LDAS FAQ
Please also check the Hydrology Disc FAQ at the NASA GES DISC for questions/answers regarding LDAS data products.
If the answer you are looking for is not found at the above FAQ or below, please contact us.
General:Q. Where can I find LIS data?
Q. Can I download LDAS data subsetted by region and/or variable?
Q. Is LDAS data provided in UTC time or local time? Does it account for daylight savings?
Q. Where can I find the GRIB tables (gribtabs) for LDAS DATA?
Q. What tools are available to help with the LDAS GRIB data?
Q. How can I convert LDAS GRIB data into ASCII?
Q. Is there a mailing list for updates on LDAS data?
GLDAS:
Q. What is the latency for GLDAS data?
Q. WGRIB gives a different variable name. Am I reading the GRIB file correctly?
Q. Does "soilm1" correspond to the top or bottom soil layer?
Q. Is GLDAS data avaiable in other than GRIB format?
Q. What data are integrated/assimilated into GLDAS?
NLDAS:
Q. Where is the latest real-time NLDAS forcing and model data?
Q. Are monthly NLDAS datasets available?
Q. Are daily NLDAS datasets available?
Q. What is the difference between NLDAS Phase 1 (NLDAS-1) and NLDAS Phase 2 (NLDAS-2)?
Q. What is the difference between NLDAS datasets at NASA and at NOAA/EMC?
Q. Does the precipitation variable include snowfall?
Q. Can you explain the components of the evaporation in the NLDAS-2 Mosaic and Noah output?
Q. What is the average surface skin temperature in the NLDAS-2 Mosaic and Noah output?
Q. Why is the canopy conductance undefined/wrong in NLDAS-2 Mosaic output after 9 March 2008?
Q. What value was used for the soil heat capacity in the Mosaic model for NLDAS-1 and NLDAS-2?
Q. Is there a mask for over CONUS for the NLDAS grid?
Q. What geographic coordinate system was used to generate the NLDAS grid?
Q. Is there a shapefile available for the NLDAS grid?
Q. Are there any tips or advice on downloading NLDAS data?
General:
Q. Where can I find LIS data?
A. The Land Information System (LIS) is a flexible land-surface modeling and data assimilation software framework developed here at NASA Goddard within the Hydrological Sciences Laboratory. GLDAS and NLDAS are specific uses of the LIS software, and these LDAS projects have produced land-surface forcing data and model output going back many decades and continuing to near real-time. Links to download GLDAS and NLDAS data are located on the right side of this FAQ. There is no "LIS data" - unless, of course, you use LIS yourself to generate your own!
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Q. Can I download LDAS data subsetted by region and/or variable?
A. Yes, it is now possible to subset both GLDAS and NLDAS datasets by region and/or by variable using the GES DISC's Mirador search tool. Just go to the Data Holdings page, and click the "Mirador Search" column for the dataset desired. For additional help, please see these news articles from the GES DISC about the on-the-fly subset service for the GLDAS and NLDAS datasets.
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Q. Is LDAS data provided in UTC time or local time? Does it account for daylight savings?
A. All LDAS data is provided in Coordinated Universal Time (UTC). There is no accounting for daylight savings. For assistance converting between UTC and a particular location by date, please visit the World Clock Time Zone Converter.
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Q. Where can I find the GRIB tables (gribtabs) for LDAS DATA?
A. GRIB tables for LDAS data (GLDAS and NLDAS) as stored on the NASA Hydrology DISC can be found on the GES DISC documentation website. For more information on how to use these gribtabs with the GLDAS and/or NLDAS data, please see the GLDAS-1 README, GLDAS-2 README, NLDAS-1 README, and NLDAS-2 README files.
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Q. What tools are available to help with the GRIB data?
A. Two extremely useful tools for processing the LDAS data in GRIB-1 format are WGRIB and grib2ctl.pl. More documentation about GRIB data can be found at NCEP. Climate Data Operators (CDO) are also useful to manipulate and analyze GRIB data. Another useful tool is NCL, which handles GRIB-1/GRIB-2 trivially. NCL has all the GRIB lookup tables built-in. Further, it has two command line utilities: "ncl_filedump", which creates a printed overview of the file contents, and "ncl_convert2nc", which converts GRIB-1/GRIB-2 to netCDF adhering to the Climate and Forecast netCDF convention.
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Q. How can I convert LDAS GRIB data into ASCII?
A. The WGRIB tool can easily convert the LDAS GRIB data into ASCII. Here's a sample command, which will dump the first record in the GRIB file named "FILENAME" into an ASCII text file named "output.txt":
wgrib -d 1 FILENAME | wgrib -text -i FILENAME -o output.txtPlease see the WGRIB documentation for additional options and assistance. LDAS datasets can also be obtained in ASCII via the GDS at the GES DISC, as detailed here.
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Q. Is there a mailing list for updates on LDAS data?
A. Yes. Yes, there is! Users of NLDAS and GLDAS datasets are encouraged to join the LDAS mailing list for updates on new data releases, new data releases, data reprocessing, and data delays or outages.
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GLDAS:
Q. What is the latency for GLDAS data?
A. Currently, GLDAS data at the NASA GES DISC is updated at about a month delay. The updates are made usually in the middle of each month.
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Q. WGRIB gives a different variable name. Am I reading the GRIB file correctly?
A. When you are not using the GLDAS GRIB table (gribtab), the names of the variables do not appear correctly. For example with GLDAS data, the rainfall rate is called 4LFTX and a WGRIB message "using NCEP-opn" appears. One way to ensure is to use KPDS values as guidance using "wgrib -v". The KPDS value for rainfall rate is 132. The PDS values are specified in Table 2 of the GLDAS-1 README and GLDAS-2 README files. Depending on the version of WGRIB and the operating system, the use of the GRIB table can be established through setting the environment and/or having a copy of gribtab in the working directory where WGRIB is issued. Please refer to "Reading the Data" section of the readme file for setting the environment. The GRIB tables are available from the GES DISC documentation website.
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Q. Does "soilm1" correspond to the top or bottom soil layer?
A. The order of soil layers in the GLDAS datasets goes from bottom to top. For example, the first record for soil moisture in NOAH is the lowest 100-200 cm, followed by the 3rd layer 40-100cm, the 2nd layer 10-40cm, then the top 0-10cm layer. This order applies to the soil temperature as well. One way to confirm is to check the values: soil moisture amounts increase as layer depths increase.
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Q. Is GLDAS data avaiable in other than GRIB format?
A. Yes, GLDAS dataset is also available in NetCDF format using the GES DISC's Mirador search tool. Just go to the Data Holdings page, and click the "Mirador Search" column for the dataset desired. Please follow the step-by-step instruction in the news article from the GES DISC about the netCDF conversion service for the GLDAS dataset.
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Q. What data are integrated/assimilated into GLDAS?
A. Data are integrated within GLDAS as static parameter fields, as meteorological forcing, and through the process of data assimilation. Parameter fields include vegetation type and properties derived from AVHRR and MODIS, soil properties from the USDA, and elevation from GTOPO30 (see GLDAS vegetation page). Meteorological forcing datasets include downscaled NOAA Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP), Air Force Weather Agency solar radiation, and NOAA Global Data Assimilation System (GDAS) air temperature, surface pressure, wind speed, land specific humidity (see GLDAS forcing page).
Currently, the only data that GLDAS integrates through data assimilation is MODIS snow cover in the 0.25 degree Noah simulation. We have developed an technique for assimilating GRACE terrestrial water storage data, but that has not been used in GLDAS. We do assimilate GRACE data into a separate stand-alone simulation of the Catchment land surface model which is then used to diagnose drought. For more information, see the NDMC website.
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NLDAS:
Q. Where is the latest real-time NLDAS forcing and model data?
A. The real-time NLDAS Phase 2 (NLDAS-2) forcing and model output data is currently in production at NOAA/NCEP/EMC. Automated scripts and programs gather and process the data needed to generate the NLDAS-2 forcing datasets. These scripts/programs run once per day, and update one day's worth (typically 12Z to 12Z) of forcing. This forcing data is then used to drive the various land-surface models to produce the model output. Scripts archive the data at the NASA GES DISC, as well as at the NCEP/EMC realtime NLDAS ftp site. The very latest available data can be found at either of the previous two links.
The NLDAS-2 forcing currently is available around 4 days behind the current date. Long-time users of NLDAS products will note that this is a slightly longer delay than the 2-3 day data latency that was typical from NLDAS Phase 1. The primary reason for this is that the NARR model data used in the generation of NLDAS-2 forcing is currently in a quasi-operational mode and not available as early as the model data used in the generation of NLDAS-1 forcing. The NLDAS-2 model output data may be delayed another day, as an entire day's worth of forcing is needed for the model simulations. Also, GDS data may be behind by another couple days, as the updated index file is generated. Occasionally, problems with the data used to create NLDAS-2 forcing and/or disk/computer issues will cause a further delay. NLDAS participants are aware of these delays and are working hard to restore the data to make it as current as available. If you need further assistance, please e-mail the NLDAS Technical Contact.
DISCLAIMER: NLDAS data should be used for research or educational purposes only. This data should NOT be relied on for operational use as data gaps can occur due to hardware failure and/or model upgrading procedures.
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Q. Are monthly NLDAS datasets available?
A. Yes, monthly NLDAS datasets for NLDAS-1 forcing, NLDAS-2 forcing, NLDAS-2 Mosaic, and NLDAS-2 Noah are now available from the GES DISC. Please see the updated NLDAS-1 README and NLDAS-2 README files for details about the creation and content of these datasets. The latest month of data will typically be available around 10-15 days after the beginning of the following month. Monthly climatology datasets are also available.
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Q. Are daily NLDAS datasets available?
A. Daily NLDAS datasets are not currently available, and there are no immediate plans to make them available. The NLDAS group has received many requests for these datasets; however, some users have requested 00Z-00Z averages, some requested 12Z-12Z averages, and some requested 00LST-00LST averages.
If you need daily averages, please download the hourly NLDAS datasets and create your own daily datasets.
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Q. What is the difference between NLDAS Phase 1 (NLDAS-1) and NLDAS Phase 2 (NLDAS-2)?
A. This is the most popular NLDAS question. Hopefully, below you can find an appropriately short yet fully detailed answer! For further information, please refer to the NLDAS-1 Forcing, NLDAS-1 Model, NLDAS-2 Forcing, and NLDAS-2 Model pages.
The most significant difference is the time frames of the datasets. NLDAS-1 is available from mid-1996 to the end of December 2007. NLDAS-2 is available from January 1979 to near real-time.
Another major difference between the two phases of NLDAS is the sources of model data and observations used to create the respective forcing datasets. NLDAS-1 uses the 40-km NCEP Eta model-based Data Assimilation System (EDAS) for the surface meteorology, while NLDAS-2 uses the 32-km NARR system. For downward shortwave radiation at the surface, NLDAS-1 uses GOES-based satellite retrievals, with EDAS data used when/where not available; NLDAS-2 uses GOES data to bias-correct the NARR shortwave radiation.
Both NLDAS-1 and NLDAS-2 use a 1/8th-degree CPC daily gauge analysis as the source of the precipitation forcing. This analysis is PRISM-adjusted and was produced by the CPC using a least squares distance weighting scheme. NLDAS-2 uses this analysis over the entire data record, while NLDAS-1 used this analysis from 2002 onward. For the period 2002 and before (in the NLDAS-1 data only), the other daily gauge analysis used was a daily 1/4th-degree CPC product, which was generated using a Cressman analysis and interpolated by the NLDAS team to 1/8th-degree using the budget bilinear method.
During the period when the 4-km hourly Doppler radar Stage II precipitation estimates are available (mid-1996 to present) over CONUS, both NLDAS-1 and NLDAS-2 first use these hourly estimates to temporally disaggregate the daily gauge analysis into hourly precipitation. If the radar estimates are not available (due to maintenance or coverage issues), the additional hourly datasets used to calculate the weights differ between NLDAS-1 and NLDAS-2. In NLDAS-1, the EDAS precipitation data is used over CONUS, Canada, and Mexico. In NLDAS-2 over CONUS or Mexico, 8-km half-hourly CMORPH satellite-retrieved estimates (2002 to present) are used if Stage II is not available. If CMORPH is unavailable, such as before 2002, the 2 X 2.5 degree CPC Hourly Precipitation Dataset (HPD) is used. If the HPD is also unavailable, then the NARR precipitation data is used. Over Canada, only NARR precipitation is used due to poor gauge coverage, with a 1-degree blending applied at the U.S.-Canada border.
The hourly estimates from the radar, CMORPH, HPD, EDAS, or NARR are only used to calculate the weights for the temporal disaggregation; the daily sum of the hourly NLDAS precipitation will be equal to the daily gauge analysis from the CPC. In addition, over CONUS, the hourly precipitation from both the NLDAS-1 and NLDAS-2 precipitation should be nearly identical when/where the hourly radar estimates are available. Small differences between NLDAS-1 and NLDAS-2 may still be present, however, owing to the 1/4th-degree daily gauge analysis used in NLDAS-1 before 2002 or to the nature of the amount/quality of precipitation gauge observations and radar estimates used between retrospective or real-time data productions. Further details about the NLDAS-2 precipitation can be found in the Appendix C section describing the NLDAS-2 forcing data.
Several significant attributes are identical between the NLDAS-1 and NLDAS-2 datasets. They are both on the 1/8th-degree NLDAS grid and available hourly. The vegetation, soils, and elevation datasets used are also identical. The same four land-surface models (Mosaic, Noah, SAC, and VIC) are also used in both NLDAS-1 and NLDAS-2, although the models underwent some upgrades and corrections between the two phases of NLDAS. For more on these model improvements, please see Section 2.3 of Xia et al. (2012, Part 1). The spin-up procedures between NLDAS-1 and NLDAS-2 do differ, owing the different time periods between them.
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Q. What is the difference between NLDAS datasets at NASA and at NOAA/EMC?
A. NLDAS datasets are available at both the NASA GES DISC (for both retrospective and real-time) and at NOAA/EMC's ftp sites for NLDAS retrospective and real-time simulations. Datasets from either NASA or from NOAA/EMC can be used, and the fields in the datasets are identical, except where noted below. The file name conventions are also different between NASA and NOAA/EMC, as well as the directory structures. Within each year, NASA uses the ordinal date while NOAA/EMC uses YYYYMMDD. The GRIB tables and a few PDS GRIB values also differ between the NLDAS datasets at NASA and at NOAA/EMC. GRIB tables and documentation for NLDAS data as stored at the NASA GES DISC are found on their documentation website.
For NLDAS-2 Mosaic, the NOAA/EMC retrospective data provides the vegetation cover as a fraction (from 0 to 1) while the real-time data from NOAA/EMC is provided as a percentage (from 0 to 100). All NLDAS-2 Mosaic vegetation cover data provided at the NASA GES DISC is in units of fraction.
For NLDAS-2 Noah, NOAA/EMC provides the albedo and the moisture availability variabes in units of fraction, while the NASA GES DISC provides these variables in units of percentage. NOAA/EMC provides the net shortwave, net longwave, and ground heat flux with signs reversed from the traditional ALMA direction for general energy balance components. For these variables at the NASA GES DISC, the signs are reversed from the NOAA/EMC data, and these three variables have the sign correct in the traditional direction. Finally, the units of the snow water-equivalent in the NOAA/EMC data are in meters, while in the NASA GES DISC data, the units are in kg/m^2 (or millimeters).
For NLDAS-1 Forcing, some PDS variable ID numbers were changed for the datasets at the NASA GES DISC to prevent confusion, as the data at NOAA/EMC uses the identical PDS ID values for several variables.
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Q. Does the precipitation variable include snowfall?
A. The precipitation fields in the NLDAS-1 Forcing files and in the NLDAS-2 Forcing "A" and "B" files represent the total precipitation, including both rain and snow. The land-surface models each have their own individual method to determine how this total precipitation reaches the land-surface (as rain, snow, or some combination). Typically, the near-surface air temperature from the forcing at the same time interval as the precipitation is used to determine rain or snowfall. The NLDAS-2 Mosaic and Noah model output files contain rainfall and snowfall fields as a result of their individual methods.
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Q. Can you explain the components of the evaporation in the NLDAS-2 Mosaic and Noah output?
A. There are four evaporation components of the total latent heat flux in the NLDAS-2 Mosaic and Noah datasets. They are as follows (with PDS ID number, variable names, full description, and units):
199:EVBSsfc:Direct evaporation from bare soil [W/m^2] 200:EVCWsfc:Canopy water evaporation [W/m^2] 198:SBSNOsfc:Sublimation (evaporation from snow) [W/m^2] 210:TRANSsfc:Transpiration [W/m^2]These variables correspond to the following evaporation components of the ALMA standard:
NLDAS names ALMA names ALMA description/units ----------- ---------- ---------------------- EVBSsfc ESoil Evaporation from bare soil [kg/m2s] EVCWsfc ECanop Evaporation from canopy interception [kg/m2s] SBSNOsfc SubSnow Total sublimation from the ground snow pack [kg/m2s] TRANSsfc TVeg Transpiration from canopy [kg/m2s]For both NLDAS-2 Mosaic and Noah, the total latent heat flux is:
121:LHTFLsfc:Latent heat flux [W/m^2]For NLDAS-2 Mosaic, these components are defined as positive in the downward direction, which is the opposite direction as the traditional ALMA standard for these variables. However, the latent heat flux for NLDAS-2 Mosaic is defined as positive in the upward direction, which means that the latent heat flux is roughly equal to the negative of the sum of these four components for NLDAS-2 Mosaic:
LHTFLsfc ~= -(EVBSsfc + EVCWsfc + SBSNOsfc + TRANSsfc)Note that the sign of the SBSNOsfc variable only switched directions within the NLDAS-2 Mosaic datasets on and after 9 March 2008.
For NLDAS-2 Noah, these components and the latent heat flux are all defined as positive in the upward direction. The latent heat flux is roughly equal to the sum of the four evaporation components:
LHTFLsfc ~= EVBSsfc + EVCWsfc + SBSNOsfc + TRANSsfcFor both NLDAS-2 Noah and Mosaic, there is another energy term that describes the snow phase-change heat flux:
229:SNOHFsfc:Snow phase-change heat flux [W/m^2]The SNOHFsfc can be thought of as roughly identical to the sum of the Qsm and Qfz terms (albeit, in different units) from the ALMA standard.
When comparing these four evaporation components and the SNOHFsfc variable between NLDAS-2 Mosaic and Noah, please reverse the sign for these five variables from NLDAS-2 Mosaic.
Additionally, another term for the total evapotranspiration (in mass units) can be found here:
057:EVPsfc:Total evapotranspiration [kg/m^2]The EVPsfc variable is in mass units and the LHTFLsfc variable is in energy units, but both represent the total upwards water flux. A unit conversion between the two variables (assuming a constant latent heat of vaporization value) shows that the values of these variables should be roughly equal.
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Q. What is the average surface skin temperature in the NLDAS-2 Mosaic and Noah output?
A. NLDAS-2 Mosaic and Noah provide the hourly instantaneous surface skin temperature (aka, the temperature at exactly 00 minutes of every hour - not the average temperature over the entire hour). The "average" here refers to the average over the entire grid box for all vegetation, bare soil, and snow skin temperatures. For more on the definition of the AVSFTsfc variable, see the "AvgSurfT" description at the ALMA standard pages.
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Q. Why is the canopy conductance undefined/wrong in NLDAS-2 Mosaic output after 9 March 2008?
A. On 9 March 2008, the NLDAS-2 Mosaic data was transferred from the retrospective stream (starting back from Jan 1979) to the real-time simulations. Unfortunately, the canopy conductance is not properly defined in the real-time NLDAS-2 production. There are no plans to correct this in the NLDAS-2 Mosaic hourly or monthly output data.
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Q. What value was used for the soil heat capacity in the Mosaic model for NLDAS-1 and NLDAS-2?
A. The value used for Mosaic in NLDAS-1 is 175,000 J m-2 K-1, while the value used for Mosaic in NLDAS-2 is 70,000 J m-2 K-1. The name of this variable in the Mosaic code is "CSOIL0". The reason for this difference is explained in detail in Section 5.3 within Robock et al. (2003).
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Q. Is there a mask for over CONUS for the NLDAS grid?
A. Yes, GrADS binary mask and control files, as well as a sample FORTRAN program to read the mask, are available here.
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Q. What geographic coordinate system was used to generate the NLDAS grid?
A. No particular geographic coordinate system was consulted when the NLDAS grid was originally configured. The grid boxes are simply 1/8th-degree boxes with the center of the lower-left grid box at 25.0625 N and 124.9375 W. The details of the grid are available here and here.
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Q. Is there a shapefile available for the NLDAS grid?
A. Yes, a shapefile (and associated files) for GIS can be found here.
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Q. Are there any tips or advice on downloading NLDAS data?
A. The wget tool is very helpful to easily downloading GRIB-1 format data from the NLDAS ftp site. An example command is found below - please check the wget options before running a similar command on your end. This example will download an entire year's worth of GRIB data (approx. 14 GB!), so proceed with caution. Here are the wget options used:
- (-A grb) = download only the GRIB (grb) files
- (--timeout=20 -t inf) = download data until a timeout occurs of 20 seconds, after which it will try again infinitely until successful
- (-nH) = disable generation of directories with the host name
- (--cut-dirs=5) = disable generation of the first 5 sub-directories on the local disk
- (-r) = turn on recursive retrieving
- (-c) = continue any partially-downloaded files
- (-o log) = output message to a local file named "log"
wget -A grb --timeout=20 -t inf -nH --cut-dirs=5 -r -c -o log \ ftp://hydro1.sci.gsfc.nasa.gov/data/s4pa/NLDAS/NLDAS_FORA0125_H.002/2009/The above example should be entered as one line; be sure to remove the "\" backslash. You can use wildcards on the command line to grab specific or multiple years/months/days of data.
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