PHYS575CSI655 Introduction to Atmospheric Physics and Chemistry Lecture Notes

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PHYS-575/CSI-655Introduction to Atmospheric
Physics and ChemistryLecture Notes 6Cloud
Microphysics Part 3
Overview of Clouds 1. Nucleation of Water
Vapor 2. Warm Clouds 3. Water Content and
Entrainment 4. Droplet Growth (Warm Clouds) 5.
Microphysics of Cold Clouds 6. Artificial
Modification of Clouds 7. Thunderstorm
Electrification 8. Cloud and Precipitation
Chemistry
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6. Artificial Modification of Clouds and
Precipitation
The microstructures of clouds are influenced by
the concentrations of CCN and ice nuclei. The
growth of precipitation particles is a result of
instabilities that exist in the microstructures
of the clouds Two main types of
instabilities (1) In warm clouds the larger
drops increase in size at the expense of the
smaller droplets due to growth by the
collision-coalescence mechanism. (2) If ice
particles exist in a certain optimum range of
concentrations in a mixed cloud, they grow
by deposition at the expense of the droplets
(and subsequently by rimming and aggregation).
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• Cloud Precipitation Modification by Cloud
  Seeding
• Introducing large hygroscopic particles or water
  drops
• into warm clouds to stimulate the growth of
  raindrops
• by the collision-coalescence mechanism.
• (2) Introducing artificial ice nuclei into cold
  clouds (which
• may be deficient in ice particles) to
  stimulate the
• production of precipitation by the ice
  crystal mechanism.
• Introducing comparatively high concentrations of
• artificial ice nuclei into cold clouds to
  reduce drastically
• the concentrations of super-cooled droplets
  and thereby
• inhibit the growth of ice particles by
  deposition and
• rimming, thereby dissipating the clouds and
  suppressing
• the growth of precipitable particles

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Artificial Modification of Warm Clouds

• Introduce
• - small water droplets (radius 30 µm)
• hygroscopic particles (e.g. NaCl)
• into the base of a cloud these then grow by
  condensation and later by
• collision-coalescence as they are carried up and
  subsequently fall through
• a cloud.
• In the late 1940s, 50s and 60s attempts were
  made to seed warm clouds
• but the results were inconclusive.
• Seeding with hygroscopic particles has also been
  used in attempts to
• Improve fog visibility, but at present the most
  effective methods for
• dissipating fogs are brute force approaches
  such as evaporating fog
• droplets by ground-based heating.

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Artificial Modification of Cold Clouds
Project Cirrus, July 1946 Cloud ice particle
seeding with frozen carbon dioxide dry
ice Because large numbers of ice crystals that a
small amount of dry ice can produce, it is most
suitable for overseeding cold clouds. When
a Cloud is overseeded it is almost completely
converted to ice particles which then evaporate
(after being transported away from the seeding
site). This has been used to clearing fog at
international airports. Seeding with silver
iodide, cupric sulfide, organic materials can be
more effective than dry ice, but are more
expensive. Artificial seeding may lead to latent
heat deposition and increased buoyancy. This may
produce convective plumes that raise the air to a
level for free convection. Seeding has also been
attempted to reduce hail damage, but inconclusive.
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7. Thunderstorm Electrification
All clouds are electrified to some degree. In
vigorous convective clouds sufficient electrical
charges are separated to produce electric fields
that exceed the dielectric breakdown of cloudy
air, resulting in intracloud lightening
discharge. Most lightening occurs in cold
clouds, although rarely with warm clouds.
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Lightning
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Distribution of Electric Charges in Simple
Thunderstorms
http//www.britannica.com/thunderstorms_tornadoes/
images/ocliwea007a4.gif
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Cloud Electrification Lightning
Onset of strong electrification follows the
occurrence of heavy precipitation in the form of
graupel or hailstones. As the particle (rimer)
falls through the the cloud it is negatively
charged due to collisions with small particles
giving rise to a negative charge in the
main charging zone of the cloud. The smaller and
thus positively charged particles are carried
upwards by updrafts to the upper regions of the
cloud. As the electrical charges are separated,
the electric field intensity increases until it
exceeds that which the air can sustain. The
resulting dielectric breakdown assumes the form
of a lightening flash that can be either (1)
within the cloud itself, between clouds, or from
the cloud to the air (cloud flashes) or (2)
between the cloud and the ground (ground flash).
http//www.geog.ucsb.edu/joel/g110_w06/lecture_no
tes/thunderstorm/agburt11_03.jpg
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Lightning and Thunder
Ground Flashes that charge the ground negatively,
originate in the lower part of the main negative
charge center in the form of a discharge called a
stepped leader which moves downward toward the
Earth in discrete steps. Each step lasts for
about 1 µs, during which time the stepped leader
advances about 50m. The time interval between
each step is about 50 µs. When the negatively
charged leader is 10-100 m from the ground, a
discharge moves upward from the ground to meet
it. when contact is made, a large number
of electrons flow downward and create a highly
luminous lightning stroke.
The lightning flash heats the air to above
30,000K. The pressure in the path increases to
10-100 atm, creating a powerful shockwave, and
further out, a sound wave known as thunder.
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The Global Electrical Circuit
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Sprites, Elves and Blue Jets
Sprites are luminous flashes that last from a few
to a few hundred µs, and extend from 90 km
altitude down to cloud tops and more than 40 km
in the horizontal. Probably generated by an
electric field pulse associated with a cloud
lightning stroke. Elves are µs-long luminous
rings that are centered over a lightning
stroke at about 90 km altitude. They are
likely due to atmospheric heating from
the electromagnetic pulse generated by the
lightning stroke. Blue Jets are luminous cones
that extend upward from the tops of
thunderstorms and provide an electrical
connection between the thunderstorm and
ionosphere.
http//www.holoscience.com/news/img/Sprites.jpg
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8. Clouds and Precipitation Chemistry
Clouds serve as both sources and sinks of gases
and particles, and they redistribute chemical
species in the atmosphere. Precipitation
scavenges particles and gases from the atmosphere
and deposits them on the surface of the Earth
(acid precipitation - acid rain).

• Transport of Particles and Gases
• Gases and particles are carried upward by the
  updrafts that feed clouds.
• Pollutants are thus transported to the upper
  portions of the atmosphere where
• solar radiation can chemically destroy and/or
  modify them.
• Evaporation of clouds enhances water vapor which
  is the source molecule for OH.
• Oxidization of DMS and SO2 by OH and the
  production of aerosol is enhanced
• near clouds.

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Cloud and Precipitation Chemistry

• Nucleation Scavenging
• Dissolution of Gases in Cloud Droplets
• Aqueous-Phase Chemical Reactions
• Precipitation Scavenging
• Sources of Sulfate in Precipitation
• Chemical Composition of Rain
• Production of Aerosol by Clouds

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Cloud and Precipitation Processes
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Aerosol Transport