Terrestrial Air Temperature and Precipitation: Monthly Climatologies

(Version 4.01)

produced and documented by

Kenji Matsuura and Cort J. Willmott
(with support from IGES and NASA)

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 4.01) released September 2, 2009


 

DATA SOURCES:

Station records that served as bases for our Terrestrial Air Temperature: 1900-2008 Gridded Monthly Time Series (Version 2.01) and Terrestrial Precipitation: 1900-2008 Gridded Monthly Time Series (Version 2.01) archives are used here to help create new gridded climatologies of monthly and annual average air temperature (T) and total precipitation (P).  These two sets of station time series were drawn primarily from recent versions of the Global Historical Climatology Network (GHCN version 2) and the Global Surface Summary of Day (GSOD) archive.  Selected averages from Legates and Willmott’s (1990a and b) long-term station averages of monthly and annual T and P also were used to help produce this new gridded archive.

SPATIAL INTERPOLATION:

Using long-term-average climatologies available over a relatively dense network of stations can increase the accuracy of spatially interpolating shorter-term-average climate variables (Willmott et al. 1996).  The long-term-average climatology used here informs our interpolations through the application of climatologically aided interpolation (CAI) (Willmott and Robeson, 1995).  CAI is described briefly below.   Long-term monthly station averages for our spatially dense station networks of T and P (our long-term base or background climatologies) were estimated for all of the stations mentioned above which had at least 10 years of observations available within the entire period of record (1900-2008).  Long-term station averages available within the Legates and Willmott climatologies—which were not co-located with any station in the 1900-2008 dataset—were added to our background climatology network.  The total number of precipitation-station time averages included within our background climatology is 57,210 and there are 40,199 for air temperature. 

 

The shorter-term climate variables that we are interested in estimating are mean monthly T and P averaged over the period from 1975 through 2005.  For those stations that had at least 10 years of observations available for all months within this time period, time averages were evaluated for each of the 12 months; thus, some of our shorter-term station averages are based on less than 30 years of record.  The total number of air-temperature stations used is 12,857, while the number of precipitation stations is 17,721. 

 

CAI was employed to interpolate each shorter-term (1975-2005) average monthly air temperature or total precipitation field.  All interpolations were made to a 0.5 degree by 0.5 degree of latitude/longitude grid, where the grid nodes are centered on 0.25 degree.  CAI operates by first differencing the variable of interest (observed at each station within the shorter-term network of stations) from a long-term climatological average available at or interpolated to each shorter-term station location.  Note that our monthly background air-temperature climatologies were created using DEM-assisted interpolation (Willmott and Matsuura 1995) while our background monthly precipitation climatologies were traditionally interpolated.  Our ‘traditional’ interpolation algorithm is based on the spherical version of Shepard’s distance-weighting method (Shepard, 1968; Willmott et al., 1985).  For each field, traditional interpolation then was performed on the station differences to obtain a gridded difference field.  Finally, the gridded difference field was added to the estimates of the long-term climatology at the same grid points to obtain an estimated shorter-term gridded field. 

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. The time-averaged cross-validation errors then were interpolated to the grid nodes to create our climatologies of the cross-validation errors.

ARCHIVE STRUCTURE:

air_temp2009.clim.gz:

Average monthly and annual air temperature, representing the period 1975 - 2005, 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 - 112

Mean Monthly Air Temperature (deg C, Jan - Dec)

12F8.1

 

air_temp2009_cv.clim.gz:

Cross-validation errors associated with air_temp2009.clim. 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 - 112

Mean Cross-Validation Errors for Monthly Air Temperature (deg C, Jan - Dec)

12F8.1

 

precip2009.clim.gz

Average monthly and annual total precipitation, representing the period 1975 - 2005, 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 - 112

Average Monthly Total Precipitation (mm, Jan - Dec)

12F8.1

 

precip2009_cv.clim.gz:

Mean Cross-validation errors associated with precip2009.clim. 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 – 112

Mean Cross-Validation Errors for Monthly Total Precipitation (mm, Jan - Dec)

12F8.1

 

SELECTED REFERENCES:

Legates, D.R. and C.J. Willmott (1990a). Mean seasonal and spatial variability in 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.

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., S.M. Robeson and M.J. Janis (1996). Comparison of approaches for estimating time-averaged precipitation using data from the USA. International Journal of Climatology, 16, 1103-1115.