Time Series of Phytoplankton Biomass Distribution over the Northwestern Pacific Area by Monthly Composite Images from Nimbus7-CZCS Data
Processed and edited by H. Fukushima,1 M. Toratani,1 S. Matsumura2 and A. Harashima3 1 Tokai University 2 National Research Institute of Far Seas Fisheries ( NRIFSF ) 3 National Institute for Environmental Studies ( NIES )
1. Introduction 1.1 What's in the CD-ROM? This CD-ROM contains monthly composite images of phytoplankton pigment concentrations in the North Western Pacific from December 1978 through June 1986, derived from about 4,000 elements of Coastal Zone Color Scanner (CZCS) data. Also included are associated images of the spectral radiance arising from the ocean body and atmosphere. These images convey information of temporal and spatial variability in marine biological/chemical processes such as the nutrient /primary production relation, as well as information of aerosol particle concentrations that may affect the oceanic processes. CZCS, aboard the Nimbus-7 satellite, was a scanning radiometer having four observation channels in the visible wavelengths. This ocean color instrument was the first to enable space-based mapping of ocean vegetation, mainly by measuring the concentration of surface phytoplankton pigment (approximately proportional to chlorophyll-a). Despite being designed with a life time of only one year, the instrument was operated over eight years, although the observation times were limited because of its experimental, "proof of concept," mission. Due to a mechanical problem, the opportunities for observation also decreased significantly from 1983. Nevertheless, CZCS collected valuable data over the global ocean, stimulating many applications and related studies.1), 12) The information included in this CD-ROM will contribute much to the requirements of researchers in ocean science, atmospheric science, and environmental management, as well as to environmental modelers and policy makers. The CZCS mission has now been succeeded by ADEOS-Ocean Color and Temperature Scanner (OCTS), after a period from 1986 to 1996 in which there were no satellite-borne ocean color sensors. The on-line monitoring and analysis of global change based on these missions are anticipated. 1.2 Satellite as a Monitoring Tool for Asian Marginal Seas The human population explosion, rapid industrialization, and increases in the use of fertilizers have strongly influenced the cycling of elements such as C, N, and P, which lead to perturbations of marine ecosystems in marginal seas. Assessment of these perturbations in the North Pacific area and the seas facing the Asian continent is particularly urgent. The United Nations Convention on the Law of the Sea (UNCLOS) states each country's responsibility in the environmental management of their Exclusive Economic Zone (EEZ). On the other hand, United Nations Environment Program (UNEP) has defined several Regional Sea Action Plans based on the cooperation of related countries. The twelfth action plan, called the Northwestern Pacific Action Plan (NOWPAP), covers the Northwestern Pacific area surrounded by the Democratic Peoples Republic of Korea, Japan, the People's Republic of China, the Republic of Korea, and the Russian Federation. Recently held inter- governmental discussion has given high priority to database construction and planning for cooperative environmental monitoring. 1.3 What's New? This data set has some forerunners: NASA/Goddard Space Flight Center (GSFC) made the first global CZCS composite data set as a joint effort with a University of Miami group,4) and some of this booklet's authors have also previously published another CD-ROM5), 15) which includes this region. Our current data set has a higher spatial resolution (3 by 4 km) compared to the NASA/GSFC composite (18 by 18 km), and has been processed with modified atmospheric corrections tuned for the local Asian dust aerosol. The new data set has also been improved in the following points. First, the 4 km binned data was composed from the full resolution data (1 km x 1 km), resulting in smoother images based on data an order of magnitude more dense. A pixel-wise ozone/pressure correction was applied to the atmospheric correction algorithm. A movie file was generated to help users efficiently comprehend the temporal- spatial variation. The last, but important, improvement was an automated pixel- wise quality check. This procedure has significantly increased the number of valid pixels compared with the earlier product, which depended on inspection by the human eye and rejection of the whole image if any part was of unsatisfactory quality. 1.4 Biogeochemical Changes Detected from the Data on this CD-ROM Users who, for example, access the movie file, will notice that the semi -enclosed seas, such as the Yellow Sea or the Seto Inland Sea, are characterized by relatively high levels of phytoplankton biomass, reflecting the corresponding high nutrient levels. Another feature that will be noticed is that the areas of phytoplankton blooms propagate northward from early spring to summer. The blooms cease in mid-summer and break out again in mid-latitudes in autumn. So the careful observer will notice the close interactions between the biological/ chemical/physical processes of the ocean, the socio-economic states of the surrounding terrestrial systems, and the climate. It will be fruitful, however, to access related studies such as the application of Sverdrup's critical depth theory14) or the construction of a database from ship-based observations,10) because the satellite data gives little information on the vertical structure of biogeochemical parameters. 2. Data Processing 2. 1 Outline of the Data Processing The NASA/GSFC provided the CZCS data set which consisted of 4,129 individual scenes (about 40 GB in total) covering the northwest Pacific region of lat 20N to 50N, long 120E to 180E. The outline of data processing is shown in Fig. 1. At first, all the scenes were corrected for atmospheric effects at the full resolution (800 m by 800 m at nadir). Cloud-contaminated pixels and "unlikely" pixels were then screened, resulting in water-leaving radiance (Lw) images for 443, 520, and 550 nm bands, and aerosol radiance (La) images for the 670 nm band. Images of the chlorophyll-like pigment concentration (concentration of chlorophyll-a + phaeopigments) were successively derived using the Lw data. All the individual data were averaged each month over the pigment and radiance data to generate the composite images. The individual processes are explained in Appendix 1. Fig. 1: Block flow diagram of data processing Individual CZCS data --> Atmospheric correction ( ECMWF Pressure/TOMS Ozone ) --> Cloud-contaminated pixel screening --> Pixel-wise quality control --> Water-leaving/ aerosol radiance and pigment concentration files --> Composite data compilation --> Monthly averaged image files 3. Files and Formats 3.1 Directories This CD-ROM has a directory structure as follows. The ROOT-directory contains the readme.txt and appendx1.txt files, which are basically identical in content to this booklet, and the appendx2.txt file showing all directories and files included in this CD-ROM. Users who have a World Wide Web browser can view the contents by opening the index.htm file. Table 1. Directory structure (shown in full form in Appendix-2 in the CD-ROM) [root] readme.txt, jreadme.txt, appendx1.txt, appendx2.txt, index.htm, jindex.htm [movie] chlmovie.mov [gif] index.gif [chlphe] [1978], ......, [1986] ( defined for each year) [colorbar] colbar.gif [binary] [chlphe] same as [gif/chlphe]-subdirectory [nlw443] same as [gif/chlphe]-subdirectory [nlw550] same as [gif/chlphe]-subdirectory [la670] same as [gif/chlphe]-subdirectory [htm] readme.htm, jreadme.htm, appendx1.htm, appendx2.htm [1978], ......, [1986] ( defined for each year ) 3.2 Movie File Chlmovie.mov is a QuickTime(R) file that demonstrates each monthly composite every 2 seconds. Users can open it with QuickTime or equivalent tools. 3.3 GIF Image Files Each subdirectory from [1978] through [1986] in the [chlphe] directory has monthly average pigment concentration image files for each year in the Compuserve(R) GIF format. These files can be displayed with web browsers, or most other graphics capable softweares . Colbar.gif in the [colorbar] directory is a color scale which should be displayed together with pigment concentration image GIF files. 3.4 Binary Pigment Image Files Files in the [chlphe] directory are average pigment concentration images in a simple raster format of 768 lines by 1536 columns, covering the study area with each pixel in a 2.21 x 2.21 minute grid. Each pixel consists of a one byte (8 bit) unsigned integer, corresponding to a digital number within the range 0 - 255. The pigment concentration estimate (P) in g/l is converted into a digital number (DN) by the following equation: DN = log10(P) * 50 + 100 Note that some digital numbers are used for particular purposes, namely, count 0 for "no data", 254 for "coast line", and 255 for cloud/land pixels. "No data" here means there was no observation or no qualified data for the pixel location (or bin) during the month. The coast line pixels were generated from the map data provided by Generic Mapping Tool.16) 3.5 Binary Radiance Image Files The subdirectories [nlw443] and [nlw550] contain averaged normalized water-leaving radiance at the 443 nm and 550 nm bands, respectively. The amount water-leaving radiance was normalized in terms of solar irradiance magnitude at the sea-surface using the following equation:7) nLw(lambda) = Lw(lambda) / t0*cos(theta0), where t0 is transmittance between the sun and the sea-surface and theta0 is the solar zenith angle. Since nLw is normalized in a similar way to the reflectance, it can be used as a substitute for the subsurface reflectance. While nLw(443) is sensitive to the pigment concentration, nLw(550) tends to be more stable in the open ocean. It responds very well, however, to suspended solids (SS) of terrigenous origin, and is useful to depict coastal regions with high concentrations of suspended solids. It also reflects the presence of "high-reflectance water" such as is found during coccolithophore blooms. La(670) image files in the [la670] directory contain aerosol radiance values at the 670 nm band. The monthly composites of aerosol radiance show seasonal and spatial variability of aerosol concentrations in the northwest Pacific, especially the high concentrations in spring due to Asian dust aerosols which originate from the Chinese desert area. Radiance value L (nLw443, nLw520, nLw550, or La670) in W/cm2/sr/nm was converted into DN as follows: DN = L (W/cm2/nm/sr) * 100, where sr is the steradian. 4. Citation and Inquiry The numerical and image data in this CD-ROM should be cited using the title of this booklet. Requests for information or further shipments of this CD- ROM should be mailed to the following address: Database Section Center for Global Environmental Research National Institute for Environmental Studies 16-2 Onogawa, Tsukuba, Ibaraki 305, Japan Fax: +81-298-58-2645, E-mail: cgerdb@nies.go.jp Further inquiries related to technical or scientific problems, as well as errors found by users, should be sent to hajime@fksh.fc.u-tokai.ac.jp (Hajime Fukushima) or harashim@nies.go.jp (Akira Harashima). 5. Acknowledgments We thank G. Feldman and his group for providing the CZCS data set and H. Gordon for providing the Rayleigh radiance look-up table. This work originates from the program "Study on the marine environmental change by global mapping of satellite ocean color data", which was supported by the grant of GERF (Global Environmental Research Fund) from Japan Environment Agency ( FY 94-96, Grant No. D3) and was achieved by the CD-ROM making team based on the Database Budget of Center for Global Environmental Research ( CGER ), NIES as follows: GERF-D3 Program: Participating Organizations (Contact person ) NIES ( A. Harashima), Nara University ( R. Tsuda ), NRIFSF ( S. Matsumura ), Chiaba University ( Y. Yasuda ), Tokai University ( H. Fukushima ), Jaspan Sea National Fisheries Research Institute ( H. Nagata ), National Research Institute for Resources and Environment ( J. Ishizaka ), Meteorological Research Institute ( M. Endoh ). CD-ROM Making Team: Perssonnels A. Harashima, Y. Satoh (NIES-Marine Environment Research Team), H. Fukushima, M. Toratani(Tokai University), S. Nakai, A. Suzuki, T. Miyazaki ( Database Section, NIES-CGER ). References 1)Barale, V. and P. M. Schlittenhardt (eds.) (1993): Ocean Colour: Theory and Applications in a decade of CZCS experience, Kluwer Academic Publishers, 367p. 2)Campbell, J., J. Blaisdell, and M. Darzi (1995): Level-3 SeaWiFS Data Products: Spatial and Temporal Binning Algorithms, SeaWiFS Tech. Report Ser., 32, NASA Tech. Memo., 104566. 3)Evans R. H. and H. R. Gordon (1994): J. Geophys. Res., 99, C4, 7293-7307. 4)Feldman, G., N. Kuring, C. Ng, W. Esaias, C. McClain, J. Elrod, N. Maynard, D. Endres, R. Evans, J. Brown, S. Walsh, M. Carle, and G. Podesta (1989):EOS, 70, 634-641. 5)Fukushima, H., M. Toratani, and T. Nakajima (1993): J. Photogram. Rem. Sens. Soc. Jp. 32(3), 34-42. 6)Fukushima, H., and M. Toratani (1997): J. Geophys. Res. (in press). 7)Gordon, H. R., D. K. Clark, J. W. Brown, O. B. Brown, R. H. Evans, and W. W. Broenkow (1983): Applied Optics, 22(1), 20-36. 8)Gordon, H. R., J. W. Brown, and R. E. Evans (1988a): Applied Optics, 27(5), 862-871. 9)Gordon, H. R., O. B. Brown, R. H. Evans, J. W. Brown, R. C. Smith, K. S. Baker, and D. K. Clark, (1988b): J. Geophys. Res., 93, 10909-10924. 10)Harashima, A. and Y. Kikuchi, (1990): EOS, 71, 314-315. 11)McClain, C. R., G. Fu, M. Darzi, and J. K. Firestone (1992): PC-SEAPAK User's Guide, Ver. 4.0, NASA Tech. Memo., 104557. 12)Mitchel, B. G. (1994): J. Geophys. Res., 99, C4, 7291-7292, and other authors' papers in Special Section in the same volume, "Ocean color from space: A Coastal Zone Color Scanner retrospective". 13)Mueller, J. L. (1988): Applied Optics, 27, 3, 438-440. 14)Obata, A., J. Ishizaka, M. Endoh (1996): J. Geophys. Res., 101, C9, 20657-20667. 15)Takagi M., H. Fukushima, I. Asanuma, and J. Ishizaka (1993): Northwestern Pacific Coastal Zone Color Scanner monthly composite, a CD-ROM publication. 16)Wessel, P., and W. H. F. Smith (1991): EOS, 72, 441.