NASA Climatology Interdisciplinary Data Collection/NASA Climatology Interdisciplinary Data Collection - Disc 2.iso/readmes/readme.czcs_opc

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Data Access

     CZCS Data

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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

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Data Set Overview

     This data set is a collection of monthly composites of ocean
     chlorophyll concentration derived from the Coastal Zone Color
     Scanner (CZCS) instrument flown aboard the Nimbus-7 satellite from
     October 1978 through June 1986. This concentration provides a
     direct measure of the abundance of phytoplankton and its
     variability in space and time over most of the world's oceanic
     regions. The CZCS data set represents the only source of
     satellite-derived, global oceanic biomass productivity, and serves
     as an important precursor to the next generation of advanced ocean
     color instruments. These include the Sea-Viewing Wide
     Field-of-View Sensor (SeaWiFS), launched on August 1, 1997, and
     future missions conducted as part of NASA's Earth Science
     enterprise.

     Sponsor
     The production and distribution of this data set are being 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 the Distributed Active Archive
          Center (Code 902.2) at the Goddard Space Flight Center,
          Greenbelt, MD, 20771, for producing the data in its
          present format and distributing them. The original data
          products were produced by the Nimbus Project Office in
          collaboration with the NASA Goddard Space Flight Center
          Space Data and Computing Division, the NASA GSFC
          Laboratory for Oceans, and the University of Miami
          Rosenstiel School of Marine and Atmospheric Science.
          Goddard's share in these activities was sponsored by
          NASA's Earth Science enterprise.

     Original Archive
     The geophysical data from which this CZCS monthly composite data
     set is derived were produced by the Nimbus Project Office in
     collaboration with the NASA Goddard Space Flight Center (GSFC)
     Space Data and Computing Division, the NASA GSFC Laboratory for
     Oceans, and the University of Miami Rosenstiel School of Marine
     and Atmospheric Science. This global processing effort was
     initiated in 1985 and completed in early 1990. See Feldman et al.
     (1989) for a complete description of the processing system used to
     generate these products. The level 3 monthly composite data
     product, with a spatial resolution of 20 km at the equator, was
     used to generate these 1 degree x 1 degree averages. The complete
     suite of CZCS-derived geophysical parameters is currently
     available from the Distributed Active Archive Center (DAAC) at
     NASA GSFC.

     Future Updates
     The CZCS data set is currently classified as a static data set,
     and is unlikely to be reprocessed in the near term due to new data
     sources, including the Ocean Color and Temperature Sensor (OCTS),
     which operated from August 1996 to May 1997, and SeaWiFS.

The Data

     Characteristics

        * Parameters: Chlorophyll (pigment) concentration, defined as
          the sum of the concentrations of chlorophyll-a and
          phaeophytin- a

        * Units: mg/m^3
        * Typical Range (monthly average):
               0.05 mg/m^3 (e.g., tropical non-coastal waters) to 30
               mg/m^3 (e.g., coastal waters, North Pacific, North
               Atlantic)

        * Temporal Coverage: November 1978 - June 1986
        * Temporal Resolution: monthly composites, monthly composites
          over temporal coverage of data set,
          and composites over temporal coverage of data set.

        * Spatial Coverage: Global Ocean
        * Spatial Resolution: 1 degree x 1 degree

     Source
     The data source for this data set was the Coastal Zone Color
     Scanner (CZCS) flown on Nimbus-7.

     Nimbus-7 was launched in October 1978 and was a research-and-
     development satellite serving as a stabilized, Earth observing
     platform for the testing of advanced systems for sensing and
     collecting data in the pollution, oceanographic, and
     meteorological disciplines. It provided an opportunity to assess
     each instrument's operation in the space environment and to
     collect a sizable body of data with the global and seasonal
     coverage needed for support of each experiment. The mission also
     extended and refined the sounding and atmospheric structure
     measurement capabilities demonstrated by experiments on previous
     Nimbus observatories.

     Nominal orbit parameters for the Nimbus-7 spacecraft are

          Launch date: 10/24/78
          Orbit: Sun synchronous, near polar
          Nominal altitude: 955 km
          Inclination: 99.3 degrees
          Nodal period: 104 minutes
          Nodal Increment: 26.1 degrees
          Equatorial crossing time: 11:50 AM (local time)

     CZCS, one of eight instruments aboard Nimbus-7, had six spectral
     bands (channels); four chiefly for ocean color, each of 20
     nanometer (nm) band width and centered at 443, 520, 550, and 670
     nanometers. These are referred to as channels 1 through 4,
     respectively. Channel 5 sensed reflected solar radiance, but had a
     100 nanometer bandwidth centered at 750 nanometers and a dynamic
     range that was more suited to land. Channel 6 operated in the 10.5
     to 12.5 micrometer region and sensed emitted thermal radiance for
     derivation of equivalent black body temperature of the sea
     surface. Channel 6 failed within the first year of the mission,
     though, and so was not used in the global processing effort.

     The following lists the primary purpose of each CZCS channel.

          433-453 nm (blue) -- chlorophyll absorption
          510-530 nm (green) -- chlorophyll concentration
          540-560 nm (yellow) -- Gelbstoffe concentration
          660-680 nm (red) -- aerosol absorption
          700-800 nm (far red) -- land and cloud detection

     CZCS was a cross-track scanning system. The Instrument Field of
     View (IFOV) of each detector was .865 mrad, yielding a resolution
     of 825 m at the satellite subpoint. The swath covered 1566 km in
     width from a maximum scan angle of approximately 40 degrees. Data
     were then transmitted to a receiving station at a rate of 800
     kbps.

     Due to the power demands of the various onboard experiments, the
     CZCS sensor was operated on an intermittent schedule. In 1981 it
     was determined that the sensitivity of the other CZCS channels was
     degrading with time; in particular channel 4. Sensitivity
     degradation was persistent and increased during the rest of the
     mission. In mid- 1984, Nimbus-7 mission personnel experienced
     turn-on problems with the CZCS system, which were related to power
     supply problems. Spontaneous shut down of the CZCS system began
     occurring as well and persisted for the rest of the mission. From
     March 9, 1986, to June 1986 the CZCS system was given highest
     priority for the collection of a contemporaneous data set of ocean
     color. It was turned off in June 1986.

     A detailed description of the CZCS instrument and the Nimbus-7
     satellite is available on the Goddard Space Flight Center
     Worldwide Web site.

The Files

     Format

        * File Size: 259200 bytes, 64800 data values
        * Data Format: IEEE floating point notation
        * Headers, trailers and delimiters: none
        * Land, water, or ice mask: land and ice (-999.9)
        * Fill value: -99.
        * Image 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 CZCS data files is

          czcs.chlrcn.1nmego.[yymm].ddd (monthly composite)
          czcs.chlrcn.1ncego.[mm].ddd (monthly climate)
          czcs.chlrcn.1nxego.ddd (data set climate)

     where:

          czcs = data product designator (CZCS)
          chlrcn = parameter name (chlorophyll concentration)
          1 = number of levels
          n = vertical coordinate, n = not applicable
          m or c = temporal period, m = monthly c = climatology
          e = horizontal grid resolution, e = 1 x 1 degree
          go = spatial coverage, g0 = global ocean
          yy = year
          mm = month
          ddd = file type designation (bin=binary, ctl=GrADS control
          file)

     Directory Path

          /data/inter_disc/biosphere/czcs_color/yyyy
          /data/inter_disc/biosphere/czcs_color/climate

     where yyyy is the 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

The Science

     Theoretical Basis of Data
     The theory of measurement behind remote sensing of oceanic
     chlorophyll is based on the fact that the content of water, be it
     organic or inorganic particulate matter or dissolved substances,
     affects its color. Ocean water, containing very little particulate
     matter, scatters light as a Rayleigh scatterer with the well known
     deep purple or bluish color of the ocean. As particulate matter is
     added to the water, the scattering characteristics are changed and
     the color is changed. Photosynthetic pigments as found in
     phytoplankton (e.g., chlorophyll-a) preferentially absorb higher
     energy blue light but reflect green light through scattering
     processes similar to those that result in the "greenness" of land
     vegetation. Thus, as the concentration of phytoplankton increases,
     ocean color shifts from blue to green. However, some
     phytoplankton, such as the various red tide organisms, can change
     the water to colors such as red, yellow, blue-green, or mahogany.
     Inorganic particulate matter in water, such as that originating
     from river discharge, has a different color from organic material,
     typically brownish in color but sometimes varying with red. By
     sensing the color with very high signal-to-noise ratios, the CZCS
     measurements provide a mechanism for analyzing that color for the
     content of the water.

     The relationship between chlorophyll content of the ocean and the
     measured CZCS radiances in the blue and green portion of the
     visible electromagnetic spectrum is described further in Ocean
     Color from Space

     Processing Sequence and Algorithms
     The algorithm used for estimating the chlorophyll content of the
     ocean from CZCS measurements involves the use of radiance ratios
     as described in Gordon et al. (1980) and Gordon et al. (1983).

     The general form of the equation is

          log(C) = a + b*log[Lw(1)/Lw(2)]

     where
          C is the chlorophyll concentration (mg/m^3)
          a,b are regression coefficients
          Lw(1),Lw(2) are the atmospherically corrected radiances for a
          pair of CZCS channels

     For CZCS chlorophyll processing, these channel pairs are taken to
     be

          (443, 550 nm), for C < 1.5 mg/m^3
          (520, 550 nm), for C > 1.5 mg/m^3

     The regression coefficients are different for the two wavelength
     pairs. They are also somewhat dependent on the type of
     phytoplankton present as well as the amount of suspended
     particulates (Viollier and Sturm 1984).

     The atmospherically corrected radiances represent the energy
     exiting at the ocean-atmosphere interface after penetrating the
     surface and being reflected back by inorganic and organic matter
     in subsurface layers. These so-called "water-leaving radiances"
     are the radiances the satellite would have observed in the absence
     of an overlying atmosphere.

     Thus, the fundamental quantity of interest, Lw, which contains
     information on the chlorophyll content of the ocean, can be
     expressed as

          Lw(i)=L(i)-La(i)=L(i)-Lr(i)-Lp(i)

     where

          L(i) is the satellite-measured backscattered radiance at
          wavelength i
          La(i) is the atmospheric contribution to the radiance at
          wavelength i
          Lr(i) is the molecular (Rayleigh) scattering contribution to
          the radiance at wavelength i
          Lp(i) is the atmospheric aerosol scattering contribution to
          the radiance at wavelength i

     The molecular and aerosol scattering contributions also
     incorporate the effect of ozone on the radiances, since the CZCS
     channels are located in that part of the visible spectrum
     containing the weak Chappuis ozone absorption band. Ozone data
     derived from the Total Ozone Mapping Spectrometer (TOMS) aboard
     the same satellite is used for this purpose. The atmospheric
     contribution to the satellite- observed radiances is on the order
     of 80% to 90% of the signal. Thus an accurate means of determining
     this contribution is required to extract meaningful information on
     ocean composition from the observed radiances. The procedure for
     performing the atmospheric correction of CZCS radiances can be
     found in more detail in Williams et al. (1985).

     Once the chlorophyll estimates have been derived from individual
     CZCS measurements, the resultant data are then binned to a fixed,
     linear latitude-longitude array of dimension 1024 lines by 2048
     pixels, corresponding to a spatial resolution of about 18 km at
     the equator. All of the individual chlorophyll values falling
     within each pixel area are averaged together over four time
     scales: daily, 5-day, monthly, and annually. As part of the
     averaging process, the input values are first screened for cloud
     and land contamination, the presence of Sun glint, abnormal values
     of water-leaving radiances, and low solar angle, among others.
     After averaging, the composite chlorophyll concentrations are
     converted to 8-bit values using appropriate scale and offset
     coefficients. Because of the poor and highly variable temporal and
     spatial sampling the level 3 CZCS averages are generally referred
     here to as composites instead of averages. The resulting Level 3
     Composite data product is one of the official CZCS archive
     products located in the Goddard DAAC, and is the data set from
     which the monthly 1 degree x 1 degree chlorophyll product is
     derived.

     A more detailed description of the CZCS data processing system is
     described in the appropriate sections of Satellite Ocean Color
     background document.

     The following steps were performed by the Goddard DAAC on the
     original Level 3 Monthly Composite data to create this data set.
     For each month and each 1 degree x 1 degree latitude-longitude
     grid box

       1. All scaled (8-bit) chlorophyll values in the 2048 x 1024
          input array located within a circular area circumscribing the
          given grid box were arithmetically averaged together to
          produce the composite value of scaled chlorophyll for that
          grid box.
       2. All resulting composite values in the 360 x 180 monthly array
          were then converted from their byte values (1 to 245) to the
          corresponding floating point chlorophyll values using the
          following transformation equation:
               Log(chlorophyll) = .012*(byte value) - 1.4
       3. Pixel values of 0 (no data) were set to -99.
       4. Pixel values of 246-251 were not used in the original file.
       5. Pixel values of 253 (land), 254 (ice), and 255 (continental
          outline) were all set to -999.9.
       6. The resulting remapped chlorophyll data (with embedded
          land/ice mask) were output to a flat IEEE binary file.
       7. Twelve monthly climatology (each month is a composite of the
          entire temporal for that month) files were derived from the
          monthly composite chlorophyll data.
       8. One composite climatology file (over temporal coverage of
          data set) was produced from the twelve monthly climatology
          files.

     Scientific Potential of Data
     CZCS data provide the only source of global measurements related
     to ocean biological productivity and its regional and temporal
     variability over both short- and long-time scales. It is an
     important source of information for a wide range of studies
     pertinent not only to ocean biology but also to physical
     oceanography and atmosphere and ocean interactions. Some
     applications of the data include

        * studies of phytoplankton dynamics (e.g., phytoplankton
          blooms) as a function of nutrient availability and the
          seasonal variation of solar energy required for
          photosynthesis (Brock et al. 1992, Wroblewski et al. 1988)

        * correlation studies relating the enhancement or suppression
          of primary production to localized, transient phenomena such
          as coastal upwelling or large-scale cyclic phenomena such as
          El Niño (McClain et al. 1984, Feldman et al. 1984)

        * use of the imagery to locate fronts, eddies coastal currents,
          and other circulation features (Deuser et al. 1988)

        * understanding of the role of marine biomass in the global
          carbon cycle and how it may relate to environmental change
          and influence global climate (Moore and Bolin 1986, Sundquist
          and Broecker (eds) 1985)

     In addition, the phytoplankton population as measured by
     corresponding oceanic chlorophyll content can act as a natural
     tracer for the presence of pollutants that suppress plant growth,
     or for subtle changes in the environment (e.g., sea surface
     temperature or salinity) that may affect phytoplankton growth.

     Validation of Data
     Not available at this revision.

Contacts

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

          EOS Distributed Active Archive Center (DAAC)
          Code 902.2
          NASA Goddard Space Flight Center
          Greenbelt, Maryland 20771
          Internet: [e-mail]
          301-614-5224 (voice)
          301-614-5268 (fax)

References

     Brock, J.C., and C.R. McClain. 1992. Interannual variability in
     phytoplankton blooms observed in the northwestern Arabian Sea
     during the southwest Monsoon. J. Geophys. Res., 97(C1):733- 750.

     Deuser, W.G., F.E. Muller-Karger, and C. Hemleben. 1988. Temporal
     variations of particle fluxes in the deep subtropical and tropical
     North Atlantic: Eulerian versus LaGrangian effects. J. Geophys.
     Res., 93(C6):6857-6862.

     Feldman, G., D. Clark, and D. Halpern. 1984. Satellite color
     observations of the phytoplankton distribution in the eastern
     equatorial Pacific during the 1982-1983 El Niño . Science,
     226(4678):1069-1071.

     Gordon, H.R., D.K. Clark, J.L. Muller, and W.A. Hovis. 1980.
     Phytoplankton pigments from the Nimbus-7 coastal zone color
     scanner: Comparisons with surface measurements. Science,
     210:63-66.

     Gordon, H.R., D.K. Clark, J.W. Brown, O.B. Brown, R.H. Evans, and
     W.W. Broenkow. 1983. Phytoplankton pigment concentrations in the
     Middle Atlantic Bight: Comparison of ship determinations and CZCS
     estimates. Appl. Opt., 22(1):20-36.

     McClain, C.R., L.J. Pietrafesa, J.A. Yoder. 1984. Observations of
     Gulf stream-induced and wind-driven upwelling in the Georgia Bight
     using ocean color and infra-red imagery. J. Geophys. Res.,
     89:3705-3723.

     Moore, B., and B. Bolin. 1986. The oceans, carbon dioxide, and
     global climate change. Oceanus, 29(4):9-15.

     Sundquist, E.T., and W.S. Broecker (eds). 1985. The Carbon Cycle
     and Atmospheric CO2 Natural Variations Archean to Present,
     American Geophysical Union, Washington, DC, 627 pp.

     Viollier, M., and B. Sturm. 1984. CZCS data analysis in turbid
     coastal water. J. Geophys. Res., 89:4977-4985.

     Williams, S.P., E.F. Szajna, and W.A. Hovis. 1985. Nimbus-7
     Coastal Zone Color Scanner (CZCS) Level 2 Data Product User's
     Guide, NASA Tech. Mem. 86202, Washington, DC.

     Wroblewski, J.S., J.L. Sarmiento, and G.R. Flierl. 1988. An ocean
     basin scale model of plankton dynamics in the North Atlantic 1.
     Solutions for the climatological oceanographic conditions in May.
     Global Biogeochem. Cycles, 2(3):199-218.

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Last update:Fri Aug 22 18:12:39 EDT 1997
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