Transcript Understanding Weather and Climate Ch 5

How Do the Clouds Form?
The global water cycle
Ocean water covers 70% of the
Earth’s surface
More than 1000 major rivers flow on the
seven continents
Mississippi river (#6)
Amazon river (#1)
Yangtze river (#4)
Water vapor flows in the atmosphere
Clouds drifts with the winds
Precipitation drives many atmospheric circulations
which in turn transport water around the world
Hadley Circulation
Walker Circulation
Monsoons
A significant fraction of the human body
is water (~75%)
The remaining:
fat, protein,
carbonhydrate,
other solids
So the water in our bodies may come from …
A stream in the Niagara Fall …
A white cloud over the tropical Pacific …
A dew drop in the Amazon rainforest …
An ice sheet in the Arctic Ocean…
Therefore we need to protect the environment because any pollution we put
into the environment may someday come back into our bodies
Movie time!
Weather: Wet
Water (H2O ) is unique on earth because it
can exist in all 3 states (phases)
• An H2O molecule
• 3 states (gas, liquid, solid)
depending on how the molecules
are connected together
• Can change from any state to
any other state. Latent heat is
consumed or released in a
phase change
e.g. Evaporation -> liberation of
water molecules, requires energy
• Saturation: equilibrium between
evaporation and condensation
Methods to achieve saturation and condensation
•
Diabatic processes – add/remove heat
– Conduction (e.g. movement of air mass over a cold surface)
– Radiation (e.g. cooling of boundary layer air by longwave
radiation)
•
Adiabatic processes - no addition/removal of heat
– Add water vapor to air
– Mix warm air with cold air
– Cooling of air parcel when it rises (because air parcel expands
when it rises, like a balloon)
1st Law of Thermodynamics  expanding air cools,
compressed warms (like a manual hand air pump).
- pDa = cv DT
Forms of Condensation:
• saturation  liquid droplets or ice crystals
• condensation/deposition  dew, frost, fog, clouds
Different types of fog found throughout the U.S.
Clouds
• Clouds are instrumental to the Earth’s energy and moisture
balances
• Most clouds form as air parcels are lifted and cooled to
saturation (Lowering temperature to dew point  cloud
formation)
• Clouds have interesting distributions
Satellite observation of clouds - Projects
• NASA’s International Satellite Cloud Climatology Project
(ISCCP) Combine the measurements of 5 geostationary and 1-2
polar orbiting satellites. 1983-Now, cloud top height and optical
depth.
• NASA’s Earth Observation System including a set of polar
orbiting satellites (A-Train), especially CloudSat (with a cloud
radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle
information, detailed vertical structure.
Static Stability and the Environmental Lapse Rate
• Static stability – refers to atmosphere’s susceptibility to being displaced
• Stability related to buoyancy  function of temperature
• The rate of cooling of a parcel relative to its surrounds determines
its ‘stability’ of a parcel
1) rate of cooling of parcel (unsaturated v. saturated)
•
unsaturated – dry adiabatic lapse rate (DALR)
•
saturated – saturated adiabatic lapse rate (SALR)
2) rate of cooling of surrounding atmosphere
•
environmental lapse rate (ELR): an overall
decrease in air temperature with height
•
ELR is measured by weather balloon. Long-term
global average ELR = 0.65oC/100m.
Three possible outcomes:
1) absolutely unstable air
2) absolutely stable air
3)
conditionally unstable air
The three types of stability
Environment
Parcel
Parcel
Parcel
Environment
Environment
Absolutely
Unstable
Absolutely
Stable
Conditionally
Unstable
What stops ‘unstable’ air masses from rising indefinitely ?
1) Entrainment
• Turbulent mixing of ambient air into parcel
• Leads to evaporation along cloud boundaries
• Evaporation uses latent heat, cooling the cloud
 reduces buoyancy
Courtesy Russ Dickerson, U. Maryland
2) Encountering a layer of stable air (inversion)
• a rising parcel may reach a stable upper air
environment
• the parcel cooling rate will exceed that of
the ambient air
• the parcel will slowly cease ascension and
come to rest at some equal temperature level
• three types: radiation, frontal, subsidence
Cloud Properties
1. Cloud top height/pressure
2. Cloud thickness (optical depth)
3. Cloud coverage
• When clouds comprise more than 9/10th of the sky = overcast
• When coverage is between 6/10th and 9/10th = broken
• When coverage is between 1/10th and 6/10th = scattered
• Cloud coverage less than 1/10th = clear
NASA’s International Satellite Cloud Climatology Project (ISCCP)
Cloud Classification - commonly used in climate research
Why do clouds constitute a wildcard for climate change?
• Clouds are good reflectors of solar
(shortwave) energy and good
absorbers of earth’s (longwave)
energy.
• The net effect (cooling or warming)
depends on the type of cloud
• In a changing climate, increases in
some types of clouds would
promote warming, while increases
in others would cause cooling
• Climate models are limited in how
well they can simulate clouds:
• We don’t know all of the
processes that cause clouds
• Clouds form at scales much
smaller than models can
resolve
• Conclusion: clouds cause probably
the largest uncertainty in model
simulations of future climate.
Stronger
Greenhouse
effect
Stronger
Albedo
effect
Summary
• Global water (hydrological) cycle
• Water Vapor Basics (names of different phase changes,
latent heat)
• Two methods of achieving saturation and condensation
(diabatic vs. adiabatic processes). Different types of
condensation - dew, frost, fog (radiation, advection,
upslope, precipitation, steam), clouds.
• Formation of clouds: 3 types of stability. Two factors
limiting the height of clouds. 3 cloud properties. 9 ISCCP
cloud types.