ATMS 591
Aerosol-Cloud-Climate Interactions
Spring Quarter 2015
Logistics
• Class webpage
• Class meets 1:30-2:50 Mondays and
Wednesdays in Room 406, ATG Building
• Instructor: Prof. Robert Wood [ATG 718, Phone 543-1203]
• Office Hours: flexible
• Prerequisites: ATM S 535: Cloud Physics and
Dynamics, or by specific permission of the instructor.
• Textbook: No required textbook
Class description
This graduate special topics class focuses on the
general topic of aerosol-cloud-climate interactions.
The class aims to cover many aspects of this topic,
including a review of the theoretical arguments linking
aerosols and clouds, the use of field and satellite data,
and process and climate models to understand the
problem, and a consideration of past and future
changes in aerosol indirect climate forcing.
ATMS 591: Learning Goals:
1. Understand the physical bases for aerosol-cloud interactions
2. Awareness of the important literature on the topic of aerosolcloud-climate interactions
3. Understand how clouds are affected by aerosols and how cloud
processes affect aerosols
4. Gain an appreciation for the concept of aerosol indirect forcing
(also known as aerosol forcing by aerosol-cloud interactions)
5. Understand how observations and models are used to assess
the climatological impacts of anthropogenic and natural aerosols.
Useful texts
• Atmospheric Science, an introductory survey, by J. M. Wallace and
P.V.Hobbs (Second Edition), Academic Press.
• A Short Course in Cloud Physics, by M. K. Yau and R. R. Rogers. Elsevier, Jan
1989, ISBN: 978-0-7506-3215-7.
• Atmospheric Chemistry and Physics: From Air Pollution to Climate Change,
2nd Edition. John H. Seinfeld, Spyros N. Pandis. ISBN: 978-0-471-72018-8,
1232 pages
• Physics and Chemistry of Clouds, by Lamb and Verlinde: Cambridge
University Press; 1st edition. Copies available in the UW Bookstore
• Clouds in the Perturbed Climate System: Their relationship to energy
balance, atmospheric dynamics, and precipitation. Edited by Jose
Heintzenberg and Robert Charlson. MIT Press, 2009. Ernst Strungman
Forum reports. ISBN 978-0-262-01287-4.
Aerosol-cloud-climate interactions
• A definition
– Two-way interactions between aerosol particles
and clouds that influence regional and global
climate
• “Two-way” interactions:
– Aerosol particles influencing cloud radiative properties,
a.k.a. aerosol indirect effects
– Clouds influencing the properties of climatically important
aerosol particles
Class organization
• Largely reading based
– We will read and discuss/critique some of the key
papers describing:
• The physical basis for aerosol-cloud-climate interactions
• How process models and observations are used to
understand aerosol-cloud interactions
• How aerosol-cloud interactions are represented in
climate models
• Past and future assessments of climate impacts of
anthropogenic aerosols
Class organization
• Projects
– Each student will carry out a project during the second half
of the quarter. The aim is to align this project to make it
useful for your own research. Examples might include:
• Literature review of topic pertinent to your research
• Use of data from your own research, or from readily obtainable
datasets to explore facets of aerosol-cloud-climate interactions
• Design simple model experiments to provide insights on physical
bases for aerosol-cloud-climate interactions
Aerosol effects on clouds:
Ship tracks in low clouds
over the Bay of Biscay
Cloud impacts on aerosols
Myriad
processes
http://asr.science.energy.gov/science
Myriad aerosol-cloud-precipitation processes
occurring within a climate model gridbox (or not)
IPCC AR5, Chapter 7, Fig 7.16
IPCC “Radiative Forcing” Diagram
Understand
physical bases
for, and model
representation of,
this entry
IPCC 5th Assessment
Report (AR5),
Summary for
Policymakers,
Fig. SPM.5, 2013
Reading list (first few)
• Twomey, S., 1977: The Influence of Pollution on the Shortwave
Albedo of Clouds. J. Atmos. Sci., 34 (7): 1149–52.
• Albrecht, B. A., 1989: Aerosols, Cloud Microphysics, and
Fractional Cloudiness. Science, 245, 1227–30.
• Ramanathan, V. Aerosols, Climate, and the Hydrological Cycle.
Science 294, no. 5549 (December 7, 2001): 2119–24.
doi:10.1126/science.1064034.
• Lohmann, U., and J. Feichter. Global Indirect Aerosol Effects: A
Review. Atmospheric Chemistry and Physics 5, no. 3 (2005):
715–37
The Twomey Effect
Twomey effect
• The paper(s) that “started it all”
– Precursor papers
• Physical basis for growth of cloud droplets dates back
to equations for diffusional growth from Maxwell
(1800s). Applied to cloud droplet growth by Kraus and
Smith (1949), Howell (1949), Squires (1952).
• Influence of composition of aerosol particles by Köhler
(1921)
• Twomey’s own prior work (e.g. Twomey 1959, Twomey
and Warner 1967) provided a clear influence of the
concentration of aerosol particles on the concentration
of cloud droplets
• Theoretical basis covered in ATMS 535
Twomey, S., and J. Warner, 1967: Comparison of
Measurements of Cloud Droplets and Cloud Nuclei. J.
Atmos. Sci., 24, 702–3
Twomey effect
• The paper(s) that “started it all”
– Actually, Twomey (1974) was the first paper suggesting
anthropogenic aerosol impacts on cloud albedo:
Twomey, S., 1974: Pollution and the Planetary Albedo. Atmospheric Environment, 8, 1251–56.
Twomey effect
• Twomey (1974)
– Actually, Twomey (1974) was the first paper suggesting
anthropogenic aerosol impacts on cloud albedo:
Twomey, S., 1974: Pollution and the Planetary Albedo. Atmospheric Environment, 8, 1251–56.
Twomey effect
a.k.a. the first aerosol indirect effect
Theoretical expression for AIE
• Response of cloud optical thickness t to change in some
aerosol characteristic property A
primary
•
feedback
Generally, because AIEs must be dominated by warm clouds
and ice clouds formed by homogeneous freezing, the
property most relevant to the problem is the cloud
condensation nucleus concentration (CCN).
• Aerosol size and composition effects can also play a role
Twomey
Albrecht
(Mostly) regulating feedbacks in stratocumulus
Regional gradients: Strong aerosol indirect
effects in an extremely clean background
Satellite-derived cloud droplet
concentration Nd
Albedo enhancement
(fractional)
low level wind
George and Wood, Atmos. Chem. Phys., 2010
Observational evidence for the Twomey effect
Painemal and Minnis (2012)
Model estimates of the two major aerosol
indirect effects (AIEs)
• Pincus and Baker (1994)
– 1st and 2nd AIEs comparable
• GCMs (Lohmann and
Feichter 2005)
1st AIE: -0.5 to -1.9 W m-2
2nd AIE: -0.3 to -1.4 W m-2
Limited investigation of factors that control the relative
importance of the two AIEs
Detecting aerosol impacts on cloud
• An observed change in cloud property C is caused by
changes due to meteorology M and aerosols A:
𝛿𝐶 =
𝜕𝐶
𝜕𝑀
𝛿𝑀 +
𝐴
meteorology-driven
𝜕𝐶
𝜕𝐴
𝛿𝐴
𝑀
aerosol-driven
• To determine aerosol-driven changes on C, one needs
to measure meteorology-driven changes
• This is a particularly arduous task
Stevens and Brenguier (2009)
Shiptracks
=0
Shipping lanes
• Shipping emissions increase along
preferred lanes
• Control clouds upstream; perturbed
clouds downstream
Klein and Hartmann (1993)
= 0.06 K-1 × 0.4 K
= 0.024
Observed f  0.02-0.03
A cloud cover increase of 0.02
represents a radiative forcing of 2 W m-2
Peters et al. (ACP, 2011)
What about ice?
de Boer et al. (2013)
Summary
• Uncertainty in equilibrium climate sensitivity largely
controlled by uncertainty in how clouds will change.
– Low clouds constitute largest source of error, but high
clouds show robust changes
• Aerosol forcing, including effects on clouds, is likely a
significant fraction of CO2 forcing.
– Aerosol-cloud interactions important for determining
overall aerosol forcing
– Low clouds primary culprits, but ice phase may be
important