Lecture 6Lecture 6Role of clouds on climate Role of clouds on climate

and climate changeand climate change

Effects of clouds on temperature Effects of clouds on temperature profilesprofiles

• Clouds absorb LW

• Clouds reflect SW

• Which effect “wins?”

• Depends on emitting T

• For low clouds, T4 ~ Ts4 ,

so SW effect is greater

• For high clouds, T4 << Ts

4 so LW effect “wins”

• High clouds: warm

• Low clouds: coolDetails are sensitive to optical properties and distributions of clouds, but remember the basic conclusions

Observed Mean Cloud AmountObserved Mean Cloud Amount

Source: http://isccp.giss.nasa.gov/

Observed Mean Cloud Top Observed Mean Cloud Top Temp.Temp.

Source: http://isccp.giss.nasa.gov/

Observed Cloud Top PressureObserved Cloud Top Pressure

Source: http://isccp.giss.nasa.gov/

Observed Mean Cloud FractionObserved Mean Cloud Fraction

• High clouds mostly due to tropical convection (Amazon, Congo, Indonesia, W. Pacific)

• Low clouds (stratocumulus) over eastern parts of subtropical ocean basins – Cold SST– Subsiding air– Strong inversion

high clouds( < 440 mb)

low clouds( > 680 mb)

all clouds

Unstable vs. stable environment

Annual Mean Annual Mean Cloud ForcingCloud Forcing• “Cloud forcing” is

defined as the difference between a “clear sky” and “all sky” measurement

• At the surface, (a) is all warming, and (b) is all cooling

• Net effect of clouds is to cool the surface, but changes can go either way

OLR

solar abs

Rnet

Global Mean Cloud Radiative Global Mean Cloud Radiative ForcingForcing

• Clouds increase planetary albedo from 15% to 30%

• This reduces absorbed solar by 48 W m-2

• Reduced solar is offset by 31 W m-2 of LW warming (greenhouse)

• So total cloud forcing is –17 W m-2

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Cloud effects on LW at TOA

Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

Cloud effects on SW at TOA

Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

Therefore,Therefore,

Clouds Clouds coolcool the surface climate. the surface climate.

Duh!?

(a) 太陽短波輻射 (100)

氣體吸收 :16

雲吸收 : 3

被空氣散射回太空 : 6

被雲反射 :20

被地面反射 : 4

地面吸收 :51

(b) 長波輻射 ( 進入太空的量 )

地面放射 :21

15 被氣體吸收 ,

6 直接進入太空

大氣放射 :38

雲放射 :26

出去短波輻射

出去長波輻射

(c) 對大氣而言 :

吸收 = 16 + 3 + 15 = 34

放射 = 38 + 26 = 64

不夠的量 = 30,

=> 必須透過地表蒸發、對流送到高處，冷却達飽和凝結，釋放潛熱(23)和可感熱 (7)加熱大氣而獲得能量補充

How might this role changes How might this role changes (current cloud forcing = -17 (current cloud forcing = -17

watt•mwatt•m-2-2) if cloudiness change ) if cloudiness change (increase or decrease) under(increase or decrease) under G.W.G.W.??

Temporal change of global mean anomaly of Temporal change of global mean anomaly of net net SW SW at TOA, Surface, and Atmos (1983 to 2011) at TOA, Surface, and Atmos (1983 to 2011)

Effect of Pinatubo

Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

(1) a decrease at the surface and TOA (atmosphere as well) produced by the Mt. Pinatubo volcanic aerosols in 1991-92;

(2) an overall increase at TOA and the surface, but not in the atmosphere

Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

(1) decreases at the surface and increases in the atmosphere, but not at TOA;

(2) In late 1990s, a small decrease at TOA and in the atmosphere and a larger increase at the surface.

Temporal change of global mean anomaly of Temporal change of global mean anomaly of net net LW LW at TOA, Surface, and Atmos (1983 to 2011) at TOA, Surface, and Atmos (1983 to 2011)

ISCCP global average of TISCCP global average of Tsfcsfc change change (1983-2009(1983-2009)

ISCCP zonally average of TISCCP zonally average of Tsfcsfc change change (1983-2009(1983-2009)

ISCCP global average of cloud amount change ISCCP global average of cloud amount change (1983-2009(1983-2009)

ISCCP zonally average of cloud amount change ISCCP zonally average of cloud amount change (1983-2009(1983-2009)

ISCCP global average of cloud top pressure ISCCP global average of cloud top pressure change change

(1983-2009(1983-2009)

ISCCP zonally average of cloud top pressure ISCCP zonally average of cloud top pressure change change

(1983-2009(1983-2009)

Cloud ClassificationCloud Classification

ISCCP global average of ISCCP global average of high-levelhigh-level cloud amount cloud amount change (1983-2009change (1983-2009)

ISCCP global average of ISCCP global average of middle-level middle-level cloud cloud amount change (1983-2009amount change (1983-2009)

ISCCP global average of ISCCP global average of low-levellow-level cloud amount cloud amount change (1983-2009change (1983-2009)

ISCCP day-time ISCCP day-time stratusstratus cloud amount change in cloud amount change in over over waterwater

ISCCP day-time ISCCP day-time stratusstratus cloud amount change in cloud amount change in over over landland

ISCCP global day-time ISCCP global day-time deep convectivedeep convective cloud cloud amount changeamount change

ISCCP day-time ISCCP day-time middle-levelmiddle-level cloud amount cloud amount change in change in ArcticArctic

ISCCP day-time ISCCP day-time deep convectivedeep convective cloud amount cloud amount change in change in AntarcticAntarctic

Trend of sea-ice coverTrend of sea-ice cover

N.H. S.H.