Global Air Temperature and Precipitation: Regridded Monthly and Annual Climatologies
(Version 2.01)
reinterpolated and documented by
Cort J. Willmott, Kenji Matsuura and David R. Legates
(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
kenjisan@udel.edu
Archive (Version 2.01) created November 18, 1998
DATA SOURCES:
Legates and Willmott's (1990a and b) station records of monthly and annual mean
air temperature (T) and precipitation (P) were used to produce this archive.
The number of stations (and oceanic grid nodes) used was 24,941 for air
temperature, and 26,858 for precipitation, respectively.
SPATIAL INTERPOLATION:
Traditional reinterpolation 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
and precipitation 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 and precipitation fields. A more robust neighbor finding
algorithm, based on spherical distance, also was developed and used.
Incorporating station-height information, 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 air temperature (or 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 air
temperature or precipitation field.
ARCHIVE STRUCTURE:
|
air_temp.clim: |
Average monthly and annual 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 Air Temperature (deg C, Jan - Dec) |
12F7.1 |
| 15 |
101 - 107 |
Mean Annual Air Temperature |
F7.1 |
|
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 Monthly Air Temperature (deg C , Jan - Dec) |
12F7.1 |
| 15 |
101 - 107 |
Cross-validation errors for Mean Annual Air Temperature |
F7.1 |
|
air_temp_dem.clim: |
Average monthly and annual 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.grid file. |
|
air_temp_dem.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.grid.
|
|
precip.clim: |
Average 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 |
Monthly Precipitation (mm, Jan - Dec) |
12F7.1 |
| 15 |
101 - 107 |
Mean Annual Precipitation |
F7.1 |
|
precip.cve.clim: |
Cross-validation errors associated with average 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 Monthly Precipitation (mm, Jan - Dec) |
12F7.1 |
| 15 |
101 - 107 |
Cross-validation errors for Mean Annual Precipitation |
F7.1 |
|
precip_corrected.clim: |
Average monthly and annual gage-corrected precipitation
interpolated to a 0.5 by 0.5 degree grid resolution. The format of
each record is the same as for the precip.grid file.
|
|
precip_corrected.cve.clim: |
Cross-validation errors associated with monthly and
annual raingage corrected precipitation interpolated to a 0.5 by
0.5 degree grid resolution. The format of each record is the same
as for the precip.cve.grid file.
|
SELECTED REFERENCES:
Legates, D. R. and C. J. Willmott (1990a) Mean Seasonal and Spatial Variability
Global Surface Air Temperature. Theoretical and Applied Climatology
, 41, 11-21.
Legates, D. R. and C. J. Willmott(1990b) Mean Seasonal and Spatial Variability
in Gauge-Corrected, Global Precipitation. International Journal of Climatology,
10, 111-127.
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.