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Clouds, Cloud Formation, and Stability. Lab 6 October 12, 2009. Condensation. Water vapor does not readily condense on its own Water has high surface tension Needs unreasonably high relative humidities or very cold temperatures (~-40 o C)

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condensation
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

slide3

Cloud Condensation Nuclei

  • CCN are described by the size of the particle
cloud condensation nuclei
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 <100%

A particle that will not serve as a CCN is called hydrophobic.

Vapor usually will condense on these for RH >100%

slide5
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

cirriform clouds
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”

stratiform clouds
Stratiform clouds

Characterized by a horizontally uniform base

Forms in stable atmospheres

May or may not precipitate

May exist at any level

Layered

cumuloform clouds
Cumuloform clouds

Large in vertical extent

May or may not precipitate

Result from vertical motion

Cumulus

“fair weather cumulus”

Cumulonimbus

“anvil cloud”

other cloud types
Other cloud types

Mammatus

Lenticular

Kelvin-Helmholtz

Cloud Streets

Severe weather clouds

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

kelvin hemholtz waves
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

cloud streets
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

shelf and roll clouds
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

wall cloud
Wall Cloud

Associated with severe thunderstorms

Indicates area of strongest updraft

The strongest tornados form here

satellite imagery
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.

visible satellite
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 don’t show as well

ir satellite
IR Satellite

Pros- available at all hours- provides an estimate of cloud-top height

Cons- lower spatial resolution- low clouds don’t show because their temperatures are close to the surface temperature

Color enhancement table often applied to bring out important temperatures

Raw

Enhanced

slide24

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
slide25

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
slide26

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
stability
Stability

Where is the stable layer?

stability28
Stability

Stable Equilibrium

If the ball is displaced it will return to it’s 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 it’s new location.

stability29
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

conditions for stability
Conditions for Stability

Absolutely Stable

Absolutely Unstable

Conditionally Unstable

stable atmosphere
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

unstable atmosphere
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

examples
Examples

Unstable

Unstable

sources of lift
Sources of Lift

4 ways to lift a parcel to the LCL

Frontal Boundary

Orographic

Convergence

Convection

slide36
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 parcel’s temperature and the environment’s 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

slide38
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 parcel’s temperature and the environment’s 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

more uses for skew t s
More Uses for Skew-T’s

Finding cloud levels

Forecasting precipitation type

more uses for skew t s41
More Uses for Skew-T’s

Finding cloud levels – useful for aviation

Clouds are likely present at three layers on this diagram. Can you find them?

more uses for skew t s42
More Uses for Skew-T’s

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