GEMS ozone

The CAMS Global Reanalysis contains a parameter (variable) called  'GEMS Total column ozone' and a parameter called 'Total column ozone'. What is the difference between the two?

Moreover, some CAMS Global datasets have two parameters (variables) called 'GEMS Ozone' and 'GEMS Total column ozone'. What is 'GEMS' in this context?

Is 'GEMS Ozone' different from the 'Ozone' in other ECMWF-produced data?

CAMS data users please use the 'GEMS Ozone' and 'Total Column GEMS Ozone'.

The CAMS Global data is one of multiple datasets produced by ECMWF's IFS model. The IFS computes two types of ozone fields:

  • 'GEMS Ozone' (param 210203)  and 'Total Column GEMS Ozone' (param 210206):
    • Uses the full tropospheric chemistry scheme (i.e. a relatively complex computation)
    • Produced specifically for CAMS
  • 'Ozone mass mixing ratio' (param 203) and 'Total column ozone' (param 206):
    • Uses a simple Cariolle stratopsheric chemistry parametrisation that is also applied in the troposphere (i.e. a relatively simple computation that is computationally cheaper and also used for ECMWF's high resolution NWP model)
    • Strictly speaking not a CAMS product, but present in CAMS data for technical reasons.

The name 'GEMS' refers to a predecessor project of CAMS.

What does 'Total Column' mean?

'Total column X' describes the total amount of X contained in the whole height of the atmosphere.


CAMS Global data differentiates horizontally by latitude and longitude, and vertically by levels. Think of the levels as multiple layers of atmosphere:

  • 'Model levels' (ml) are defined by sequence, counted from level 1 on top of the atmosphere, to level 60 at the bottom (i.e. at the orography).  For example, to get Ozone data at the surface of the orography, use model level 60.
  • 'Pressure levels' (pl) are defined as all data points where pressure is identical, for example a 500 hPa level, a 400 hPa level, and so on.

Some data however is not differentiated vertically, for example the 'Total column' parameters, or 2m temperature. These parameters by definition have a single value in the vertical. In the data archive such 2-dimensional data is classified as 'levtype=sfc'.

So 'GEMS Ozone' refers to GEMS Ozone at a particular model level or pressure level. The unit is kg/kg. 'Total Column GEMS Ozone' refers to GEMS Ozone integrated over the whole height of the atmosphere. The unit is kg/m**2.