South American Precipitation:
1960-1990 Gridded Monthly Time Series

(Version 1.02)

interpolated and documented by

Scott R. Webber and Cort J. Willmott
(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 27, 1998

STATION DATA SOURCE:

Monthly-total precipitation (P) for 4,505 stations from Webber and Willmott's (1998) "South American Precipitation: Station Records Archive (version 1.02)" were used to produce this archive. Work on Webber and Willmott's (1998) station- record data set 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.

Incorporating a climatology, from a relatively-dense network of stations, can further increase the accuracy of spatially interpolating time series of monthly climate variables (Willmott and Robeson, 1995). Climatologically-aided interpolation (CAI) of monthly precipitation, therefore, was employed. Briefly, monthly precipitation at each station (from the time series) is first differenced from monthly climatological averages available at or interpolated to the station locations. Traditional interpolation then is performed on the station differences to obtain a gridded difference field. Finally, the gridded difference field is added to traditionally interpolated estimates of climatology at the same grid points.

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.trad.ts.tar:


     Monthly precipitation totals for the years 1960-90 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)


precip.trad.cve.ts.tar:

     Cross-validation errors associated with monthly
     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)


precip.cai.ts.tar:

     Monthly precipitation for the years 1960-90 interpolated 
     with Willmott and Robeson's (1994) Climatologically-aided
     algorithm to a 0.5 by 0.5 degree grid resolution.  The
     format for each record is the same as for the
     precip.trad.ts.tar files.


precip.cai.cve.ts.tar:

     Climatologically-aided precipitation cross-validation errors
     for the years 1960-90 interpolated to a 0.5 by 0.5 degree grid
     resolution.  The format of each record is the same as for
     the precip.trad.cve.ts.tar files.


SELECTED REFERENCES:

Shepard, D. (1968) A two-dimensional Interpolation function for irregularly-spaced Data. Proceedings, 1968 ACM National Conference, 517-523.

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

Willmott, C. J. and S. M. Robeson (1995) Climatologically Aided Interpolation (CAI) of Terrestrial Air Temperature. International Journal of Climatology, 15, 221-229.

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., 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, 12, 5-16.