Example data: MERRA-2 3-d, 3-Hourly, Instantaneous, Pressure-Level, Assimilated Meteorological Fields (M2I3NPASM_5.12.4). The downloaded daily mean temperature at 750 hPa is shown in Figure 1.
Figure 1: Example of daily mean air temperature over US at 875 hPa on Jan 1 2020.
Prerequisites:
A user must be registered with Earthdata Login and be authorized to access data at the NASA GES DISC. Please read more about this here: GES DISC data access.
Procedure:
1. Start at the GES DISC home page, https://disc.gsfc.nasa.gov/.
2. Search for the data collection name (M2I3NPASM).
You may find the data collection that contains the variable of interest by searching for the variable name in the MERRA-2 file specification document: https://gmao.gsfc.nasa.gov/pubs/docs/Bosilovich785.pdf.
3. Click “Subset/Get Data” below the product name, as shown in Figure 2.
Figure 2: Example of data collection search result page.
4. Set options to get the daily mean:
- Download Method: To select the method, select "Get File Subsets using the GES DISC Subsetter."
- Refine Date Range: Select a date range: for the first-time user, start with a couple of days for testing. In this example, we use January 1-3, 2020.
- Refine Region: In this example, we select data over the USA (-125.96, 24.542, -73.929, 49.855). For requesting regional data check the box: Use "Refine Region" for geo-spatial subsetting.
- Select variables: In this example, we select air_temperature and relative_humidity_after_moist.
- Select Dimension: To select vertical levels, the default is for all levels; in this case, we choose six levels. For two-dimensional data, this option is not shown.
- Select Time Range: Check the option "mean" to get daily mean data. Please do not select any hour in "From" and "To." The default is to get data at all time steps of the day.
- Regrid: Use the default option to keep the original MERRA-2 spatial resolution at 0.5x0.625 degrees (lat x lon).
- Output format: Select the data file format as 'netCDF'.
Figure 3: Example of the subsetting interface.
5. Click "Get Data" to get the data download page.
- Save the "Download Links List" to a file and follow the "Download Instructions" to download selected data.
- You may click on "Selected Parameters" to see all subsetting options used.
- You may change the options by clicking on "Refine Parameters."
Figure 4: Example of the data download page.
Additional Info:
MERRA-2 data collections may be found by searching for variable of interest names from the file specification document: https://gmao.gsfc.nasa.gov/pubs/docs/Bosilovich785.pdf.
More MERRA-2 documents, FAQs, and reference articles may be found at https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/docs/.