Increase of the satellite retrieved aerosol optical depth due to the enhanced illumination optical depth due to the enhanced illumination of cloud-free areas in the vicinity of clouds

Alexander Marshak (GSFC)

Thanks to: Guoyong Wen (UMBC)L i R (GSFC)Lorraine Remer (GSFC)

Jim Coakley (OSU)

March 26, 2007 Alexander Marshak 1

How do clouds affect aerosol retrieval?aerosol retrieval?

clouds are complex and “satellite analysis may be affected clouds are complex and satellite analysis may be affected by potential cloud artifacts” (Kaufman and Koren, 2006);

Both• cloud contamination (sub-pixel clouds)• cloud adjacency effect (a clear pixel with in the vicinity of clouds)cloud adjacency effect (a clear pixel with in the vicinity of clouds)may significantly overestimate AOT.

But they have different effects on the retrieved AOT: But they have different effects on the retrieved AOT: while cloud contamination increases “coarse” mode, cloud adjacency effect increases “fine” mode.

March 26, 2007 Alexander Marshak 2

Aerosol-cloud photon interaction

0

Thick clouds0

Thin clouds

500

1000ow

1000

row

1500

2000

ro

50

60

0 20 40 60 80

0.373

0.383

0.392

0.402

.47u

m)

Aerosol Optical Thickness (0.47um)

0 1000 2000

2000

col

0.0 0.2 0.4 0.6ASTER_B3N_BL2100x2490_bin

0 500 1000 1500 2000col

0.000 0.125 0.250 0.375 0.500 0.625ASTER B3N TL2100x2490 bin20

30

40

50

Y (

km)

0.335

0.344

0.354

0.364

0.373

osol

Opt

ical

Thi

ckne

ss (

0.4

March 26, 2007 Alexander Marshak 3

ASTER B3N TL2100x2490 bin

0 20 40 60 80X (km)

0

10

0.306

0.316

0.325 Aer

os

Aerosol-cloud radiative interactionThi k l d 15Thin clouds, <τ>=7 Thick clouds, <τ>=15

14

0 2 4 6 8 10 12 14

45.2

50.0

Cloud Optical Depth

14

0 2 4 6 8 10 12 14

45.3

50.0

Cloud Optical Depth

6

8

10

12

Y (

km)

20.9

25.8

30.6

35.5

40.3

Clo

ud O

ptic

al D

epth

6

8

10

12

Y (

km)

21.8

26.5

31.2

35.9

40.6

Clo

ud O

ptic

al D

epth

0 2 4 6 8 10 12 14X (km)

0

2

4

1.5

6.4

11.2

16.1

0 2 4 6 8 10 12 14X (km)

0

2

4

3.0

7.7

12.4

17.1

AOT0.66=0.1

enhancement: Δρ= ρ3D-ρ1D

March 26, 2007 Alexander Marshak 4

Δρ ~ 0.004Δτ ~0.04 or ~40%

Δρ ~ 0.014Δτ ~0.14 or ~140%

from Wen et al. 2007

Contributors to the cloud enhancement

0.05

h t

• Rayleigh scattering• Aerosols

0.04

ecta

nce

AOT=0.1

enhancement: Δρ= ρ3D-ρ1D

• Aerosols• Surface reflectance 0.03

ent o

f Ref

lect

AOT=0.5

AOT=1.0

0.01

0.02

nhan

cem

en

For dark surfaces, the

0.00

0.01Enh

,enhancement only weakly

depends on the AOT

March 26, 2007 Alexander Marshak 5

0.0 0.1 0.2 0.3 0.4Surface Albedo

0.00

If Rayleigh scattering is the main source for the enhancement in the source for the enhancement in the

vicinity of clouds then

we retrieve larger AOT and, namely, and, namely,

the “fine” mode fraction

However, it is not clear yet how much the observed aerosol grows (in the vicinity of clouds) comes fromgrows ( n the v c n ty of clouds) comes from

• the 3D cloud effects in the retrievalsl ph si s ( h midifi ti n)

March 26, 2007 Alexander Marshak 6

• real physics (e.g., humidification)

Enhancement of radiance near cloudsthe twilight zoneg

Cumulus clouds over Atlantics

March 26, 2007 Alexander Marshak 7

Fig. 2 from Koren et al., GRL (in press)

How to (realistically) remove the 3D l d ff t l ?cloud effect on aerosols?

The enhancement is defined as the difference between the two radiances:

• one is reflected from a broken cloud field with the scattering Rayleigh layer above it• and one is reflected from the same broken cloud field but with the Rayleigh l h i ti ti b t tt ilayer having extinction but no scattering

Rayleigh layerRayleigh layer

March 26, 2007 Alexander Marshak 8

Broken cloud layer

Enhancement (θ0=60o, θv=0o)

0.10.47 μm

0.03

0.0350.65 μm

0.06

0.08

cem

ent

0.02

0.025

emen

tAc = 1.0Ac = 1.0

0.02

0.04

enha

nc

0.01

0.015

enha

nce

Ac = 0.5

A = 0 3

Ac = 0.5

A = 0 3

00 10 20 30 40 50

average cloud optical thickness

0

0.005

0 10 20 30 40 50

average cloud optical thickness

Ac = 0.3 Ac = 0.3

average cloud optical thickness average cloud optical thickness

The enhancement vs <τ> for three Ac=1.0, 0.5 and 0.3; Ac=1 corresponds to the pp approximation. Dots (correspond to the thin and thick clouds) and dash lines (correspond to the two Brazilian scenes) are from the

March 26, 2007 Alexander Marshak 9

and dash lines (correspond to the two Brazilian scenes) are from the Wen et al. (2007) MC calculations.

Angstrom exponent

0.47 μm vs 0.65 μm 0.65 μm vs 0.84 μm

p ll t d p ll t d

2

nt 2

2.5

nt

very polluted very polluted

1

1.5

stro

m e

xpon

e

1.5

2

trom

exp

onen

pollutedpolluted

0.5

Ang

s

0.5

1

Ang

sclean

clean

00 10 20 30 40 50

average cloud optical depth

00 10 20 30 40 50

average cloud optical depth

March 26, 2007 Alexander Marshak 10

Three cases: clean, polluted and very polluted.

Conclusions

• 3D cloud enhancement only weakly depends on AOT; • 3D cloud enhancement only weakly depends on AOT; molecular scatt. is the key source for the enhancement;

h h h f f f l • The enhancement increases the fraction of fine aerosol mode;

• Retrieved AOT can be corrected for the 3D radiative effects;

March 26, 2007 Alexander Marshak 11

Airborne aerosol observations in the i i it f l dvicinity of clouds

March 26, 2007 Alexander Marshak 12Courtesy of Jens Redemann

Stochastic model of a broken cloud field

Clouds follow the Poisson distr. and are defined by• average optical depth, <τ>

l d f ti A• cloud fraction, Ac• aspect ratio, AR = hor./vert.

March 26, 2007 Alexander Marshak 13

AR = 2 Ac = 0.3 AR = 1