Data Atmosphere GNV180

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The NASA/Total Ozone Mapping Spectrometer (TOMS) Ozone Image Data 1978- 1991 CD-ROM product was prepared by researchers at the NASA/Goddard Space Flight Center (GSFC) in Greenbelt, Maryland USA using Version 6.0 data provided by the Goddard Ozone Processing Team. The CD-ROM product is dated July 1991 and contains nearly 12.5 years of daily and monthly NASA/TOMS data, as well as software to read and display the data, color palette files and boundary map files for overlay on the ozone images.

The Nimbus-7 platform follows a sun-synchronous polar orbit to measure total in-column ozone over the entire world for every 24 hour-period. This ozone measurement is inferred from differential absorption of scattered sunlight in the ultraviolet range, using a ratio of two wavelengths (e.g. 312 and 331 nm.) to perform the calculation. While the TOMS instrument takes measurements within a 50 km.-square IFOV, the data provided on the CD-ROM have been resampled to a global grid (cylindrical equidistant projection) of one degree latitude by 1.25 degrees longitude. There are also two Northern and Southern polar hemispherical views provided in an orthographic map projection. All of these are byte images containing a range of values from 16 to 191, which correspond to scaled Dobson units (DUs) of 100 to 650, DU being a layer of thickness in the atmosphere by which ozone is measured.

GRID has extracted individual images for the Global and two Polar Hemispherical views on a monthly basis for the entire time period covered by the CD-ROM product, that is Nov. 1978 through March 1991 (see below). The resulting arrays are 190 rows (lines, records) by 320 columns (elements, pixels, samples) for the Global views, and 156 rows/columns square for the Polar Hemispherical views. Any time sequence within the available period can be supplied on demand, for one, two or all three of the geographic areas in question. While a single year of (12) Global arrays comprises 730 Kbs., one year of (12) North or South Polar Hemispherical arrays comprises 292 Kbs.

The major use of the NASA/TOMS Ozone monthly image data is for the purpose of visualization rather than analysis, although NASA does consult the CD-ROM product first when questions arise concerning possible features in the ozone field (Dr. Rich McPeters, personal communication). The CD-ROM product and the GRID-extracted arrays are particularly useful for comparative viewing of ozone levels through time. However, there are some problems with these image data which prospective users should be aware of. One is that "edge effects" exist at transition zones where areas of diminishing sunlight affect the data values, and also as a result of the interpolation done on the original data to produce the resampled images. Another is that apparent discontinuities (or two-row shifts) in the last four pixels or columns of data are observable at the edge of many global arrays, but this problem has not been scientifically confirmed/documented.

The original source of the NASA/TOMS Ozone monthly image data (CD-ROM product for 1978-91) is as follows: National Space Science Data Ctr.; Code 933; NASA/GSFC; Greenbelt, MD 20771 USA. The scientific contacts for the product are Drs. Richard McPeters, Arlin Krueger and Patricia Guimaraes, all of NASA/GSFC Code 916. Dr. McPeters in particular has been instrumental in replying to GRID questions concerning these data. The reference for the CD-ROM product is the booklet of the same name which accompanies it; a better scientific reference for the NASA/TOMS Ozone data is as follows: Fleig, Albert J., A. J. Krueger, P. K. Bhatia et al. 1988. "Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) User's Guide", NASA Reference Publication 0000 (Draft), NASA/Goddard Space Flight Center, 80 pages.

The NASA/TOMS Ozone Monthly data are arranged as follows on the CD-ROM image data product. The TOMS Monthly image data files are found under the directory "MONTHLY", within a series of annual sub-directories "Yyy", where "yy" is the year in two digits. An annual sub-directory contains 12 monthly average ozone images, and within each of these are found the Global and two Polar Hemispherical views in a single 360 row by 340 column array (122 kb). Within each monthly average image, the three sub-images are found in the following row/column coordinate locations:

Southern Hemisphere image = rows 4 to 159 (total of 156 rows); (upper left) cols. 3 to 158 (total of 156 columns). Northern Hemisphere image = rows 4 to 159 (total of 156 rows); (upper right) cols. 183 to 338 (total of 156 columns). Global lat./long. image = rows 171 to 360 (total of 190 rows); (lower half) cols. 11 to 330 (total of 320 columns).

In other words, the following rows and columns are ALWAYS empty of data: rows 1 to 3, and 160 to 170; and columns 1 & 2, and 339-340.

Depending on the month (season), rows 171 to 195 may or may not be occupied by data, as well as rows 336 to 360. This is related to difficulties in winter-time data acquisition over the Northern and Southern hemispheres, respectively, as the TOMS instrument cannot measure ozone in the absence of sunlight. The same applies in terms of the centers of the hemispherical halves (North and South) images, which take on the appearance of "donuts" during the winter months.

The following computer processing steps were used to extract the data. First, the Monthly image data files were FTPed to the IBM 4381 mainframe computer. There the CMS routine 'PICK' was used to extract the three data subsets (global, northern and southern hemispheres) from each of the monthly average data files, using the following series of commands:

PICK Myrmo IMG A OZyrmonG/N/S IAX1 A (input/output file names) (FROMCOL 11 FROMROW 171 TOCOL 330 TOROW 360 for Global image. (FROMCOL 183 FROMROW 4 TOCOL 338 TOROW 159 for N. Polar image. (FROMCOL 3 FROMROW 4 TOCOL 158 TOROW 159 for S. Polar image.

The first file name & type represent the original input Monthly image data file from the CD-ROM, and the second file name & type represent the output image on the IBM 4381 ('G' standing for "global", 'N' for "Northern" hemisphere and 'S' for "Southern" hemisphere in each case. The fromcol, fromrow, tocol and torow parameters show the subsets extracted repetitively for all years/months of data available.

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