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COMPARISON OF CLOUD AMOUNTS FROM MODIS, TRMM, ISCCP AND SAGE

COMPARISON OF CLOUD AMOUNTS FROM MODIS, TRMM, ISCCP AND SAGE with comments on satellite intercalibration Patrick Minnis, Louis Nguyen NASA Langley Research Center Sunny Sun-Mack, Yan Chen SAIC Dave Doelling AS&M, Inc. Pi Wang STX 2006 Meeting of GEWEX Cloud Assessment, Madison, WI

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COMPARISON OF CLOUD AMOUNTS FROM MODIS, TRMM, ISCCP AND SAGE

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  1. COMPARISON OF CLOUDAMOUNTS FROM MODIS, TRMM, ISCCP AND SAGE with comments on satellite intercalibration Patrick Minnis, Louis Nguyen NASA Langley Research Center Sunny Sun-Mack, Yan Chen SAIC Dave Doelling AS&M, Inc. Pi Wang STX 2006 Meeting of GEWEX Cloud Assessment, Madison, WI June 6-7, 2006

  2. Cloud Products - ISCCP - SAGE - solar occultation (250 km path length) any cloud betwixt sensor & sun will cause a cloud response - sparse measurements (1 for each terminator passage) - subvisible ( < 0.03) & opaque ( > 0.03) - MODIS Atmosphere Team (1-km Terra & Aqua MODIS) - Collection 4, February 2000 - December 2005 - CERES (1-km Terra & Aqua MODIS, sampled to 2 km) - Collection 4, February 2000 - December 2005

  3. Summary of SAGE Tropical Measurements, 1985-1999 • Opaque cloud frequency decreasing, z > 12 km • Subvisible cloud frequency increasing, z > 12 km

  4. Another view of SAGE Tropical Measurements, 1985-1999 Opaque Cloud decreases above 12 km Subvisual cloud increases Wang et al., JClim, submitted

  5. Comparisons with SAGE-II

  6. RH300 Trends from NCEP Reanalysis - Remember Warren’s decreasing cirrus trend!

  7. Ice/High cloud amount Comparison from 1998-2003 37N-37S Ocean I - ISCCP T - Terra CERES A - Aqua CERES V - VIRS Land Good agreement in magnitude over land Divergent trends over ocean Combo Time in months since December 1997

  8. Conclusions from Last Meeting (2005) • ISCCP high cloud trends appear consistent over land with surface and other recent datasets(should use 22 or more yrs) - decent correlation with humidity • ISCCP high cloud trends are variant over ocean - divergent from sfc and recent clouds - weak correlation with NCEP RH • Decreasing humidity suggests decrease in cirrus cloudiness - SAGE analysis suggests that the response includes thinner clouds in Tropics & drop in mean height rather than simple decrease in cloud amount • Cause of UTH drop? Is it real?

  9. Other Cloud Amount Comparisons

  10. CERES GLOBAL SEASONAL CYCLE OF CLOUD AMOUNT, 2003 Open symbols refer to cloud fraction with retrievable pixels 0.634 0.609 0.625 0.606 Very similar cycles except for June & July when Aqua < Terra 4% of cloudy pixels do not accommodate retrieval models!

  11. CERES day CERES vs ISCCP Cloud Amount Jul. 2000 ISCCP mean

  12. Monthly mean cloud fractions from surface, ISCCP, and CERES Terra MODIS

  13. CERES MODIS CLOUD PRODUCTS ARE DIFFERENT THAN THE MODIS TEAM PRODUCTS • Different masks (use different channels, thresholds, etc.) • Different radiative transfer - different ice/water models - different atmospheric properties - different interpretive models • Different processing systems => differences in products

  14. DAYTIME CLOUD AMOUNTS, Terra, October 2003 CERES Ed2 MOD08 MODIS Team CERES vs MODIS Team interpretation of MODIS data

  15. NIGHT CLOUD AMOUNTS, Terra, October 2003 CERES Ed2 MOD08 MODIS Team

  16. DAYTIME CLOUD AMOUNT DIFFERENCE, MOD08 - CERES Terra, October 2003 CERES fewer clouds over ITCZ & eastern Antarctica more clouds in Arctic & western Antarctic

  17. Total cloud amount Comparison from 1998-2003 37N-37S Time in months since December 1997 I - ISCCP T - Terra CERES A - Aqua CERES V - VIRS M08- MOD08

  18. DAYTIME MONTHLY MEAN CLOUD AMOUNTS, MOD08 - CERES 30°N - 30°S Trends: CERES: 0.017/decade MODIS: 0.048/decade

  19. DAYTIME MONTHLY MEAN CLOUD AMOUNTS, MOD08 - CERES 30°N - 60°N + 30°S - 60°S Trends: CERES: 0.005/decade MODIS: 0.017/decade

  20. DAYTIME MONTHLY MEAN CLOUD AMOUNTS, MOD08 - CERES 60°N - 90°N + 60°S - 90°S Trends: CERES: 0.079/decade MODIS: 0.059/decade

  21. TOTAL CLOUD TOP PRESSURE, Terra, October 2003 mb CERES Ed2 MOD08 MODIS Team

  22. MOD08 cloud pressures generally larger than CERES especially in Tropics where difference is ~ 75 mb

  23. WATER DROPLET EFFECTIVE RADIUS, Terra, October 2003 µm CERES Ed2 MOD08 MODIS Team

  24. TOTAL OPTICAL DEPTH DIFFERENCE, MOD08 - CERES Terra, October 2003 Differences large only in polar regions - different retrieval methods over snow

  25. MOD08 cloud optical depth less than or equal to CERES Midlatitude difference is ~ 2

  26. SUMMARY • Significant differences between CERES & MODIS interpretation of clouds • Some areas of agreement (opt depths over ocean) • Each dataset still under evaluation • Many changes ahead for Collection 5 (CERES Edition 3)

  27. CALIBRATION MONITORING Calibration… • May or may not affect cloud fraction • Will affect cloud optical depth • Can produce misleading trends • Critical for producing physical quantities - modelers can be misled by biased parameters Proposed method for consistent intercalibration: • Use self-calibrated instruments as references - MODIS on Aqua & Terra, VIRS on TRMM • Intercalibrate with moderately tight matching constraints over ocean • Determine trends in each satellite with deep convective cloud targets • Provide correction factors for differences in filter functions

  28. APPROACH • EXAMINE RELATIVE TRENDS IN “CALIBRATED” IMAGER CHANNELS - Terra-VIRS - Aqua-VIRS - MODIS vs CERES SW • Transfer calibration to other satellites - use methods of Minnis et al. (2002, JTech)

  29. EXAMINE RELATIVE TRENDS IN IMAGER CHANNELS Terra-VIRS, VISIBLE Compute slope for each month VIRS Version 5a Version 6

  30. EXAMINE RELATIVE TRENDS IN IMAGER CHANNELS VISIBLE VIRS vs Terra VIRS vs Aqua • Aqua brighter (1-2%) than Terra • VIRS V5a appears to be ok, V6 seems to have added a trend!

  31. MORE VISIBLE CHANNEL COMPARISONS Terra vs Aqua • Aqua brighter (1-2%) than Terra in direct comparison • Confirms VIRS V5a conclusion

  32. INTERCALIBRATIONS Comparison of CERES SW and MODIS 0.635 µm, Jan 2000 - Mar 2005 Slope of CERES vs MODIS: SW vs 0.64 µm Aqua Terra No trend for Terra; apparent trend for Aqua Terra & Aqua MODIS may trend relative to each other Terra darker than Aqua by 1.2% at start of 2003

  33. Comparison of CERES SW and Terra MODIS 0.635 µm, Jan 2000 - Jul 2005 • Terra discontinuity at day 1407, gains were tweaked. • Any trends caused by sudden change in gain

  34. Deep Convective Cloud Radiances Corrected to Overhead Sun, VIRS V5a, 1998-2003 Monthly pdf SZA correction factor T11i < 205.0 K, SZA < 40°, VZA < 40°, 10° < RAA < 170°, (T11) < 1.0 K, and () < 0.02 i

  35. Aqua MODIS Trend 0.635 µm, July 2002 - Jul 2005 Aqua vs CERES Aqua Deep Convective • Aqua stable as a rock! • CERES is degrading!

  36. Terra MODIS Deep Convective Trend 0.635 µm, Jan 2000 - Oct 2005 • Terra shows apparent trend • Discontinuity causing it • CERES is not degrading!

  37. TRMM VIRS Deep Convective Trend 0.64 µm, Jan 1998 - Oct 2005 • V5a stable as Aqua • VIRS lunar calibration used for V6 - not helpful!

  38. Back to VIRS vs MODIS 0.64 µm, Jan 2000 - Jul 2005 VIRS vs Terra VIRS vs Aqua Theoretical slope over ocean is 1.048 • V6 DCC correction aligns VIRS with both MODIS • Aqua -VIRS in nearly perfect agreement with theory - Terra too dark!

  39. Transfer of MODIS to NOAA-16 Vis Calibration Using Polar Crossings

  40. Transfer of AVHRR Vis Calibrations Back in Time Use DCC approach to confirm/adjust relative trends after anchoring to MODIS/VIRS from Doelling et al., 2001 (AMS Sat Met Conf)

  41. Summary Comments • Careful intercalibration supplemented by deep convective cloud relative calibrations can provide a reliable calibration record - that will link historical and current satellites - easily applied to both polar & geosynchronous satellites • Need to account for spectral differences - theory is probably most practical way - empirical, more difficult, is more accurate

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