COMPARISON OF CLOUD AMOUNTS FROM MODIS, TRMM, ISCCP AND SAGEwith comments on satellite intercalibration

Patrick Minnis, Louis NguyenNASA Langley Research CenterSunny Sun-Mack, Yan ChenSAICDave DoellingAS&M, Inc.Pi WangSTX2006 Meeting of GEWEX Cloud Assessment, Madison, WIJune 6-7, 2006

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

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

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

Comparisons with SAGE-II

RH300 Trends from NCEP Reanalysis- Remember Warrens decreasing cirrus trend!

Ice/High cloud amount Comparison from 1998-200337N-37SI - ISCCPT - Terra CERESA - Aqua CERESV - VIRSTime in months since December 1997Good agreement in magnitude over landDivergent trends over oceanLandOceanCombo

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 amountCause of UTH drop? Is it real?

Other Cloud Amount Comparisons

0.6340.6090.6250.606CERES GLOBAL SEASONAL CYCLE OF CLOUD AMOUNT, 2003Open symbols refer to cloud fraction with retrievable pixelsVery similar cycles except for June & July when Aqua < Terra4% of cloudy pixels do not accommodate retrieval models!

CERES vs ISCCPCloud Amount Jul. 2000CERES dayISCCP mean

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

Table 1. Monthly mean cloud amounts in percent from surface observations (Hahn and Warren, 1999), CERES, and ISCCP (Rossow and Schiffer, 1999).

Domain

Surface

(1971-96)

Terra MODIS Ed 2

12/2002, 6/2003

Aqua MODIS Ed 1a

12/2002, 6/2003

ISCCP D2

(1984-98)

60S - 60N, December

60.9

60.3

61.2

68.2

90S - 90N, December

61.9

60.6

61.4

68.2

60S - 60N, June

58.6

58.6

58.9

66.6

90S - 90N, June

60.0

60.3

59.8

66.6

CERES MODIS CLOUD PRODUCTS ARE DIFFERENT THAN THE MODIS TEAM PRODUCTSDifferent 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

CERES vs MODIS Team interpretation of MODIS data

MOD08MODIS TeamCERES Ed2NIGHT CLOUD AMOUNTS, Terra, October 2003

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

Total cloud amount Comparison from 1998-200337N-37SI - ISCCPT - Terra CERESA - Aqua CERESV - VIRSM08- MOD08Time in months since December 1997

DAYTIME MONTHLY MEAN CLOUD AMOUNTS, MOD08 - CERES30N - 30STrends: CERES: 0.017/decadeMODIS: 0.048/decade

Trends: CERES: 0.005/decadeMODIS: 0.017/decadeDAYTIME MONTHLY MEAN CLOUD AMOUNTS, MOD08 - CERES30N - 60N + 30S - 60S

Trends: CERES: 0.079/decadeMODIS: 0.059/decadeDAYTIME MONTHLY MEAN CLOUD AMOUNTS, MOD08 - CERES60N - 90N + 60S - 90S

MOD08MODIS TeamCERES Ed2TOTAL CLOUD TOP PRESSURE, Terra, October 2003mb

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

MOD08MODIS TeamCERES Ed2WATER DROPLET EFFECTIVE RADIUS, Terra, October 2003m

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

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

SUMMARY Significant differences between CERES & MODIS interpretation of cloudsSome areas of agreement (opt depths over ocean)Each dataset still under evaluationMany changes ahead for Collection 5 (CERES Edition 3)

CALIBRATION MONITORINGCalibration

May or may not affect cloud fractionWill affect cloud optical depthCan 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 oceanDetermine trends in each satellite with deep convective cloud targets Provide correction factors for differences in filter functions

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)

EXAMINE RELATIVE TRENDS IN IMAGER CHANNELSTerra-VIRS, VISIBLECompute slope for each monthVIRS Version 5a Version 6

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

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

INTERCALIBRATIONSComparison of CERES SW and MODIS0.635 m, Jan 2000 - Mar 2005Slope of CERES vs MODIS: SW vs 0.64 mNo trend for Terra; apparent trend for AquaTerraAquaTerra & Aqua MODIS may trend relative to each otherTerra darker than Aqua by 1.2% at start of 2003

Comparison of CERES SW and Terra MODIS0.635 m, Jan 2000 - Jul 2005 Terra discontinuity at day 1407, gains were tweaked. Any trends caused by sudden change in gain

Deep Convective Cloud RadiancesCorrected to Overhead Sun, VIRS V5a, 1998-2003T11i < 205.0 K, SZA < 40, VZA < 40, 10 < RAA < 170, (T11) < 1.0 K, and () < 0.02 iSZA correction factorMonthly pdf

Aqua MODIS Trend0.635 m, July 2002 - Jul 2005Aqua vs CERESAqua Deep Convective Aqua stable as a rock! CERES is degrading!

Terra MODIS Deep Convective Trend0.635 m, Jan 2000 - Oct 2005 Terra shows apparent trend

Discontinuity causing it

CERES is not degrading!

TRMM VIRS Deep Convective Trend0.64 m, Jan 1998 - Oct 2005 V5a stable as Aqua VIRS lunar calibration used for V6- not helpful!

Back to VIRS vs MODIS0.64 m, Jan 2000 - Jul 2005VIRS vs AquaVIRS vs TerraTheoretical 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!

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

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

Summary CommentsCareful 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 satellitesNeed to account for spectral differences - theory is probably most practical way- empirical, more difficult, is more accurate