Transcript Clouds, Cloud Formation, and Stability

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
-3
●Over Ocean: 300-600 cm
3
7
-3
●Over land: 10 – 10 cm
●
–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
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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