CERES Beta1 ISCCP-D2like Data Quality Summary

The purpose of this document is to inform users of the accuracy of this data product as determined by the CERES Science Team. The document summarizes user applied revisions (e.g. Rev1), key validation results, provides cautions where users might easily misinterpret the data, provides links to further information about the data product, algorithms, and accuracy, and gives information about planned data improvements. This document also automates registration in order to keep users informed of new validation results, cautions, or improved data sets as they become available.

User applied revisions are a method CERES uses to identify improvements to existing archived data products that are simple for users to implement, and allow correction of data products that would not be possible in the archived versions until the next major reprocessing 1 to 2 years in the future. All revisions applicable to this data set are noted in the section User Applied Revisions to Current Edition.

This document is a high-level summary and represents the minimum information needed by scientific users of this data product.

Table of Contents

• Nature of the ISCCP-D2like Product
• User Applied Revisions to Current Edition
• Cautions and Helpful Hints
• Accuracy and Validation
• References
• Web Links to Relevant Information
• Expected Reprocessing
• Referencing Data in Journal Articles

Nature of the ISCCP-D2like Product

The Monthly Gridded Cloud Averages (ISCCP-D2like), archival data product contains 3-hourly (GMT based) monthly and monthly 1° gridded regional mean cloud properties as a function of 18 cloud types, similar to the ISCCP D2 product, where the cloud properties are stratified by pressure, optical depth and phase. There will be 3 ISCCP-D2like datasets depending on the source of clouds. See Table 1 for ISCCP-D2like filenames and products.

• A dataset for Terra-MODIS and Aqua-MODIS using input from the CERES Single Satellite Footprint (SSF) product. The CERES based MODIS retrievals are not the official MOD0X or MYD0X products available at the Goddard DAAC.
  • The MODIS cloud retrievals are separated into daytime and nighttime products. The daytime retrievals are based on both visible and IR MODIS channels, whereas the nighttime are based on IR only. The SZA threshold between day and night is 82°.
• A 5-satellite, daytime 3-hourly GMT, 8km nominal resolution, geostationary-only cloud product limited to 60°N to 60°S.
• A future merged Terra, Aqua and GGEO ISCCP-D2like product is also planned, where the MODIS retrievals will take precedence over the GGEO retrievals and it will combine the Terra morning (10:30 AM local equator crossing time) and Aqua afternoon (1:30 PM) MODIS cloud observations. The 2-channel GGEO retrievals will be somewhat modified in order to more closely resemble the MODIS retrievals. Also it is very likely that the GGEO nighttime retrievals will be incorporated in the merged product.

All CERES ISCCP-D2like cloud products are based on the same cloud type definitions, independent of the cloud source, however there are slight changes in the read programs. The Edition version will have identical read codes for simplicity.

The CERES ISCCP-D2like cloud product was designed to closely emulate the NASA-GISS ISCCP-D2like products, so that they meet the needs of the climate community and can easily be incorporated into GCMs, ISCCP simulators and other climate modeling studies. The only major difference is that the CERES ISCCP-D2like are on the CERES 1° nested grid, whereas ISCCP is on a 2.5° equal area grid. The cloud retrievals have been stratified into 18 cloud types based on cloud top pressure, optical thickness and phase, similar to the ISCCP daytime 15 cloud types. See Figure 1 for the ISCCP-D2like cloud types. The GEO product contains cloud amount, effective pressure, effective temperature, optical depth, IWP/LWP, just as ISCCP provides, however for the MODIS retrievals particle size and IR emissivity properties are also provided for the Terra/Aqua products. See examples of the monthly mean products. The ISCCP-D2like variables are listed in Table 2. For the Terra or Aqua daytime datasets, the cloud amount (frequency) of each of the 42 ISCCP D1 types (6 optical depth by 7 cloud top pressure bins) is also given in Figure 2.

Figure 1: ISCCP-D2like 18 Cloud Types

Stratify clouds into optical depth, cloud top pressure and cloud phase bins ISCCP-D2like adds a liquid category to each of the 3 high cloud bins

Figure 2: ISCCP-D2like 42 Cloud types for MODIS cloud amounts

The user of the Terra or Aqua only datasets should be aware that this is not a temporally interpolated dataset and only reflects the cloud conditions at the Terra (daytime 10:30 AM and nighttime 10:30 PM) or Aqua (daytime 1:30 PM and nighttime 1:30 AM) sampling times. However the CERES_MODIS cloud retrievals are superior to the geostationary 2-channel based cloud retrievals. The Terra or Aqua only datasets are best used in conjunction with CERES fluxes and instantaneous comparisons. The geostationary 3-hourly based dataset samples the complete daytime diurnal cycle, however the cloud retrievals incorporate only a visible and IR channel common to all geostationary satellites for spatial consistency. This dataset can be used wherever ISCCP D2 products are currently used. A merged dataset is planned in order to take advantage of the MODIS superior cloud retrievals and the geostationary diurnal retrievals.

The monthly 3-hourly means are averaged from the daily measurements during that time block. The monthly means are averaged from the sampled monthly 3-hourly means. No temporal interpolation is performed to estimate the cloud properties in the unsampled bins. All cloud properties are weighted by cloud fraction in the averaging process. The ISCCP-D2like monthly cloud property means differ from the SRBAVG2 (GGEO+MODIS clouds) or SRBAVG3 (MODIS-only clouds) monthly means, since they have been instantaneously temporally averaged between measurements. See the CERES SRBAVG Quality Summary.

For Terra and Aqua only the instrument in cross-track scan mode is processed and following the Instrument Scan Modes table. There is only one MODIS instrument on either Terra or Aqua, however the MODIS pixel level retrievals are first spatially averaged over the CERES footprint and then the footprints are averaged into the CERES 1° grid. The RAPS mode SSF product does not provide uniform spatial sampling.

The Terra and Aqua MODIS cloud retrieval process and quality has been very well outlined in the Terra or Aqua DQS. There is some information of the GGEO cloud retrieval quality in the SRBAVG DQS. The GGEO process is briefly described here. The geostationary radiances are first cross-calibrated against MODIS, which is well calibrated, over ocean to eliminate possible calibration drifts. The GGEO cloud mask is based on clear-sky threshold, which rely on the GSFC GEOS-4 assimilated model predicted clear-sky temperatures and the MODIS based monthly clear-sky albedo maps. The geostationary daytime cloud retrievals assumes a 10µm liquid radii and a 60µm diameter ice particle distributions similar to the ISCCP model. An IR emissivity is then inferred from the corresponding optical depth retrieval, in order to determine the effective cloud temperature and based on the GEOS-4 temperature profiles, places the cloud at the proper pressure level. A threshold of 253°K is used to discriminate ice clouds from liquid clouds. At the current time no geostationary nighttime retrievals (available in the SRBAVG product) are used in the ISCCP-D2like product, and may be used in the merged ISCCP-D2like product, after effective normalization of GGEO clouds with MODIS has been completed.

User Applied Revisions for Current Edition

The purpose of User Applied Revisions is to provide the scientific community early access to algorithm improvements, which will be included in future Editions of the CERES data products. The intent is to provide users simple algorithms along with a description of how and why they should be applied in order to capture the most significant improvements prior to their introduction in the production-processing environment. It is left to the user to apply a revision to data ordered from the Atmospheric Science Data Center. Note: Users should never apply more than one revision. Revisions are independent and the latest, most recent revision to a data set includes all of the identified adjustments.

ISCCP-D2like Beta1

There are no revisions at this time.

Cautions and Helpful Hints

The CERES Science Team notes several cautions regarding the use of CERES-ISCCP-D2like Beta1 data:

• Currently there are no reported errors in the CERES ISCCP-D2like product

• The instantaneous regional geostationary retrievals are first averaged into 4 static (SFC-700mb, 700-500mb, 500-300mb, 300-100mb) pressure levels as an intermediate product. A gamma distribution, which uses the log and linear optical depth, is assumed to back out optical depth for each layer in order to determine the ISCCP-D2 cloud type, the algorithm is given in the reference section under Kato.

• The Terra and Aqua SSF MODIS cloud retrieval algorithm is not the same, since the 1.6µm is not available on the Aqua instrument and the greatest differences occur over the poles, where improvements in the Aqua retrievals have been made. More information in the Aqua SSF DQS.

Accuracy and Validation

The primary goal of the ISCCP-D2like product is to provide climate quality monthly mean cloud properties consistent with the CERES observed fluxes. The input hourly MODIS cloud properties are obtained from the CERES SSF product, which has been well validated and summarized in the Terra and Aqua SSF Data Quality Summary. The input 3-hourly geostationary cloud properties are obtained from an internal GGEO product, however the GEO cloud properties are summarized in the SRBAVG DQS. (Users should consult the SSF and SRBAVG Data Quality Summary for the accuracy associated with the cloud properties.

Future validation efforts will be performed when deriving the merged ISCCP-D2like product in order to produce GEO cloud properties consistent with MODIS cloud properties. Comparisons with ISCCP, and MOD0X products over a 5-year time period will be performed. The Data Quality Summary will be updated as these become available.

References

The first two references describe the CERES_MODIS cloud retrieval algorithm. The third reference explains the cloud optical depth gamma distribution assumption. The last reference outlines the GGEO cloud retrieval Layered Bispectral Threshold Model (LBTM) algorithm and is a subset of the MODIS algorithm. Further references of validation and quality of MODIS and GEO cloud retrievals can be found on the CERES reference page from the NASA Langley Cloud and Radiation Group Home Page.

Baum, B. A.; Welch, R. M.; Minnis, P.; Stowe, L. L.; Coakley, J. A., Jr.; Titlow, J.; Tovinkere, V.; Heck, P. W.; Trepte, Q.; Doelling, D. R.; Mayor, S.; Berendes, T.; Han, Q.; Christopher, S. A.; Kuo, K.-S.; Penaloza, M.; Logar, A.; and Davis, P.: Imager Clear-Sky Determination and Cloud Detection (Subsystem 4.1). "Clouds and the Earth's Radiant Energy System (CERES) Algorithm Theoretical Basis Document, Volume III: Cloud Analyses and Determination of Improved Top of Atmosphere Fluxes (Subsystem 4)(Subsystem 4)" (PDF), NASA RP 1376 Vol. 3, edited by CERES Science Team, December, 1995, pp. 43-82.

Baum, B. A.; Minnis, P.; Coakley, J. A., Jr.; Wielicki, B. A.; Menzel, P.; Titlow, J.; Tovinkere, V.; Heck, P. ; and Mayor, S.: Imager Cloud Height Determination (Subsystem 4.2). "Clouds and the Earth's Radiant Energy System (CERES) Algorithm Theoretical Basis Document, Volume III: Cloud Analyses and Determination of Improved Top of Atmosphere Fluxes (Subsystem 4)" (PDF) NASA RP 1376 Vol. 3, edited by CERES Science Team, December, 1995, pp. 83-134.

Kato, S.; Rose, F. G.; Charlock, T. P.; 2005: Computation of Domain-Averaged Irradiance Using Satellite-Derived Cloud Properties. J. of Atmos. and Oceanic Tech., 22, pp 146-164

Minnis, P.; Smith, W. L., Jr.; Garber, D. P.; Ayers, J. K.; and Doelling, D. R.: Cloud Properties Derived From GOES-7 for the Spring 1994 ARM Intensive Observing Period Using Version 1.0.0 of the ARM Satellite Data Analysis Program. NASA RP 1366, August 1995, 59 pp

Expected Reprocessing

A publishable, "Edition" version of the CERES ISCCP-D2like Terra/Aqua and GGEO products is scheduled for released in January 2009. A merged Terra/Aqua/GGEO CERES ISCCP-D2like product is planned for Spring 2009. Also a CERES ISCCP-D2like Terra/Aqua with the associated CERES fluxes is planned for late 2009. A Notification of any changes will be sent to registered users.

Attribution

These data are NOT for publication.

Feedback and Questions

For questions or comments on the CERES Quality Summary, contact the User and Data Services staff at the Atmospheric Science Data Center.

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