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Chapter 11: lightning
this girl is charged !!
Source: Halliday, Resnick, and Walker, Fundamentals of Physics
Lightning is due to in-cloud electric charge separation.
This separation can only be understood in view of cloud processes.
Lightning Conductor
large
electric
potential
small
electric
potential
Source: Viemeister, The Lightning Book
lifecycle of a
lightning strike
pilot leader
stepped leader
return stroke
textbook,
Fig. 11.28
Lightning Strike Frequency
Ground strikes /km2/yr
Source: Ackerman and Knox, Meteorology
lightning occurs mostly over land
reasons:
1. CAPE and convective updrafts are weaker for marine thunderstorms
2. the marine environment has fewer CCN and fewer ice nuclei.
review
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Final will cover
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chapter 7 global winds
chapter 9 airmasses and fronts
chapter 10 extratropical cyclones
chapter 11 thunderstorms and tornadoes
Final is similar in format and value to the two mid-semester tests
Summary of chapter 7: how winds blow around the globe
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General circulation model
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Hadley cell, Ferrel cell, Polar cell
trade winds, mid-lat westerlies, polar easterlies
Polar front; mid-latitude heat transfer by transient airmass advection
jet stream above the polar front (thermal wind)
effect of continents and seasons
ocean currents, and upwelling
Summary of chapter 9: airmasses and fronts
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airmass classification
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cT
cP
mT
mP
frontal classification
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cold fronts
warm fronts
stationary fronts
occluded fronts
Summary of chapter 10: how a mid-latitude frontal disturbance works
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Fronts exist, may strengthen, and may evolve
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Polar front (initial)
Developing phase (open wave)
Mature phase (occlusion first forms)
Dissipating stage (low far into the cold air)
This evolution is tightly connected to the jet stream
Evolution of a frontal disturbance
Chapter 10 review cont’d
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The jet stream is consistent with a large horizontal temperature gradient (the
atmosphere is baroclinic).
The jet stream has waves, called Rossby waves
These waves may first form in the lee of mountains (lee cyclogenesis)
These waves propagate, and are unsteady
The shorter waves are important for weather at the surface, because
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UL divergence causes uplift, and cyclogenesis near the surface.
These waves, in turn, are affected by the low-level cyclogenesis.
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UL divergence occurs ahead of the Rossby trof
Warm advection ahead of the surface low builds the UL ridge
Cold advection behind the surface low deepens the UL trof.
The evolution of midlatitude frontal disturbances is understood by the synergy between
UL wave evolution, and LL cyclone evolution (baroclinic instability).
Finally, the raison d’étre of these frontal disturbances is to transfer heat poleward …
fast
fast
slow
Note the advection of cold and warm airmasses
Chapter 11: review
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All thunderstorm types require static instability
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The more instability, the stronger the updraft, and the more severe the storm can
be
Three types exist:
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The distinction between these types is based on wind shear
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Thunderstorms are sometimes organized on the mesoscale as large storm clusters
(maybe squall lines)
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Low-level warm, humid air
Upper-level cool air
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Ordinary (air mass, single-cell)
Multicell
Supercell
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Ordinary storms: little shear  short-lived
Supercell storms: strong shear -> long-lived
Chapter 11: review cont’d
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Supercell storms are marked by
– Rotating updraft
• Due to the tilting of horizontal spin (from the wind shear)
• Strengthened by vortex stretching
– Separate downdraft
– Sometimes also
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• a hook echo
• a bounded weak-echo region
• A v-notch
Most tornadoes, and all severe ones (F3-F5), are spawned by
supercell storms
The supercell ‘mesocyclone’ may spawn a tornado by the same
mechanisms:
– Vortex tilting
– Vortex stretching
Tornadoes are classified from F0 to F5, according to intensity
Lightning results from charge separation in a thunderstorm
– supercooled droplets need to collided with graupel/hail