Investigations of Artifactsin the ISCCP Datasets

William B. Rossow

July 2006

CLIMATOLOGY

ERBE VS ISCCP-FD at TOA

LW

SW

Comparison of Three Cloud ClimatologiesISCCP, SOBS, HIRS

ISCCP

ISCCP

HIRS-W

HIRS-W

SOBSSOBS

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

RADIANCE CALIBRATION EFFECTS

Calibration Effect onTotal Cloud Amount

EstimatedUncertainty

Cloud Type Variations

CIRRUS HIGH

CUMULUS LOW

Cloud Type Variations

CIRRUS HIGH

CUMULUS LOWSTRATUS

CONVECTIVE

CONCLUSIONS

Calibration Effects

on ISCCP Total Cloud Amount < 0.5%

on ISCCP Cloud Type Amounts < 1%

DETECTION SENSITIVITY

Monthly Mean Anomalyof IR-Marginal Cloud Amount

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 !!

SATELLITE VIEW ANGLE CHANGES

Cloud Detection Variation with View Angle

∂ Cloud Amount / ∂ Mue = 25%

Changes of Cloud Amount and Cosine Satellite Zenith Angle

Correlations of Monthly Mean Changes ofLow and Upper Cloud Amounts and MUE

LOW LOW

HI VAR LO VAR

UPPER UPPER

HI VAR LO VAR7%, 10%

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

SAMPLING & COVERGAGEEFFECTS

Scatterplot of Cloud Amount AnomaliesSurface Observations vs ISCCP

“Global” Northern Hemisphere Land

Percentage Coverage of Earth by ISCCP

Average = 92%Max = 98%Min = 70%

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

MAIN CONCLUSIONSRadiance 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

OPEN ISSUESRadiance 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

BACKUP SLIDES

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!!

NOTES1. 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