Monthly total-precipitation records for 8,818 independent, land-surface weather stations located north of 43 N were used to produce this gridded archive. Some 2336 station records were drawn from version 2 of the Global Historical Climatology Network (Peterson and Vose, 1998), while 6426 station records were obtained from the Atmospheric Environment Service/Environment Canada. Fifty-six more Russian station records were acquired through collaboration with the State Hydrometeorological Institute, St. Petersburg, Russia.

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 used.

Incorporating station climatologies, 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 (in the time-series network) was first differenced from monthly climatological averages available at or interpolated to the time-series station locations. Traditional interpolation then was performed on the station differences to obtain a gridded difference field. Finally, the gridded difference field was added to interpolated estimates of climatology at the same grid points.

To indicate (roughly) the spatial interpolation errors, station-by-station cross validation was employed (Willmott and Matsuura, 1995). One station was removed at a time, and precipitation was 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 was made, the removed station was 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 gridded precipitation field.

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

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., 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.