THE NCEP NCAR 40-YEAR REANALYSIS PROJECT

The NCEP and NCAR are cooperating in a project (denoted ''reanalysis'' ) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring commun ities. This effort involves the recovery of land surface, ship, rawins onde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is ke pt unchanged over the reanalysis period 1957-96. This eliminates perce ived climate jumps associated with changes in the data assimilation sy stem. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art gl obal data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global s ystem implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database ha s been enhanced with many sources of observations not available in rea l time for operations, provided by different countries and organizatio ns. The system has been designed with advanced quality control and mon itoring components, and can produce 1 mon of reanalysis per day on a G ray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a ''quick look'' CD -ROM (one per year) with six tropospheric and stratospheric fields ava ilable twice daily, as well as surface, top-of-the atmosphere, and ise ntropic fields. Reanalysis information and selected output is also ava ilable on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A spec ial CDROM, containing 13 years of selected observed, daily, monthly, a nd climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depend ing on the degree to which they are influenced by the observations and /or the model. For example, ''C'' variables (such as precipitation and surface fluxes) are completely determined by the model during the dat a assimilation and should be used with caution. Nevertheless, a compar ison of these variables with observations and with several climatologi es shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predi ctability studies and for monitoring the quality of the observing syst ems. The 40 years of reanalysis (1957-96) should be completed in early 1997. A continuation into the future through an identical Climate Dat a Assimilation System will allow researchers to reliably compare recen t anomalies with those in earlier decades. Since changes in the observ ing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the p eriods immediately after the introduction of new observing systems, su ch as new types of satellite data. NCEP plans currently call for an up dated reanalysis using a state-of-the-art system every five years or s o. The successive reanalyses will be greatly facilitated by the genera tion of the comprehensive database in the present reanalysis.