Recent updates to the atmospheric aerosol modelling of the ECMWF IFS in support to CAMS

Abstract

<p>The Integrated Forecasting System (IFS) of ECMWF is used within the Copernicus Atmosphere Monitoring Service (CAMS) to provide global analyses and forecasts of atmospheric composition, including aerosols as well as reactive trace gases and greenhouse gases.</p><p>The aerosol model of the IFS, IFS-AER, is a simple sectional-bulk scheme that forecasts seven species:  dust, sea-salt, black carbon, organic matter, sulfate, and  since July 2019, nitrate and ammonium.  The main developments that have been recently carried out, tested and are now contemplated for implementation in the next operational version (known as cycle 48r1) are presented here.</p><p>The dry deposition velocities are computed as a function of roughness length, particle size and surface friction velocity, while wet deposition depends mainly on the precipitation fluxes. The parameterizations of both dry and wet deposition have been upgraded with more recent schemes, which have been shown to improve the simulated deposition fluxes for several aerosol species. The impact of this upgrade on the skill scores of simulated aerosol optical depth (AOD) and surface particulate matter concentrations against a range of observations is very positive.</p><p>The simulated surface concentration of nitrate and ammonium are frequently strongly overestimated over Europe and the  United States in the current version of the IFS. Nitrate, ammonium, and their precursors nitric acid and ammonia, were evaluated against a range of ground and remote data and it was found that the recently-implemented gas-particle partitioning scheme is too efficient in producing nitrate and ammonium particles.</p><p>A series of small-scale changes, such as adjusting nitrate dry deposition velocity, direct particulate sulphate emission, and limiting nitrate/ammonium production by the concentration of mineral cations, have been implemented and shown to be effective in improving the simulated surface concentration of  nitrate and ammonium.</p><p>The representation of secondary organic aerosol (SOA) in the IFS has been overhauled with the introduction of a new SOA species, distinct from primary organic matter, with anthropogenic and biogenic components. The implementation of this new species leads to a significant improvement of the simulated surface concentration of organic carbon. An evaluation of simulated SOA concentrations at the surface against climatological values derived from observations using Positive Matrix Factorisation (PMF) techniques also shows a reasonable agreement.</p>