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The following procedures describe how to compute the pressure and geopotential on model levels, geopotential height and geometric height.
Pressure on model levels
In ERA5, pressure is provided at the surface, but not on individual model levels. However, pressure on model levels (p_ml) is shown in Figure 1, and can be computed using the procedure described below.
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http://www.umr-cnrm.fr/gmapdoc/spip.php?article157
Illustrations of model levels, model half levels and model layers
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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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Please note, this procedure is an approximation to the calculation performed in the IFS (which also takes account of the effects of cloud ice and water and rain and snow).
Prerequisites to calculating Geopotential on model levels
You will need:
- A computer running Linux
- Python3
The CDS API installed; Your computer must be set up for downloading ERA5 model level data (from the 'reanalysis-era5-complete' dataset, stored in ECMWF's MARS catalogue) through the CDS API. For more details, please follow the instructions here (step B).
- The ecCodes library to read and write data.
Step 1: Download input data
First we must retrieve the required ERA5 data. We need:
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- 'tq_ml.grib' (a GRIB file containing temperature and specific humidity)
- 'zlnsp_ml.grib' (a GRIB file containing geopotential and log of surface pressure).
Step 2: Compute geopotential on model levels
We then use a Python script to compute geopotential (z) for all model levels:
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Alternatively, there is a customer-supplied script (which runs on Microsoft Windows) that computes geopotential on model levels for a specific location. This script was written for the ERA-Interim dataset, but can be adapted to ERA5. Please see the article ERA-Interim: compute geopotential on model levels for details.
Geopotential height
In ERA5, and often in meteorology, altitudes (the altitude of the land and sea surface, or specific altitudes 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) of the land and sea surface in metres, 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.
Geometric height
The geometric height or altitude (alt) is given by:
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where Re is the radius of the Earth. This geometric height is relative to mean sea level and it is assumed that the Earth is a perfect sphere - for more information see ERA5: data documentation - spatial reference systems.
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This document has been produced in the context of the Copernicus Climate Change Service (C3S).The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.The users thereof use the information at their sole risk and liability. For the avoidance of all doubt, the European Commission and the European Centre for Medium-Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |
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