View
4
Download
0
Category
Preview:
Citation preview
An Introduction to Twomey’Effect Guillaume Mauger
Aihua Zhu
Mauna Loa, Hawaii on a clear day Mauna Loa, Hawaii on a dusty day
Non-selectivescattering.
Rayleighscattering
Miescattering
The impact ofclouds
Clouds have two competing influences in theradiation budget:
1) They reflect solar radiation and prevent itfrom reaching the surface of earth, similar to anumbrella, and preventing the warming of theatmosphere through absorption of theradiation.
2) They absorb the infrared radiation escapingfrom the surface of the earth and thus trap thewarmth in atmosphere, similar to a greenhouse.
Low cloud
Middle cloud
High cloud
Cloudtypes
Different impacts of differentclouds
Low level clouds: large albedo, weak absorption of infrad radiation-----coolingUpper level clouds: weak albedo, strong absorption of infrad radiation----warming
is the extinction efficiency, is given by Mietheory, nearly equal to 2is the extinction efficient
For solar wavelengths and for realistic (polydisperse)drop distributions we can eliminate the integrationand adopt the simple formula
Where N is the drop concentration per cubic centimeterand can be any representative of mean or model radius
Opticalthickness
Extinction efficiencies: general behaviour anddeviations
Size dependence ofthe extinction efficiencyfactors forhomogeneousspherical particles. Theinfluence of variationsof the real part (upperpanel) and imaginarypart (lower panel) ofthe refractive index isillustrated.Typical refractiveindex m of clouddroplet is:m=1.333-1.96*10-9i
The liquid water content W is given by the volume-meanradius W (in number)
=W (in mass)=
ρwater W (in number)
A water content of 1/3 gm-3 (which is typical at least oflower level clouds) was adopted to calculate values ofextinction coefficient kE (cm-1) and optical thickness perkilometer of cloud depth hrNkm
22πτ =22 rNkE π=
Parameters needed to give theextinctionOptical thickness:
es ββω /0 =
∫∫+
−
+
−= 1
1
1
1
)(
)(
duuP
duuuPg
Single scatteringalbedo:
Asymmetry factor:
hrNkm22πτ =
ase βββ +=)cos(θ=uWhere P(u) is the phase function
is the extinction coefficientis absorption opticalthickness
)1( 0ωττ −=a
Increase pollution (pollution index x) will increase thealbedo if dS/dx is positive:
=dxdS
because:
so we need to prescribeand
)1( 0ωττ −=aBecause:
¬Clean air over the tropical oceans 10-100 cm-3¬Continental concentrations run from 500-1000cm-3;¬Moderate to heavy pollution has values >5000cm-3
Typical cloud nucleiconcentration
The values 0.01 and 0.1 represent a reasonable and anextreme value of , respectively, for the absorptionkm-1 depth in continental conditions when N=1000cm-3
∫ ∫ ΦΦΦ==↑
πµµµµ
πµµ
2
0
1
0 000
0 ),;,(1*)0(
)( ddRF
Fr dif
*)/(),;0(),;,( 000 FIR r µµπµµ Φ=ΦΦ
∫==↑ 1
0 0002 )(2*)0(
µµµπ
drFa
fr
Spherical (or global) albedo:As=
Spherical albedo eliminates geometric variables and gives a reasonablyrepresentative global value of albedo, It represents the fraction ofincident radiation reflected by a sphere covered by a layer of theprescribed properties.
According to Liou, spherical albedo can becalculated as:
Frequency distributions ofthe reflectances at 1,535nm versus reflectances at754 nm determined duringthe ACE-2 experiment.Isolines of geometricalthickness (H) and dropletnumber concentration (N)demonstrate the higherreflectance in pollutedcloud if normalised by asimilar geometricalthickness (Brenguier et al.2000).
Indirect Forcingof Aerosols
Flow chart showing theprocesses linking aerosolemissions or production withchanges in cloud optical depthand radiative forcing. Barsindicate functional dependenceof the quantity on top of the barto that under the bar. Symbols:CCN (Cloud conden-sationnuclei); CDNC (Cloud dropletnumber concentration); IN (Icenuclei); IP (Ice particles); OD(Optical depth); HC(Hydrometeor concentration); A(Albedo); fc (Cloud fraction); Dc(Cloud optical depth); tc(Radiative forcing).
¬Pollution may increase or decrease the brightnessof clouds depending on the optical thickness of theclouds and the way in which cloud nucleusconcentration varies with absorption opticalthickness.¬In all but the thickest clouds the pollutionincreases the albedo. Since most of the earth’scloud cover is not very thick this result suggest thatthe planetary albedo also will increase with increaseof pollution.
conclusion
Recommended