undercuts and chokes off the updraft.
Photo by Moller
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Multicell Thunderstorms
• Life cycle of any one of the cells of a multicell thunderstorm is like any air-mass thunderstorm.
• The life cycle of the multicell is much different due to the interaction of the cells one with another.
• The key to the long life of the multicell is the development of the gust front.
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Shelf cloud – some examples
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Fig. 11.11
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Multicell Thunderstorms
• Shelf Cloud often indicates rising air over the gust front.
• New cells develop in front of the storm.
• Gust front maintained by the cool downdrafts.
• Gust front appears like a mesoscale cold front.
• Same effects of pressure check, temperature drop, and wind shift upon passage.
• Outflow boundary is the remnant of a gust front.
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Multicell - cont'doutflow boundary as seen by ground-based radar
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Photo by Moller
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MeteorologyToday Quit
Chapter Menu
Thunderstorm Movement
1 Developing cell2 Mature thunderstorm3 Dissipating thunderstorm
Upper-level wind
Dry air
Warm, moist surface air
EW
S
N1
1
1
2
23
Multicell storms move slightly to the right of the upper-level wind
Multicell movement
youngcell
oldcell
Photo by Doswell
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south north
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Why do new cells form mainly on the southern side ?
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Supercell Thunderstorms• occur most frequently in the southern
Great Plains in spring.• compared to single cells, supercells are:
– rare– longer-lived– larger– organized with separate up- and
downdrafts.
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LP
photo credit:Nguyen
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Photo by Bill McCaul
low-precipitation supercells
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LP supercell
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photo credit:Nguyen
HP
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weak echo region (WER) in a supercell storm
• As the storm intensifies, the updraft becomes stronger and more erect.
• The result are:– the development of mid-level
echo overhang (WER)– a tighter reflectivity gradient
(hail is most common just north of the WER)
– a shift in cloud top position (right above the WER)
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Weak vs strong thunderstorm
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supercell - seen by ground-based radar
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Cross-section thru supercell - note BWER
54,000 ft topsNW SE
Note: lowest 7,000 ft cannot be seen by radarbecause of the curvature of the earth.
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Supercell Thunderstorms
• Supercells only form when strong upper-level winds blow, and a significant change in wind direction occurs, generally southeasterly at the surface and WSW in the upper troposphere.
• Wall clouds may be found below the storm’s cloud base -- significant rotation may be seen (tornado spawning area).
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Thunderstorm evolution and shear
• no shear
• strong shear
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Supercell formation in a clockwise-turning hodograph
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thunderstorm classification:a summary