Data Access

ISCCP C2 Cloud Data

Cloud fraction	Cloud top pressure	Cloud optical thickness
Cloud top temperature	Clear sky surface reflectance	Clear sky surface temperature

Readme Contents

Data Set Overview

Sponsor

Original Archive

Future Updates

The Data

Characteristics

Source

The Files

Format

Name and Directory Information

Companion Software

The Science

Theoretical Basis of Data

Processing Sequence and Algorithms

Scientific Potential of Data

Validation of Data

Contacts

Points of Contact

References

Data Set Overview

Clouds play an important role in the climate change by affecting the heat exchange between the Sun, Earth and atmosphere. Also, they modulate the solar irradiance incident on the Earth's surface (the insolation) and this affects the productivity of plants both on land and in the water as well as the surface temperature and heat budget. Numerous efforts to form cloud climatologies from both surface and satellite observations have been made. At present the most important of these is the ongoing International Satellite Cloud Climatology Project (ISCCP), established in 1982 as part of the World Climate Research Program (WCRP). A combination of satellite-measured radiances, ice/snow cover dataset and TOVS atmospheric temperature/humidity are used by ISCCP to produce a global dataset on cloud and surface variables. Operational data collection and processing for ISCCP have been underway since July 1983. An overview of the project and the data products is given in Rossow and Schiffer (1991); the algorithm and its effectiveness are described by Rossow and Garder (1993a&b) while Rossow et al. (1993) compare the resulting products to other cloud climatologies. The ISCCP C2-series data products, briefly described here(more extensively in Rossow et al. (1996)), are gridded data averaged over each month. These data (spanning over the period July 1983 to June 1991) are originally produced on an equal area map grids which has a constant 2.5 degree latitude increments and varaiable longitude increments ranging from 2.5 degree at the equator to 120 degree at the pole The Goddard DAAC has regridded these dataset to 1x1 degree equal angle grid for inclusion in the Interdisciplinary data collection.

Sponsor
The production and distribution of this data set are funded by NASA's Earth Science enterprise. The data are not copyrighted; however, we request that when you publish data or results using these data please acknowledge as follows:

The authors wish to thank William B. Rossow, and the Goddard Institute for Space Studies (GISS),New York, NY, USA, for the production of this data set, and the Distributed Active Archive Center (Code 902) at the Goddard Space Flight Center, Greenbelt, MD, 20771, for putting these data in their present format and distributing them. These distribution activities were sponsored by NASA's Earth Science enterprise.

Original Archive
The original ISCCP C2 cloud dataset was produced by the Goddard Institute for Space Studies (GISS) at New York, NY. This data set in its original format can be obtained from NASA Langley Research Center, Distributed Active Archive Center. This is the long term archive for the data and also the source for detailed information concerning the ISCCP C-series and other data products.

Note: The format of the data in the GISS and LaRC archives is different than that stored at the Goddard DAAC. The Goddard DAAC regridded the original equal area grid to a 1 by 1 degree (lat/lon) equal angle grid for only a subset of total parameters. For more details see Processing Sequence and Algorithms.

Future Updates
Since the improved ISCCP cloud-products (series-D) are now being produced at GISS, the production of C-product for data beyond June 1991 has stopped. However, additional years of the ISCCP D-2 data are being processed at GISS and Goddard DAAC maintains a subset of the D2 Cloud Data (regridded to 1x1 degree equal angle grid) as a part of the other Interdisciplinary data collection products.

The Data

The ISCCP C2 monthly mean data presented here is on 1x1 degree latitude-longitude world grid that starts at (89.5N, 179.5W) and runs eastward and southward to latitude 89.5 S.

The original ISCCP C2 (monthly mean) and C1 (daily) products were calculated on an approximately equal area world grid ( equivalent to an area of a 2.5x2.5 degree latitude-longitude grid at the equator). The latitude band widths were fixed to 2.5 degree and grid size along the longitude was varied to provide an integer number of cells in a latitude zone and grid cell area as close to an equatorial grid cell as possible. Map grids started from south pole to north pole. In each latitude zone, all longitudes were indexed in order from the Greenwich meridian eastward(0 - 360 degree) before going to the next latitude zone. Here we have interpolated a subset (6 out of the original 132 parameters) of the equal-area monthly mean product to a 1x1 degree grid for easy comparison to the other Interdisciplinary Data Collection products.

Characteristics

• Parameters:

  Parameter	Description	Range	Units
  cldfrc	mean cloud fraction	0 - 100	percent
  cldprs	mean cloud top pressure	35 - 985	millibars
  cldtau	mean cloud optical thickness	0.09 - 119	dimensionless
  cldtmp	mean cloud top temperature	190 - 310	Kelvin
  srfref	mean clear sky surface reflectance	0 - 1	fraction
  srftmp	mean clear sky surface temperature	199 - 325	Kelvin

• Temporal Coverage: July 1983 - June 1991
• Temporal Resolution: Gridded monthly means
• Spatial Coverage: Global
• Spatial Resolution: 1 degree x 1 degree equal angle grid

Source
ISCCP was established in 1982 as part of the World Climate Research Programme (WCRP) to collect and analyze satellite radiance measurements to infer the global distribution of clouds, their properties, and their diurnal, seasonal, and interannual variations.

The first phase of the project produced the gridded, 3-hourly (stage C1) and monthly (stage C2) datasets (Rossow, and Schiffer 1991, Rossow et al. 1989). A subset of the monthly mean C2 data, consisting of six parameters and covering the period of July 1983 to June 1991 (regridded and reformated), is available as part of the Interdesciplinary Dataset Collection.

There are four principle product levels:

• Reduced Resolution Radiance Data(B3)
  These data are a reduced resolution version (in both time and space) of the original visible and infrared images from all of the operational weather satellites. The data consist of radiances (4-7 km pixels) spaced at 30 km interval, every 3 hours, from the individual satellites. The calibration and navigation data are also appended (Rossow et al., 1987). When the D algorithms were introduced, a new (BT) calibration table product was also added.

• Pixel Level Cloud Product(CX)
  These products consist of calibrated radiances and viewing geometry, cloud detection results, cloud and surface properties from radiative analysis for individual satellite at a 30 km resolution.

• Gridded Cloud Product(C1)
  These are spatial averages of the 3-hourly CX quantities and statistical summaries on a global equal area grid ( approximate 280 km by 280 km cell size). These products are obtained by merging the results from all satellites. The atmosphere and surface properties from TOVS are appended as well.

• Climatological Summary Product(C2)
  The C2 products are monthly averages of C1 quantities and statistics including the data sets corresponding to eight 3-hourly monthly means( periods centered on, 0, 3, 6, 9, 12, 15, 18, and 21 hours UTC) for each product at equal area grid. There are a total of 132 parameters for each map grid cells in the original C2 data set. We have chosen 6 of these and only the monthly means (average of the eight 3-hour monthly mean sets) to include in the Interdisciplinary Data Collection.

The resulting datasets and analysis products are being used to improve understanding and modeling of the role of clouds in climate, with the primary focus being the elucidation of the effects of clouds on the radiation balance. These data can also used to support many other cloud studies, including understanding of the hydrological cycle.

Data are collected from the suite of weather satellites operated by several nations and processed by several groups in government agencies, laboratories, and universities. For each operational satellite, a Satellite Processing Center (SPC) collects the raw satellite data and sends it to the Global Processing Center (GPC). The Correlative Data Center coordinates the delivery of other satellite and conventional weather data to the GPC. The Satellite Calibration Center (SCC) normalizes the calibration of the geostationary satellites with respect to a polar orbiter satellite standard. All ISCCP data products are archived at the NASA Langley Research Center, Distributed Active Archive Center.

The satellites involved in the C-version products are listed in the table below:

>

Temporal and Regional Coverage
Satellite	Sensor	C1 start	C1 end	Longitudes
NOAA-7	AVHRR	07/01/83	01/31/85	global (1)
NOAA-8	AVHRR	10/01/83	06/24/84	global
NOAA-9	AVHRR	02/01/85	11/08/88	global
NOAA-10	AVHRR	11/17/86	08/30/91	global
NOAA-11	AVHRR	10/18/88	6/30/91	global
GOES-5	VISSR	07/01/83	07/30/84	15W-135W
GOES-6	VISSR	07/01/83	01/21/89	75W-165E (2)
GOES-7	VISSR	04/26/87	6/30/91	15W-135W (3)
METEOSAT-2	MIR	07/01/83	08/11/88	60W-60E
METEOSAT-3	MIR	08/11/88	01/25/91	60W-60E (4)
METEOSAT-4	MIR	06/19/89	06/30/91	60W-60E
GMS-1	VISSR	01/21/84	06/30/84	160W-80E
GMS-2	VISSR	07/01/83	09/27/84	160W-80E
GMS-3	VISSR	09/27/84	12/04/89	160W-80E
GMS-4	VISSR	12/04/89	06/30/91	160W-60E

Notes: NOAA-7, 9 & 11 were afternoon satellites, while NOAA 8 & 10 were morning satellites.

1. Global coverage required 24 hours.
2. For better coverage of seasonal weather events, GOES-6 was moved to 98W on August 30, 1984, to 108W on November 22, 1984, back to 98W on July 28, 1986, and to 135W from April 1987 until its failure on 1/21/89.
3. For better coverage of seasonal weather events, GOES-7 is positioned between 98W and 108W, with a position of 98W during the tropical convective season.
4. METEOSAT-3 was positioned to 50W longitude in early August 1991 to supplement the coverage of GOES-7. This move was necessary in order to provide coverage of Atlantic weather events during the absence of a GOES East Satellite.

The ISCCP subset presented here consists of 576 datafiles (96 monthly mean data filess per parameter x 6 parameters). Though size of single data file is only .259 MB but if whole eight years worth of data is downloaded it would require ~150 MB of disk storage.

Format

• File Size: 259200 bytes, 64800 data values
• Data Format: IEEE floating point notation
• Headers, trailers, and delimiters: none
• Fill value: -999.99
• Continent mask: none (data valid over land and water)
• Orientation: North to South

  Start position: (179.5W, 89.5N)

  End position: (179.5E, 89.5S)

Name and Directory Information

Naming Convention:

The file naming convention for the monthly files is

isccp_c2.pppppp.1nmegg.[yymm].ddd

where

isccp = data product designator(isccp_c2)

pppppp = parameters(cldfrc,cldtmp,cldprs,cldtau,srftmp,srfref

1 = number of levels(=1)

n = pressure levels for vertical coordinate, n=not applicable

m = temporal period, m = monthly

e = horizontal grid resolution, e = 1 x 1 degree

gg = spatial coverage, gg = global (land and ocean)

yy = year

mm = month

ddd = file type designation, (bin=binary, ctl=GrADS control file)

Directory Path:

/data/inter_disc/radiation_clouds/isccp_c2/pppppp/yyyy/

where pppppp is the parameter and yyyy is year.

Companion Software

Several software packages have been made available on the CIDC CD-ROM set. The Grid Analysis and Display System (GrADS) is an interactive desktop tool that is currently in use worldwide for the analysis and display of earth science data. GrADS meta-data files (.ctl) have been supplied for each of the data sets. A GrADS gui interface has been created for use with the CIDC data. See the GrADS document for information on how to use the gui interface.

Decompression software for PC and Macintosh platforms have been supplied for datasets which are compressed on the CIDC CD-ROM set. For additional information on the decompression software see the aareadme file in the directory:

software/decompression/

Sample programs in FORTRAN, C and IDL languages have also been made available to read these data. You may also acquire this software by accessing the software/read_cidc_sftwr directory on each of the CIDC CD-ROMs

Theoretical Basis of Data

The ISCCP cloud algorithm starts with independent visible and infrared estimates as to whether a region is clear, cloud covered or on the border and therefore uncertain. The two estimates are then merged to make a final clear or cloudy judgment. At night only the infrared algorithm can be used. Key features of the cloud detection algorithm are

• use of space and time radiance variation tests over several different space and time domains to account for the global variety of cloudy and clear characteristics
• estimation of clear radiance values for every time and place, and
• use of radiance thresholds that vary with type of surface and climate regime, (Rossow and Garder, 1993a).

The ISCCP products include the infrared estimates for both day and night observations but the bispectral products and the cloud optical thickness estimate are available only during the day. The bispectral algorithm is more accurate. Therefore a correction is made to the infrared only nighttime cloud products when the mean diurnal total cloud fraction is calculated. The correction is determined from a comparison of daytime bispectral and infrared only cloud fractions. The ISCCP products include low, mid, and high altitude cloud fractions, and several daytime cloud types based on cloud height and brightness. It also includes the 3-hour as well as the diurnal means. Here we include only the C2 monthly mean diurnal cloud fraction and optical thickness together with the cloud top temperature and pressure. The mean clear sky reflectance and skin temperature are also included.

Processing Sequence and Algorithms
ISCCP collects visible (~0.6 micrometer)and infrared (~11 micrometers) data from several geostationary weather satellites and from the National Oceanic and Atmospheric Administration (NOAA) operational meteorological satellites which are in Sun-synchronous near polar orbits. The data collection started with July 1, 1983 and is still continuing. The geostationary satellites on a three hourly basis monitor the Earth from 60 N to 60 S latitude and each covers 120 degrees of longitude. Those involved are the GOES 5-7, METEOSAT 2-5, GMS 1-4, and the INSAT series. Ideally these would have given full global coverage from 60 N to 60 S, but due to numerous problems 3-hourly coverage is not always available in some regions. This is particularly true around India. The NOAA 7,9 & 11 satellites each observe all regions on the Earth at least twice a day. Normally there are two operating at any one time, one crosses the Equator about 7:30 AM & PM local time and the other crosses in the early afternoon and morning. The Polar regions are viewed several times a day. The project used the afternoon satellites, NOAA 7,9&11, as calibration standards to ensure a uniform calibration among the numerous satellites used. The original C-version algorithms (Rossow and Garder, 1993a) were used to produce cloud products for the period (July 1983-June 1991). The current version of the ISCCP C2-series was released in 1991. Since then the C-series data has gone through an extensive review and a number of improvements have been recommended. The D-series data was developed as a result of this process. Validation of the products and comparison with other cloud climatologies are given by Rossow and Garder (1993b) and Rossow et al. (1993). Some improvements both in the calibration corrections and in the cloud algorithms have been made and a new D-series of cloud products are now being produced (Rossow et al., 1995). Eventually the D-series products will cover the entire period July 1, 1983 to June 2000.

Rergridding from an equal area world grid to 1 x 1 degree equal angle grid

For consistency with the other data sets in the Goddard DAAC's Interdisciplinary Data Collection, the original ISCCP C2 data for a subset of parameters were extracted from the ISCCP anonymous FTP site and reformatted at the Goddard DAAC from the original 8 bit quantities (scaled integer values) into 32-bit floating point quantities (unscaled values) and regridded to 1 x 1 degree equal angle grid from their original approximate 280 km x 280 km equal area grid maps.

In the regridding process the original data at equal area grid with constant 2.5 degree latitudinal increments and variable longitudinal increments, ranging from 2.5 degree at the equator to 120 degree at the pole were replicated as many time as needed to produce a 1 by 1 degree latitude-longitude equal angle product. A weighted average was used in cases where two equal area grid cells overlapped a single one degree grid cell. This weighting was based on the percent area each original equal area grid cells covered in the 1 degree area. Changes in grid area due to changing latitude were taken into consideration in this procedure. This regridding method is different from the one used by ISCCP to convert their data from approximate 280 km x 280km equal area to 2.5 by 2.5 degree equal angle. Their method did not use a weighted average in cases where grid cells overlapped, but instead chose one of the grid cell values over another. As a result the values from the statistical files in the original dataset can not be related to all of the grid values in this regridded dataset.

Also, the south to north orientation was reversed, and for each latitude zone, data along the longitude was made to start from 180 west going towards east,again for conformity to the existing criteria, and gif images, created from the resultant files, were visually inspected to assure that the data was free of artifacts introduced by these procedures.

Scientific Potential of Data
The monthly mean summary of the data set given here can be used for many types of climate studies including:

• Study of global, seasonal, and interannual cloud variability (Rossow et al., 1993)
• Correlation with other climate parameters (Kyle et al., 1995)
• Investigation of global energy transport (Sohn and Smith, 1992).

Validation of Data
The validation of the data is discussed in some detail in Rossow and Garder 1993b with some additional details given in Rossow and Garder (1993a) and Rossow et al. (1993). The following summary of error estimates is given in Table 1 in Rossow et al. (1993):

Summary of error estimates
Surface Temperature - open water	<2 K
Surface Temperature - land	4 K
Surface Temperature - sea ice	4 K
Surface Reflectance - open water	3%(except in glint regions)
Surface Reflectance - snow-free land	3%-5%
Surface Reflectance - snow cover	10%
Surface Reflectance - sea ice	10%
Cloud amount	<10%

ISCCP new product( D-series) was produced in part as a result of intensive research done on the C-series data, in which over 200 research articles have been written.

Similar research is continuing with the D-series product and includes comparison of observations from an on-going series of field experiments. Rossow et al. (1996) discuss the differences between the ISCCP C series and D series algorithms, as well as on-going validation efforts being made on the D-series product. The ISCCP project has documented known and fixed data errors in their dataset.

Points of Contact
For information about or assistance in using any DAAC data, contact

EOS Goddard Distributed Active Archive Center (DAAC)

Code 902

NASA Goddard Space Flight Center

Greenbelt, Maryland 20771

e-mail: daacuso@daac.gsfc.nasa.gov

301-614-5224 (voice)

301-614-5268 (fax)

For questions about ISCCP science, contact

Dr. William B. Rossow

NASA Goddard Institute for Space Studies

2880 Broadway

New York, NY 10025 USA

e-mail: clwbr@giss.nasa.gov

(212) 678-5567

The long term archives for the ISCCP data products are at:
(Not including the Stage CX 30 km resolution data)

ISCCP Central Archives

National Climatic Data Center

Federal Building 151 Patton Ave.

Asheville, NC 28801-5001

email: satorder@ncdc.noaa.gov

(704) 271-4800 (option #5) (voice)

(704) 271-4876

(Including the Stage CX 30 km resolution data)

Langley DAAC

Mail Stop 157B

NASA Langley Research Center

Hampton, VA 23681-0001

e-mail: userserv@eosdis.larc.nasa.gov

telnet eosdis.larc.nasa.gov

(804) 864-8656 (voice)

(804) 864-9807 (fax)

Brest, C.L., and W.B. Rossow, 1992. Radiometric calibration and monitoring of NOAA AVHRR data for ISCCP. Int. J. Remote Sens., 13:235-273.

Brest, C.L., W.B. Rossow, and M.D. Roiter, 1996. Update on ISCCP calibration for visible and infrared radiances. J. Atmos. Ocean. Tech., (submitted).

Kyle, H. L., M. Weiss, and P. Ardanuy, 1995. Cloud, surface temperature, and outgoing longwave radiation for the period from 1979 to 1990, J. Climate, 8:2644-2658.

Rossow, W. B., and R. A. Schiffer, 1991. ISCCP cloud data products, Bull. Amer. Meteor. Soc., 72:2-20.

Rossow, W. B., and L. C. Garder, 1993a. Cloud detection using satellite measurements of infrared and visible radiances for ISCCP, J. Climate, 6: 2341-2369.

Rossow, W. B., and L. C. Garder, 1993b. Validation of ISCCP cloud detection, J. Climate, 6: 2370-2393.

Rossow, W. B., and Y.-C Zhang, 1995. Calculation of surface and top of atmosphere radiative fluxes from physical quantities based on ISCCP data sets: 2. Validation and first results, J. Geophys. Res., 100:1167-1197.

Rossow, W. B., E. Kinsella, A. Wolf, and L. Garder, 1987. International satellite Cloud Climatology Project (ISCCP) Description of Reduced Resolution Radiance Data. In, WMO/TD No. 58, (eds), World Meteorological Organization, Geneva, 143 pp.

Rossow, W.B., L.C. Garder, and A.A. Lacis, 1989. Global seasonal cloud variations from satellite radiance measurements. Part I: Sensitivity of Analysis. J. Climate, 2:419-458.

Rossow, W. B., A. W. Walker, and L. C. Garder, 1993. Comparison of ISCCP and other cloud amounts, J. Climate, 6:2394-2418.

Rossow, W. B., A. W. Walker, D. E. Beuschel, and M. D. Roiter, 1996. International Satellite Cloud Climatology Project (ISCCP): documentation of new cloud datasets, 115 pages, available on internet at : http://isccp.giss.nasa.gov/documents.html

Sohn, B. J., and E. A. Smith, 1992. Global energy transports and the influence of cloud on transport requirements: A satellite analyses, J. Climate, 5:717-734.

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