SEMIANALYTICAL CLOUD RETRIEVAL ALGORITHM AND ITS APPLICATION TO DATA FROM MULTIPLE

OPTICAL INSTRUMENTS ON SPACEBORNE PLATFORMS:

SCIAMACHY, MERIS, MODIS, SeaWiFS AND GOME

Alexander A. Kokhanovsky, Vladimir V. RozanovWolfgang von Hoyningen-Huene, John P. Burrows

Institute of Remote Sensing, Bremen UniversityP. O. Box 330440 Bremen, Germany

alexk@iup.physik.uni-bremen.de

PARAMETERS to be retrieved:

1. Cloud optical thickness /5-100/

2. Cloud top height /0.5-10km/

3. Cloud cover /0-1/

4. Cloud albedo/0.3-0.8/

5. Liquid water path /50-200 /

6. Thermodynamic phase /ice, water or mixed clouds/

7. Average size of droplets/crystals

2gm

PHYSICAL BASIS

• Bi-spectral measurement of TOA reflectance to get cloud optical thickness (visible), cloud droplet radius (IR)

• Spectrally resolved measurements (approximately, 50 spectral points in the oxygen A-band 758-772nm) to get CTH, CBH

• The ratio R(1550)/R(1670) for the ice/water cloud discrimination

758 760 762 764 766 768 7700.0

0.1

0.2

0.3

0.4

0.5

0.6

symbols - approximationlines - SCIATRAN

15

12

9

6

3

1

refle

ctio

n f

un

ctio

n

wavelength, nm

Exponential approximation

0 0 0 0 0

0 0 0

0

0.05, 4

, , , = , , -

, , , , , , / cos reflection function

, , reflection function of a semi-infinite cloud

spherical albedo of a semi-infinite cloud

spherica

c

c

R R r r K K

R I F

R

r

r

0

0 0

l albedo of a finite cloud

( ) escape function

Functions , , , , , ( ) are given by simple

analytical equations ( ., , 2003)

K

r r R K

Kokhanovsky et al JGR

a i b r

R R TRT

Table 1. Characteristics of selected space instruments, related to measurements of the backscattered light in the oxygen A-band (7550-7750 ). 

  

 

  

Instrument Platform Year Spectral interval,

Spectral resolution,

Spatial resolution, km2

GOME ERS-2 1995 5760-7940

3.3 40*320

SCIAMACHY ENVISAT 2002 6040-8050

4.8 30*60

MERIS ENVISAT 2002 7600 

25 0.3*0.3or1.1*1.1

GLI ADEOS-II 2002 7630 

80 

0.25*0.25 and1.0*1.0

POLDER ADEOS-II 2002 76337651

100400

6.0*7.0   

A

A

A

Typical SCIAMACHY spectra in the oxygen A-band

Cloud top height determination from SCIAMACHY

WWW.IUP.PHYSIK.UNI-BREMEN.DE

VALIDATION

MULTIPLE TECHNIQUES

4 6 8 10 12 14 16 18 20

2

4

6

8

10

(52N,4.9E)

(53.2N,5.8E)

(52.9N,5.7E)

Cabauw

*

clou

d to

p he

ight

, km

longitude, degrees

505

10

X

Cloud top height determination from GOME data using oxygen A-band information as compared to

ATSR-2 IR retrievals (ERS-2 satellite)

23.07.1995, orbits 50723095, 50723114, ERS-2

0 100 200 300 400 500 600 700 8000

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

24000

fr

eq

ua

en

cy

particle absorption length, m

CONCLUSION

• SACURA has great potentials in cloud remote sensing

AcknowledgementsK. Bramstedt, M. BuchwitzR. de Beek, K.-U. Eichmann

W. Lotz, T. Nauss S. Noel, A. Rozanov

H. Schroeter, M. VountasEuropean Space Agency DLR(grant 50EE0027)

and NASA

The retrieval techniques1: The look-up-table approach

00 0

0

d , ,cos =- , , +B , ,

d optical thickness

zenith solar angle

zenith observation angle

relative azimuth

reflected light intensity

B source function

II

I

0

Z

O

2: The semi-analytical approach

0 0 0 0 0

0 0 0

0

0,

, , , = , , -

, , , , , , / cos reflection function

, , reflection function of a semi-infinite cloud

spherical albedo of a semi-infinite cloud

spherical alb

R R r r K K

R I F

R

r

r

0

0 0

edo of a finite cloud

( ) escape function

Functions , , , , , ( ) are given by simple

analytical equations ( ., , 2003)

K

r r R K

Kokhanovsky et al JGR

Optical characteristics of clouds

Trishchenko and Liu, 2001

Optical thickness

ISCCP dataSurface observations

Kokhanovsky, 2003

The physics behind the semi-analytical approach

Satellite

0 0 0

0 0 0

1

, , , , , ,

, , , = , , - , ,

r t

R R T

R R T

Sun

1

2

1

0 0 0

0 0 0 0 0

0 0

0 0

, , , = , , - , ,

3, , , 1 cos

71.07 0.75 (1 )

0.85 asymmetry parameter (water clouds)

4

3(1- ) , , , ,mes

R R T

T tK K K

t g

g

K K

g R R

The cloud optical thickness determination

The accuracy of the semi-analytical asymptotic theory0.65 m ,2003Kokhanovsky and Rozanov

The accuracy of the semi-analytical asymptotic theory ,2003Kokhanovsky and Rozanov 1.55 , 6

efm a m

The accuracy of the semi-analytical asymptotic theory ,2003Kokhanovsky and Rozanov 1.55 m

Cloud top height determination from a satellite

The geometry of the problem

a i b rR R T R T

Nadir observation, solar zenith angle – 20 degreesCloud optical thickness –20.

The accuracy of the forward model

Cloud top height determination from a satellite

The accuracy of the forward model

Cloud top height determination from a satellite

01 , 5.5 , 60l km h km

Cloud top height determination from a satellite The physical principle behind the retrieval

0250 , 5, 60l m

Preliminary results !

Cloud geometrical thickness determination from a satellite

0 5.5 , =20, 60h km

Table 1. Characteristics of selected space instruments, related to measurements of the backscattered light in the oxygen A-band (7550-7750 ). 

  

 

  

Instrument Platform Year Spectral interval,

Spectral resolution,

Spatial resolution, km2

GOME ERS-2 1995 5760-7940

3.3 40*320

SCIAMACHY ENVISAT 2002 6040-8050

4.8 30*60

MERIS ENVISAT 2002 7600 

25 0.3*0.3or1.1*1.1

GLI ADEOS-II 2002 7630 

80 

0.25*0.25 and1.0*1.0

POLDER ADEOS-II 2002 76337651

100400

6.0*7.0   

A

A

A

Typical SCIAMACHY spectra in the oxygen A-band

Cloud top height determination from SCIAMACHY

PMD1 23-Jan-2003 4700/4701

University of Bremen, IUP/IFE, Michael.Buchwitz@iup.physik.uni-bremen.de

Cloud top height determination from SCIAMACHY

Cloud top height determination from SCIAMACHY

Cloud top height determination from GOME data using oxygen A-band information as compared to

ATSR-2 IR retrievals (ERS-2 satellite)

23.07.1995, orbits 50723095, 50723114, ERS-2

24.03.1998, 80324174, ERS-2

4 6 8 10 12 14 16 18 20

2

4

6

8

10

(52N,4.9E)

(53.2N,5.8E)

(52.9N,5.7E)

Cabauw

*

clou

d to

p he

ight

, km

longitude, degrees

5253

5455

5657

0

2

4

6

8

10

12

14

24

68

1012

14161820

Cabauw

*

505

10

X

Conclusions

• Most of important cloud parameters can be retrieved using spectral top of atmosphere reflectance measured by SCIAMACHY. The calibration of the instrument should be improved for this, however.

• Cases of inhomogeneous clouds can bias retrieval results considerably. This should be clarified in future research.

• The information on clouds obtained on a global and regional scale should enhance our studies of climate change, including anthropogenic influences on cloud microphysical and optical properties.

•JGR, 2003, 2004; IEEE TGRS, 2004; JQSRT, 2004; IEEE TGRS Letters, 2004

AcknowledgementsK. Bramstedt, M. Buchwitz

R. de BeekW. von Hoyningen-Huene

S. Noel, M. Vountas----

European Space Agency DLR(grant 50EE0027)