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Ocean Pigment Concentration from CZCS

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

References

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), launchedon August 1, 1997, and future missions conducted as part of NASA's Mission to Planet Earth program.

Sponsor
The production and distribution of this data set are being funded by NASA's Mission To Planet Earth program.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 Mission to Planet Earth program.

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.bin (monthly composite)
czcs.chlrcn.1ncego.mm.bin (monthly climate)
czcs.chlrcn.1ncego.bin (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
bin = file type designation, binary

Directory Path

http://daac.gsfc.nasa.gov/data/inter_disc/biosphere/czcs_color/yyyy
http://daac.gsfc.nasa.gov/data/inter_disc/biosphere/czcs_color/climate

where yyyy is the year.

Companion Software
Sample programs in FORTRAN and IDL languages are available to read these data.You may also acquire this software by accessing the Goddard DAAC anonymous FTP site in the directory:

software/inter_disc_software

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 CZCSmeasurements, the resultant data are then binned to a fixed, linearlatitude-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 areaare averaged together over four time scales: daily, 5-day, monthly, andannually. As part of the averaging process, the input values are firstscreened for cloud and land contamination, the presence of Sun glint,abnormal values of water-leaving radiances, and low solar angle, amongothers. After averaging, the composite chlorophyll concentrations areconverted to 8-bit values using appropriate scale and offsetcoefficients. Because of the poor and highly variable temporal andspatial sampling the level 3 CZCS averages are generally referred hereto as composites instead of averages. The resulting Level 3 Compositedata product is one of the official CZCS archive products located inthe 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 3Monthly 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 monthlycomposite chlorophyll data.
  8. One composite climatology file (over temporal coverage of data set) was producedfrom 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 Ni&ntildeo (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.

 

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 Ni&ntildeo . 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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