Clouds and Climate: Cloud Response to Climate Change

Clouds and Climate: Cloud Response to Climate Change

SOEEI3410Ken Carslaw

Lecture 5 of a series of 5 on clouds and climate• Properties and distribution of clouds• Cloud microphysics and precipitation• Clouds and radiation• Clouds and climate: forced changes to clouds• Clouds and climate: cloud response to climate

change

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Content of this Lecture

• The importance of cloud feedbacks: Climate sensitivity

• Cloud radiative forcing• Factors affecting clouds• Cloud feedback in climate models


Reading

• Section 7.2.2 Cloud Processes and Feedbacks of IPCC 2001– http://www.grida.no/climate/ipcc_tar/wg1/271.htm

http://www.grida.no/climate/ipcc_tar/wg1/271.htm


Climate Sensitivity

• Climate sensitivity determines the global temperature when a radiative forcing is applied


Climate Sensitivity

• DT = change in global mean temperature• Q = radiative forcing (W m-2)• l = climate sensitivity (W m-2 K-1)

lQT D


Sensitivity of Climate Models

Sensitivity to forcing from doubled CO2 (~4 Wm-2)

Summer 2002

NC

AR

GFD

L

2xC

O2 S

ensi

tivity

(K)


Cloud Changes and Climate Sensitivity

1/l=4.2 K Wm-2

1/l=1.8 K Wm-2

% Change in low cloud amount per 1K temperature change


Change in “Cloud Radiative Forcing”

• Clouds cause a net cooling effect on climate (net -20 Wm-2 forcing (equivalent to about 8*CO2)– All models agree on sign (+/-) of CRF

• Cloud feedback is about how CRF changes as greenhouse gases increase– Models disagree greatly on this

• Some clouds warm, some cool. DT depends on which clouds change


Humidity and Temperature

• Increased T• Increased water

vapour in atmosphere• Increased

cloudiness?

• NO

• Relative humidity is the relevant quantity

Overall increase in atmospheric water vapour

Overall increase in atmospheric water vapour and temperature

100% RH


Cloud Radiative Forcing (CRF)

• Factors that determine CRF (or, what does a climate model need to get right?)– Cloud location (solar intensity)– Depth/thickness– Coverage– Drop/ice concentrations

How would SW and LW impact on climate change for these two cloud field?


CRF, dependence on location, thickness and height

0 50 100 150 -40

-20

0

20

40

liquid water path (g m-2)

DTs (

K)

Winter 5o N

lowmed

high

cloud height

Equilibrium surface temperature change due to presence of different clouds

0 50 100 150 -40

-20

0

20

40

liquid water path (g m-2)

DTs (

K)

Winter 65o N

lowmed

high


Reasons for Cloud Changes

• Large-scale dynamics/circulation– Global circulation changes in response to changes in ocean

circulation, changes in ocean-atmosphere T contrast, etc

• Thermodynamic/cloud-scale changesChanges to: – vertical T profile, – atmospheric stability, – turbulence structure of boundary layer,– water substance transport– aerosol

• Very difficult to separate in observations


Circulation/Dynamical Changes

Tropicalconvection

Tradewindcumulus

Sub-tropical St/Sc

Hadley/Walkercirculation

Equator 30oN

• Cloud fields are determined by large-scale circulation

• Non-local response

• El Nino


Observed Clouds With Temperature

• Observations from the International Satellite Cloud Climatology Project

• Clouds become optically thinner (less reflective) at higher temperatures

• +ve or –ve feedback?-60 -40 -20 0 20 40 60

-0.15

-0.1

0.05

0

0.1

latitude

d lo

g(op

tical

dep

th)/d

T

Ocean low clouds


Observed cloud with temperature: Tropical Cirrus

25 26 27 28 29 300

0.05

0.1

0.15

0.2

sea surface temperature (K)

Clo

ud A

mou

nt

slope = 10-20% change per 1 K SST

observations

• Japan’s Geostationary Meteorological Satellite

• 11 and 12 mm wavelength radiometer

• 130oE-170oW, 30oS-30oN (Pacific)

• 260 K brightness temperature product is a measure of “high thin cloud” – cirrus

• Cirrus cover decreases with increasing SST

Richard Lindzen, MIT



The Adaptive Infrared Iris as a Climate Change Regulator

warm ocean cold ocean

more IR to space

less cirrus

more rain

less watertransport

less water vapour


Problems With the Infrared Iris Idea

• A hotly debated climate feedback• See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html

http://www.gsfc.nasa.gov/topstory/20020915iristheory.html


Net Cloud Feedbacks in GCMs

-3

-2

-1

0

1

2

3

Cha

nge

in C

RF

(W m

-2)

Different models

SW

LW

netCOOLING

WARMING

Doubled CO2 experiments


Difficulties

• Different types of clouds have different effects and may change in different ways – many separate problems

• Some aspects of clouds (thickness, ice content) are difficult to observe

• Sub-grid scale problems• Effects of temperature and circulation can be confused• Changes observed on short time scales (e.g., El Niño)

may not always be good indicators of climate change-induced changes


Questions for this lecture

• On slide 6, what could explain the wide range of climate model sensitivities to doubling of CO2?

• Based on slide 6, what would happen to our climate if the coverage of high thin cirrus clouds increased (a) at the equator, (b) 65oN? What explains the difference?

• On slide 14, explain whether the data indicate a positive or negative climate feedback.

• For the first model shown on slide 19 explain what cloud changes could account for the changes in global mean SW and LW cloud radiative forcing.


Competition

• Take a photograph of a cloudy scene. Send it to me with a detailed explanation of what the clouds are doing to climate.

• The winner will be decided based on beauty and complexity of the cloud scene and accuracy of the explanation

• Closing date: end of term• Prize: A large tin of chocolates

Documents

Clouds and Climate: Cloud Response to Climate Change