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Clouds
Liquid water mixing ratio
Liquid water density of clouds
l  wl  air
Cloud droplet distribution
Number density N (D):
the number of droplets per nit
volume (concentration) in an
interval D + ΔD
wl 
mass liquid
mass dry air
l 
mass liquid
volume of dry air
mass of a droplet  l 6 D 3
Liquid water content
L  6  l N i ( Di ) Di3
Cloud-Radiation-climate feedback
Cooling effect
Warming effect
Satellite
View of
Clouds
Geostationary
Satellites
Polar
orbit
satellite
St & Sc
Stratus and stratocumulus
Transition
Trade cumulus
Intertropiccal Convergence Zone (ITCZ)
NASA: The Earth Radiation Budget Experiment (ERBE)
It measures the energy budget at
the top of the atmosphere.
Energy budget at the top of atmosphere (TOA)
Incoming solar radiation 340 W/m2
Reflected SW radiation
Q1= 50 W/m2
Fictitious
climate
system
No clouds
Incoming solar radiation 340 W/m2
Reflected SW radiation
Q= 100 W/m2
shortwave cloud forcing
dQ=Q1-Q=-50 W/m2 (cooling)
Emitted LW radiation
F1= 270 W/m2
Emitted LW radiation
F= 240 W/m2
longwave cloud forcing
dF=F1-F=30 W/m2 (warming)
Present
climate
system
with clouds
SW cloud forcing = clear-sky SW radiation – full-sky SW radiation
LW cloud forcing = clear-sky LW radiation – full-sky LW radiation
Net cloud forcing (CRF) = SW cloud forcing + LW cloud forcing
Current climate: CRF = -20 W/m2 (cooling)
But this does not mean clouds will damp global warming! The impact
of clouds on global warming depends on how the net cloud forcing
changes as climate changes.
Direct radiative forcing due to doubled CO2, G = 4 W/m2
  0  positive cloud feedback

CRF
G
  0  zero cloud feedback
  0  negative cloud feedback
e.g. If the net cloud forcing changes from -20 W/m2 to
-16 W/m2 due to doubling CO2, the change of net
cloud forcing CRF  - 16 - (-20)  4 W/m 2 will add
to the direct CO2 forcing. The global warming will
be amplified by a fact of 2.
Cloud radiative effects depend on cloud distribution, height,
and optical properties.
Low cloud
High cloud
Tc
Tc
Tg  Tc
Ta
Tc  Ta
Tg
SW cloud forcing dominates
LW cloud forcing dominates
In GCMs, clouds are not resolved and have to be
parameterized empirically in terms of resolved
variables.
water vapor (WV)
cloud
surface albedo
lapse rate (LR) WV+LR
ALL
Cloud formation
Two processes, acting together or individually, can lead to air
becoming saturated: cooling the air or adding water vapor to
the air. But without cloud nuclei, clouds would not form.
without
cloud
nuclei
with
cloud
nuclei
How precipitation forms
Frictional force
Frictional force = Gravitational force
Terminal velocity
Gravitational force
Bergeron Process
Supercooled water: To make the transition from a liquid to
the lattice structure of ice, some foreign particles, ice nuclei,
are needed to initiate freezing. Until the nuclei form, liquid
water can exist far below the freezing point. In fact, pure
water droplets can remain in liquid form near -40F.
Mixed phase clouds
Ice crystal: molecules more
organized, difficult to escape
Supercooled droplet: molecules less
organized, easy to escape
Es  e  E
condensate
evaporate
At a certain condition, cloudy air is unsaturated to supercooled
water droplet, but is saturated to ice crystal, leading to evaporation
of supercooled water droplet and growth of ice crystal.
1. Supercooled water droplets are readily to freeze if they
impact an object.
2. Enlarged crystals are easy to break up into fragments
serving as freezing nuclei.
Collision-Coalescence Process
(a) Collision
-Larger drops fall faster than
smaller drops, so as the drops fall,
the larger drops overtake the
smaller drops to form larger drops
until rain drops are formed.
-In a cloud with cloud droplets
that are tiny and uniform in size:
-The droplets fall at a similar
speed and do not Collide.
-The droplets have a strong
surface tension and never
combine even if they collide.
(b) Coalescence
-The merging or "sticking together" of
cloud droplets as they collide.
Droplets may:
1.Stick together/Merge
2.Bounce off one another
3. Airstream repelling
4. Coalescence is enhanced when
droplets have opposite electrical charges.
Aerosol feedback
Direct aerosol effect: scattering, reflecting, and absorbing
solar radiation by particles.
Primary indirect aerosol effect (Primary Twomey effect):
cloud reflectivity is enhanced due to the increased
concentrations of cloud droplets caused by anthropogenic
cloud condensation nuclei (CNN).
Secondary indirect aerosol effect (Second Twomey effect):
1. Greater concentrations of smaller droplets in polluted
clouds reduce cloud precipitation efficiency by restricting
coalescence and result in increased cloud cover,
thicknesses, and lifetime.
2. Changed precipitation pattern could further
affect CCN distribution and the coupling between
diabatic processes and cloud dynamics.
Cloud classification
High level clouds at heights of 5-13 km
Cirrus, Ci
Fibrous, threadlike, white
feather clouds of ice
crystals, whose form
resembles hair curls.
Cirrostratus, Cs
Milky, translucent cloud
veil of ice crystals, which
sometimes causes halo
appearances around moon
and sun.
Cirrocumulus, Cc
Fleecy cloud; Cloud banks
of small, white flakes.
Medium level clouds at heights of 2-7 km
Altocumulus, Ac
Grey cloud bundles, sheds
or rollers, compound like
rough fleecy cloud, which
are often arranged in banks.
Altostratus, As
Dense, gray layer cloud,
often evenly and opaquely,
which lets the sun shine
through only a little.
Low level clouds at heights of 0-2 km
Stratocumulus, Sc
Cloud plaices, rollers or
banks compound dark
gray layer cloud.
Stratus, St
Evenly grey, low layer cloud,
which causes fog or fine
precipitation and is
sometimes frazzled.
Clouds with large vertical extending at heights of 0-13 km
Cumulus, Cu
Heap cloud with flat basis
in the middle or lower level,
whose vertical development
reminds of the form of
towers, cauliflower or cotton.
Cumulonimbus, Cb
In the middle or lower level
developing thundercloud,
which mostly up-rises into
the upper level.
Nimbostratus, Ns
Rain cloud. Grey, dark
layer cloud, indistinct
outlines.
Other appearances
Kelvin-Helmholtz
Wave
lightning
halo
aurora
borealis
sun dog
rainbow
haze
dawn
Convective plume structures, skewness, and Kurtosis
Narrow branch of updraft compensated by broad branch of downdraft
skewed structure
Normal distribution
1
2 
( x   )2
exp[
]
2 2
Skewed distribution
Skewness
w 3
( w 2 )3 / 2
Normal distribution
w 3
( w 2 )3 / 2
0
Kurtosis
w 4
( w 2 ) 2
3
Normal distribution
w 4
( w 2 ) 2
3 0
Atmospheric Radiation Measurement Program
http://www.arm.gov/
http://plot.dmf.arm.gov/plotbrowser/
Cloud measurement
FSSP (forward scattering spectrometer probe)
The FSSP is of the general class of instruments called optical
particle counters (OPCs) that detect single particles and size
them by measuring the intensity of light that the particle scatters
when passing through a light beam.
Chart of FSSP
Prism
Scattering
Photodetector
Module
Dump spot
Airflow
He-Ne Hybrid Laser
FSSP
Optical Array probe
It uses an array of photodiodes to measure the size of hydrometeors
from the maximum width of their shadow as they pass through a
focused He-Ne laser beam. The shadow is cast onto a linear diode
array and the total number of occulted diodes during the airflow's
passage represents the size of droplets. The size is categorized into
one of 60 channels and this information is sent to the data system
where the number of particles in each channel is accumulated over
a preselected time period. The optical array probe is a particle sizing
instrument, not a liquid water content probe. If liquid water content
information is desired, some fairly loose assumptions must be made
with regard to the phase, habit, and density of the particles.
Optical Array probe
Older techniques
Use a slide with oil, such as vaseline, on it, expose it to air flow. The
soft oil captures the water droplets. Then, one may take photos and
read through a microscope to obtain the droplet size distribution.
Hotwire probes
Droplet impinging
on this wire
The hotwire will cause the water droplet impinging on the wire to
evaporate, which will cool the hot wire. To keep the probe at a
constant temperature, an electric current must be applied that will
be proportional to the liquid water content evaporated.
Ceilometer
Ceilometer is an instrument for the measurement of cloud base.
The device works day or night by shining an intense beam of light
(often ultraviolet) at overhead clouds. Reflections of this light
from the base of the clouds are detected by a photocell in the
receiver of the ceilometer. The height can be determined using
the emitted and received light.
Two basic types of ceilometers:
The transmitter fixed to direct its
beam vertically. The receiver is
stationed a known distance away.
The parabolic collector of the
receiver continuously scans up
and down the vertical beam,
searching for the point where the
light intersects a cloud base
Scanning receiver
Rotating transmitter ceilometer
The transmitter rotates while the receiver is pointing vertically
and coplanar with the rotating projector beam. Clouds
encountered with the light beam produce a backscattered
signal, which is detected by the receiver. Cloud height is then
determined by
triangulation.
Laser ceilometer
It has a transmitter and a receiver, and it measures the
time of return from scattering of the laser off from the
cloud; it’s ideal because it can work during the day and
night, but it only gives cloud observations from overhead.
Low power laser ceilometer
It uses an eye-safe, low-power laser to transmit light pulses to
the cloud base, up to 40,000 feet (13 km).
Instrumentation
Latest version W-band (95 GHz)cloud radar
Millimeter Wave Cloud Radar (3
X-band scanning ARM precipitation radar
Vaisala Ceilometer