METR125: Physical Meteorology
Nucleation of water vapor on
condensation
Wallace and Hobbs, Sections 6.1 and 6.4
What besides water vapor do we need to make a
cloud?
Cold Cloud Processes
Homogeneous Nucleation of Droplets;
Kelvin’s Equation
Cloud Condensation Nuclei.
Warm Clouds.
Growth of Drops by Condensation
Atmospheric Aerosols
Warm Cloud
Processes
Heterogeneous Nucleation of Droplets;
Köhler Curves
Courtesy: Steve Platnick, NASA
Growth of Drops by Collisions.
Ice Nuclei and Ice Crystal in Clouds
Growth of Ice Particles in Clouds
REVIEW
Rain Drops, Cloud Droplets, and CCN
Review
• Clouds form when air becomes
supersaturated wrt pure, flat liquid water
(or ice, in some cases)
• Supersaturation most commonly occurs in
the atmosphere when air parcels ascend,
resulting in expansion and cooling (WH 2.6)
• Water vapor condenses onto aerosols
forming a cloud of small water droplets
Aerosol
Theory
• But do we really need (Andy) aerosol to
make a cloud droplet? Can we make cloud
via condensation without the aid of
aerosols*?
*homogeneous or spontaneous nucleation
(last lecture)
Very little, and take 24 hrs to grow to size of rain drop
Theory
• Kelvin’s formula can be used to
– calculate the radius r of a droplet which will be
in (unstable) equilibrium with air with a given
water vapor pressure e
– determine the saturation vapor pressure e over a
droplet of specified radius r
• r = 0.01 micrometers requires a RH of 112.5%
• r = 1.0 micrometer requires a RH of 100.12%
Curvature Effect
• Supersaturations that
develop in natural clouds
due to the adiabatic ascent
of air rarely exceed 1%
(RH=101%),
• Consequently, droplets do
not form in natural clouds
by the homogeneous
nucleation of pure water…
Theory
• …droplets do form in natural clouds by the
heterogeneous nucleation process
• on atmospheric aerosols
Yes!
Solution Effect
• Once an aerosol particle
replaces a water molecule in
the droplet, the equilibrium
vapor pressure (e.g., number
of water vapor molecules
required to surround the
droplet to maintain
equilibrium) decreases
• Therefore, water droplet can
keep from evaporate at lower
vapor pressure
Aerosols
• PARTICLES, liquid or solid,
SUSPENDED IN THE ATMOSPHERE
• DIAMETERS MEASURED IN
MICRONS – ONE MILLIONETH OF A
METER.
– Too small to be seen individually (0.1 – 1 μm)
Size and shape of soot and sulfate aerosols in the atmosphere.
Credit: Mihály Pósfai, Arizona State University
Sources of Aerosols : Wildfire
in CA August 31, 2009
http://photojournal.jpl.nasa.gov/catalog/PIA12190
Sources of Aerosols: Volcanic Eruption
Mt. St. Helens,
Washington, April 8,
1980
• The most important components of volcanic
degassing are H2O, CO2 and sulfur as SO2,
H2S and SO4- in varying fractions
depending on the magma type
Dust storm in the Gobi Desert, China
Satellite: Terra
Date : 3/9/2013
Resolutions:
1km (691.1KB)
500m (2.3 MB)
250m (5.6 MB)
Bands Used: 1,4,3
http://modis.gsfc.nasa.gov/gallery/individual.php?db_date=2013-03-17
Sources of Aerosols– Urbanizations
Skyscrapers in downtown Seoul in March 2006 are
shrouded by yellow dust storms blowing in from
China's Gobi desert.
Aerosol effects on Clouds
• Video
http://www.met.sjsu.edu/metr112videos/MET%20112%20Video%20LibraryMP4/clouds/
The Clouds Makers.mp4
aerosol particles have a range of sizes
from nanometers to micrometers,
a range of shapes, and
different chemical compositions
clouds cover roughly half of the earth's
surface; yet any individual cloud is
composed of billions of individual droplets
or crystals, each of which
started its life as an aerosol particle.
Size of Aerosol Particle
• Following the original work of Whitby
(1978) and Hoppel (1988), it has been
accepted to name these modes "nucleation
mode" (particles with Dp < 0.01 µm ),
"Aitken mode" (0.01 < Dp < 0.1 µm)),
"accumulation mode" (0.1 < Dp < 1 µm),
and "coarse mode" (Dp > 1 µm)
Solution Droplets
Note that the previous lecture discussion
is valid for a pure water drop
• if a droplet is comprised of a solution it can be in equilibrium with the
environment at a much lower RH -->
• this explains the formation of haze
• This process of condensation will
grow drops , but not to precipitation
sizes (~ 2 mm)
Q: So, if a droplet grows
to some size by
condensation, how can
it continue to grow to
precipitation size???
(to be answers in next lecture)
QUESTION FOR THOUGHT:
• Haze particles can form when the relative
humidity is less than 100%. Are these haze
particles pure water droplets or solution
droplets? Why?
numerator represents ___________
Curvature Effect
denominater represents____________ Solute Effect
Köhler Peak
Must know
Fig. 1 (Andreae and Rosenfeld, 2008)
• Fig. 1. Köhler curves showing the equilibrium water vapor
supersaturation at 293 K for droplets of pure water (dotted
curve) and for droplets containing various masses of
dissolved (NH4)2SO4 (solid curves) vs. diameter of the
droplet (Seinfeld and Pandis, 1998). The water vapor
supersaturation, S (%)=(p / p0 − 1)100, where p is the
partial pressure of the water vapor and p0 is the saturated
vapor pressure over a plane surface of water at this
temperature. In the indicated example, an ambient water
vapor S of 0.15% (dashed line) exceeds the critical value
for all ammonium sulfate aerosols with dry diameter
≥ 0.1 μm. These aerosols will therefore activate and grow
into cloud droplets, whereas smaller aerosols remain as
unactivated haze particles. Droplets below their
corresponding equilibrium curve will shrink by
evaporation whereas those above will grow by
condensation (the indicated droplets correspond, for
example, to a dry diameter of 0.05 µm).
http://www.sciencedirect.com/science/article/pii/S0012825208000317
Right of the peak:
• CCN is activated right of Kohler peak
• Continue to grow as long as water vapor is
available
• Become cloud droplet
FYI
FYI
Köhler Curve
How is amount of
solution
change water droplet
formation?
Give example.
How different solution
change
water droplet
formation?
Give example.
5,6,
2,3,4
3,6
Droplet activated
• Find it in text book P214
CCN activation
• the CCN size distribution is dominated by
the growing nucleation mode (Aitken
Mode) rather than by the accumulation
mode
• The activation of CCN determines the
initial number of cloud droplets
Water Droplet Growth - Condensation
FYI
Growth slows down with increasing droplet size:
large droplets :
G s 
dr
~ env
dt
r
Since large droplets grow slower, there is a narrowing of the size distribution
with time.
R&Y, p. 111
PHYS 622 - Clouds, spring ‘04, lect.4, Platnick
Class Participation
rdry= 0.1
0.22
0.48 m
Assumes supersaturation=0.05%, p=900 hPa, T=273K
Water Droplet Growth - Condensation
Diffusional growth summary:
• Accounted for vapor and thermal fluxes to/away from droplet.
• Growth slows down as droplets get larger, size distribution narrows.
• Initial nucleated droplet size distribution depends on CCN spectrum & ds/dt
seen by air parcel.
• Inefficient mechanism for generating large precipitation sized cloud drops
(requires hours). Condensation does not account for precipitation
(collision/coalescence is the needed for “warm” clouds - to be discussed).
Sinks of Aerosols
• Aerosols are removed from the atmosphere by dry and wet
processes.
• For particles below Dp < 0.1 µm the dominant dry removal
mechanism involves diffusion to the surface, a process
which becomes less efficient as particle size increases.
• Coarse particles (Dp > 1 µm) settle gravitationally, a
process which becomes less efficient as particle size
decreases.
• In the range 0.1 < Dp < 1 µm, dry removal is very slow,
these particles are removed mainly by growth to cloud
drops during cloud formation and subsequent removal
from the atmosphere in precipitation.
• The following Slides will be discussed in
future lecture
Aerosol forcing has big uncertainty in current climate change research
Example of Effects of Aerosols
• They scatter visible sunlight:
– The sky looks dirty or hazy
– Creates brilliant orange and red
skies at sunrise and sunset
– Creates a large aureole around the
sun or full moon
Some act as nuclei formation of cloud
drops or ice crystals
Photo taken by Carlye Calvin
© University Corporation for Atmospheric Research
Photo shows a sunset made possible by the aerosols in the air.
Photo taken by Carlye Calvin with copyright at
the University Corporation for Atmospheric Research.
Aerosol Radiative Forcing
1) Direct forcing
Scattering and
absorption of solar and
terrestrial radiation by
aerosols
2) Indirect forcing
Effects of aerosols on the
optical properties and
lifetime of clouds
0o C
surface
Cloud drop
Rain drop
Ice crystal
Ice precipitation
• An important type of primary particle is so-called
"soot", emitted from combustion of carbonaceous
fuels. It consists of black carbon containing
material which has not been fully oxidized in the
combustion process,
• Soot is formed in the combustion of carbonaceous
fuels as particles with a diameter 5 to 20 nm
July 2005
Satellite observations
Aerosol Distributions over Land and Ocean have evident differences
January
January
20062006
Seasonal variation of aerosol optical thickness (AOD) is evident
Key Understanding Related to Aerosol Effects
Much is still to be learned about how aerosols affect climate.
(IPCC 2007 modified from Haywood and Boucher, 2000)
The aerosols that act as nuclei for formation of cloud
drops are called cloud condensation nuclei (CCN).
An increase in the number of condensation nuclei increases
the number of cloud droplets per unit volume, and decreases
the droplet size. This increases the amount of sunlight
reflected by the cloud. Similarly, fewer condensation nuclei
mean fewer and larger cloud droplet per unit volume. Thus
cumulus clouds over land (where there are a lot of
condensation nuclei) have more small droplets than clouds
forming over the open ocean, where there are usually fewer
nuclei.
• According to traditional Köhler theory, a critical
supersaturation (RH > 100%) exists above which water
vapour condensed in an uncontrolled way and cloud
droplets form. This critical supersaturation depends on
particle size and chemical composition. Once a drop of
sufficient volume is formed, gaseous species like SO2 can
dissolve and be oxidized in the aqueous phase.
• On average, nine out of ten clouds evaporate rather than
precipitate. When the droplets evaporate, larger particles
reform as a result of the additional oxidized material, e.g.
sulfate (Birmili, et al., 1999; Yuskiewicz, et al., 1999).
Importance of particle size
The dynamics of particles (biological and nonbiological)
in the air is governed mainly by
particle physical characteristics of which size is
one of the most important
Particle size matters!
Without aerosols, without clouds
Zonal Mean Cloud Effective Radius
(M. D. King, S. Platnick et al. – NASA GSFC)
July 2006 (Collection 5)
Aqua
Why are cloud droplet sizes different
over land and ocean?
AEROSOL!
Class Discussion
• http://www.nasa.gov/vision/earth/environm
ent/ny_air_prt.htm
• Answer questions:
What is the role of urban aerosol on clouds
droplet size and rainfall?
Aerosol decreases surface insolation
surface
Total solar radiation decreased by aerosol= 20Wm-2
Based on M-D. Chou’s radiative transfer model
(Jin, Shepherd, and King, 2005, JGR)
Key Understanding Related to Aerosol Effects
Much is still to be learned about how aerosols affect climate.
(IPCC 2007 modified from Haywood and Boucher, 2000)
Cloud Fraction
Aerosol effects on Cloud Lifetime Effect
Aerosol Number
Sekiguchi et al. 2003
Rosenfeld et al. 2006
• Satellite studies show aerosol loading is positively correlated
with low cloud fraction
• Interpreted as a result of longer cloud lifetimes caused by
increasing aerosol number and reduced drizzle rates
Class participation
•
1.
2.
3.
4.
Open notes quiz
What is the typical size of aerosol?
What are the sources for aerosol?
What is the Aerosol direct effect?
What is aerosol indirect effect?