Air masses and fronts
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Transcript of Air masses and fronts
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Air masses and fronts
Textbook, Chapter 9
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Discussion How do precipitation systems form?
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How does this precipitation form?
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How does this precipitation form?
mou
ntai
ns
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How does thisprecip form?
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How does this storm form?
let’s revisit the lifting mechanisms …
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Five Mechanisms
1. Convective ascent
Cumulus clouds …
spontaneous buoyant
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The 4 other mechanisms are forced ascent …
(a) over a mountain (b) or a front
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(c) Convergence into surface low
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(d) Uplift forcedby jet stream
Upper-level divergence
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Areas of jet-stream driven ascent
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Cloud & precipitation formation mechanisms
• Buoyant ascent [bubble ascent]• Forced ascent [layer ascent]
a) Orographicb) Frontalc) Low-level convergence (friction)d) Upper-level divergence (jet stream)
• Many clouds are the result of several simultaneous processes.
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1. Buoyant ascent
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• We’ve talked about stability and parcel ascent (Chapter 4, Lab 7).
• Remember that in a conditionally unstable atmosphere, a cloud will continue to rise as long as its core temperature exceeds that of the surroundings
1. Buoyant ascent
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cumulus growth
stable
environmental lapse rate stable
conditionally unstable
stable
condi-tionally unstable
moistdry
Cumulus humilis Cumulus congestus Cumulonimbus
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Lifting up a mountain may cause destabilization, …which is why Cu clouds first form there
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Cloud & precipitation formation mechanisms
• Buoyant ascent [bubble ascent]• Forced ascent [layer ascent]
a) Orographicb) Frontalc) Low-level convergence (friction)d) Upper-level divergence (jet stream)
• Many clouds are the result of several simultaneous processes.
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(a) Orographic clouds
Altocumulus lenticularis, Greenland
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Quantifying the Foehn effect
The Foehn or Chinook effect
(web link)
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Chinook wall cloud
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Pop quiz: when a good westerly wind blows and heavy snowfall occurs over the Cascades, then in Yakima on the east side it
will be ……. than in Seattle on the west side.
A: sunnier but coolerB: sunnier, warmer, drier, and windierC: sunnier, cooler, more damp, and calmerD: cloudier, cooler, more damp, and windier.
Seattle Yakima
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Cloud & precipitation formation mechanisms
• Buoyant ascent [bubble ascent]• Forced ascent [layer ascent]
a) Orographicb) Frontalc) Low-level convergence (friction)d) Upper-level divergence (jet stream)
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(b) Uplift along fronts
A front is a sudden transition between airmasses
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Fronts separate airmasses
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continental maritime
• cP
• cT
• mP
• mT
Types of airmasses
polar
tropical
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Fig. 9.3
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Fig. 9.4
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Air masses have distinct lapse rates, and therefore also distinct cloud and precipitation characteristics
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Pop quiz: what airmass is
this ?
cPcTmPmT
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Pop quiz: what airmass is this ?
cPcTmPmT
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Pop quiz: what airmass is this ?
cPcTmPmT
Hint: this happens in winter
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Warm fronts
The cold air recedes …
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Warm front clouds
Where is the cold air??
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Fig. 9.16
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Cold fronts
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Cold front clouds
light rain or dry
(1) stable air ahead of cold front
(2) unstable air aheadof cold front
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Fig. 9.14
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Occluded fronts
Stage I Stage II
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Cross section through an occluded front
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Fig. 9.18
(western US)
(eastern US)
Cold occlusion
Warm occlusion
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3D view of fronts during the evolution of a frontal disturbance
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drylines
Thunderstorms, sometimes severe ones, can be triggered along the dryline.
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Case study: winter storm of 10-13 Dec 2000
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Cloud & precipitation formation mechanisms
• Buoyant ascent [bubble ascent]• Forced ascent [layer ascent]
a) Orographicb) Frontalc) Low-level convergence (friction) d) Upper-level divergence (jet stream)
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(c) Low-level convergence into lows or trofs
1) Tropical lows (hurricanes) extract their energy from latent heat release
2) Mid-latitude lowsextract most of their energy from the jet stream
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LOW-LEVEL CONVERGENCEINTO A TROF
Tropical cyclogenesis
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Cross section thru a tropical cyclone
LOW-LEVEL CONVERGENCE
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Check out some hurricane animations
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Tropical cyclones have different names in different oceans
There spin is always cyclonic, i.e. ccw in the north and cw in the south
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• Where do hurricanes form?– Away from the equator– Where the SST is at least
80F
August SST
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Hurricane Effects:Wind: causes most damage, but limited mortality.
Storm surge: a historical killer, now mitigated by evacuation.
Rain and Inland Flooding: take more lives than wind, but do less damage.
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coas
tal
Hurricane mortality and damage, United States 1900-2008
Hurricanes are covered in the textbook in Chapter 8
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Lows, and convergence, uplift, clouds, and precipitation, also occur at higher latitudes.
What energizes them?
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Answer: the jet stream !!
• Buoyant ascent [bubble ascent]• Forced ascent [layer ascent]
a) Orographicb) Frontalc) Low-level convergence (friction)d) Upper-level divergence (jet stream)
THIS WILL BE COVERED IN CHAPTER 10
End of Chapter 9