![]() NETCDF:"GLDAS_NOAH025_3H.4":Tair_f_inst - Air Temperature (K) NETCDF:"GLDAS_NOAH025_3H.4":Psurf_f_inst - Surface Pressure (Pa) NETCDF:"GLDAS_NOAH025_3H.4":Qair_f_inst - Specific humidity (Kg/Kg) So for PySEBAL we need the following subdatasets from a single GLDAS file: The codes of the parameters are listed in the GLDAS user manual (Table 3.1). ![]() These subdataset names are used in the further steps to process individual parameters.įor example, NETCDF:"GLDAS_NOAH025_3H.4":Tair_f_inst is the name of air temperature grid at 06:00 on and we will use this name to extract/process this single grid. In the metadata, under Subdatasets: all the parameters provided as subdatasets are listed. # Display metadata of the 'GLDAS_NOAH025_3H.4' # change directory to where you have downloaded the GLDAS data using the belowĬd /mnt/path/to/youroutputfolder/2019/144 Now let us see how to read the metadata of this file in command line using gdal tools. Nc4 represents the extension and format of the data, in this case netCDF4 GLDAS_NOAH025_3H represents the name, spatial resolution (025 means 0.25 degree resolution) and temporal resolution (3H means three hour data)Ī20190524 represents the date of acquisitionĠ600 represents the time (in this case 06:00 GMT) the parameters are computed forĠ21 represents the version of GLDAS data, in this case 2.1 The GLDAS file name follows a particular structure. Because 144 is the corresponding Julian date of 24 May in 2019. After the download, you will find this data inside /mnt/path/to/youroutputfolder/2019/144. Let us now see how to process a single NetCDF file (.nc4) dowloaded from GLDAS using the previous step and perform the required conversions for PySEBAL as explained in the beginning of this exercise.įor example let us consider the GLDAS data representing 06:00 hours on which also happens to be a date when there is a Landsat acquisition over our study area - MIS. ![]() Step 3: Processing single gldas data file To install the package open MobaXterm and an Ubuntu session. Instead we will use a python package called gldas to download data from a START date to END date. As the GLDAS data is three hourly, for the Landsat satellite which has an acquisition time of ~07:30 GMT over Urmia Lake Basin, an average of GLDAS data at 06:00 and 09:00 GMT are computed.Īs the temporal resolution of GLDAS data is three hours, downloading GLDAS data manually is not a practical approach. Compute instantaneous SWdown, Ws, Tair, P and Rh maps representing the time of acquisition of satellite image.Compute daily averages of SWdown, Ws, Tair, P and Rh.Convert specific humidity to relative humidity following the description here.Convert the unit of GLDAS pressure from Pa to Milli bar (Mb).Convert GLDAS air temperature from Kelvin to Deg C.To match the requirements for PySEBAL following conversion steps have to be performed: Further GLDAS provide specific humidity and PySEBAL require relative humidity as input. The units at which GLDAS provide air temperature (K) and Pressure(Pa) are different.
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