North America Land Data Assimilation System (NLDAS)
The North American Land Data Assimilation System (NLDAS) provides precipitation, land-surface states (e.g., soil moisture and surface temperature), and fluxes (e.g., radiation and latent and sensible heat fluxes) by integrating observations from numerous sources combined with land-surface modeling (Mitchell et al., 2004). Phase 2 of NLDAS comprises hourly data from Jan 1979 to present (with a 2- to 5-day lag) at 1/8th-degree grid spacing over the contiguous United States and parts of Canada and Mexico.
NLDAS integrates a large quantity of observation-based and model reanalysis data to drive offline (not coupled to the atmosphere) land-surface models (LSMs), enabled by the Land Information System (LIS) (Kumar et al., 2006; Peters-Lidard et al., 2007). Observations of daily precipitation gauges are integrated with Doppler radar and CMORPH precipitation estimates to produce hourly precipitation values. NLDAS forcing drives four land-surface models: NASA's Mosaic, NOAA's Noah, OHD's SAC, and Princeton's implementation of VIC. Currently, only Mosaic model output is available via the GES DISC, with the others to be added in the coming year.
NLDAS Phase 2 is a collaboration project among several groups: NCEP Environmental Modeling Center (EMC), NASA's Goddard Space Flight Center (GSFC), Princeton University, the NWS Office of Hydrological Development (OHD), the University of Washington, and NCEP's Climate Prediction Center (CPC). NLDAS is a core project with support from NOAA's Climate Prediction Program for the Americas (CPPA). The NASA/GSFC group led the development of the algorithm to generate the forcing data and produced this data for the retrospective period (January 1979 - December 2007); this group also generated the retrospective Mosaic model simulation. The University of Washington and Princeton University developed the VIC model and the Princeton group generated the retrospective period VIC model simulation. NCEP/EMC, in collaboration with the University of Washington, made improvements to the Noah model; NCEP/EMC also generated retrospective period model simulations for Noah and OHD's SAC model. FTP access to the near real-time forcing data and output from the Noah, SAC, and VIC models can currently be accessed from NCEP EMC. In addition to FTP access, the forcing data and model output are also available through the GrADS Data Server (GDS) at the GES DISC.
More information is available at the Land Data Assimilation Systems (LDAS) and Land Information System (LIS) websites, as well as the NASA/GSFC's and NCEP/EMC's NLDAS drought websites. NLDAS drought monitoring products support the National Integrated Drought Information System (NIDIS).
Global Land Data Assimilation System (GLDAS)
The goal of the Global Land Data Assimilation System (GLDAS) is to generate optimal fields of land surface states and fluxes by integrating satellite- and ground-based observational data products, using advanced land surface modeling and data assimilation techniques (Rodell et al., 2004). GLDAS drives multiple, offline (not coupled to the atmosphere) land surface models, integrates a huge quantity of observation based data, and executes globally at high resolutions (2.5° to 1 km), enabled by the Land Information System (LIS) software package (Kumar et al., 2006).
A vegetation-based “tiling” approach is used to simulate sub-grid scale variability, with a 1 km global vegetation dataset as its basis. Soil and elevation parameters are derived from high resolution global datasets. Observation-based precipitation and downward radiation products and the best available analyses from atmospheric data assimilation systems are employed to force the models.
Intercomparison and validation of these products is being performed with the aim of identifying an optimal forcing scheme. Data assimilation techniques for incorporating satellite based hydrological products, including snow cover and water equivalent, soil moisture, surface temperature, and leaf area index, are now being tested and implemented. The output fields support several current and proposed weather and climate prediction, water resources applications, and water cycle investigations. GLDAS has resulted in a massive archive of modeled and observed, global, surface meteorological data, parameter maps, and output which includes 1° and 0.25° resolution 1979-present simulations of the Noah, CLM, Mosaic, and VIC land surface models. The project is funded by NASA's Energy and Water Cycle Study (NEWS) Initiative. More information is available at the Land Data Assimilation Systems (LDAS) and Land Information System (LIS) web sites.
Kumar, S. V., C. D. Peters-Lidard, Y. Tian, P. R. Houser, J. Geiger, S. Olden, L. Lighty, J. L. Eastman, B. Doty, P. Dirmeyer, J. Adams, K. Mitchell, E. F. Wood, and J. Sheffield, 2006: Land Information System - An interoperable framework for high resolution land surface modeling, Environ. Modelling and Software, 21, 1402-1415.
Mitchell, K.E., D. Lohmann, P.R. Houser, E.F. Wood, J.C. Schaake, A. Robock, B.A. Cosgrove, J. Sheffield, Q. Duan, L. Luo, R.W. Higgins, R.T. Pinker, J.D. Tarpley, D.P. Lettenmaier, C.H. Marshall, J.K. Entin, M. Pan, W. Shi, V. Koren, J. Meng, B.H. Ramsay, and A.A. Bailey, 2004: The multi-institution North American Land Data Assimilation System (NLDAS): Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system. J. Geophys. Res., 109, D07S90, doi:10.1029/2003JD003823.
Peters-Lidard, C.D., P.R. Houser, Y. Tian, S.V. Kumar, J. Geiger, S. Olden, L. Lighty, B. Doty, P. Dirmeyer, J. Adams, K. Mitchell, E.F. Wood and J. Sheffield, 2007: High-performance Earth system modeling with NASA/GSFC's Land Information System. Innov. Sys. and Soft. Eng., 3(3), 157-165.
Rodell, M., P. R. Houser, U. Jambor, J. Gottschalck, K. Mitchell, C.-J. Meng, K. Arsenault, B. Cosgrove, J. Radakovich, M. Bosilovich, J. K. Entin, J. P. Walker, D. Lohmann, and D. Toll, 2004: The Global Land Data Assimilation System. Bull. Amer. Meteor. Soc., 85 (3), 381–394.