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Cloud trends from GOME, SCIAMACHY and OMI

Ping Wang, Mark Kroon, Piet Stammes, Ronald van der A

KNMI, De Bilt, the Netherlands

OMI science meeting, Helsinki, 24-27 June 2008

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Overview• Importance of clouds• Satellite instruments • Method • Global frequency distributions of cloud properties• Trends in global cloud properties • Conclusions

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Importance of clouds for climate

1. Clouds dominate the radiative budget of the atmosphere:

Clouds contribute about 75 % to Earth’s albedo. Height of clouds determines their temperature.

2. Clouds play a central role in the hydrological cycle.

3. Current understanding of clouds is limited:

Global climate models have to be validated with global observations of clouds.

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Satellite spectrometersINSTRUMENT /SATELLITE SPECTRAL RANGE PIXEL SIZE

GOME / ERS-2: 240 – 800 nm 320x40

km2

SCIAMACHY / Envisat: 240 – 2380 nm 60x30 km2

GOME-2 / Metop-A: 240 – 800 nm80x40 km2

OMI/AURA: 270 – 500 nm24x13 km2

Spectral resolution: 0.2 - 0.4 nm

SCIAGOME/GOME-2

O2 A

OMI

O2-O2Raman

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Spectral resolution: 0.4 nm

Spectral resolution: 1 pm

O2 A-band simulation

O2 A-band measurement

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Cloud retrieval algorithm FRESCOReality Retrieval model

ceff1-ceff

•Geometrical cloud fraction•Cloud optical thickness•Cloud top pressure•Cloud bottom pressure•Cloud phase•………

FRESCO algorithm produces:• Effective cloud fraction: ceff• Cloud pressure pc

pcAc = 0.8

ps, As

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Simulation of O2 cloud pressure

Sneep et al. JGR 2008

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O2 A-band cloud results compared to ISCCP data

Data selection:

GOME (FRESCO): 1996-2003SCIAMACHY (FRESCO): 2003-2005

ISCCP D2 data: 1996-2005

Area: 60° N – 60° STime: 10:OO hr local time

Monthly averages

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Comparison between GOME and ISCCP

ISCCP 09:00

land ocean

1997

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Time series of global mean cloud pressure

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Time series of NH and SH cloud pressure

GOME

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OMI global mean cloud pressure

Whole Globe (60S,60N) OMICloud Pressure COL3

660680700720740760780800

Time (months)

Clou

d Pr

essu

re

OMI O2O2OMI Raman

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OMI global mean effective cloud fraction

Whole Globe (60S,60N) OMICloud Fraction COL3

0.280

0.290

0.300

0.310

0.320

0.330

0.340

Time (months)

Clou

d Fr

actio

n

OMI O2O2OMI Raman

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Global mean cloud pressure 1996-2008

cloud pressure

620

640

660

680

700

720

740

1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145

month from January 1996

Pc [h

Pa]

gome Pcscia PcOMI O2O2 Pc

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Global mean effective cloud fraction 1996-2008

effective cloud fraction

0.25

0.26

0.27

0.28

0.29

0.3

0.31

0.32

0.33

0.34

1 12 23 34 45 56 67 78 89 100 111 122 133 144

month from January 1996

c_ef

f gome cscia cOMI O2O2 c

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O2-O2 cloud fraction across track

July 2007

The larger ceff at the edges of the swath might contribute to the larger global mean ceff

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O2-O2 ceff, Pc vs. lat.

July 2007

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Conclusions• O2 absorption provides unique height information

about clouds, since visible light penetrates into clouds.• O2 methods are complementary to IR methods which

give mostly the top of the cloud.• O2 cloud pressure has a clear bimodal distribution

unlike ISCCP. • The global average O2 A-band cloud pressure has a

clear seasonal dependence, which is missing in ISCCP.. OMI O2-O2 global averaged cloud pressure and

effective cloud fraction show less seasonal variation than GOME and SCIA FRESCO cloud products.

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Whole Globe (60S,60N) OMICloud Pressure ECS2

600620640660680700720740760780800

Time (months)

Clou

d Pr

essu

re

OMI O2O2OMI Raman

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Whole Globe (60S,60N) OMICloud Fraction ECS2

0.1750.1950.2150.2350.2550.2750.2950.3150.335

Oct-04

Dec-04Feb

-05Ap

r-05Jun-0

5

Aug-0

5Oct-

05

Dec-05Feb

-06Ap

r-06Jun-0

6

Aug-0

6Oct-

06

Dec-06Feb

-07Ap

r-07Jun-0

7

Aug-0

7Oct-

07

Time (months)

Clou

d Fr

actio

n

OMI O2O2OMI Raman

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Trend in global cloud pressure from GOME and SCIAMACHY

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Use of O2 absorption band:direct measure of cloud pressure

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oceanland

Cloud pressure distributions from GOME

Global mean, 1997

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• O2-O2 FRESCO comparison Maarten et al. 2008• FRESCO+ cloud fraction and pressure monthly average,

perhaps not all the data• Differences in the FRESCO and OMI cloud average.• Cloud effect on climate• ISCCP data new version?• How to explain the seasonal variation• Check the SZA dependence?• Make FRESCO cloud trend plot using excel ?