Hello,
I am comparing ERA5 model level data to radiosondes at specific lat-lon coordinates and have a question about the associated Earth model in the dataset.
The dataset I am working with has the following attribute for the grid:
// global attributes:
:DATA_SOURCE = “ECMWF: ERA5 GRIB 1 spherical harmonics (T639) and reduced Gaussian grid data (N320).” ;
:DATA_INTERPOLATION = “CISL RDA: ERA5 GRIB 1 N320/T639 data interpolated (or synthesized) to 1280 longitude x 640 Gaussian latitude GRIB 1 data using emoslib-4.3.9 (compiled with cmake with grib_api-1.14.7 and fftw-3.3.4 support for gnu/gfortran-4.4.7).” ;
I want to make sure that I understand the documentation (faceconfluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation#ERA5:datadocumentation-SpatialgridSpatialGrid) correctly.
The IFS assumes that the underlying shape of the Earth is a perfect sphere, of radius 6371.229 km, with the surface elevation specified relative to that sphere. The geodetic latitude/longitude of the surface elevation datasets are used as if they were the spherical latitude/longitude of the IFS.
ERA5 data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the major/minor axes) and in the vertical it is referenced to the EGM96 geoid […]
For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km (note, this is inconsistent with what is actually used in the IFS), as defined in the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17.
For data in GRIB2 format the earth model is a sphere with radius = 6371.2229 km (note, this is consistent with what is actually used in the IFS), as defined in the WMO GRIB2 specifications, section 2.2.1, Code Table 3.2, Code figure 6.
For data in NetCDF format (i.e. converted from the native GRIB format to NetCDF), the earth model is inherited from the GRIB data.
Based on the text above, I am wondering whether I need to convert the lat-lon coordinates given in the ERA5 model level output to WGS84 coordinates before comparing data to specific sounding locations. I understand that the ERA5 0.25 degree product is referenced with respect to WGS84, but I am not sure whether the model level dataset is modified from the original IFS grid that is spherical.
Thank you so much!
Tobias