Clouds,%20Cloud%20Formation,%20and%20Stability

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Clouds, Cloud Formation, and Stability

• Lab 6
• October 12, 2009

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Condensation

• Water vapor does not readily condense on its own
• Water has high surface tension
• Needs unreasonably high relative humidities or
  very cold temperatures (-40oC)
• Cloud condensation nuclei are needed to aid
  condensation

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Cloud Condensation Nuclei

• CCN are described by the size of the particle

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Cloud Condensation Nuclei

• Aerosol a fine suspended solid or liquid
  particle in a gas
• Cloud droplets can form on both insoluble and
  soluble particles
• A particle that will serve as CCN is called
  hygroscopic or hydrophillic
• Vapor may condense at RH lt100
• A particle that will not serve as a CCN is called
  hydrophobic.
• Vapor usually will condense on these for RH gt100

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CCN

• Sources are dust, volcanoes, factory smoke,
  forest fires, sea salt
• Over Ocean 300-600 cm-3
• Over land 103 107 cm-3
• More in urban areas, less in rural
• Aerosol concentrations decrease with height
• Very light, stay suspended for a long time

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Cirriform Clouds

• Usually exist above 16,000 feet
• Generally thin, sometimes partially translucent
• Comprised of ice crystals
• Absorb longwave radiation, but are bright and
  reflective (have a high albedo)
• Rarely precipitate
• Virga
• Cirrus (Ci)
• Called mares tails

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Cirrus
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Stratiform clouds

• Characterized by a horizontally uniform base
• Forms in stable atmospheres
• May or may not precipitate
• May exist at any level
• Layered

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Stratus
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Nimbostratus
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Cumuloform clouds

• Large in vertical extent
• May or may not precipitate
• Result from vertical motion
• Cumulus
• fair weather cumulus
• Cumulonimbus
• anvil cloud

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Fair weather cumulus
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Cumulonimbus
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Other cloud types

• Mammatus
• Lenticular
• Kelvin-Helmholtz
• Cloud Streets
• Severe weather clouds

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Mammatus clouds

• Precipitation evaporates out of anvil
• Evaporation cools the air and it sinks
• If drops are large, mammatus will be long lived

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Lenticular Clouds

• Stationary, lens-shaped clouds over mountains at
  high altitude
• Stable, moist air flows over mountain, creating a
  large scale standing wave
• Indicates region of turbulence

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Kelvin-Hemholtz Waves

• Form when two parallel layers of air are moving
  at different speeds and in different directions
• Upper layer is usually faster
• Very short lived

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Cloud Streets

• Form due to horizontal rolls in the atmosphere
• Also due to uneven surface heating
• Clouds form over updrafts in rolls
• Occurs more frequently over the ocean

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Shelf and Roll Clouds

• Low, horizontal, wedge-like cloud
• Shelf Attached to Parent Storm
• Roll Removed from Parent Storm
• Formation is due to gust front from thunderstorms

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Wall Cloud

• Associated with severe thunderstorms
• Indicates area of strongest updraft
• The strongest tornados form here

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Satellite Imagery

• Visible imagery essentially a black and white
  camera on a satellite. Measures brightness in the
  visible spectrum.
• Infrared imagery measures infrared radiance from
  the object (ie, the surface or cloud top) it is
  pointed at. From blackbody theory, the
  temperature of the object can be found since
  temperature changes with height, the cloud-top
  height can then be estimated.

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Visible Satellite

• Pros- good at showing low clouds and fog-
  available in high spatial resolution
• Cons- only works in daylight- clouds can be
  confused with reflective features like snow-
  optically thin clouds like cirrus dont show as
  well

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IR Satellite

• Pros- available at all hours- provides an
  estimate of cloud-top height
• Cons- lower spatial resolution- low clouds
  dont show because their temperatures are close
  to the surface temperature
• Color enhancement table often applied to bring
  out important temperatures

Raw
Enhanced
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Clouds and Satellite Imagery

• The bright, puffy areas in the visible image on
  the right are cumulus and cumulonimbus clouds
  (the cumulonimbus are fuzzier around the edges).
  Notice how the cloud tops over the Front Range
  are cold in the IR imagery

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Cirrus in Visible vs. IR

• Because cirrus are cold and optically thin
  (meaning the sun can be seen through the cloud),
  they are more easily seen in the IR than the
  visible

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Low clouds/fog in visible vs. IR

• Because low clouds are bright and warm, they are
  easily seen in the visible, but not the IR

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Stability
Where is the stable layer?
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Stability

• Stable Equilibrium
• If the ball is displaced it will return to its
  original position
• Unstable Equilibrium
• If the ball is displaced it will accelerate away
  from the equilibrium point
• Neutral Equilibrium
• If the ball is displaced it will stay in its new
  location.

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Stability

• In the atmosphere we can use the environmental
  temperature and dew point profile to determine
  the stability of a given sounding
• In an stable atmosphere, a displaced parcel will
  return to its original position
• In an unstable atmosphere, a displaced parcel
  will continue to move in the direction it was
  pushed

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Conditions for Stability

• Absolutely Stable
• Absolutely Unstable
• Conditionally Unstable

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Stable Atmosphere

• Vertical motion is suppressed
• This can be produced by an inversion, which can
  be caused by
• Cooling of the surface at night
• Subsiding air (frequently associated with a ridge
  of high pressure)
• The tropopause is very stable due to the
  inversion caused by ozone in the stratosphere
• This means that storms cannot penetrate into the
  stratosphere

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Unstable Atmosphere

• Buoyant parcels are accelerated upward
• As they rise and cool, they are still warmer than
  the environment since the environment is cooling
  faster than the adiabatic lapse rate
• Larger instabilities lead
• to larger updrafts
• Large updrafts lead to
• the formation of
• cumulonimbus clouds
• and thunderstorms

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Examples
Unstable
Unstable
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Lifting a Parcel
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Sources of Lift

• 4 ways to lift a parcel to the LCL
• Frontal Boundary
• Orographic
• Convergence
• Convection

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CAPE

• CAPE Convective Available Potential Energy
• CAPE is the energy available to a rising parcel
  to accelerate it
• On a Skew-T, CAPE is proportional to the area
  between the parcels temperature and the
  environments when the parcel is warmer
• CAPE gives an upper limit on how high updraft
  speeds can get in a severe storm
• High values of CAPE are associated with the
  possibility of strong convection

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CAPE
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CIN

• CIN Convective INhibition
• This is the energy that must be overcome in order
  to lift a parcel to its LFC
• On a Skew-T, CIN is proportional to the area
  between the parcels temperature and the
  environments when the parcel is colder
• Large values of CIN will prevent the formation of
  storms, but often the presence of some CIN can
  add strength to a storm if this energy is overcome

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CAPE and CIN
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More Uses for Skew-Ts

• Finding cloud levels
• Forecasting precipitation type

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More Uses for Skew-Ts

• Finding cloud levels useful for aviation

Clouds are likely present at three layers on this
diagram. Can you find them?
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More Uses for Skew-Ts

• Forecasting precipitation type

The 00C isotherm in this skew-T shows that the
precipitation will fall through a layer which is
above freezing, thus implying that freezing rain
is possible