Roger Davies, Physics Department, The University of Aucklandhttp://www.physics.auckland.ac.nz/uoa/professor-roger-davies

Radiative-Convective Modelling of the Earth’s Climate:The Effect of Observed Changes in Cloud Amounts

2

climate physics at Auckland

• observations– MISR on Terra (global)

• albedos, heights, winds

– cloud heights– ‘effective’ height,

• H´ = H – <H>

– decadal time series of H´ – correlations of H´ with SOI (El

Niño/La Niña)– hints of decreasing H

• theory– equilibrium climate sensitivity– global balance of energy– radiative transfer

• shortwave (albedo)• longwave (greenhouse effect)• gases• clouds

– cloud physics– convection– radiative-convective

equilibrium (RCE)• with detailed clouds

NZIP_11

3

MISR: Multiangle Imaging SpectroRadiometer

• 9 fixed view angles– +70.5° to –70.5°– reflected solar radiances (4 bands)

• climate data records– self-consistent: 5/2000 – present

• deseasonalized interannual variations– albedo anomalies

• onboard calibration ≈1% relative radiometric accuracy

– height anomalies• geometrically (stereo) derived: no

calibration drift

Terra: 10:30 am sun-synchronous, pole-pole, 14.6 orbits per day

4NZIP_11

5

300 km

NZIP_11

6NZIP_11

7NZIP_11

8

Cloud-Top Heights measured by MISR

• stereo pattern-matching from the A-cameras (±26°)– measurements with horizontal resolution 275 m: heights every 2.2 km– effective height: max contrast in SW reflectivity, may include surface– rms instantaneous height uncertainty: ≈500 m (validated)– O(108) measurements per month, globally at 10:30 am local time– global sampling error: ≈30 m/month; ≈8 m/year

• March 2000 — February 2010: the first 10 years

• consistent climate data record of cloud heights– insensitive to radiometric calibration– high resolution measurements– uniform technique from pole-pole (no view angle effects)

• deseasonalized anomalies, globally and regionally

NZIP_11

9

Why bother with global cloud heights?

• Cloud-Climate feedback• Changes in effective height, H

– emitted longwave radiation to space controls temperatures at z > H• this temperature is fixed for a given albedo, in equilibrium

– if H decreases, the convective layer is reduced• results in lower equilibrium surface temperature

• In radiative-convective equilibrium (RCE)– height increase implies warming, decrease implies cooling

• Over 1 decade, CO2 forcing ≈0.28 W m-2 (IPCC)– In RCE this is equivalent to an increase of ≈28 m in global average

effective height

NZIP_11

10

-70 -60 -50 -40 -30 -20 -10 0 10 200

5

10

15

20

the simplest of equilibrium climate models

temperature (°C)

altit

ude

(km

)

convection

altitude controlled by the greenhouse effect

temperature controlled by albedo

-70 -60 -50 -40 -30 -20 -10 0 10 200

5

10

15

20

the simplest of equilibrium climate models

temperature (°C)

altit

ude

(km

)

convection, with lapse rate G

he altitude controlled by the greenhouse effect

Tmin temperature controlled by albedo, a

NZIP_11

11

-70 -60 -50 -40 -30 -20 -10 0 10 200

5

10

15

20

the simplest of equilibrium climate models

temperature (°C)

alti

tud

e (k

m)

raising tropopause height by 154m (≈2xCO2) increases surface temperature by ≈1°C

NZIP_11

12

100 1000-3

-2

-1

0

1

2

3

Series1

The Effect of Carbon Dioxide on Equilibrium Surface Temperature

CO2 concentration (parts per million)

Ch

ang

e o

f E

qu

ilib

riu

m S

urf

ace

Tem

per

atu

re °

C

change CO2 onlykeep clouds constantkeep albedo constantkeep water vapour constant

NZIP_11

13

100 1000-3

-2

-1

0

1

2

3

Series1

The Effect of Carbon Dioxide on Equilibrium Surface Temperature

CO2 concentration (parts per million)

Ch

ang

e o

f E

qu

ilib

riu

m S

urf

ace

Tem

per

atu

re °

C

change CO2, water vapourand low cloud fractionand cloud height by ±300 m/K

with water vapour feedback at 8%/Kand cloud fraction at 5%/K

NZIP_11

14

Annual Mean Effective Height

meters

NZIP_11

15

Interannual rms Fluctuation of Cloud-top Heights

meters

NZIP_11

16

Correlation of anomalies in cloud-top height with anomalies in sea level pressure

r

sea level pressure from NCEP reanalysis

cloud-top heights from MISRMar 2000–Feb 2010

NZIP_11

17

Correlation of anomalies in cloud-top height with anomalies in surface temperature

r

surface temperature from NCEP reanalysis

cloud-top heights from MISRMar 2000–Feb 2010

NZIP_11

18

Correlation of anomalies in cloud-top height with anomalies in Southern Oscillation

Index r

SOI from Australian Bureau of Meteorology

cloud-top heights from MISRMar 2000–Feb 2010

NZIP_11

19

2000 2002 2004 2006 2008 2010

-1500

-1000

-500

0

500

1000

1500

-15

-10

-5

0

5

10

15

Central Pacific, –Indonesia, –SOI

Year

Hei

gh

t an

om

alie

s (m

)

Indonesian heights invertedSOI inverted

NZIP_11

20

2000 2002 2004 2006 2008 2010-100

-80

-60

-40

-20

0

20

40

60

global cloud height anomalies

year

hei

gh

t an

om

aly

(m)

12-month running meansampling error ±8 m

overall decrease: 45 ±5 m max departure: –80 minterannual rms: 26 m

NZIP_11

21

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

mean annual global stereo height distributionclouds+surface (12/05-6/08)

WITHOUT THIN CIRRUS

all

cloud only

fraction/km

heig

ht

(km

)

Total fractions

surface: 0.366non-surface: 0.634

low (0-3 km): 0.363 mid (3-7 km): 0.144 high (>7 km): 0.128

NZIP_11

22NZIP_11

23NZIP_11

NZIP_11 24

relative trapping of longwave emission from the surface

(adapted from Kiehl and Trenberth)

clouds 45%

water vapour 33%

carbon dioxide 15%

others 7%

NZIP_11 25

Equilibrium Surface Temperatures: with constant albedo

Observed 288 K

No Atmosphere 255 K

Model, no clouds 278 K

Model, observed clouds 291 K

Model, no high clouds 286 K

Model, observed clouds, 50% CO2 290 K

26

100 1000-3

-2

-1

0

1

2

3

Series1

The Effect of Carbon Dioxide on Equilibrium Surface Temperature

CO2 concentration (parts per million)

Ch

ang

e o

f E

qu

ilib

riu

m S

urf

ace

Tem

per

atu

re °

C

change CO2 and water vapourkeep relative humidity constantkeep clouds constantkeep albedo constant

with water vapour feedback @8%/1°C

NZIP_11

27

100 1000-3

-2

-1

0

1

2

3

Series1

The Effect of Carbon Dioxide on Equilibrium Surface Temperature

CO2 concentration (parts per million)

Ch

ang

e o

f E

qu

ilib

riu

m S

urf

ace

Tem

per

atu

re °

C

change CO2, water vapourand low cloud fraction

with water vapour feedback at 8%and cloud fraction at 5%/degree

NZIP_11

28

100 1000-5

-4

-3

-2

-1

0

1

2

3

4

5

Series1

The Effect of Carbon Dioxide on Equilibrium Surface Temperature

CO2 concentration (parts per million)

Ch

ang

e o

f E

qu

ilib

riu

m S

urf

ace

Tem

per

atu

re °

C

change CO2 onlykeep clouds constantkeep albedo constantkeep water vapour constant

Last Glacial Maximum

Maximum Arctic ice-albedo feedback

NZIP_11

29

Standard Deviation of Annual Average of Cloud-top Heights

meters

Pacific box: 30°S–30°N, 100°E–230°E (130°W)Area: 1/6 of globe

NZIP_11

30

2000 2002 2004 2006 2008 2010

-150

-100

-50

0

50

100

150

200

250

Local Height Anomalies: Pacific Box (30°N–30°S, 100°E–130°W)rest of world

Year

Hei

gh

t an

om

aly

(m)

NZIP_11

31

2000 2002 2004 2006 2008 2010

-100

-80

-60

-40

-20

0

20

40

60

80

Normalized Height anomalies: Pacific Box/6, Remainder*5/6, Total

Year

Hei

gh

t an

om

aly

(m)

NZIP_11

32NZIP_11

33

In Summary

• MISR cloud heights provide a useful climate data record• additional CO2 forcing over the last decade ≈ +0.28 W m-2

– equivalent to an increase in effective emission altitude of ≈28 m in RCE

• globally averaged effective height shows this level of fluctuation at the annual level (26 m rms)– heights have decreased by ≈45 m over the last decade

• the major event of the decade is coincident with the 2007-8 La Niña– offsetting height changes between Indonesia and Central Pacific– teleconnections elsewhere appear to dominate the global result

NZIP_11