This simulation was forced with National Oceanic and Atmospheric Administration (NOAA)/Global Data Assimilation System (GDAS) atmospheric analysis fields (Derber et al., 1991), the disaggregated Global Precipitation Climatology Project (GPCP) V1.3 Daily Analysis precipitation fields (Adler et al., 2003 Huffman et al., 2001), and the Air Force Weather Agency’s AGRicultural METeorological modeling system (AGRMET) radiation fields. The GLDAS-2.1 simulation started on Januusing the conditions from the GLDAS-2.0 simulation. The GLDAS-2.1 products supersede their corresponding GLDAS-1 products. The GLDAS-2.1 data are archived and distributed in NetCDF format. The data product contains 36 land surface fields from January 2000 to present. The monthly data product was generated through temporal averaging of GLDAS-2.1 Noah 3-hourly data simulated with the Noah Model 3.6 in Land Information System (LIS) Version 7. This data product, reprocessed in January 2020, is GLDAS-2.1 Noah monthly 0.25 degree data from the main production stream and it is a replacement to its previous version. Once the GPCP Version 1.3 data become available, the GLDAS-2.1 data products are processed in the main production stream and are removed from the Early Products archive. Since the GPCP Version 1.3 data have a 3-4 month latency, the GLDAS-2.1 data products are first created without it, and are designated as Early Products (EPs), with about 1.5 month latency. GLDAS-2.1 data products are now available in two production streams: one stream is forced with combined forcing data including GPCP version 1.3 (the main production stream), and the other stream is processed without this forcing data (the early production stream). The choice of forcing data, as well as DA observation source, variable, and scheme, vary for different GLDAS-2.2 products. GLDAS-2.2 product suites use data assimilation (DA), whereas the GLDAS-2.0 and GLDAS-2.1 products are "open-loop" (i.e., no data assimilation). GLDAS-2.1 is forced with a combination of model and observation data from 2000 to present.
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GLDAS-2.0 is forced entirely with the Princeton meteorological forcing input data and provides a temporally consistent series from 1948 through 2014. NASA Global Land Data Assimilation System Version 2 (GLDAS-2) has three components: GLDAS-2.0, GLDAS-2.1, and GLDAS-2.2.