Download Understanding Weather and Climate Ch 7

Clouds and rain formation
Review of last lecture
• 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.
The most common atmospheric
circulation structure
H
L
Radiation
Cooling
or No
Heating
Convection
Heating
Latent/Sensible
Conduction
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Satellite observation of clouds
• 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.
Clouds
• Clouds consist of many tiny water
droplets (~10 μm) and/or ice particles
• Most clouds form as air parcels are
lifted and cooled to saturation
• Clouds have a wide range of height
and thickness
• Clouds are instrumental to the Earth’s
energy and moisture balances
Level of saturation/condensation
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 both good reflectors
of solar radiation (cooling effect)
and good absorbers of earth
emitted longwave radiation
(warming effect).
• The net effect (cooling or
warming) depends on the type of
cloud
• In a changing climate, increases
in high thin clouds would promote
warming, while increases in low
thick clouds would cause cooling
• Climate models have difficulties
in simulating clouds, especially
low thick clouds (stratocumulus)
• Conclusion: Clouds cause the
largest uncertainty in model
simulations of future climate.
Stronger
warming effect
Stronger
cooling effect
Video: The climate wild card
• https://www.youtube.com/watch?v=Py1dE
FKuJJU
Global distribution of precipitation
Precipitation formation - cloud drop growth
• Not all clouds precipitate due to their small
sizes and slow fall rates
• Balance between gravity and frictional
drag  eventually become equal to
achieve terminal velocity VT, which is
proportional to the square root of cloud
drop radius VT=c r0.5 ,where r is drop
radius and c is a constant.
• For a cloud drop to fall, its terminal
velocity must exceed the vertical velocity
of the upward-moving air parcel.
Otherwise it will be carried up.
• Cloud drop growth is required for
precipitation to form
Fgravity
Fdrag
Mechanisms for cloud drops to grow larger
1. Collision Coalescence (warm clouds, T > 0 C, form rain)
2. Bergeron Process (cool/cold clouds, T < 0 C, form snow)
Cold Clouds
Cool Clouds
1. Collision Coalescence: Growth in Warm Clouds
• Process begins with larger collector drops
which have higher terminal velocities
• Collector drops collide with smaller drops and
merge with them (coalesce). Coalescence
efficiency is generally very high, indicating
that most collisions result in the two drops
joining.
• If collector drop is too big: compressed air
beneath falling drop forces small drops aside
• If collector drop is too small (same size as
other drops) it will fall at same speed and no
collision will occur
• So, collection efficiency is greatest when the
size of collector drop is slightly larger than the
size of the other drops
• After the collector drops become large, the
larger one among them can serve as a “supercollector” to collide with other collector drops
Raindrop shape and maximum size
• Determined by competition between surface
tension and frictional drag. Frictional drag is
larger at the bottom than at the top
• Small drop (<0.08in): frictional drag <<
surface tension  Sphere shape
• Medium-size drop (0.08in<size<0.25in):
frictional drag approaches surface tension 
Parachute shape
• Large drop (>0.25in): frictional drag at bottom
> surface tension  Split (The surface tension
at the top allows the raindrop to remain more
spherical while the bottom gets more flattened
out.)
• Maximum drop size of about 0.25in or 5 mm
Summary
• Formation of clouds: 3 types of stability. Two factors
limiting the height of clouds.
• 3 cloud properties. 9 ISCCP cloud types. Why do clouds
constitute a wildcard for climate change?
• Forces acting on a cloud/rain droplet. Terminal velocity.
How does it change with cloud drop radius?
• Growth mechanisms for rain and snow
• Formation of rain: coalescence process (the collector is
larger than the cloud droplets but not too large)
Works cited
• http://www.edudemic.com/study-finds-most-people-thinkcloud-computing-is-run-on-actual-clouds/
• http://hyperphysics.phyastr.gsu.edu/hbase/electric/diph2o.html
• http://nyffetyff.deviantart.com/art/Raindrop-189805290
• http://www.its.caltech.edu/~atomic/snowcrystals/photos/p
hotos.htm
• http://www.crh.noaa.gov/unr/?n=06-04-99_pg1
• http://www.clker.com/clipart-cartoon-sun.html
• http://pmm.nasa.gov/node/145