Cloud and Precipitation Chemistry

Introduction to Atmospheric Physics and Chemistry
Lecture Notes #6
Cloud 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
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.
6. Artificial Modification of Clouds and Precipitation
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
(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).
Cloud & Precipitation Modification by Cloud Seeding
(1) 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.
(3) 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
Artificial Modification of Warm Clouds
- 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 1940’s, 50’s and 60’s 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.
Cloud Seeding with Silver Iodide
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
Seeding has also been attempted to reduce hail damage, but inconclusive.
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.
Distribution of Electric Charges in Simple Thunderstorms
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).
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.
The lightning flash heats the air to
When the negatively charged leader is
above 30,000K. The pressure in the
10-100 m from the ground, a discharge
path increases to 10-100 atm, creating
moves upward from the ground to meet it. a powerful shockwave, and further out,
when contact is made, a large number of a sound wave known as thunder.
electrons flow downward and create a
highly luminous lightning stroke.
The Global Electrical Circuit
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.
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.
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
Cloud and Precipitation Processes
Aerosol Transport
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