Read Me

	TOMS Version 8 May 22, 1996 Gridded O3 Data: 1978-1993 INTRODUCTION This 2 CD-ROM set contains the latest version (version 8) of ozone data from the TOMS (Total Ozone Mapping Spectrometer) instrument onboard the Nimbus 7 spacecraft. Data covering the entire Nimbus 7 TOMS lifetime, November 1, 1978 through May 6, 1993, are given as daily files of gridded data, as zonal means, and as monthly averages. Software is included to image the data. For scientific investigators, a similar CD-ROM containing TOMS surface reflectivity values is planned. A CD-ROM of version 8 data from the Meteor 3 TOMS instrument (August 1991 - December 1994) will be produced this summer. HARDWARE REQUIREMENTS For PC: The data on this CD-ROM are ASCII format and can be accessed by most systems. The image display software requires Microsoft Windows and a VGA or better graphics card that can support at least 256 colors. For best performance, a 486 or faster machine with a math co-processor is recommended. For Mac: Because the software is designed for a multi-platform environment, it has been written to the ISO 9660 Standard. All filenames are given in the 8.3 format. For best performance, a 68030 or faster machine is recommended. Graphic support for at least 256 colors is recommended. OVERVIEW OF THIS CD-ROM The first CD-ROM, designated OPT_004A, contains daily and monthly gridded data for the period November 1978 through 1987. The second CD-ROM, designated OPT_004B, contains similar data for 1988 through May 1993, along with daily and monthly zonal means, TOMS "overpass" data for 371 individual ground locations, and daily GIF images. The data are described in more detail in the DATA FILES section. Each CD-ROM contains a DOCUMENT subdirectory that has a full copy of the TOMS User's Guide in Adobe Acrobat format. Each also contains a SOFTWARE subdirectory with PC and MAC subdirectories containing platform-specific software to image or view the data. THE OZONE MEASUREMENT The Nimbus 7 spacecraft was in a south-to-north, sun-synchronous polar orbit so that it was always close to local noon/midnight below the spacecraft. Thus, ozone measurements were taken for the entire world every 24 hours. TOMS directly measures the ultraviolet sunlight scattered by the Earth's atmosphere. Total column ozone is inferred from the differential absorption of scattered sunlight in the ultraviolet range. Ozone is calculated by taking the ratio of two wavelengths (312 nm and 331 nm, for example), where one wavelength is strongly absorbed by ozone while the other is absorbed only weakly. The instrument has a 50 kilometer square field of view at the sub-satellite point. TOMS collects 35 measurements every 8 seconds as it scans right to left producing approximately 200,000 ozone measurements daily. These individual measurements vary typically between 100 and 650 Dobson Units (DU) and average about 300 DU. This is equivalent to a 3 mm (about a 10th of an inch) thick layer of pure ozone gas at NTP conditions. THE DATA FILES Gridded Daily: The individual TOMS measurements have been averaged into grid cells covering 1 degree of latitude by 1.25 degrees of longitude. The 180x288 ASCII data array contains data from 90S to 90N, from 180W to 180E. Each ozone value is a 3 digit integer (see sample). LECT is the local equator crossing time. Both CDs example: \y87\m8710\ga871008.n7t sample data: Day: 281 Oct 8, 1987 Production V70 NIMBUS-7/TOMS OZONE Asc LECT: 11:35 AM Longitudes: 288 bins centered on 179.375 W to 179.375 E (1.25 degree steps) Latitudes : 180 bins centered on 89.5 S to 89.5 N (1.00 degree steps) 144144144144144144144144144144144144144144144144144144144144144144144144145 145145145149149149149161161161161149149149149149149149149149149149149149149 149149149149149149149149149149142142142142143143143143143143143143143143143 143140140140140140140140140140140140140140140140140140140140140140140140140 140140140140140140140140140140140140140140140140140140140140148148148148148 148148148148148148148148148148148148148148148148148148148148148148148148148 148148147147147147147147147147145145145145143143143143143143143143143143143 143143143143143143143143143143143143143143143143143143143143143143143143143 143143143143143143143143153153153153153153153153153153153153153153153153161 161161161160160160160160160160160160160160160146146146146144144144144144144 144144144144144144144144144144144144144144144144144144144144144144144144144 144144144144144144144144144144144144144 lat = -89.5 Gridded Monthly Average: Data in the same format as the gridded daily, but averaged for each month. For each grid cell at least 20 days of data must be good for the average to be computed. Both CDs example: \y79\gm7903.n7t GIF Image: Each day of data has a corresponding GIF image. Each image is 640x480 pixels (full screen in standard VGA) at 256 colors. North polar, south polar, and Aitoff projections are shown. 2nd CD only example: \images\iy79\im7903\ni790301.gif Overpass Data: These files contain "overpass" data, the best match single TOMS observation each day, for 371 ground locations worldwide. (Dobson and other science sites, along with major cities) See README file in \overpass for list of sites. 2nd CD only example: \overpass\ntoms035.n7t Zonal Means: Daily and monthly averaged data in 5ø latitude zones. At least 75% of possible data must be present for the mean to be given. 2nd CD only example: \zonalmen\zonalmon.n7t example: \zonalmen\daily\zonal_79.n7t PROBLEMS WITH THE DATA Polar Night: Because TOMS measures ozone using scattered sunlight, it is not possible to measure ozone when there is no sun (in the polar regions in winter). Consequently, maps of the Antarctic ozone hole for August and September, for example, will always have areas of missing data due to polar night. Missing Data: During 1978/1979 the TOMS instrument was turned off periodically to conserve power, including a 5 day period (6/14-6/18) in June 1979. On many days, data were lost due to missing orbits or other problems. Edge Effects: At the transition between good and flagged or missing data, an "edge effect" will appear, consisting of a few pixels of apparently low ozone values. These are artifacts generated when the original data were interpolated to produce the stored image data. A second type of edge effect will be noticed in the polar plots consisting of apparent discontinuities in the ozone field. Since TOMS takes 24 hours to map the entire Earth, near the dateline the ozone measurements have been taken almost 24 hours apart. If the ozone is changing with time, this will produce the observed discontinuity. High Terrain: The ozone reported is total column ozone to the ground. Over high mountains (the Himalayas, the Andes) low ozone will be noticed relative to surrounding low terrain. This is not an error. SOFTWARE DESCRIPTION 1. LVIEW and JPEGVIEW Lview 3.1 is a freeware imaging program for Windows which can easily view GIF and JPG images. JpegView is a commonly used image viewer for the Macintosh. It can read JPG, GIF and other image formats. 2. THE VISUANALYZE LITE PROGRAM VisuAnalyze Lite is designed to provide the user with a means of viewing the daily ozone data. Once loaded, the program will display a window consisting of a pull down menu and three boxes: one each for the image, the data table and the site information selected by the cursor. The menu allows you to control the viewing process and has a number of controls over the screen image. North Orthographic, South Orthographic, and Hammer-Aitoff projections are supported. An overlay provides grid and continental maps for easy reference. Palette permits the user to select between different color patterns to aid in image analysis. VisuAnalyze Lite allows the opening of as many windows as memory permits. There are minor differences between the MAC and PC versions. The included version is limited to viewing TOMS data; for more information on the program or for information on obtaining the unlimited version contact: Bharat Thacker Research and Professional Services (RPS) 5711 Sarvis Avenue, Suite 510 Riverdale, Maryland 20737 USA (301) 699-7771 or (313) 278-7719 DIRECTORY STRUCTURE CDROM #1 - OPT_004A root \DOCUMENT (TOMS User's guide) \SOFTWARE (Source Code Templates) \MAC \ACROBAT (MAC Acrobat reader) \JPEGVIEW (MAC .GIF viewer) \VISANLZE (MAC imaging software) \PC \ACROBAT (PC Acrobat reader) \LVIEW (PC .GIF viewer) \VISANLZE (PC imaging software) \Yxx (Month average grid files) \Myymm (Daily grid files) CDROM #2 - OPT_004B root \DOCUMENT (TOMS User's guide) \SOFTWARE (identical to 1st CD) \IMAGES \Iyyy \IMyymm (Daily GIF images) \OVERPASS (by location number - see README file) \Yxx (Month average grid files) \Myymm (Daily grid files) \ZONALMEN (Monthly average zonal means) \DAILY (Daily average zonal means) DOCUMENTATION There are README files on the CD-ROM for all of the software under the appropriate directory. A complete copy of the TOMS User's Guide is on the CD-ROM in PDF Format. Adobe Acrobat readers are included in the subdirectories \SOFTWARE\PC\ACROBAT and \SOFTWARE\MAC\ACROBAT. Free Adobe Acrobat readers for other platforms can be found on the Adobe homepage at: http://www.adobe.com INTERNET SERVICES NASA/Goddard Space Flight Center's Laboratory for Atmospheres has a Web Site devoted to Atmospheric Science. This site contains ozone related information including aircraft missions, spacecraft, tropospheric data, atmospheric modeling, and more. The URL address of this site is: http://hyperion.gsfc.nasa.gov News about the TOMS data, the upcoming Earth Probe and ADEOS TOMS missions, and any software updates will appear on the TOMS homepage. For Internet access to the TOMS Home Page use URL address: http://jwocky.gsfc.nasa.gov Direct data access is available via an anonymous FTP account: ftp jwocky.gsfc.nasa.gov. logon: anonymous password: your e-mail address cd pub/nimbus7 CONTACTS Dr. Richard McPeters (science) NASA Goddard Space Flight Center, Code 916 Greenbelt, Maryland 20771 USA mcpeters@wrabbit.gsfc.nasa.gov (301) 286-3832 (301) 286-1754 (fax) Eric Beach (general) Hughes STX 7701 Greenbelt Road, M/S 301H Greenbelt, Maryland 20771 USA beach@qhearts.gsfc.nasa.gov (301) 286-4526 (301) 286-1754 (fax)