Please also check the LDAS FAQ at the NASA Land Data Assimilation Systems for questions/answers regarding LDAS data products.
What are the GLDAS products?
The Global Land Data Assimilation System (GLDAS) data products consist of time series of land surface state (e.g., soil moisture and surface temperature) and flux (e.g., evaporation and sensible heat flux) parameters simulated by four land surface models (CLM, Mosaic, Noah and VIC). The 1.0 degree resolution data range from 1979 to present for the four models. The 0.25 degree data cover 2000 to present for the NOAH model. For more information about GLDAS, see http://ldas.gsfc.nasa.gov/.
What are the differences among LDAS, NLDAS, and GLDAS?
The Land Data Assimilation Systems (LDAS) are being developed to enable more accurate reanalysis and forecast simulations by numerical weather prediction (NWP) models. Specifically, these systems will reduce the errors in the stores of soil moisture and energy which are often present in NWP models and which degrade the accuracy of forecasts. The LDAS are being implemented at 1/8 degree (about 15 kilometer) resolution across North America (NLDAS) and at 1/4 degree resolution globally (GLDAS). The systems are currently forced by terrestrial (NLDAS) and space-based (GLDAS) precipitation data, space-based radiation data, and numerical model output. NLDAS is currently running retrospectively and in near real-time. For more information about LDAS, see http://ldas.gsfc.nasa.gov.
What is the GRIB format? How can I interpret it?
The GRIB (GRIdded Binary) format is a mathematically concise data format commonly used in meteorology to store historical and forecasted weather data. For more details about the GRIB format, please see http://www.nco.ncep.noaa.gov/pmb/docs/on388/
WGRIB, GrADS, or other GRIB readers are required for reading the GLDAS data. WGRIB is a program to manipulate, inventory, and decode GRIB files. The source code and installation instructions for WGRIB are available from
http://www.cpc.ncep.noaa.gov/products/wesley/wgrib.html
The Grid Analysis and Display System (GrADS) is an interactive desktop tool for easy access, manipulation, and visualization of earth science data. The format of the data may be either binary, GRIB, NetCDF, or HDF. The documentation and software for GrADS can be found at http://grads.iges.org/grads/
Why should I use GRIBTAB files to decode the GRIB files?
The GLDAS products were created using the GRIB format, but a specific parameter table was used in the encoding process. To read the GLDAS GRIB data, it is necessary to download the GRIBTAB and set the environmental variables first before using any GRIB readers! For more information, please see ftp://hydro1.sci.gsfc.nasa.gov/data/s4pa/GLDAS_V1/README.GLDAS.pdf.
What are the land surface models used in GLDAS?
Currently, GLDAS drives four land surface models: Mosaic, Noah, the Community Land Model (CLM), and the Variable Infiltration Capacity (VIC). More information about the models is available at the Land Data Assimilation Systems (LDAS) and Land Information System (LIS) Web sites.
Can I obtain the source codes for the land surface models?
Yes. The source codes for the land surface models can be obtained from the Land Information System (LIS) website. The LIS source code currently includes 3 land surface models: CLM, Noah and VIC.
Can I obtain the forcing dataset that was used in GLDAS?
Forcing data are atmospheric inputs to the land surface models, including precipitation, radiation, and surface wind, temperature, pressure and humidity. They can be obtained from http://ldas.gsfc.nasa.gov/gldas/GLDASforcing.php.
Can I obtain the land surface parameter data sets used in GLDAS?
Land surface parameters are properties of the land surface (e.g., soil, land cover, and topography) that change at a time step of a day or longer. They can be obtained from http://ldas.gsfc.nasa.gov/gldas/GLDASvegetation.php .
What is the GrADS data server?
The GrADS Data Server (GDS, formerly known as the GrADS-DODS Server) is a stable, secure data server that provides subsetting and analysis services across the Internet. The core of the GDS is OPeNDAP, a software framework used for data networking which makes local data accessible to remote locations. GDS subsetting capability allows users to retrieve a specified temporal and/or spatial subdomain from a large dataset, eliminating the need to download everything simply to access a small relevant portion of a dataset. The GDS analysis capability allows users to retrieve the results of an operation applied to one or more datasets on the server. Examples of analysis operations include basic math functions, averages, smoothing, differencing, correlation, and regression; the GDS supports any operation that can be expressed in a single GrADS expression.
Can you give an example of retrieving GLDAS data via GDS?
Users can retrieve GLDAS data from a GDS server using analysis tools such as GrADS, Ferret, Matlab, or IDL. Below is an example of a GrADS script to access the GDS server and draw the "Layer 10 soil moisture" (soilmoist10) parameter from the CLM model.
'reinit'
'sdfopen http://hydro1.sci.gsfc.nasa.gov/dods/GLDAS_CLM10SUBP_3H'
'set lon -180 180'
'set lat -60 90'
'set gxout grfill'
'set grads off'
'd soilm10'
'run cbarn'
'draw title GLDAS CLM 3-Hourly 1.0 degree Average Layer 10 Soil Moisture\ on Jan 2, 1979 at 00Z[k]'
How to retrieve GLDAS and NLDAS data via GDS as ASCII text?
The GDS can provide subsets of any data set it serves, in ASCII comma-delimited format. To retrieve a subset, enter a URL of the form http://gds-base-url/dataset.ascii?constraint.
The constraint portion of the URL should be an OPeNDAP constraint expression. Here are some basic constraints:
- A constraint of the form "var" will request the complete contents of the variable.
- A constraint of the form "var[a:b]" will return the subset of the variable defined by a and b.
- A constraint of the form "var[a:n:b]" will return every n th element of the subset defined by a and b, starting with element a.
For subsets of variables with multiple dimensions, each dimension must have a constraint. So a constraint for a subset of a three-dimensional variable would appear as var[a1:b1][a2:b2][a3:b3] or var[a1:n1:b1][a2:n2:b2][a3:n3:b3].
For more information, please visit GrADS-DODS Server - User's GuideGDS.
1. An example for GLDAS (An example for NLDAS is also provided below.)
Here is an example for retrieving GLDAS NOAH Model 0.25 Degree 3-Hourly Data, subsetted by parameter and geo-region, as ASCII text, http://hydro1.sci.gsfc.nasa.gov/dods/GLDAS_NOAH025SUBP_3H.ascii?evap[0:2][389:392][417:418].
The result is as below.
evap, [3][4][2]
[0][0], 9.999E20, 9.999E20
[0][1], 9.999E20, 9.999E20
[0][2], 1.571E-5, 9.999E20
[0][3], 1.464E-5, 1.357E-5
[1][0], 9.999E20, 9.999E20
[1][1], 9.999E20, 9.999E20
[1][2], 6.4900023E-6, 9.999E20
[1][3], 5.5800024E-6, 5.9800022E-6
[2][0], 9.999E20, 9.999E20
[2][1], 9.999E20, 9.999E20
[2][2], 4.0999976E-6, 9.999E20
[2][3], 3.4199975E-6, 3.5399976E-6
time, [3]
730175.0, 730175.125, 730175.25
lat, [4]
37.375, 37.625, 37.875, 38.125
lon, [2]
-75.625, -75.375
The expression evap[0:2][389:392][417:418] is an array expression; the numbers are array indexes, starting from 0; and "evap" is the variable name for "total evapotranspiration kg/m^2/s."
The dimension information is listed at the end of the results page. For this example, they are time, latitude, and longitude.
The time indexes, "730175.0, 730175.125, and 730175.25," are for days referenced from 00z01Jan0001. For this GLDAS NOAH Model 0.25 Degree 3-Hourly Data, corresponding time steps are 00Z24Feb2000, 03Z24Feb2000, and 06Z24Feb2000.
Here are the formulas for computing latitude, longitude, and index.
lat or lon to index:
latIndex = [lat - (-59.875)]/0.25
lonIndex = [lon - (-179.875)]/0.25
index to lat or lon:
lat = latIndex*0.25 + (-59.875)
lon = lonIndex*0.25 + (-179.875)
Information about GLDAS parameters and their corresponding variable names, as well as dimension information, can be found via GDS at http://hydro1.sci.gsfc.nasa.gov/dods/GLDAS_NOAH025SUBP_3H.info. More information about other GLDAS data sets are available at http://hydro1.sci.gsfc.nasa.gov/dods/.
2. An example for NLDAS
Here is an example for retrieving NLDAS-2 Mosaic Model 0.125 Degree Hourly Data, subsetted by parameter and geo-region, as ASCII text, http://hydro1.sci.gsfc.nasa.gov/dods/NLDAS_MOS0125_H.002.ascii?arainsfc[0:4][80:82][330:333].
The result is as below.
arainsfc, [5][3][4]
[0][0], 1.7018, 1.8356, 1.714, 1.515
[0][1], 2.383, 1.9492, 1.502, 1.4482
[0][2], 2.1638, 1.7084, 1.7128, 1.6206
[1][0], 1.8774, 2.0668, 1.9701, 1.7782
[1][1], 2.5921, 2.1661, 1.7055, 1.6803
[1][2], 2.3214, 1.8743, 1.9219, 1.8595
[2][0], 1.9778, 2.1998, 2.1181, 1.9308
[2][1], 2.8006, 2.363, 1.8781, 1.8675
[2][2], 2.5706, 2.0942, 2.166, 2.1133
[3][0], 1.7215, 1.9087, 1.8323, 1.6653
[3][1], 2.4163, 2.0321, 1.61, 1.596
[3][2], 2.1993, 1.7856, 1.8408, 1.7904
[4][0], 1.4244, 1.5505, 1.4612, 1.3039
[4][1], 1.8815, 1.552, 1.2063, 1.1733
[4][2], 1.6083, 1.2801, 1.2941, 1.2346
time, [5]
722452.0, 722452.0416666666, 722452.0833333334, 722452.125, 722452.1666666666
lat, [3]
35.063, 35.188, 35.313
lon, [4]
-83.688, -83.563, -83.438, -83.313
The result can be explained in the same way described in the example for GLDAS.
Information about NLDAS parameters and their corresponding variable names, as well as dimension information, can be found via GDS at http://hydro1.gsfc.nasa.gov/dods/NLDAS_MOS0125_H.002.info. More information about other NLDAS data sets are available at http://hydro1.gsfc.nasa.gov/dods/.
What is the height of GLDAS near surface wind, temperature, and specific humidity?
The height of GLDAS near surface wind, temperature, and specific humidity is varies across model outputs depending on the choice of forcing data. For the GLDAS data (forced by GDAS) at GES DISC NASA, the wind height is at 10 m, temperature and specific humidity are at 2m.
