1. Investigations of Artifacts in the ISCCP Datasets William B. Rossow July 2006

2. CLIMATOLOGY

3. ERBE VS ISCCP-FD at TOA LW SW

4. Comparison of Three Cloud Climatologies ISCCP, SOBS, HIRS ISCCP HIRS-W SOBS

5. Factors that Could Cause Spurious Cloud Amount Changes Changes in Radiance Calibration Changes in Cloud Property Distribution Changes in Satellite Viewing Geometry Changes in Sampling Distribution and Coverage

6. RADIANCE CALIBRATION EFFECTS

7. Calibration Effect on Total Cloud Amount Estimated Uncertainty

8. Cloud Type Variations CIRRUS HIGH CUMULUSLOW

9. Cloud Type Variations CIRRUS HIGH CUMULUSLOWSTRATUS CONVECTIVE

10. CONCLUSIONS Calibration Effects on ISCCP Total Cloud Amount < 0.5% on ISCCP Cloud Type Amounts < 1%

11. DETECTION SENSITIVITY

12. Monthly Mean Anomaly of IR-Marginal Cloud Amount

13. CONCLUSIONS Inconsistencies among ISCCP, HIRS and SOBS could be explained by shifting of optical thickness distribution of low-level clouds to smaller values that surface observers identify as Cumulus Note Tselioudis et al. 1992 !!

14. SATELLITE VIEW ANGLE CHANGES

15. Cloud Detection Variation with View Angle ∂ Cloud Amount / ∂ Mue = 25%

16. Changes of Cloud Amount and Cosine Satellite Zenith Angle

17. Correlations of Monthly Mean Changes of Low and Upper Cloud Amounts and MUE LOW HI VARLO VAR UPPER HI VARLO VAR 7%, 10%

18. CONCLUSIONS ISCCP results contain spurious regional variations associated with varying satellite zenith angle Global, Long-term changes in Zenith Angle might explain as much 1% changes in Total Cloud Amount BUT pattern of variations does not match pattern of Cloud Amount changes

19. SAMPLING & COVERGAGE EFFECTS

20. Scatterplot of Cloud Amount Anomalies Surface Observations vs ISCCP “Global” Northern Hemisphere Land

21. Percentage Coverage of Earth by ISCCP Average = 92% Max = 98% Min = 70%

22. CONCLUSIONS ISCCP is the only dataset that BOTH directly resolves the diurnal cycle (except for SOBS over land) and covers the whole globe Therefore, sampling and diurnal aliasing are NOT problems for ISCCP Need to Investigate Effects of Trends in Water/Land and Day/Night Coverage Ratios

23. MAIN CONCLUSIONS Radiance Calibration – ISCCP CA uncertainty < 0.5%, Not a problem for SOBS but could be problem for other datasets Detection Sensitivity – ISCCP CA trend in marginally detectable cloud type could explain inconsistencies with HIRS and SOBS but requires more careful comparisons View Angle Effects – ISCCP effects on CA up to 1% but pattern not consistent with long-term changes Sampling – Not a problem for ISCCP, but is a problem for SOBS Diurnal Aliasing – Not a problem for ISCCP, but is a problem for drifting polar orbiters and might (possibly) be a problem for other polar orbiters

24. OPEN ISSUES Radiance Calibration – Angle dependent calibration effects and more subtle spectral-angle effects have not been checked as yet for ISCCP, Other Satellite Datasets need to complete calibration studies Detection Sensitivity – More careful comparisons of ISCCP, SOBS and HIRS are required to explain the “trend” inconsistencies over LONG- TERM View Angle Effects – Uncertainties can be reduced by improving treatment of angle dependence (Especially in retrieving physical properties) Sampling – Needs to be checked for some other datasets Diurnal Aliasing – Problem for polar orbiters but may only cause long- term effects if orbits drift but this is not known for sure

26. BACKUP SLIDES

27. Current Cloud Property Data Sets (Quantity±instantaneous error, mean uncertainty, source) Cloud Cover ± 15%, 5%, satellite, surface weather obs. Cloud Top Temperature ± 3-6K, 2K, satellite Cloud Top Height ± 0.5-1.9 km, 0.3 km, satellite Cloud Optical Thickness ± 25%, 10%, satellite Cloud Particle Size ± 2  m (liquid), ± 10  m (ice), 1  m (liquid), 10  m (ice), satellite Cloud Water Path ± 15% (liquid), ± 200% (ice), 10% (liquid), 100% (ice), satellite Cloud Base Temperature ± 3-6K, 2K, surface obs. Cloud Base Height ± 0.5-1 km, 0.3 km, surface obs. What’s Left to Do?  Cloud Vertical Structure!!

29. NOTES 1. Finish Diurnal Aliasing tests 2. Investigate effect of trend in Water/Land Coverage Ratio 3. Investigate effect of Day/Night Sampling Difference and trend in the Coverage Ratio 4. Investigate effects of Spectral Response Differences on results 5. Investigate Scan-Angle Dependence of Calibrations