Aerosol-cloud-precipitation Interaction

METR215: Advanced Physical Meteorology
Professor Menglin Jin
•Basic aerosol-cloud processes
•Current key hypotheses
•Focus on Lifetime effect in buffered system
What is aerosol optical thickness? What is it at 0.55 micrometer?
What are the typical values for this variable?
Aerosol Direct Effect:
Scattering/absorbing SW/LW
Indirect Effect: serve as CCN
0o C
Cloud drop
Rain drop
Ice crystal
Ice precipitation
Aerosol decreases surface insolation
Total solar radiation decreased by aerosol= 20Wm-2
Based on M-D. Chou’s radiative transfer model
(Jin, Shepherd, and King, 2005, JGR)
Aerosol reduce surface insolation
for extreme polluted days
Solar Radiation (Wm )
Jin, Shepherd and Zheng 2010
Observed aerosol reducing cloud droplet size
Jin and Shepherd 2008, JGR
Different Aerosol Effects
(from various papers)
• Biomass burning (sugar cane fields): This can greatly reduce or
shut-off precipitation processes because of slowed down collision
and coalescence, mainly due to the substantial increases in CCN
concentrations from the smoke. The increased CCN concentrations
leads to smaller sized drops, which have smaller collections kernels.
Paper and pulp mills: These emissions change the size
spectrum by introducing large and giant CCN, while not altering the
small CCN concentrations. This increases the collision/coalescence
efficiency, leading to enhanced precipitation (also maybe due to
additional moisture emitted from such mills).
Ship tracks: The smoke stack and emissions from a ship can
change the number of CCN particles that then change the cloud
structure. There is more liquid water in these clouds, higher CCN
concentrations, and therefore these clouds reflect more solar
energy. These clouds can also be somewhat deeper than
surrounding clouds. Drizzle processes maybe reduced or shut off
(which may explain the unexpected higher liquid water contents).
• Daniel Rosenfeld
Rocky Mountains, CO
GRL 2003
Droplet diameter [mm]
Droplet diameter [mm]
Crystal concentration [cm-3]
Droplet concentration [cm-3]
Crystal diameter [mm]
Crystal diameter [mm]
The difference between the cloud clear air equivalent anthropogenic aerosol sulfate concentrations on the
two days is nearly an order of magnitude, but in absolute terms it is only 1 mg m-3. Astonishingly, this
small amount of aerosol can reduce the snowfall rate up to 50%.
Evidence is presented to demonstrate the possible magnitude of the secondary indirect aerosol effect on
precipitation rates from cold mixed-phase clouds in mountainous regions where a seeder-feeder cloud
couplet is present. Changes as small as 1 mg m-3 in CCN aerosol concentration can cause significant
changes in cloud properties and precipitation efficiencies. (Quoted from Borys et al., GRL 2003).
(1) Maritime and Rural aerosols
Clouds from clean maritime air develop precipitation
efficiently. After interacting with rural aerosols, the
clouds are less efficient in developing precipitation.
(2) Urban air pollution
Ho Chi Minh
The blue color indicates detrimental effect of urban air
pollution on the precipitation in the clouds.
(3) Smoke from forest fires
Another case of detrimental effect of the
interaction of Clouds with biomass burning smoke
on the precipitation in the clouds can be seen in
the blue color over Sumatra and Kalimantan.
The TOMS aerosol index can be seen below:
The effect of aerosols on precipitation in clouds was
calculated from the data of the image above. The warm
colors represent efficient precipitation processes, while the
cold colors represent suppressed precipitation, due to the
pollution. The scale is the maximal cloud top temperature
[0C] required for onset of precipitation.
So, does air pollution suppress or enhance overall rainfall
amount from convective clouds?
Observations and model simulations show that always
clouds with more small CCN will rain less for a given
maximum vertical development.
Simulations show that in warm base clouds elevating the
onset of precipitation can lead to longer time of cloud
growth before downdrafts take over, and hence this
dynamic feedback causes greater vigor and secondary
formation of clouds, leading to more overall precipitation.
This is Rosenfeld’s view. Not necessarily true
Confounding Influences on Observations of Cloud Cover
Apparent increases in aerosol optical depth
in partly cloudy regions.
Interactions in a Buffered System
Shallow clouds are a crucial part of the earth’s climate system. They radiate
in the longwave at approximately the same temperature as the surface but
their stark albedo contrast with the dark underlying ocean/land exerts a
significant shortwave cooling on the climate system. These clouds also play
an important role in the development of deeper convection the details of
their treatment in climate models influence climate sensitivity. The effect of
aerosol particles on these clouds has been the subject of intense scrutiny
for a number of decades. Traditionally aerosol influences have been viewed
in terms of their effect on the cloud albedo or on the cloud lifetime. While
there is abundant observational evidence for aerosol-induced cloud
brightening, the evidence for the proposed increase in cloudiness or
“lifetime” is ambiguous. In this presentation we will make the case that this
ambiguity stems, at least in part, from the fact that the aerosol-cloudprecipitation system is buffered. This means that the response of the cloud
to a forcing is weaker than would have been expected had internal
mechanisms not been accounted for. A number of different examples of this
buffering capacity will be presented. A strategy that includes field
experiment and modeling work aimed at improving our understanding of the
system will be proposed.
shallow cumulus convection,
Shallow water clouds (Kaufman et al. )
• only a few hundred
meters thick
• an increase in shallow
cloud cover by only 0.04
is enough to offset 2-3°K
of greenhouse warming
• By reflecting sunlight
back to
• space, stratiform clouds
are “the vast climate
refrigerator of the tropics
and subtropics
Longitudinal dependence of the shallow cloud fraction (Left) and droplet effective radius
(Right) for the northern tropical Atlantic with dust intrusions (Upper) and southern tropical
Atlantic with smoke intrusion (Lower).
Kaufman Y J et al. PNAS 2005;102:11207-11212
©2005 by National Academy of Sciences
Aerosol Effect in a Buffered System
Stevens and Feingold 2009, Nature
• 1. What is Lifetime effect (LE) hypothesis?
• 2. Why LE is important to shallow martine
cloud system?
• 3. What is the authors’ theory? (paragrahy
Stevens and Feingold 2009, Nature
• satellite observations
– What solid observations on cloud droplet size
vs. aerosols? (open cellular, fig. 2)
– Global magnitude
– Cloud fraction
Stevens and Feingold 2009, Nature
• Two line of arguments related satellite
observations for LE
– The artefact argument
Stevens and Feingold 2009, Nature
• A Buffered System
- negative feedback
- result of negative feedback