1. Cloud Trends and Anomalies Observed by MISR
R. Davies
Physics Department
The University of Auckland, New Zealand
7/7/06
GEWEX Cloud Workshop

2. MISR: Multi-angle Imaging Spectro-Radiometer
9 view angles at Earth surface:
70.5º, 60º, 45.6º, 26.1º forward/ aftward
and 0º
Multiple spectral bands at each angle:
MISR/Terra: 446, 558, 672, 866 nm
400-km swath: 9-day global coverage
275 m km sampling
7 minutes to observe each scene
at all 9 angles
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3. MISR : observation concept
9 view angles at Earth surface:
+70.5º to –70.5º
stereo match to get Reflecting Layer Reference Altitude (RLRA) every 2.2 km (90% coverage)
10-day area-weighted zonal averages of mean RLRA (>100 independent samples)
similarly, coverage by height
independent of radiometric calibration
consistent processing since 3/2000
Terra: 10:30 am sun-synchronous
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4. first: seasonal cycles
globally averaged cloud fraction
10-day means (≈130 independent orbits)
area weighted zonal averages
avoids overweighting of polar latitudes by Terra
stratified by height
surface (≈clear)
low (<3 km)
middle (3–7 km)
high (> 7 km)
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6. i.e. global mean cloud fraction > 0.638 at 10:30 am
0.362±0.001
i.e. global mean cloud fraction > 0.638
at 10:30 am
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7. 0.321±0.001
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8. 0.1545±0.0004
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9. 0.161±0.001
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10. second: global cloud anomalies
deseasonalized 10-day anomalies
mean year anomaly with expected sampling error
linear regression
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18. third: global implications
how do cloud albedos change?
what about cloud heights?
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19. MISR : observation concept
9 view angles at Earth surface:
+70.5º to –70.5º
multiple spectral bands at each angle: 446, 558, 672, 866 nm
on-board calibration system: ≈1% relative accuracy
‘expansive’ albedo integrated over the view from 30 km, averaged over swath width (300 km)
energy-weighted, area-weighted, 10-day zonal averages (>100 independent samples)
consistent (reprocessed) data from 5/2000 (discontinuously) to present
Terra: 10:30 am sun-synchronous
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20. green band albedo
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24. spectral albedo summary
all MISR bands show similar anomalies
regional anomalies are consistently larger
sampling uncertainty (1s)
0.003 (10-day, global)
(12-month, global)
interannual albedo changes <0.001 (0.3%)
overall difference 2005–2000
<0.2%
<0.3 W m-2 (equivalent broadband absorption)
well within the expected radiometric uncertainty
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27. thru 8/05
global ‘trend’ –11±2 m/yr
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29. height and fraction summary
MISR has the ability to produce consistent records of reflecting layer reference altitude and coverage by height
interannual differences
sampling error of ±11 m in height, ±0.002 in coverage
difference of –44 ±11 m between first and last full year
consistent with a reduction in tropical high cloud (ITCZ) –70 ±12 m/yr
these differences exceed the sampling uncertainty
unlike the albedo time series, the height differences are unaffected by radiometric calibration
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30. global ‘trend’ –5±3 m/yr thru 5/06 preliminary 7/7/06
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31. wrap up: MISR
can characterize the seasonal variability of cloud fraction (+height and albedo)
can limit the secular trend to less than ≈0.001/yr in global total cloud fraction
interannual variability in global total cloud fraction ≈0.001
larger relative changes in high cloud amount, with measurable interannual changes in mean cloud top height ≈10m globally, ≈100 m in tropics
effective height change > albedo change (radiative forcing)
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33. Cloud reconstruction Obtaining cloud top height and cloud position
(Xg,Yg)
Distance between a features seen with 2 cameras: ∆D
(Xc,Yc) D
CTH
2) Cloud top height calculation:
CTH = ∆D / [tan (2 -1)]
3) Parallax correction :
Xc = Xg + CTH * tan() * sin (  )
Yc = Yg + CTH * tan() * cos (  )
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