1. Cloud trends from GOME, SCIAMACHY and OMI
OMI science meeting, Helsinki, June 2008
Cloud trends from GOME, SCIAMACHY and OMI
Ping Wang, Mark Kroon,
Piet Stammes, Ronald van der A
KNMI, De Bilt, the Netherlands

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

3. 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.

4. Satellite spectrometers
INSTRUMENT /SATELLITE SPECTRAL RANGE PIXEL SIZE
GOME / ERS-2: – 800 nm 320x40 km2
SCIAMACHY / Envisat: 240 – 2380 nm 60x30 km2
GOME-2 / Metop-A: – 800 nm 80x40 km2
OMI/AURA: – 500 nm 24x13 km2
Spectral resolution: nm
SCIA
GOME/GOME-2
OMI
Raman
O2-O2
O2 A

5. O2 A-band simulation O2 A-band measurement Spectral resolution: 1 pm
Spectral resolution: 0.4 nm

6. Cloud retrieval algorithm FRESCO
Reality
Retrieval model
Ac = 0.8 pc
ceff
1-ceff
Geometrical cloud fraction
Cloud optical thickness
Cloud top pressure
Cloud bottom pressure
Cloud phase
………
ps, As
FRESCO algorithm produces:
Effective cloud fraction: ceff
Cloud pressure pc

7. Simulation of O2 cloud pressure
Sneep et al. JGR 2008

8. O2 A-band cloud results compared to ISCCP data
Data selection:
GOME (FRESCO):
SCIAMACHY (FRESCO): ISCCP D2 data:
Area: 60° N – 60° S Time: 10:OO hr local time
Monthly averages

9. Comparison between GOME and ISCCP
land
ocean
1997
ISCCP 09:00

10. Time series of global mean cloud pressure

11. Time series of NH and SH cloud pressure
GOME

12. OMI global mean cloud pressure

13. OMI global mean effective cloud fraction

14. Global mean cloud pressure 1996-2008

15. Global mean effective cloud fraction 1996-2008

16. O2-O2 cloud fraction across track
The larger ceff at the edges of the swath might contribute to the larger global mean ceff
July 2007

17. O2-O2 ceff, Pc vs. lat.
July 2007

18. 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.

22. Trend in global cloud pressure from GOME and SCIAMACHY

23. Use of O2 absorption band: direct measure of cloud pressure

24. Cloud pressure distributions from GOME
land
ocean
Global mean, 1997

25. 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 ?