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In ERA5, pressure is provided at the surface, but not on individual model levels. However, an illustration of pressure on model levels (p_ml) is shown in Figure 1, and can be computed for particular dates and times using the procedure described below.
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The model half-level pressure (p_half) as shown , illustrated in Figure 2, is given by:
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The pressure on model levels (p_ml) is shown , illustrated in Figure 1, and is given by the mean of the pressures on the model half levels immediately above and below:
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For more details about the vertical discretisation, please see Part-iii Dynamics and numerical procedures, section 2.2 and the FULL-POS documentation at:
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Figure 1. An illustration of IFS model levels, showing | Figure 2. An illustration of model half-levels and model layers. The pressure |
Geopotential on model levels
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In ERA5, and often in meteorology, altitudes heights (the altitude height of the land and sea surface, or specific altitudes heights in the atmosphere) are not represented as geometric altitude (in metres above the spheroid), but as geopotential height (in metres above the geoid, which is represented by the mean sea level in ERA5). However, ECMWF archive the geopotential (in m2/s2), not the geopotential height.
To obtain the geopotential height (h) in metres (of the land and sea surface in metressurface or at particular heights in the atmosphere), simply divide the geopotential ( geopotential at the surface is called orography in the Climate Data Store (CDS)) by the Earth's gravitational acceleration, 9.80665 m/s2. This geopotential height is relative to mean sea level - for more information see ERA5: data documentation. Note, in the Climate Data Store (CDS), geopotential at the surface is called orography.
Geometric height
The geometric height or altitude (alt) is given by:
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