ATMS 591Aerosol-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 aerosol-

cloud-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

http://asr.science.energy.gov/science

Myriad processes

Myriad aerosol-cloud-precipitation processes occurring within a climate model gridbox (or not)

IPCC AR5, Chapter 7, Fig 7.16

IPCC “Radiative Forcing” Diagram

IPCC 5th Assessment Report (AR5), Summary for Policymakers, Fig. SPM.5, 2013

Understandphysical bases for, and model representation of, this entry

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

• 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

primary feedback

Twomey

Albrecht

(Mostly) regulating feedbacks in stratocumulus

Regional gradients: Strong aerosol indirect effects in an extremely clean background

George and Wood, Atmos. Chem. Phys., 2010

Albedo enhancement (fractional)

Satellite-derived cloud droplet concentration Nd

low level wind

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:

• To determine aerosol-driven changes on C, one needs to measure meteorology-driven changes

• This is a particularly arduous task

meteorology-driven aerosol-driven

Stevens and Brenguier (2009)

Shiptracks

= 0

Shipping lanes• Shipping emissions increase along

preferred lanes• Control clouds upstream; perturbed

clouds downstream

Peters et al. (ACP, 2011)

Observed f 0.02-0.03

= 0.06 K-1 × 0.4 K = 0.024

Klein and Hartmann (1993)

A cloud cover increase of 0.02 represents a radiative forcing of 2 W m-2

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