NWP | PGW | Order of Pressure levels of GRIB file

NWP | Pseudo-Global-Warming (PGW) hands-on | ERA5 | GFS

NWP | ERA5 | GRIB1 and GRIB2

In GRIB files, the order of pressure levels is important because many tools expect them to be sorted either in ascending (from surface to top of the atmosphere) or descending (from top of the atmosphere to surface) order. Here's how you can ensure the order of pressure levels and reverse them using tools like cdo or grib_set.

---

Step 1: Check the Current Order of Pressure Levels

To inspect the order of pressure levels in the GRIB file, use the following commands:

1. Using grib_ls:

   grib_ls -p level era5_with_signal.grib

   This lists the levels (level) in the GRIB file. Check if they are in ascending or descending order.

2. Using cdo:

   cdo showlevel era5_with_signal.grib

   This outputs the pressure levels in the order they appear in the file.

---

Step 2: Ensure Ascending or Descending Order

If the pressure levels are not in the desired order, you can reorder them using cdo.

Ensure Ascending Order (Surface to Top of Atmosphere)

To reorder pressure levels in ascending order (e.g., 1000 hPa → 850 hPa → ... → 10 hPa):

cdo vertsort era5_with_signal.grib era5_sorted_ascending.grib

The vertsort operation ensures that the levels are sorted in ascending order.

---

Ensure Descending Order (Top of Atmosphere to Surface)

To reorder pressure levels in descending order (e.g., 10 hPa → 100 hPa → ... → 1000 hPa), use the vertsort operation and then reverse the order:

cdo invertlev era5_with_signal.grib era5_sorted_descending.grib

The invertlev operation reverses the order of the vertical levels.

---

Step 3: Verify the New Order

After reordering the levels, verify the result:

1. Using grib_ls:

   grib_ls -p level era5_sorted_ascending.grib
   grib_ls -p level era5_sorted_descending.grib

2. Using cdo:

   cdo showlevel era5_sorted_ascending.grib
   cdo showlevel era5_sorted_descending.grib

Ensure that the levels are in the desired order.

---

Step 4: For NetCDF Files

If you are working with a NetCDF file (era5_with_signal.nc) and need to reorder the pressure levels:

Ascending Order

Use cdo vertsort for NetCDF files as well:

cdo vertsort era5_with_signal.nc era5_sorted_ascending.nc

Descending Order

Use cdo invertlev to reverse the order:

cdo invertlev era5_with_signal.nc era5_sorted_descending.nc

---

Additional Tips:

1. Regrid if Necessary: If the pressure levels are missing or inconsistent, you can interpolate them to a standard set using cdo intlevel. For example:

   cdo intlevel,1000,850,500,250 era5_with_signal.nc era5_regridded_levels.nc

2. Metadata Consistency: Ensure that the typeOfLevel (e.g., isobaricInhPa) is correctly set, especially after reordering or interpolating levels. Use grib_set if needed:

   grib_set -s typeOfLevel=isobaricInhPa era5_sorted_ascending.grib era5_final.grib

---

Summary of Commands:

1. Check the current order of levels:

   grib_ls -p level era5_with_signal.grib
   cdo showlevel era5_with_signal.grib

2. Ensure ascending order (surface to top):

   cdo vertsort era5_with_signal.grib era5_sorted_ascending.grib

3. Ensure descending order (top to surface):

   cdo invertlev era5_with_signal.grib era5_sorted_descending.grib

4. Verify the new order:

   grib_ls -p level era5_sorted_ascending.grib
   cdo showlevel era5_sorted_ascending.grib

5. For NetCDF files:

   cdo vertsort era5_with_signal.nc era5_sorted_ascending.nc
   cdo invertlev era5_with_signal.nc era5_sorted_descending.nc

References

1. Climate Data Operators (CDO) Tutorial (recommend)