The CAMS Global Reanalysis contains a parameter (variable) 'GEMS Total column ozone' and a parameter 'Total column ozone'. What is the difference between the two?
In some CAMS Global datasets I see the parameters (variables) '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'.
- '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.