National Oceanic and Atmospheric Administration (NOAA), National Environmental Satellite, Data, and Information Service(NESDIS)
Return to Home Page
1. INTRODUCTION
This is Volume 2 of a two-CD-ROM set. It contains reregistered and ancillary data not discussed here. For information on the ancillary data, click above on "Return to Home Page" then scan down that page for links to documentation on specific ancillary data sets.
Volume 1 of this set contains a global 5-year top-of-the-atmosphere (TOA) clear-sky monthly climatology over land produced from the Advanced Very High Resolution Radiometer (AVHRR). This was done within the Global Vegetation Index (GVI) project in the Satellite Research Laboratory of the Office of Research and Applications, National Environmental Satellite Data and Information Service (NESDIS), National Oceanic and Atmospheric Administration (NOAA). The GVI project was sponsored by the NOAA Climate and Global Change Program for the purpose of developing improved operational GVI products that are useful for global climate studies.
The data on this CD-ROM are the third generation GVI D-level products described in the GVI User's Guide (Kidwell, 1994) and Gutman et al. (1995), the latter included on this CD. The products are given at 0.15o resolution in a rectangular latitude/ longitude (equal angle) projection. They include 5-year means and standard deviations for each month of visible (0.63 um) and near-infrared (0.85 um) reflectances (@1, @2), normalized difference vegetation index (NDVI=(@2-@1)/(@1+@2)), thermal IR (11 and 12 um) brightness temperatures (T4, T5), precipitable water index (PWI=T4-T5), and observational solar zenith angle.
A number of auxiliary files are included to facilitate data analysis. The satellite based files are the Quality Description (QD) files, an Auxiliary Mask (AM) file, and a 10'-resolution remapped version of NDVI. The non-satellite datasets contain soil and vegetation maps, topography, precipitation, etc. (see the content for details).
The QD files are monthly and include 5-year averages of the number of cloud-free observations (weeks), the biases of satellite scan angle, and the snow masks based on the visible reflectance and 11 um temperature data. The AM files are stationary (independent of the month) and contain land/sea tag, political boundaries and two surface type masks based on maximum (Vmax) and minimum (Vmin) annual NDVI values characterizing low annual variability in NDVI (Vmax-Vmin<0.2) for evergreen (Vmax>0.45) and deserts (Vmax<0.2). Descriptions of the QD and AM files are given in Appendix 1.
The data processing is briefly described below. Its full description is given in Gutman et al. (1995) and is provided on this CD-ROM in the directory "document". Programs to read the data on several platforms are provided in Appendix 2.
2. BRIEF DATA PROCESSING DESCRIPTION
The processing comprises three stages: B (weekly), C (monthly), and D (climatology) (see Gutman et al. 1995 for the nomenclature and data products generation).
The processing of weekly data (B3.0/W15 - B3.2/W15) was based on the GVI weekly composite raw data (counts) -- the second generation of GVI. The processing steps included: 1) conversion of counts in AVHRR thermal channels to T4 and T5 and correction for calibration non-linearity; 2) calculation of reflectances in the solar channels, @1 and @2, using the updated (Pathfinder) calibration, which accounted for sensor degradation, correction for Sun-Earth distance and normalization to overhead sun; 3) cloud/clear identification using T4-thresholds dependent upon month, region, and viewing angle. The NDVI=(@2-@1)/(@2+@1) and PWI=(T4-T5) were calculated at the B-level to retain accuracy. All B-product values were packed into 8 bits. Note that no atmospheric or angular corrections were applied to these data, hence no surface geophysical variables have been derived. This is yet to be done (see details in Gutman et al. 1995).
The monthly processing (B3.2/W15 - C3.2/M15) flow included the following steps: 1) QC flags were applied on a weekly basis, resulting in data gaps, 2) each quantity was averaged over one month (five weekly composites) for each map cell to partially fill in the data gaps and reduce some of the angular variability (the weeks that spill over into the next month were used for both the current and the previous months); 3) bi-linear spatial interpolation was applied to the missing data areas with persistent cloudiness in monthly averaged images; 4) 3x3 map cell running average smoothing was done to partially account for the imperfection of cloud screening, to filter out atmospheric and angular variabilities, and to compensate for different number of observations used in monthly averaging and for random spatial sampling from the original Global Area Coverage (GAC) data into the GVI map cells. The monthly time series (C3.2/M15) of all the aforementioned variables for each individual year of the period from April 1985 - September 1994 are available at NOAA National Climatic Data Center (NCDC; e-mail: satorder@ncdc.noaa.gov).
The 5-year TOA AVHRR climatology (D3.2/M15) on this CD has been developed from the C3.2/M15 monthly fields of TOA @1, @2, NDVI, T4, T5, and PWI using data from April 1985 to December 1987 (NOAA-9), and from January 1989 to March 1991 (NOAA-11).
3. DISC ORGANIZATION, CONTENT, NAMING CONVENTIONS
These CD-ROMs have several directories discussed below.
3.1 Directory "data"
This directory contains has four main subdirectories which make up the general structure of the CD-ROM, including "avhrrcli" (on Disc 1), "avhrrclrg" (on Disc 2), "ncillary" (on Disc 1), and "vector" (on Disc 2).
3.1.1 Subdirectory "avhrrcli"
The directory "avhrrcli" contains the main reason for this collection: the authors' data. These data are outlined below, and more fully described in documentation contained in the "/document/avhrrcli" directory (see below).
Directory "avhrrcli" contains subdirectories:"average", "standev", and "qualflag".
The subdirectories "average" and "standev" contain respectively the 5-year monthly means and standard deviations of AVHRR reflectances (@1 and @2) and temperatures (T4 and T5), NDVI, PWI, sensor scan angle (sca) and the solar zenith angle (sza). The 8-bit files can be converted to floating point values using:
Conversion Naming convention
@1(i) = 45.0 * i / 255.0 + 5.0 ch1
@2(i) = 35.0 * i / 255.0 + 15.0 ch2
T4(i) = 76.0 * i / 255.0 + 250.0 ch4
T5(i) = 76.0 * i / 255.0 + 250.0 ch5
ndvi(i) = 0.8 * i / 255.0 - 0.1 ndvi
pwi(i) = 7.0 * i / 255.0 - 2.0 pwi
sca(i) = 110.0 * i / 255.0 - 55.0 sca
sza(i) = 50.0 * i / 255.0 + 20.0 sza
r1(i) = 4.0 * i / 255.0 ch1
r2(i) = 4.0 * i / 255.0 ch2
r4(i) = 3.0 * i / 255.0 ch4
r5(i) = 3.0 * i / 255.0 ch5
rndvi(i) = 0.1 * i / 255.0 ndvi
rpwi(i) = 0.5 * i / 255.0 pwi
rsca(i) = 26.0 * i / 255.0 sca
rsza(i) = 8.0 * i / 255.0 sza
Subdirectory "qualflag" contains quality description flags, describing characteristics of the monthly composites. Appendix 1 below describes the categories of quality description flags.
3.1.2 Subdirectory "avhrrclrg"
This directory, contained on Disc 2, contains selected data from avhrrcli, re-registered to a 10-minute latitude-longitude grid. These data register with the data contained in "ncillary". This re-registration is performed for integrated study with the wide variety of ancillary data contained on Disc 2..
3.1.3 Subdirectory "ncillary"
The subdirectory "ncillary" (on Disc 2) contains information concerning soils, vegetation, etc. from the Global Ecosystems Database and TerrainBase produced by NGDC.
3.1.4 Subdirectory "vector"
The subdirectory "vector" contains the ancillary vector information about dlg, grass, IDRISI, and Spatial Data Transfer Standard (SDTS) format.
3.2 Directory "document"
This directory contains the documentation for the data. Its subdirectories describe all the databases used to assemble this CD-ROM. A description of the contents for each subdirectory can be found in the files aareadme.1st and aareadme.htm in the root directory.
3.3 Directory "software"
This directory contains three subdirectories: "freeform", "geovu", and "html". This directory exists on both Volume 1 and Volume 2. The "html" subdirectory is different on the two CDs, but these differences should not concern the user.
3.3.1 Subdirectory "freeform"
The directory "freeform" contains FreeForm utilities, which the user may find useful for format conversion and documentation for several types of data. Documentation is available in the \geovu\freeform\document directory. Current documentation is updated regularly at the Website of NOAA's National Geophysical Data Center. Currently, that website is http://www.ngdc.noaa.gov/seg/freeform/freeform.html.
3.3.2 Subdirectory "geovu"
The directory "geovu" contains GeoVu browse and visualization software, which has been adapted for use on these CD-ROMs. GeoVu is provided for PCs, Macintoshes, and for Sun and Silicon Graph- ics UNIX workstations. Documentation and sample data are also provided. Under the GeoVu directory for each hardware platform (for example, PC), there are installation instructions. Make sure that the gut_gvi1.men, gut_gvi2.men, and avhrrcli.lst files are copied from the CD to the directory you create on your com- puter for running GeoVu. For example, if you create a directory called c:\geovu on your PC to contain GeoVu software, and your CD-ROM is drive e, enter the commands:
copy e:\software\geovu\pc\gut_gvi1.men c:\geovu
copy e:\software\geovu\pc\gut_gvi2.men c:\geovu
copy e:\software\geovu\pc\avhrrcli.lst c:\geovu
to copy the files from the CD to your GeoVu software directory on your computer.
The geovu package is a general data access tool developed by NGDC to display grids and images, display and plot tabular data sets, and examine related documentation during the data selection process.
3.3.3 Subdirectory "html"
The directory "html" contains much of the "plastic Website" on the CD-ROMs. You may ignore this directory, unless you want to "sleuth" the design of the CD.
3.4 Naming conventions
For the averages and standard deviations the naming convention is: varmon.img, where "var" is the variable, defined above (var=ch1, ch2, ...; e.g., pwi is the precipitable water index), and "mon" is the first three letters of the month (e.g., mon=jan, feb, ...)
The "qualflag" directory contains 12 monthly QD files monqd.img, where "mon" is as above. The "qualflag" directory also contains an auxiliary mask (AM) file maskam.img which represents fields that do not change with month (see Appendix 1 for details).
ACKNOWLEDGMENTS
The GVI project has been supported by the NOAA Climate and Global Change Program. Without its continuous funding, the generation of the 5-year global AVHRR climatology over land would not have been possible. Critical reviewing by the NOAA program manager Dr. A. Gruber motivated this work. are acknowledged for the development of the processing and visualization system. Contributions in the project by J. Powers (NOAA/NESDIS), M. Halpert and C. Ropelewski (NOAA Climate Prediction Center), and P. Schultz, R. Hucek and L. Rukhovetz, D. Sullivan of Research Data Systems Corporation (RDC) are acknowledged. This work was accomplished when one of the authors (A.I.) held National Research Council Associateship at the Satellite Research Laboratory, NOAA/NESDIS, on leave from the Marine Hydrophysics Institute, Sevastopol, Ukraine. David Hastings, assisted by David Schoolcraft, edited these CD-ROMs.
The requested form of acknowledgment for use of the data on these
CD-ROMs is as follows:
Gutman, G., D. Tarpley, A. Ignatov, and S. Olson, 1995: Global
Monthly AVHRR climatology over land: Clear-sky top-of-the-
atmosphere variables. Ed. D. Hastings. United States Dept. of
Commerce, National Oceanic and Atmospheric Administration, National
Environmental Satellite, Data, and Information Service,
National Geophysical Data Center, Boulder, Co 80303, USA.
References
Gutman, G., D. Tarpley, A. Ignatov, and S. Olson, 1995: The
enhanced NOAA Global Land Dataset from the Advanced Very High
Resolution Radiometer. Bulletin of the American Meteorological
Society, 76, 1141-1156,
Kidwell, K., 1994: Global Vegetation Index User's Guide. U.S.
Dept. of Commerce, National Oceanic and Atmospheric Administration,
National Environmental Satellite, Data, and Information
Service, National Climatic Data Center, Satellite Data Services
Division, 126 pp.
Fig. 1: The organizational structure of CD-ROM.
Fig. 1 shows the NDVI 5-year climatology for July. Despite the
short 5-year base period, the areas with high interannual vegetation
variability are clearly depicted on the map of standard
deviations and are of particular interest to climatologists. The
areas in central Siberia, southeast Australia, and northeast
Brazil show high interannual NDVI variability, the latter being
attributed to droughts caused by El Nino Southern Oscillation
during the 5 years.
Fig. 2 shows the completion of the annual cycle of the mean NDVI
from Fig. 1. The areas in white (T4< 270 K) are classified as
stable snow and in grey (270 K <T4< 280 K) as transitional
(unstable) snow, both having @1 < 20 %.
Fig. 3 shows the July climatology of PWI, and @1, T4.
APPENDIX 1: QUALITY DESCRIPTION (QD) FILES
The 8-bit QD files were generated for each month to describe some of the inherent statistical characteristics. Each of the 8 bits is a 5-year average of the corresponding bit for each month of each individual year. The following table describes how each bit was assigned:
BIT NUMBER CRITERIA/COMMENT BIT VALUE ---------- ---------------- --------- 1 nobs = 0 or 1 (mostly cloudy) 1, else 0 2 nobs = 2 or 3 (moderately cloudy) 1, else 0 3 nobs = 4 or 5 (mostly clear) 1, else 0 4 |sca| < 20 deg (near nadir) 1, else 0 5 sca > 20 deg (forward scatter bias) 1, else 0 6 sca < -20 (back scatter bias) 1, else 0 7 T4 < 270K and @1>20% (stable snow) 1, else 0 8 270K<T4<280K and @1> 20% (unstable snow) 1, else 0
AUXILIARY MASKS (AM) FILE
An 8-bit AM file was generated for delineating land, political boundaries, and masking two surface types with low annual variability: evergreen vegetation and desert. The following table describes each bit:
BIT NUMBER CRITERIA/COMMENT BIT VALUE 1 land location (no coast used) 1, else 0 2 state borders,inland water 1, else 0 3 evergreen (Vmax-Vmin under 0.2 and Vmax over 0.45) 1, else 0 4 desert (Vmax-Vmin and Vmax both under 0.2) 1, else 0 5 blank 6 blank 7 blank 8 blank
where Vmax and Vmin are annual maximum and minimum NDVI,respectively.
APPENDIX 2: READING THIRD GENERATION GVI IMAGES I. INTRODUCTION
This CD-ROM contains global Top-of-the Atmopshere (TOA) images and products derived from Advanced Very High Resolution Radiometer (AVHRR). These products were developed on a Silicon Graphics Inc. (SGI) workstation using the C language as part of the Global Vegetation Index (GVI) project by the Climate Research and Applications Division (CRAD) Land Surface Team of the Office of Research and Applications (ORA). The data on this CD-ROM consists of D-Level products, which are described in the GVI Users Guide (Kidwell, 1994). In order to ensure easy portability across all platforms, the data was scaled to 8-bits or 1-byte. Each uncompressed image is made up of 2500 columns and 904 rows of data, with 0.144 degrees (nominally 0.15 degrees) latitude/longitude resolution, giving a total size of 2260000 bytes/image. The purpose of this section is to read the 2500 x 904 image.
Each image is read by rows, starting with row 1 at 75 N, and ending with row 904 at 55 S. Every row begins at 180 W longtiude, and each sucessive map cell would be 0.144 degrees east of the previous map cell. Therefore, the upper left corner of the image starts at 75 N, 180 W, and ends in the lower right corner of the image, at 55 S, 180 E. The physical range of the data is between 0 and 255, with all ocean values containing the value 0.
Since the binary imagery is character data, you cannot
browse through it. Howver, the character data can be easily
converted into integer values. The following is a list of
FORTRAN and C programs that can be used as a guide for reading
the files. Each of these programs will convert the character
data into integer values. Reading the 8-bit imagery is somewhat
platform specific. The coding is broken down by language and
platform.
NOTE: THE OUTPUT DATA IS BEING WRITTEN TO UNIT=6, THE SCREEN.
Return to Global AVHRR-Derived Climatologies Home Page
file://cdrom.html
Revised: 25 February 1997