Cloud Condesation Nuclei

1
Cloud Condesation Nuclei

• Particles upon which water vapor condenses to
  form droplets. They are activated and grow by
  condensation to form cloud droplets.
• CCN depends on super-saturation rate (0.11),
  particle size and the solubility and wettability
  of the particles.
• For example, to serve as a CCN at 1
  supersaturation, completely wettable but water
  insoluble particles need to be at least 0.1 µm
  in radius, whereas soluble particles can serve as
  CCN at 1 supersaturation even if they are as
  small as 0.01 µm in radius. Most CCN consist of
  a mixture of soluble and insoluble inorganic and
  organic components (called internally-mixed
  nuclei).

2
Observing CCN

• The concentrations of CCN active at various
  supersaturations can be measured with a thermal
  gradient diffusion chamber, or other devices
  based on similar principles. This device
  consists of a flat chamber in which the upper and
  lower horizontal plates are kept wet and
  maintained at different temperatures, the lower
  plate being several degrees colder than the upper
  plate. By varying the temperature difference
  between the plates, it is possible to produce
  maximum supersaturations in the chamber that
  range from a few tenths of 1 to a few percent.
  Small water droplets form on those particles that
  act as CCN at the peak supersaturation in the
  chamber. The concentration of these droplets can
  be determined by photographing a known volume of
  the chamber and counting the number of droplets
  visible in the photograph, or by measuring the
  intensity of light scattered from the droplets.
  By repeating the above procedure with different
  temperature gradients in the chamber, the CCN
  supersaturation spectrum can be determined.

3
Conventional Wisdom

• Sea salt is the predominant constituent of CCN
  with D gt 1µm
• For 0.1 µm lt D lt 1 µm, the main component is
  thought to be sulfate, which may be present as
  sulfuric acid, ammonium sulfate, or from
  phytoplankton produced dimethylsulfide (see
  Charlson et al., Nature, 326, 655-661).

4

• Cloud condensation nucleus spectra in the
  boundary layer from measurements near the Azores
  in a polluted continental air mass (brown line),
  in Florida in a marine air mass (green line), and
  in clean air in the Arctic (blue line). Data
  from Hudson and Yum (2002).

5
Activity Spectrum

• Let Nc be the number of particles per unit
  volume that are activated to become cloud
  droplets.
• Data from cloud chamber measurements are often
  parameterized as
• Nc C (S-1)k
• where C and k are parameters that depend on air
  mass type.
• Rogers gives
• Maritime air 30 lt C lt 300 cm-3 0.3 lt k lt 1
• Continental air 300 lt C lt 3000 cm-3 0.2 lt k lt
  2
• Thus, for the same saturation ratio, one would
  expect to find small numbers of CCN per unit
  volume in maritime air and large numbers per unit
  volume in continental air.

6
Aerosol
It presents 3 modes -  nucleation   radius
is between 0.002 and 0.05 mm. They result from
combustion processes, photo-chemical reactions,
etc. -  accumulation  radius is between 0.05
mm and 0.5 mm. Coagulation processes. -
 coarse  larger than 1 mm. From mechanical
processes like aeolian erosion.  fine 
particles (nucleation and accumulation) result
from anthropogenic activities, coarse particles
come from natural processes.
0.01
0.1
1.0
10.0
7
Another Naming Convention

• Nucleation mode - Aitken Nuclei
• r lt 0.1 µm
• Accumulation or coagulation - Large nuclei
• 0.1 µm lt r lt1 µm
• Thought to be most important in natural cloud
  formation
• Coarse Particle Mode - Giant Nuclei
• r gt 1 µm

8
Aerosols in the Atmosphere

• Abundance and size
• Aerosol concentration is highly variable in space
  and time. Concentrations are usually highest near
  the ground and near sources.
• A concentration of 105 cm-3 is typical of
  polluted air near the ground, but values may
  range from 2 orders of magnitude higher in very
  polluted regions to several lower in very clean
  air.
• Radii range from 10-7 cm for small ions to more
  than 10 µm (10-3 cm) for the largest salt and
  dust particles.
• Small ions play almost no role in atmospheric
  condensation because of the very high
  supersaturations required for condensation.
• The largest particles, however, are only able to
  remain airborne for a limited time

9
Aerosol Types and Origin

• Aerosol particles larger than about 1 mm in size
  are produced by windblown dust and sea salt from
  sea spray and bursting bubbles
• Aerosols smaller than 1 µm are mostly formed by
  condensation processes such as conversion of
  sulfur dioxide (SO2) gas (released from volcanic
  eruptions) to sulfate particles and by formation
  of soot and smoke during burning processes.
• After formation, the aerosols are mixed and
  transported by atmospheric motions and are
  primarily removed by cloud and precipitation
  processes.

10
Origins of Atmospheric Aerosols

• 1. Condensation and sublimation of vapors and the
  formation of smokes in natural and man-made
  combustion
• 2. Reactions between trace gases in the
  atmosphere through the action of heat, radiation,
  or humidity
• 3. The mechanical disruption and dispersal of
  matter at the earths surface, either as sea
  spray over the oceans, or as mineral dusts over
  the continents
• 4. Coagulation of nuclei which tends to produce
  larger particles of mixed constitution

11

• Evidences of Aerosol Impact on
• Cloud and Precipitation
• Observations from satellites, aircraft, radars
  and surface sampling
• Theoretical studies by numerical simulations
• Climatological studies

12
Ship Track Formation
N 40 cm-3 W 0.30 g m-3 re 11.2 µm
N 100 cm-3 W 0.75 g m-3 re 10.5 µm
Borrowed from Michael King
13
Ship tracks off the Washington coast

• Adding CCN makes clouds with more, smaller
  droplets.
• These clouds are whiter, reflect more sunlight ?
  net cooling

14
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15
Ramanathan, V., P. J. Crutzen, J. T. Kiehl, and
D. Rosenfeld, 2001 Aerosols, Climate and the
Hydrological Cycle. Science, 294, 2119-2124.
16
Re gt 15 mm
TReff 15
17
Suppression of Rain and Snow by Smoke and Air
Pollution
VIRS painting yellow pollution tracks in the
clouds over South Australia, due to reduced
droplets size.
This smoke stack of a mining complex well inland
Canada causes the pollution track originating at
the white asterisk
Rosenfeld D., 1999 TRMM Observed First Direct
Evidence of Smoke from Forest Fires Inhibiting
Rainfall. Geophysical Research Letters. 26, (20),
3105-3108. Rosenfeld D., 2000 Suppression of
Rain and Snow by Urban and Industrial Air
Pollution. Science, 287 (5459),
1793-1796. Contact Prof. Daniel Rosenfeld The
Hebrew University of Jerusalem, Jerusalem 91904,
Israel daniel_at_vms.huji.ac.il phone
-972-2-6585821 fax -972-2-6512372

• VIRS retrieved effective radius does not exceed
  the 14 mm precipitation threshold in polluted
  clouds within area 2 in the Australia image.

PR shows precipitation as white patches only
outside the pollution tracks, although clouds
have same depth.
TMI shows ample water in the polluted clouds
PR shows bright band in clean clouds. Therefore,
pollution suppressed rain and snow in polluted
clouds.
18
PR H-Z
VIRS T-Re
TRMM VIRSPR, Kwajalein, 1998 11 05 0251
19
Clear day Visibility ??? km NCN 500 cm-3 BC
0.2 mg m-3
Smoke haze Visibility 800 m NCN 10000 cm-3
BC 7 mg m-3
20
The Green Ocean Maritime clouds over the
Amazon
Note the shallow precipitating clouds, extensive
warm rainout, glaciation at Tgt-10oC, and few
lightning
VIRS T-Re
TRMM VIRSPR, Amazon, 1998 04 13 1628
21
Clear day Visibility ??? km NCN 500 cm-3 BC
0.2 mg m-3
Smoke haze Visibility 800 m NCN 10000 cm-3
BC 7 mg m-3
22
The Green Ocean turns dry Smoky clouds over
the Amazon
Note that clouds do not precipitate before
reaching height of 6.5 km or 12oC isotherm,
while containing ample cloud water.
Rosenfeld D. and W. L. Woodley, 2002 Closing the
50-year Circle From Cloud Seeding to Space and
Back to Climate Change Through Precipitation
Physics. In press, Meteorological Monographs,
AMS.
TOMS Aerosol Index 13 September 1998
VIRSPR, Amazon, 1998 13 SEP 1415
VIRS T-Re
23
The classification scheme of convective clouds
into microphysical zones according to the shape
of the temperature effective radius relations
General
Maritime
-40
-40
Note that in extremely continental clouds re at
cloud base is very small, the coalescence zone
vanishes, mixed phase zone starts at Tlt-15oC, and
the glaciation can occur at the most extreme
situation at the height of homogeneous freezing
temperature of 39oC. In contrast, maritime
clouds start with large re at their base,
crossing the precipitation threshold of 14 mm
short distance above the base. The deep rainout
zone is indicative of fully developed warm rain
processes in the maritime clouds. The large
droplets freeze at relatively high temperatures,
resulting in a shallow mixed phase zone and a
glaciation temperature reached near 10oC
Glaciated
-30
-30
Glaciated
-20
-20
C
C
o
o
-10
-10
T
T
Mixed Phase
Mixed Phase
Rainout
0
0
Rainout
Coalescence
10
10
Diffusional growth
Coalescence
20
20
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
r
r
m
m
m
m
eff
eff
Continental - moderate
Continental - extreme
-40
-40
Glaciated
-30
-30
Glaciated
Mixed Phase
-20
-20
C
C
Mixed Phase
o
-10
T
o
-10
T
0
Coalescence
0
Diffusional growth
10
10
Diffusional growth
20
20
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
r
m
m
r
m
m
eff
eff
24
The Aral Sea water management failure and
ecological disaster
25
Clouds forming in salt-containing dust
Dust storm in the Aral sea, 11 May 1998
26
Cloud Physics Instrumentation of the WMI Lear Jet
FSSP-100 2-47 mm OAP-2DC 50-800 mm 2DP 0.4-6.4
mm DMT hot wire LWC Temperature Dew
Point GPS Ball Variometer Avionics
27
Observing storm clouds that keep their water
aloft This ordinary looking cloud is composed
mostly of small liquid water droplets at
temperatures as cold as -37.5oC, which is the
coldest that was ever documented in cloud by
in-situ scientific measurements. The highly super
cooled liquid water, which amounted to 1.8 gram
per cubic meter, remained as small droplets in
the cloud instead of turning back as
precipitation. The cloud was measured by a cloud
physics aircraft in west Texas on 13 August 2000,
at flight level of 32,000 feet (Woodley and
Rosenfeld, 2000).
28
Vertical profiles of maximum values of (a) cloud
water content (CWC), (b) the mean volume diameter
and (c) droplet concentration observed in the
control run at 250 m below the growing cloud top,
presented on the background of the aircraft
observations (Rosenfeld and Woodley, 2000), shown
in green (CWCgt0.2 gm-3 and black (CWClt0.2 gm-3).
The blue and red squares denote model calculated
values for the low and high CCN concentrations.
The black square in the concentration panel (c)
denotes the model ice concentrations.
29
Seeding clouds by large hygroscopic particles
seems to be successful (results from recent
experiments in Mexico and South Africa)

• Seeding by 0.5µm hygroscopic salt particles
  (mostly KCl) from flares on the wing of a plane
  at the base of the clouds.
• Earlier experiments had targeted higher, colder
  parts of clouds with the intent of forming more
  ice particles.

30
Not Seeded
790 mb NS 2.4 g m-3 12 S
761 mb NS 2.2 g m-3 6 S
730 mb NS 2.3 g m-3 76 S
31
Seeded
768 mb S 2.4 g m-3 8 S
758 mb S 2.2 g m-3 17 S
740 mb S 3.0 g m-3 25 S