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FAQ's

Q. What is the latency for GLDAS data?
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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.
 

Q. WGRIB gives a different variable name. Am I reading the GRIB file correctly?
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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 file. 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.

Q. Does "soilm1" correspond to the top or bottom soil layer?
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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.
 

Q. What data are integrated/assimilated into GLDAS?
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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.
 

Q. What are the known issues with the GLDAS-1 datasets?
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A. There are several known issues with the GLDAS-1 datasets. These issues will be resolved in GLDAS-2.1, which will replace GLDAS-1:

  • GLDAS-1 all models Forcing fields: highly uncertain during 1995-1997.
  • GLDAS-1 all models SWdownsfc: Unrealistic patterns over China, Southern Europe, and Canada appear from time-to-time from 2001 onwards, especially prominent during June-December 2002.
  • GLDAS-1 0.25-degree Noah Rainfsfc: Granularity of adjacent grid cell maxima and minima appears from time-to-time for 2001 onwards when the disaggregated CPC's CMAP precipitation fields are used. It is an artifact of the disaggregation method. 
  • The issues with the forcing will then affect the soil moisture, runoff, fluxes, etc. in the model output in the regions listed.

Please also check the FAQ page at the Hydrology GES DISC.