Gridded Monthly Tropical Pacific Rainfall Estimates using the PACRAIN Database

The Pacific Rainfall (PACRAIN) Database is a searchable repository of in-situ rainfall observations initiated and maintained by the University of Oklahoma and supported by a research grant from the National Oceanic and Atmospheric Administration (NOAA) Climate Program Office/Climate Observation Division.  PACRAIN records originate from a variety of sources and are currently disseminated in individual files containing daily or monthly rainfall totals for fixed points.  It is well known that comparisons of areally averaged rainfall estimates (derived from climate model output, for example) to such point observations are subject to unnecessary sampling errors.

To provide an analysis and validation tool for areally averaged rainfall products for the tropical Pacific, monthly gridded estimates have been derived from PACRAIN records using standard geostatistical techniques (e.g., Kriging). These values are computed at sufficiently small horizontal scales that averaging them yields a numerical block estimate suitable for direct comparison with typical gridded output of a climate model (or other areally averaged product.)  The final output has been cross validated by generating estimates for, and comparing to, observations that are randomly selected and omitted from all aspects of the interpolation process.

A full description of the PACRAIN database and a full listing of its sources is given.  The procedure and associated FORTRAN code is explained in detail along with its attendant assumptions and data selection strategies.  The geographic coverage of PACRAIN grids is depicted graphically and some interesting monthly grids are shown.  The results of the gridded product’s cross validation are mapped, as are the error variance values, which are obtained in conjunction with the procedure itself.