South American Air Temperature:
Gridded Monthly and Annual Climatologies

(Version 1.01)

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 1.01) created November 24, 1998

STATION DATA SOURCES:

Monthly and annual climatic-mean air temperatures were calculated for 348 stations from version 2 of the Global Historical Climatology Network (Peterson and Vose, 1998). These climatic means were then merged with 931 climatic station means drawn from Legates and Willmott's (1990) monthly and annual air-temperature climatology archive.

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 averages of air temperature 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 air-temperature fields. A more robust neighbor finding algorithm, based on spherical distance, also was developed and used.

Incorporating elevational influences, through an average air-temperature lapse rate, can further increase the accuracy of spatially interpolating average air temperature (Willmott and Matsuura, 1995). Digital-elevation-model- or DEM-assisted interpolation of air temperature, therefore, was employed. Briefly, station air temperature is first "brought down" to sea level at the average environmental lapse rate (6.5 deg C/km). Traditional interpolation is performed on the adjusted-to-sea-level station air temperatures. Then, the gridded sea-level air temperatures are brought up to the DEM-grid height, again, at the average environmental lapse rate.

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 the air temperature 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 air temperature field.

ARCHIVE STRUCTURE:

air_temp.clim:

     Monthly- and annual-mean air temperature interpolated to a
     0.5 by 0.5 degree grid resolution (centered on 0.25 degree). 
     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-Mean Air Temperature  12F7.1
                         (deg C, Jan-Dec)
     15        101-107   Annual-Mean Air Temperature   F7.1
                         (deg C)


air_temp.cve.clim:

     Cross-validation errors associated with air temperatures
     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 Temperature (deg C, Jan-Dec)
     15        101-107   Cross-validation errors for   F7.1
                         Annual Air Temperature (deg C)


air_temp.dai.clim:

     Monthly- and annual-mean air temperature interpolated with
     Willmott and Matsuura's (1995) DEM-assisted algorithm to a
     0.5 by 0.5 degree grid resolution.  The format for each
     record is the same as for the air_temp.clim file.


air_temp.dai.cve.clim:

     DEM-assisted air-temperature cross-validation errors
     interpolated to a 0.5 by 0.5 degree grid resolution.  The
     format of each record is the same as for the
     air_temp.cve.clim file.

SELECTED REFERENCES:

Legates, D. R. and C. J. Willmott (1990) Mean Seasonal and Spatial Variability Global Surface Air Temperature. Theoretical and Applied Climatology, 41, 11-21.

Peterson, T. C. and R. S. Vose (1997) An Overview of the Global Historical Climatology Network Temperature Database. Bulletin of the American Meteorological Society, 78, 2837-2849.

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

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, 12, 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.