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 (~-40oC)

● Cloud condensation nuclei are needed to aid condensation

Cloud Condensation Nuclei

•CCN are described by the size of the particle

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%

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●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”

Cirrus

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

Stratus

Nimbostratus

Cumuloform clouds

●Large in vertical extent●May or may not precipitate●Result from vertical motion●Cumulus–“fair weather cumulus”●Cumulonimbus–“anvil cloud”

“Fair weather” cumulus

Cumulonimbus

Other cloud types●Mammatus●Lenticular●Kelvin-Helmholtz●Cloud Streets●Severe weather 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

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

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●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●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

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

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

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

Where is the stable layer?

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.

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●Absolutely Stable

●Absolutely Unstable

●Conditionally Unstable

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●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

Unstable

Unstable

Lifting a Parcel

Sources of Lift●4 ways to lift a parcel to the LCL–Frontal Boundary–Orographic–Convergence–Convection

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

CAPE

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

CAPE and CIN

More Uses for Skew-T’s●Finding cloud levels●Forecasting precipitation type

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’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