Global AVHRR-Derived Land Climatology on CD-ROM

by Garik Gutman, Dan Tarpley, Alexandr Ignatov, and Steve Olson

Volume editors David Hastings and David Schoolcraft

We are pleased to announce an advancement in representing climatological information derived from NOAA's Advanced Very High Resolution Radiometer (AVHRR) sensor. A set of two CD-ROMs contains:

1. Monthly mean values of:
   • Calibrated Normalized Difference Vegetation Index (NDVI) from AVHRR derived as (Channel2Reflectance- Channel1Reflectance)/(Channel2Reflectance+Channel1Reflectance),
   • Precipitable Water Index (PWI) from AVHRR derived as (Channel4Temperature-Channel5Temperature),
   • Reflectances from AVHRR Channels 1 & 2,
   • Temperatures from AVHRR Channels 4 & 5,
   • Sensor Scan Angle,
   • Solar Zenith Angle (above the horizon),
2. Standard deviations of these parameters,
3. Quality description flags show factors which may affect reliability of the data as a function of location and time.
4. Ancillary environmental data (such as soils, vegetation, ecosystems, topography and climate) from NGDC.

The primary data (the first three items above) are valuable for many uses. They were derived from operational NOAA data by the authors. The ancillary data (item 4 above) provide additional opportunity for multidisciplinary comparisons. They were developed by many scientists, cited in the documentation of the CD-ROM.

Data compilation

Each parameter (such as NDVI) is presented as a “climatological” mean for each calendar month. Thus there is a mean NDVI for January (and for each other calendar month), based on all selected NDVIs that met quality control criteria for dates in that month selected as follows:

Input data were NOAA's operational Global Vegetation Index weekly Plate Carree grids for AVHRR Channels 1, 2, 4, and 5, sensor scan angle, and solar zenith angle. (AVHRR Channel 3 is often omitted, because of its spectral and noise characteristics.) Several corrections and quality control checks were made to the data. From the channel data NDVI and PWI were computed. The data were then scaled to 8-bit values. Information to rescale the data to their original units is provided on the CD-ROM.

Each parameter was initially computed for weekly periods during April 1985 through September 1994. Of this period April 1985 through December 1987 and January 1989 through March 1991 were determined to be the most reliable, being relatively early in the lives of satellites NOAA-9 and NOAA-11 respectively. Early in the lives of NOAA Polar Orbiting Environmental Satellites, sensors and orbital timings have had relatively little time to degrade. The means were thus computed for the 60 months just noted. Standard deviations were computed for the same periods, to demonstrate the spatial-temporal variability of each parameter over the study period. Separate quality description flags note such factors as snow-water-desert-forest cover, effects of cloudiness on the availability of clear observations, forward- or back-scatter effects on significantly off-nadir observations, etc.

Accessing the Data

The CD-ROMs are organized to facilitate:

• Quick browsing of data and documentation using the World Wide Web browser of the user's choice. The CD-ROMs are a form of “plastic Website.”
• Quick porting to various types of software, including image processing and geographic information systems (GIS).