Chapters 5 and 6
Cloud and Aerosol Physics
Goals:
Understand …
• Homogeneous and heterogeneous nucleation of cloud droplets
• The central role of aerosol in cloud physics
• Precipitation formed in warm and cold clouds
• Saturation and super saturation in the atmosphere
• Super cooling of cloud droplets
• Formation of ice crystals in cirrus clouds and ice fog
• Cloud electrification
Figures, etc, from Wallace and Hobbs unless otherwise stated.
Aerosol
Atmosphere as a Photochemical
Reactor: OH The Hydroxyl Radical
Section 5.4: Tropospheric Aerosol
SOLAR
INFRARED
Aerosol Number Distributions for air source: Continental (red curve)
Marine (blue curve) and urban polluted (black curve)
Aitken nuclei: Particle counted
with a very high super saturation,
100% or so, also known as
condensation nuclei (CN).
Cloud condensation nuclei (CCN)
are large CN that are counted with
supersaturations of only around a
percent or so, as found in the
atmosphere.
Particle
Surface
Area and
Dynamics
(rough idea)
What fraction are CCN
at various supersaturations?
Continental (red curve)
Marine (blue curve)
and urban polluted
(black curve)
UNR CIMEL SUNPHOTOMETER AEROSOL DATA
AEROSOL SPECTRAL OPTICAL DEPTH
UNR CIMEL SUNPHOTOMETER AEROSOL DATA
Coarse mode
‘Retrieved’ Aerosol Volume Distributions
Fine mode
Chemistry of Stratospheric Sulfate Layer (strong volcanic
eruptions): A form of ‘natural’ geoengineering, aerosol
scatter solar radiation to space
Aerosol removed by tropospheric folds and jet stream dynamics: locations where
stratospheric air is forced to mix down to the troposphere.
Stratospheric Aerosol Optical Depth at wavelength 1 micron
(40 km to 2 km above tropopause)
Volcanos
Tropospheric Folds
Chapter 6: Cloud And Aerosol Microphysics
CCN size is
25x too
large
Summary In Two Images
Water droplet coalescence
to form raindrops.
Ice crystals/snowflakes at top;
Mid level mixed clouds, ice consumes water droplets
Melt to raindrops near the surface
from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/cold_clouds.html
Cloud Types
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
Chapter 6: First topic: Homogeneous nucleation of cloud
droplets (not how most cloud droplets are formed)
Droplets with R < r evaporate:
with R > r grow since the energy
diminshes. NOTE: e > es is needed
for growth.
Growth begets growth,
shrink begets droplet shrink:
Unstable equilibrium
Another View …
How Many Water Molecules in a Droplet?
Relative Humidity and Supersaturation
at Equilibrium Above a Droplet (with
respect to a flat surface of water)
Unrealistically large …
super saturations this high are not found
Cloud droplets can’t get
past the small size this way:
Need another mechanism
Relative Humidity and Supersaturation
at Equilibrium Above a Droplet (with
respect to a flat surface of water)
Unrealistically large …
super saturations this high are not found (broader range of size)
300%
280%
260%
e/es (%)
240%
220%
Cloud droplets can’t get
past the small size this way:
Need another mechanism
200%
180%
160%
140%
120%
100%
0.001
0.01
r (um)
0.1
1
æ
ö
ç
÷
es
2
s
ç
÷
=expç÷
e
ç r nkT ÷
è
ø
æ
ö
ö
ç
÷
÷
es
=exp - s S ÷÷ =expçç- Surface Tension Energy÷÷
e
ç
÷
3/2N kT ÷
Thermal Energy
ç
÷
æ
ç
ç
ç
ç
è
Relative probability molecules are in the
ø
è
vapor phase for a flat surface compared
S =Droplet surface area.
with a curved surface of radius r and area N = number of molecules in the droplet.
S.
ø
Relative Humidity and Supersaturation
at Equilibrium Above a Droplet (with
respect to a flat surface of water)
Inside water molecules ‘hold’ the surface molecules less strongly for
droplets compared with a flat surface.
How to Overcome Problem of High Supersaturation Needed
for Homogeneous Nucleation
Nucleation
s.v.p. over curved surface > s.v.p. over flat surface
svp  r 
 1 2 
1.2 109
 exp 
  1
svp   
r

RT
r
 w 
For typical molecule, r ~ 0.6 nm, would need RH=740% (SS=640%)
In atmosphere maximum SS observed is ~1%
Condensation must be on particles with r ~ 100nm
Radius
Name
Conc. (cm-3)
SS
< 0.1 µm
Aitken nuclei
10,000
1%
0.1 - 1 µm
large nuclei
100
0.1%
> 1 µm
giant nuclei
1
0.01%
Haze and RH Hysteresis
Haze and
RH
Hysteresis
150 nm diameter dry aerosol:
Start dry at 30% RH:
Increase to 80% RH:
Deliquescence:
Growth for RH>80%:
Decrease RH to 38%:
Efflorescence:
Dry for RH<38%
Raoult’s Law (Wikipedia)
Limitations of Raoult’s Law
Raoult’s Law Example: Fewer water
molecules are above a solution droplet
Köhler Theory: Add dissolved ions to reduce the number of water
vapor molecules escaping the solution droplet (impure water droplet)
From Lord Kelvin, pure water
From Raoult’s Law
From Köhler, salty water
Köhler Curves for Different Salts: NaCl and (NH4)2SO4:
1 ion unit = 1 million ions
Note the change
of scale above
100%
Haze Droplet and Activated Cloud Droplet for Ambient
RH=100.4%
Another Look at Köhler Curves
slides from http://www.met.sjsu.edu/~clements/met60_lecture/lecture11_cloud_microphysics.ppt
Heterogeneous Nucleation
Hygroscopic CCN are particularly effective
condensation initiators

Generally made of soluble salts
When droplet forms, solution has a much
lower vapor pressure than pure water
 Condensation begins when RH < 100%
Droplet growth requires supersaturations of
less than 1%

Such supersaturations are achieved in updrafts
Köhler Curves
Give the equilibrium droplet size for a
given RH.
“Saturation ratio” = RH/100
Köhler
Curves
10-19 g
10-18g
10-17g
Numbers indicate mass
of dissolved salt (NaCl)
Suppose RH =
100.1%
10-19 g
10-18g
10-17g
Droplets grow until they
reach equilibrium
radius
10-19 g
10-18g
10-17g
Droplets grow until they
reach equilibrium
radius
10-19 g
10-18g
10-17g
Droplets grow until they
reach equilibrium
radius
10-19 g
10-18g
10-17g
Droplets grow until they
reach equilibrium
radius
10-19 g
10-18g
10-17g
Droplets grow until they
reach equilibrium
radius
Typical cloud
droplet radius
10-19 g
10-18g
10-17g
Droplets grow until they
reach equilibrium
radius
Droplet Growth
If ambient RH < value at peak of curve,
droplets stop growing when much smaller
than typical cloud drop
They are called haze droplets
10-19 g
10-18g
10-17g
Suppose RH =
100.3%
10-19 g
10-18g
10-17g
Droplets growing on
smaller nuclei behave
as before
10-19 g
10-18g
10-17g
Look at largest nucleus
10-19 g
10-18g
10-17g
10-19 g
10-18g
10-17g
10-19 g
10-18g
10-17g
10-19 g
10-18g
10-17g
10-19 g
10-18g
10-17g
10-19 g
10-18g
10-17g
Droplet keeps growing!
Droplet “Activation”
If ambient RH > peak value, droplet grows
indefinitely
Once droplet has gotten “over the hump”,
it is said to be activated.
Cloud Condensation Nuclei (CCN):
Measurement Method, Thermal Diffusion Cloud Chamber
Cloud Condensation Nuclei (CCN):
Typical Measurements
Data from Hudson and Yun,
JGR, 2002 (DRI)
Warm Clouds (everywhere > 0 C)
Measurements of Cloud Droplets
Schematic
of pod mounted
instrument
Warm Cloud Microphysics Example
Warm Cloud Comparison
Cloud Aerosol
Indirect Effect:
Clouds with the
same liquid
water path, but
more CCN and
more cloud
droplets, have
higher albedo
than clouds with
lesser CCN and
fewer cloud
droplets.
(Twomey
hypothesis)
Cloud Effective Radius From Satellite
Retrievals
Generally smaller cloud droplets over land than over ocean.
Ship Tracks (ships emit huge numbers
of CCN): Bright lines from numerous
small cloud droplets.
Often present off the California coast
Cloud Liquid Water Content (LWC): Adiabatic LWC
Entrainment
of Dry Air
Causes Local
Evaporation
and Reduced
Cloud LWC
Nonattainment of Adiabatic LWC
Growth of Cloud Droplets in Warm Clouds:
Condensation and Collision-Coalescence
Growth of Cloud Droplets in Warm Clouds:
Condensational Growth Near Cloud Base
Small and Large Droplet Fall Speed
Terminal
velocity of
drops
For laminar flow, at terminal velocity
drag force=weight
mg  6 rv
3
4
mg
3  r w g
v

6 r
6 r
v
2 w g 2
r
9 
Faster Falling Droplets Collect Slower Moving Little Ones
• Giant CCN and/or turbulent mixing may produce large collector drops.
• Precipitation falls out when collector drop terminal fall speed exceeds the updraft.
• Large collector droplets can break into smaller ones during impacts.
Cold Clouds: Ice Crystal Microphysics
Key Idea: Vapor pressure for water is
greater than for ice below 0 C.
From http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter07/habits.gif
Ice Nuclei
are Often
Much Less
Prevalent
than Ice
Crystals
Ice
Multiplication
Cloud Structure