South American Precipitation:
Gridded Monthly and Annual Climatologies

(Version 1.02)

interpolated and documented by

Cort J. Willmott and Scott R. Webber
(with support from NASA's Seasonal to Interannual ESIP)

For additional information concerning this archive,
please contact us at:

Center for Climatic Research
Department of Geography
University of Delaware
Newark, DE 19716
(302) 831-2294

or

webber@udel.edu


Archive (Version 1.02) created October 28, 1998

STATION DATA SOURCE:

Climatic means of monthly- and annual-total precipitation for 5,315 stations from Willmott and Webber's (1998) "South American Monthly Precipitation: Station Climatology Archive (version 1.02)" were used to produce this archive. Work on Willmott and Webber's (1998) station climatology is ongoing, and it is unavailable for distribution at this time.

SPATIAL INTERPOLATION:

Traditional interpolation was accomplished with the spherical version of Shepard's algorithm, which employs an enhanced distance-weighting method (Shepard, 1968; Willmott et al., 1985). Station precipitation values were interpolated to a 0.5 degree by 0.5 degree of latitude/longitude grid, where the grid nodes are centered on 0.25 degree. The number of nearby stations that influence a grid-node estimate was increased to an average of 20, from an average of 7 in earlier applications. This resulted in smaller cross-validation errors (see below) and visually more realistic precipitation fields. A more robust neighbor finding algorithm, based on spherical distance, also was developed and used.

SPATIAL CROSS VALIDATION:

To indicate (roughly) the spatial interpolation errors, station-by-station cross validation was employed (Willmott and Matsuura, 1995). One station is removed at a time, and precipitation is then interpolated to the removed station location from the surrounding nearby stations. The difference between the real station value and the interpolated value is a local estimate of interpolation error. After each station cross validation is made, the removed station is put back into the network. To reduce network biases on cross-validation results, absolute values of the errors at the stations were interpolated to the same spatial resolution as the precipitation field.

ARCHIVE STRUCTURE:

precip.clim:

     Monthly and annual precipitation totals interpolated to a
     0.5 by 0.5 degree grid resolution.  The format of each
     record is:

     Field     Columns   Variable                 Fortran Format

     1         1-8       Longitude (decimal degrees)   F8.3
     2         9-16      Latitude (decimal degrees)    F8.3
     3-14      17-100    Monthly Precipitation (mm,    12F7.1
                         Jan-Dec)
     15        101-107   Annual Precipitation (mm)     F7.1


precip.cve.clim:

     Cross-validation errors associated with monthly and annual
     precipitation interpolated to a 0.5 by 0.5 degree grid
     resolution.  The format of each record is:

     Field     Columns   Variable                 Fortran Format

     1         1-8       Longitude (decimal degrees)   F8.3
     2         9-16      Latitude (decimal degrees)    F8.3
     3-14      17-100    Cross-validation errors for   12F7.1
                         Monthly Precipitation (mm, Jan-Dec)
     15        101-107   Cross-validation errors for   F7.1
                         Annual Precipitation (mm)

SELECTED REFERENCES:

Shepard, D. (1968) A two-dimensional Interpolation function for irregularly-spaced Data. Proceedings, 1968 ACM National Conference7-523.

Willmott, C. J., C. M. Rowe and W. D. Philpot (1985) Small-Scale Climate Maps: A Sensitivity Analysis of Some Common Assumptions Associated with Grid-point Interpolation and Contouring. American Cartographer, 5-16.

Willmott, C. J. and K. Matsuura (1995) Smart Interpolation of Annually Averaged Air Temperature in the United States. Journal of Applied Meteorology, 34, 2577-2586.

Willmott, C. J. and S. R. Webber (1998) South American Precipitation: Station Climatology Archive (internal document).