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Cloud Formation. Matt Rogers ( rogers@atmos.colostate.edu) Department of Atmospheric Science Colorado State University AIM Workshop 25 July 2006 Anchorage, Alaska. Water Cycle Revisited. Sources of water for clouds come from evaporation and transpiration

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cloud formation

Cloud Formation

Matt Rogers (rogers@atmos.colostate.edu)

Department of Atmospheric Science

Colorado State University

AIM Workshop

25 July 2006

Anchorage, Alaska

water cycle revisited
Water Cycle Revisited

Sources of water for clouds come from evaporation and transpiration

Sinks of water for clouds come in the form of precipitation and evaporation

What are the mechanisms that happen in between?

thirty seconds of thermodynamics
Thirty Seconds of Thermodynamics

Temperature is a measure of molecular energy - if a collection of molecules has a lot of energy, odds are, it’ll have a higher temperature as well (and vice versa.)

Gases behave (well, ideally, at least) independently of each other

If matter in the form of a liquid gains enough energy (which we call the latent heat of evaporation) it may undergo a phase change and become a gas.

If matter in the form of a gas loses enough energy (which we call the latent heat of condensation) it may turn back into a liquid.

water saturation
Water Saturation

Thought experiment - which beaker likely has

a warmer temperature? Why?

The temperature of a gas (say, water vapor) is closely related to its energy

The amount of water vapor and liquid water will depend on the total energy of the system - obeying the laws of thermodynamics and staying in equilibrium

describing saturation
Describing Saturation

In terms of a saturation vapor pressure, which is simply the

water vapor pressure (for a given temperature and

atmospheric pressure) that maintains equilibrium.

In terms of the Relative Humidity, which is the fraction of the

current vapor pressure (whatever it might be) to the saturation

vapor pressure it would have if it were in equilibrium.

Or, in terms of a dewpoint temperature, which is the

temperature that the vapor (at its current vapor pressure)

would have to be cooled to to reach its saturation vapor

pressure.

Nothing here depends on the dry air!

slide6
Dew
  • Surfaces cool strongly at night
    • Strongest on clear, calm nights
  • If a surface cools below the dew point, water condenses on the surface and dew drops are formed
frost
Frost
  • If the temperature is below freezing, the dew point is called the frost point
  • If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals
  • The resulting crystals are known as frost, hoarfrost, or white frost
cloud droplet formation
Cloud droplet formation
  • If the air temperature cools below the dew point (RH > 100%), water vapor will tend to condense and form cloud/fog drops
  • As with dew and frost, cloud drop formation prefers to condense on a surface of some sort - we call these particles cloud condensation nuclei (CCN)
  • CCN surfaces offer a locally lower saturation vapor pressure that facilitates condensation
  • The most effective CCN are water soluble.
  • Without these particles clouds would not form in the atmosphere
    • RH of several hundred percent required for pure water drop formation
aerosol sources
Aerosol Sources
  • Terrestrial Sources
    • Dust/sand/dirt particles
    • Smoke - volcanic, fires, and pollution (sulfates)
    • Pollens and spores
  • Oceanic Sources
    • Sea Salts
  • Chemical sources
    • Photodissociation
    • Heterogeneous/Homogeneous chemistry
cloud formation mechanisms
Cloud Formation Mechanisms

…can be anything that causes water vapor cool and condense

Direct cooling…

…lifting of an airmass…

cloud formation mechanisms1
Cloud Formation Mechanisms

…can be anything that causes water vapor cool and condense

…buoyant lifting…

…or mechanical forcing

cloud classification
Cloud classification

Clouds are classified by height, appearance, precipitation,

and category of vertical development

  • High Clouds - generally above 16,000 ft at middle latitudes
    • Main types - Cirrus, Cirrostratus, Cirrocumulus
  • Middle Clouds – 7,000-16,000 feet
    • Main types – Altostratus, Altocumulus
  • Low Clouds - below 7,000 ft
    • Main types – Stratus, stratocumulus, nimbostratus
  • Clouds of Vertical Development
    • Main types – Cumulus, Cumulonimbus
  • Nimbo- and -nimbus prefix/suffix
    • Denotes precipitation
low clouds
Stratus

Uniform, gray

Resembles fog that does not reach the ground

Usually no precipitation, but light mist/drizzle possible

Stratocumulus

Low lumpy clouds

Breaks (usually) between cloud elements

Lower base and larger elements than altostratus

Nimbostratus

Dark gray

Continuous light to moderate rain or snow

Evaporating rain below can form stratus fractus

Low Clouds
middle clouds
Altocumulus

<1 km thick

mostly water drops

Gray, puffy

Differences from cirrocumulus

Larger puffs

More dark/light contrast

Altostratus

Gray, blue-gray

Often covers entire sky

Sun or moon may show through dimly

Usually no shadows

Middle Clouds
high clouds
High clouds

White in day; red/orange/yellow at sunrise and sunset

Made of ice crystals

Cirrus

Thin and wispy

Move west to east

Indicate fair weather

Cirrocumulus

Less common than cirrus

Small, rounded white puffs individually or in long rows (fish scales; mackerel sky)

Cirrostratus

Thin and sheetlike

Sun and moon clearly visible through them

Halo common

Often precede precipitation

High Clouds
cirrostratus
Cirrostratus

Cirrostratus with Halo

vertically developed clouds
Cumulus

Puffy “cotton”

Flat base, rounded top

More space between cloud elements than stratocumulus

Cumulonimbus

Thunderstorm cloud

Very tall, often reaching tropopause

Individual or grouped

Large energy release from water vapor condensation

Vertically developed clouds
mechanically forced clouds
Mechanically Forced Clouds

Lenticularis

Wave clouds

clouds why we care
Clouds - Why We Care
  • Clouds transport energy from one area to another
    • Evaporation takes latent heat out of warm surface waters
    • Condensation releases same latent heat into atmosphere in a different location
    • Heat has been ‘carried’ by the water inside a cloud
latent heat release
Latent Heat Release

An average thunderstorm

contains several thousand

metric tons of water

Condensing 1 kg of water

releases ~ 2.26x106 J of

latent heat energy

An average thunderstorm containing around 1500 tons of water will release 3.45 billion Joules of energy

clouds why we care1
Clouds - Why We Care
  • Clouds affect the radiation budget by reflecting visible light from the sun (thus cooling the planet) and by trapping infrared radiation from the surface (thus warming the planet)
  • Reflection/trapping behavior depends on type of cloud…
salient tidbits
Salient Tidbits
  • In the infrared
    • Clouds absorb radiation from below, re-emit at the temperature of the cloud
      • Cirrus - very cold, emit little radiation
      • Stratus - very warm, basically emit like the surface
  • In the visible
    • Clouds reflect radiation based on the amount of cloud droplets
      • Cirrus - thin, not as reflective as thicker water clouds
      • Stratus - thicker, many water droplets, highly reflective
high clouds1
High Clouds

Thin, cold ice clouds reflect less sunlight

Extremely cold, emits infrared at colder temperatures, prevents warmer surface infrared from escaping to space

NET EFFECT: Warming

low clouds1
Low Clouds

Very thick water clouds reflect large amounts of sunlight

Very near the surface, temperature of the cloud effectively the same as surface. Infrared radiation is therefore about the same - almost like the cloud wasn’t there!

NET EFFECT: Cooling

challenges
Challenges
  • Climate modelling with clouds - need to get both type and amount correct, which is difficult
    • Overestimating high clouds - too much warming
    • Overestimation low clouds - not enough
  • Understanding cloud feedbacks
    • What does CO2 warming do to cloud populations?
    • How does increased aerosol pollution affect cloud types and amounts?
  • Observations
    • Layered cloud structures not seen from satellites