Precipitation, mean monthly (IIASA climate database)

The IIASA Climate Database was created at the International Institute for Applied System Analyses (IIASA; Laxenburg, Austria) by Rik Leemans and Wolfgang P. Cramer to represent current global climate. There are three variables included in the Database: average monthly cloudiness, precipitation and temperature; and 12 monthly values per variable. These values were calculated from existing historical weather records of a diverse nature, but with the common feature that most cover at least five years during the period between 1930 and 1960. The weather records from up to eight different sources were standardized, ranked in quality, selected, interpolated and smoothed to fit a one-half degree (.5 ) latitude/longitude terrestrial grid surface (there are no values for non-land areas). The three variables have been treated in somewhat different fashion in their processing at GRID, as explained below.

The areas with the best data coverage are Europe, the USA, southern Canada, East Asia and Japan, while Africa and Australia have less complete coverage. High latitude, arid and mountainous zones exhibit the least coverage, especially Siberia, northern Canada, South America, China, Mongolia and the Tibetan Plateau. Despite certain data gaps and inconsistencies, the IIASA Climate Database is considered appropriate for use at least at regional scales and above, in various applications relating to agriculture, biogeography, ecology, geography and especially vegetation models. The full and proper reference to the Database is: Leemans, Rik and Wolfgang P. Cramer, 1991. The IIASA Database for Mean Monthly Values of Temperature, Preicipitation and Cloudiness of a Global Terrestrial Grid. IIASA, Laxenburg, Austria, RR-91-18, 62 pages.

The original IIASA Climate Database is distributed by Leemans and Cramer in tabular form (a series of ascii files, with binary conversion program) on diskettes. There are three tables (one for cloudiness, precipitation and temperature variables) each having a long series of data records with 14 values as follows: longitude, latitude, 12 monthly values (January to December). GRID-Geneva has converted these tables into separate monthly data files with a standard image format. That is, for each of the three variables/12 months there exists a 360-row (line, record) by 720-column (element, pixel, sample) array of values which can be manipulated as an image. The original data values have been preserved by storing them in four-byte real (floating point) or two-byte integer arrays, where the geographic location (center point) of each pixel is known. GRID has also produced simplified one-byte image arrays for all three variables' data files, which are generalized versions for portrayal on most image display systems, rather than being suitable for analysis.

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Precipitation Data Set

The IIASA mean monthly precipitation data set consists of nearly 6100 records of data derived from at least five existing sources (Leemans and Cramer, p. 12). The major global patterns can be clearly seen, and the data set is comparable in quality to a similar database used by the authors for evaluation of their results (see above, p. 15). Values are in terms of millimeters, and thus for many areas of the world during maximum periods, four-digit integers are required to express the amounts. The authors first attempted to apply a height correction based on an adiabatic lapse rate from a separate data set of altitudinal values. Because of its complicated nature, this gave unsatisfactory results and so was not implemented in the current data set.

The GRID versions of this data set include 12 monthly average precipita- tion data files in two-byte (16-bit) image format, to accomodate values above 255, and 12 one-byte (eight-bit) data files with precipitation values compressed into a 0-255 range. Both sets of data files have image arrays of 360 rows (lines, records) by 720 columns (elements, pixels, or samples), and cover the entire globe from 90 degrees North latitude and 180 degrees West longitude, to 90 degrees South latitude and 180 degrees East longitude. Non-land areas (the oceans) have a data value of 9999. The data files are in the Plate Carree (Simple Cylindrical) projection, a particular form of the Equirectangular. This projection is described in a book entitled "Map projections used by the U. S. Geological Survey, Geological Survey Bulletin 1532 (second ed.), U. S. Government Printing Office, Washington D.C., 1982" p. 89, or by request directly from GRID.