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The NCEP/NCAR Reanalysis Project is a joint project between the National Centers for Environmental Prediction (NCEP, formerly "NMC") and the National Center for Atmospheric Research (NCAR). The goal of this joint effort is to produce new atmospheric analyses using historical data (1948 onwards) and as well to produce analyses of the current atmospheric state (Climate Data Assimilation System, CDAS). The Laboratoire de Météorologie Dynamique (Paris, France) is mirroring the Web site; the present service benefits from this effort.

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The Web site of the project

2001 BAMS Paper (Bulletin of the American Meteorological Society)

I have not yet finished to collect precise information on accuracy. Assessment is a very difficult task because there is no well established reference of this kind. The report WCRP # 112 "intercomparison and validation of ocean-atmosphere energy flux fileds" stress this difficulty. It concludes that NCEP/NCAR products has substantial errors in surface radiation fields, reflecting problems both with regard to clouds and clear sky radiation. Comparisons were made with the Satellite Surface Radiation Budget (SRB) products; the latter may have a positive bias but offer more realistic patterns. For most latitudes, the RMSE between the NCEP/NCAR re-analysis and the SRB is more than 20 W/m2 for the monthly means of the net shortwave flux (nswrs). As for downward shortwave flux (dswrf), the NCEP/NCAR is 10 % higher.

New algorithms were devised to improve the accuracy of the estimates of the shortwave (SW) flux but at the expenses of the estimates of the longwave flux (LW) (see Figure). Precipitation estimate was improved, too.