Update Copy of Aerosols and Greenhouse Gases

Aerosols and Greenhouse Gases

The IFS can consider the effects of several greenhouse gases and aerosol species that impact forecasts via their interaction with short-wave and long-wave radiation, which can heat or cool the atmosphere and the surface. Aside from water vapour, the greenhouse gases considered are carbon dioxide, ozone, methane, nitrous oxide and four CFC compounds. The aerosol types considered are sea salt, desert dust, organic matter, black carbon, and ammonium sulphate. In the case of sea salt and dust, three different sized particles are represented. Several of the aerosol species are hydrophyilic, which means that they would swell as the relative humidity increases. This makes the aerosol more optically thick and if this process is activated in the model it can act to reduce visibility in humid conditions.

A prerequisite to a reliable treatment of the interaction of greenhouse gases and aerosols with radiation is that their global distribution is well represented. There are two different configurations of the IFS that are used operationally, and which represent their global distribution in different ways:

• CAMS forecasts. In the model configuration used to produce air-quality forecasts for the Copernicus Atmosphere Monitoring Service (CAMS), including the CAMS reanalysis, gases and aerosols are represented by prognostic variables. This means that they are advected around with the model winds, and source and sink processes are represented. In the case of gases, fluxes from anthropogenic sources, vegetation, wetlands and ocean are all taken into account, as well as chemical reactions between gases. In the case of natural aerosol, the dependence of surface emissions on wind speed is represented (e.g. dust raised by wind and sea salt from breaking waves). Anthropogenic aerosol emissions such as those from urban sources and biomass burning make use of a database of surface sources.  In addition to aerosol particles being advected by the mean wind, they are also affected by vertical diffusion and convective lofting, sedimentation, dry deposition, and wet deposition by large-scale and convective precipitation.
• All other forecasts: Since prognostic variables are computationally expensive, in all other configurations of the IFS (HRES*, ENS, extended-range and seasonal forecasts, and ECMWF Reanalysis products), gases and aerosols are represented by a monthly-mean climatology. In the case of the different greenhouse gases, the climatology varies with month, latitude and height, but not longitude.  In the case of the various aerosol species, the climatology varies with month, latitude and longitude, but 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 but with a slight tuning to account for known deficiencies in certain locations of the globe.  Note that before the introduction of IFS Cycle 43R3 in July 2017, an older aerosol climatology was used that considered fewer aerosol species and did not include the dependence of optical properties on relative humidity.  Atmospheric chemistry is not yet included in the IFS.  Users should also be aware aerosol is not considered in the cloud microphysics (e.g. condensation nuclei) within the operational IFS - the fact that this may be important for weather is worth stressing although the impact is difficult to assess.

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}