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The IFS assumes the Earth is a perfect sphere, but 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 over land but over ocean it is referenced to the mean sea level, with the approximation that this is assumed to be coincident with the geoid. For more information on the relationship between mean sea level and the geoid, see for example Gregory et al. (2019)
Earth model
For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km, as defined in the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17.
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| Dataset name | Observation type | Measurement |
|---|---|---|
| SYNOP | Land station | Surface Pressure, Temperature, humidity |
| METAR | Land station | Surface Pressure, Temperature, humidity |
| DRIBU/DRIBU-BATHY/DRIBU-TESAC/BUFR Drifting Buoy | Drifting buoys | 10m-wind, Surface Pressure |
| BUFR Moored Buoy | Moored buoys | 10m-wind, Surface Pressure |
| SHIP | ship station | Surface Pressure, Temperature, wind, humidity |
| Land/ship PILOT | Radiosondes | wind profiles |
| American Wind Profiler | Radar | wind profiles |
| European Wind Profiler | Radar | wind profiles |
| Japanese Wind Profiler | Radar | wind profiles |
| TEMP SHIP | Radiosondes | Temperature, wind, humidity profiles |
| DROP Sonde | Aircraft-sondesRadiosondes | Temperature, wind profiles |
| Land/Mobile TEMP | Radiosondes | Temperature, wind, humidity profiles |
| AIREP | Aircraft data | Temperature, wind profiles |
| AMDAR | Aircraft data | Temperature, wind profiles |
| ACARS | Aircraft data | Temperature, wind profiles, humidity |
| WIGOS AMDAR | Aircraft data | Temperature, wind profiles |
| TAMDAR | Aircraft data | Temperature, wind |
| ADS-C | Aircraft data | Temperature, wind |
| Mode-S | Aircraft data | Wind |
| Ground based radar | Radar precipitation composites | Rain rates |
| Anchor | ||||
|---|---|---|---|---|
|
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