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Investigations of Artifacts in the ISCCP Datasets

William B. Rossow July 2006. Investigations of Artifacts in the ISCCP Datasets. CLIMATOLOGY. ERBE VS ISCCP-FD at TOA. LW. SW. Comparison of Three Cloud Climatologies ISCCP, SOBS, HIRS. ISCCP. HIRS-W. SOBS. SOBS. HIRS-W. ISCCP. Factors that Could Cause Spurious Cloud Amount Changes.

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Investigations of Artifacts in the ISCCP Datasets

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

  2. CLIMATOLOGY

  3. ERBE VS ISCCP-FD at TOA LW SW

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

  5. Factors that Could CauseSpurious 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 onTotal Cloud Amount Estimated Uncertainty

  8. Cloud Type Variations HIGH CIRRUS CUMULUS LOW

  9. Cloud Type Variations HIGH CONVECTIVE CIRRUS CUMULUS STRATUS LOW

  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 ofLow and Upper Cloud Amounts and MUE LOW LOW HI VAR LO VAR UPPER UPPER HI VAR LO 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 & COVERGAGEEFFECTS

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

  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

  25. BACKUP SLIDES

  26. 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!!

  27. 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

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