Chapter 11: lightning

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Chapter 11 lightning
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this girl is charged !!
Source Halliday, Resnick, and Walker,
Fundamentals of Physics
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Lightning is due to in-cloud electric charge
separation.This separation can only be
understood in view of cloud processes.
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Lightning Conductor
large electric potential
small electric potential
Source Viemeister, The Lightning Book
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lifecycle of a lightning strike
pilot leader
stepped leader
return stroke
textbook, Fig. 11.28
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Lightning Strike Frequency
Ground strikes /km2/yr
Source Ackerman and Knox, Meteorology
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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.
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review

• Final will cover
• 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

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Summary of chapter 7 how winds blow around the
globe

• General circulation model
• 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

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Summary of chapter 9 airmasses and fronts

• airmass classification
• cT
• cP
• mT
• mP
• frontal classification
• cold fronts
• warm fronts
• stationary fronts
• occluded fronts

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Summary of chapter 10 how a mid-latitude frontal
disturbance works

• Fronts exist, may strengthen, and may evolve
• 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

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Evolution of a frontal disturbance
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Chapter 10 review contd

• 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
• UL divergence occurs ahead of the Rossby trof
• UL divergence causes uplift, and cyclogenesis
  near the surface.
• These waves, in turn, are affected by the
  low-level cyclogenesis.
• 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 détre of these frontal
  disturbances is to transfer heat poleward

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fast
fast
slow
Note the advection of cold and warm airmasses
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Chapter 11 review

• All thunderstorm types require static instability
• Low-level warm, humid air
• Upper-level cool air
• The more instability, the stronger the updraft,
  and the more severe the storm can be
• Three types exist
• Ordinary (air mass, single-cell)
• Multicell
• Supercell
• The distinction between these types is based on
  wind shear
• Ordinary storms little shear ? short-lived
• Supercell storms strong shear -gt long-lived
• Thunderstorms are sometimes organized on the
  mesoscale as large storm clusters (maybe squall
  lines)

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Chapter 11 review contd

• 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
• 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