Aerosols and Greenhouse Gases

IFS can consider the effects of several greenhouse gases and aerosol species which affect forecasts.  Their interaction with short-wave and long-wave radiation can heat or cool the atmosphere and the surface.  The greenhouse gases considered are:

• • water vapour,
  • carbon dioxide,
  • ozone,
  • methane,
  • nitrous oxide,
  • four CFC compounds.

The aerosol types considered are:

• • sea salt (three different sized particles),
  • desert dust (three different sized particles),
  • organic matter,
  • black carbon,
  • ammonium sulphate.

Several of the aerosol species are hydrophyilic. This means  they swell as relative humidity increases which makes the aerosol more optically thick.  If this process is activated in the model it can act to reduce visibility in humid conditions. 

A reliable treatment of the interaction of greenhouse gases and aerosols with radiation requires the global distribution to be well represented.  Two different configurations of the IFS are used operationally.  These represent the global distribution in different ways:

• CAMS forecasts. The CAMS model produces air-quality forecasts for the Copernicus Atmosphere Monitoring Service (CAMS), including the CAMS reanalysis.  Gases and aerosols are represented by prognostic variables which  are advected around with the model winds. Source and sink processes are also represented.  The CAMS model takes into account:
  • fluxes of gases from anthropogenic sources, vegetation, wetlands and ocean.
  • chemical reactions between gases.
  • surface aerosol emissions dependent on wind speed (e.g. dust raised by wind and sea salt from breaking ocean waves).
  • anthropogenic aerosol emissions (e.g. from urban sources and biomass burning).  This uses a database of surface sources.
  • aerosol particles are advected by the mean wind.
  • aerosol particles are affected by vertical diffusion, convective lofting, sedimentation, dry deposition.  Also wet deposition by large-scale and convective precipitation.

• All other IFS forecasts (HRES, ENS, extended-range and seasonal forecasts, and ECMWF Reanalysis products).  Gases and aerosols are represented by a monthly-mean climatology because prognostic variables are computationally expensive.  The climatology of:
  • the different greenhouse gases varies with month, latitude and height, but not longitude.
  • the various aerosol species varies with month, latitude and longitude,.  The vertical structure of the aerosol mass mixing ratio follows a simple exponential decrease with height. 

For both gases and aerosols, the climatology has been derived from the CAMS reanalysis.  However, slight tuning is applied to account for known deficiencies in certain locations of the globe.

Note: In the current IFS:

• • atmospheric chemistry is not yet included.
  • aerosol is not considered in the cloud microphysics (e.g. condensation nuclei).  This may be important for weather but the impact is difficult to assess.

Before the introduction of IFS Cycle 43R3 in July 2017, an older aerosol climatology was used.  This considered fewer aerosol species and did not include the dependence of optical properties on relative humidity. 

Users are advised to keep themselves updated about changes to the radiation scheme through the ECMWF Newsletter and web site.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• Read more on aerosols and greenhouse gases as considered within the IFS.

_{Amended/Updated 22/03/21 - Clarification}