Description

This data set contains the forcing data for Phase 1 of the North American Land Data Assimilation System (NLDAS-1). The data are in 1/8th degree grid spacing and range from 29 Sep 1996 to 31 Dec 2007. The temporal resolution is hourly. The file format is WMO GRIB-1. The chief source of NLDAS-1 forcing is NCEP's Eta model-based Data Assimilation System (EDAS) [Rogers et al., 1995], a continuously cycled North American 4DDA system. It utilizes 3-hourly analysis-forecast cycles to derive atmospheric states by assimilating many types of observations, including station observations of surface pressure and screen-level atmospheric temperature, humidity, and U and V wind components. EDAS 3-hourly fields of the latter five variables plus surface downward shortwave and longwave radiation and total and convective precipitation are provided on a 40-km grid, and then interpolated spatially to the NLDAS grid and temporally to one hour. Last, to account for NLDAS versus EDAS surface-elevation differences, a terrain-height adjustment is applied to the air temperature and surface pressure using a standard lapse rate (6.5 K/km), then to specific humidity (keeping original relative humidity) and downward longwave radiation (for new air temperature, specific humidity). The details of the spatial interpolation, temporal disaggregation, and vertical adjustment are presented by Cosgrove et al. (2003). GOES-based solar insolation (Pinker et al., 2003) provides the primary insolation forcing (shorwave down at the surface) for NLDAS-1. GOES insolation is not retrieved for zenith angles below 75 degrees and so is supplemented with EDAS insolation near the day/night terminator. Last from the GOES-based product suite, Photosynthetically Active Radiation (PAR) and surface brightness temperature fields are included in the NLDAS-1 forcing files. NLDAS-1 precipitation forcing over CONUS is anchored to NCEP's 1/4th degree gauge-only daily precipitation analyses of Higgins et al. [2000]. In NLDAS-1, this daily analysis is interpolated to 1/8th degree, then temporally disaggregated to hourly values by applying hourly weights derived from hourly, 4-km, radar-based (WSR-88D) precipitation fields. The latter radar-based fields are used only to derive disaggregation weights and do not change the daily total precipitation. Last, convective precipitation is estimated by multiplying NLDAS-1 total precipitation by the ratio of EDAS convective to EDAS total precipitation. The Convective Available Potential Energy (CAPE) is the final variable in the forcing data set, also interpolated from EDAS. The data set applies a user-defined parameter table to indicate the contents and parameter number. The GRIBTAB file (http://disc.sci.gsfc.nasa.gov/hydrology/grib_tabs/gribtab_NLDAS_FORA_hourly.001.txt) shows a list of parameters for this data set, along with their Product Definition Section (PDS) IDs and units. For more information, please see the README Document at ftp://hydro1.sci.gsfc.nasa.gov/data/s4pa/NLDAS/README.NLDAS1.pdf.

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