regimes within the first aerosol indirect effect · – thi l dthin clouds—ht ?heterogeneous? •...
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Regimes within the First Regimes within the First Aerosol Indirect Effect
Mark A. Miller, Maureen DunnMary Jane Bartholomew Pavlos KolliasMary Jane Bartholomew, Pavlos Kollias
Brookhaven National Laboratory
Thanks: Pete Daum, Mike Jensen, Andy Vogelmann, Christine Chiu,
Dave TurnerDave Turner
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Theory of the First Aerosol Theory of the First Aerosol Indirect Effect
• In an adiabatic ascent, an increase in the droplet number concentration at constant liquid water mixing ratio results in a liquid water mixing ratio results in a decrease in the average droplet radius and an increase in the reflection of incoming
li h (T 1974 1991)sunlight (Twomey, 1974, 1991).• Key issues for implementation in real
atmosphere:atmosphere:– Isolated parcel (no mass or heat exchange)– Adequate supersaturation (updraft velocity) q p ( p y)
to activate new Cloud Condensation Nuclei (CCN)
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K Q i !Key Question!
• Does the adiabatic cloud droplet nucleation process, at constant p ,liquid water path, actually determine the optical properties of p p pthe cloud?
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A Basic ExperimentA Basic Experiment
rewmax
NN
2NFOV-τ
<200 m
#/cm
3
FMWR-LAERI-L
Cloud Chamber
%supersaturation[f(wmax)]
95-GHz-wmax
Cloud Chamber
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The One-Third LawThe One-Third Law
• A reduction in the global in the global value of reby 2-μm by 2 μm would offset the doubling Liu and Daum, 2002
-indirect warming effectgof CO2 (Slingo 1990)
g
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High LWPHigh LWP
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Low LWPLow LWP
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LWP and UpwellingLWP and Upwelling
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Updraft Limitations?Updraft Limitations?
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S d C l iSummary and Conclusions• Observed LWP-dependent regimes within
the First Aerosol Indirect Effect R d th t “O thi d L ” b • Recommend that “One-third Law” be applied selectively in coastal stratus– may require modification to account for may require modification to account for
subadiabatic clouds– Otherwise, an overestimate of AIE
N d d • Need more extensive data set• New technique for number density is
promisingpromising
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THE END
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Jensen et al. (2007)5-years MODIS Terra—300x300 km boxAlgorithm to remove cloud edge pixels>30,000 data points
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LWP RegimesLWP Regimes
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Droplet Microphysics in GCMsDroplet Microphysics in GCMs“One-Third Power Law”
Liquid Water Path[PROGNOSTIC]
Shortwave Transmission
1 3L⎡ ⎤
Shortwave TransmissionDepends on re and L
eLr C
N zβ ⎡ ⎤= ⎢ ⎥Δ⎣ ⎦Light Scattering
Properties of Cloud Droplets
Cloud Droplet Number Density[Sometimes PROGNOSTICRelative
Dispersion
Cloud Droplets
[sometimes parameterized]Dispersion
(sigma/mean)[Parameterized]
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Results from GResults from G--1 Flights During MASE1 Flights During MASE
Increasing aerosol concentrations observed with increasing Increasing aerosol concentrations observed with increasing g gg gdroplet concentrations and a decrease in the effective droplet concentrations and a decrease in the effective radiusradius..
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LWP d D i lLWP and Drizzle
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M i S lMarine Stratocumulus
J. Norris1954-1992Synoptic Ship
Observations0 10 20 30 40 50 %
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Aerosol Shortwave Indirect Aerosol Shortwave Indirect Effects
• First (or Twomey Effect): “We can with some confidence predict that clouds in the future will contain more droplets per unit volume and, other things being equal, these droplets per unit volume and, other things being equal, these droplets must become smaller” Twomey, S., 1991 Aerosols, clouds, and radiation. Atmos. Env., 25A, 2435-2442. (originally proposed in 1974)
• Second (or Albrecht Effect): “Increases in aerosol concentrations over the oceans may increase the amount of low level cloudiness through a reduction in drizzle—a process low level cloudiness through a reduction in drizzle—a process that regulates the liquid water content and energetics of shallow marine clouds” Albrecht, B.A., 1989: Aerosols, cloud microphysics, and fractional cloudiness. Science, 245, 1227-p y1230.
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Th Adi b i Cl d M d lThe Adiabatic Cloud Model
• Many past studies use an adiabatic cloud model to compute the adiabatic cloud droplet number density, which is subsequently linked to effective radius to examine the Aerosol First Indirect Effectexamine the Aerosol First Indirect Effect– Penner et al., 2005; Feingold et al., 200X;
Brengiuer et al., 2005; ect.Br ng u r t a ., 5; ct.– Assume the nucleation properties of the
aerosol particles
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Eff f MEffects of Mixing
• Homogeneous versus heterogeneous mixing– AIE impacts apparently different
Thi l d h t ?– Thin clouds—heterogeneous?• Wood: Cancellation of aerosol indirect
effects through cloud thinningeffects through cloud thinning– Cloud base height is critical factor– Zcb<400 m feedbacks cancel Twomey– Consistent with low LWP results
• SGP: New results that support homogeneous mixing in clouds with higher homogeneous mixing in clouds with higher LWP
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MODIS Analyses: Effective cloud diameterMODIS Analyses: Effective cloud diameter
RiftRift
SheetSheet
Rift Rift ZonesZones
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re vs LWP response varies with air mass
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T lf d J W (1996)Telford, J.W.(1996)
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Results from GResults from G--1 Flights During MASE1 Flights During MASE
Increasing cloud droplet concentrations observed Increasing cloud droplet concentrations observed Increasing cloud droplet concentrations observed Increasing cloud droplet concentrations observed coincidently with a decrease in the drizzle coincidently with a decrease in the drizzle concentration.concentration.
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erHomogeneous
Mi iHeterogeneous mixing
e
erMixing
Extreme case
Secondary activation
Enhanced growth
Underlying mechanism
Faster Mixing Uniform evaporation
Nucleation Coalescence
aN and aL aN=1 aN = aL aN > aL aN < aL
Mixing function
Mixing does not change N
but reduce the sizes
Stronger mixing results
in more droplets
Stronger mixing results in less but
bigger droplets
er
er
sizes droplets,
Response of
Depending on ab and aL
independent of aL
decreases with decreasing aL
increases with decreasing aL
( )1/3e ea Lr rβα α= e ear rβα=
1/3
Le ea
N
r rβααα⎛ ⎞
= ⎜ ⎟⎝ ⎠
1/3
Le ea
N
r rβααα⎛ ⎞
= ⎜ ⎟⎝ ⎠Formula
AIE Effect ? No change More AIE effect
Less AIE effecteffect
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Impacts of Cloud Dynamics on Impacts of Cloud Dynamics on p yp yCloud MicrophysicsCloud Microphysics
re
N=288
10.1 Adiabaticity
Adiabaticity, αAdiabaticity, αα α = 1: adiabatic= 1: adiabaticα α < 1: sub< 1: sub--adiabaticadiabatic
Bins of CloudBins of CloudThicknessThickness
10.1 Adiabaticity
Blue: 50Blue: 50--600m600mGreen: 600Green: 600--1000m1000m
τ
Collaborators: B.G. Kim, Steve Schwartz10.1 Adiabaticity
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Impacts of Cloud Dynamics on Impacts of Cloud Dynamics on p yp yCloud MicrophysicsCloud MicrophysicsBins of Adiabaticity
Blue: 0.1-0.9 reGreen: 0.9-1.5
Cloud Depth100 1000Cloud Depth100
R2=0.49
R2 0 92τ R2=0.92
Collaborators: B.G. Kim, Steve Schwartz100 Cloud Depth 1000
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The Entity-Type The Entity Type Entrainment Mixing Process
J.W. Telford
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b t t d
Stage 1
subsaturated
1. Droplets evaporate into subsaturated air2 C li
cloud
2. Cooling3. Descending vortex4. Saturation
saturates
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Stage 1 Stage 2
CCN( )( )0 0c ic edV V C C n+ + +
well-mixedsaturates
1. Reduction in size and number of entrained droplets prior to saturation2. Droplets enter turbule after saturation with no change in size3. Drastic changes in number concentration from parcel to parcelg p p
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Stage 1 Stage 2
CCN( )( )0 0c ic edV V C C n+ + +
well-mixedsaturates
1 Evaporation to smaller sizes than at same pressure during undiluted ascent1. Evaporation to smaller sizes than at same pressure during undiluted ascent2. Same amount of water has to evaporate from less droplets3. Continued entrainment of larger undiluted drops adds a supply of larger drops4. Parcel contains a wide range of diameters
Stage 3
descent
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Stage 1 Stage 2
CCN( )( )0 0c ic edV V C C n+ + +
well-mixedsaturates
1 Rise to accommodate newly descending parcels1. Rise to accommodate newly descending parcels2. Few biggest droplets acquire a disproportionate fraction of new water3. Large droplets formed4. Mocks giant nuclei
Stage 3 Stage 4
descent re-ascent
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Observables Checklist• Stage 1
– Reduced reflectivity in pockets near cloud top– Considerable variability in reflectivity near cloud topConsiderable variability in reflectivity near cloud top
• Stage 2 and 3– Large variations in number density from parcel to parcel
Descent– Descent– Lower reflectivity and wider spectral width in the
downdrafts relative to the updrafts• Stage 4• Stage 4
– Possible enhanced reflectivity in some updrafts due to the presence of exceptionally large droplets.
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288 3
Continental288 cm-3Number
Density
d159 cm-3
Mean andStandardDeviation
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At a Given Wavelength A Cloud Particle At Different Wavelengths
ar ar
At Different Wavelengthsd
to R
ada
d to
Rad
a
Ret
urne
d
Ret
urne
d
Ener
gy R
Ener
gy R
E E
Size of Cloud Particle Radar Wavelength
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F d l R l i hiFundamental Relationships
( )( )0
1/
2 1g
D Igττ↑
−=
+Shortwave Albedo
( )2 1 g τ+ −
3 L
2eQ ≈Assume
How much total liquid in the cloud?
Liquid Water Path (LWP)
32 w e
Lr
τρ
=How much total liquid in the cloud?
How much surface area does the liquid have?
Effective Radius--Mean radius for scattering
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R l f Hi h LWPResults for High LWP
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R l f Hi h LWPResults for High LWP
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Remotely Sensing the Cloud Remotely Sensing the Cloud Droplet Distribution
Mode Radius??
Width??
+Hei
ght
Width??
++R d E h
H
Particle Size+
Broadband IrradianceRadar EchoIntensity
Total Liquid Water(Micr v R di m t r)
Number Concentration??
(Solar Radiometer)
(Microwave Radiometer)
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Eff i R diEffective RadiusLight Scattering C S tiCross-Section~ r2
Mean
r2
r2-weighted
Particle Size
r weighted arithmetic mean radius