ACCORD aerosols

ACCORD is a NWP consortium for development of limited-area short-range numerical weather prediction (NWP) system, representing 26 European and North-African national weather services. Various cloud microphysics and radiation schemes available in ACCORD framework are being developed to account for aerosol impact on the cloud, precipitation and fog formation as well as on the direct and indirect aerosol effects on radiation transfer. As a short-range limited area NWP system, ACCORD does not aim at fully integrated representation of weather-aerosol-chemistry dynamics but relies on external climatological and near-real-time aerosol data, currently obtained from Copernicus Atmosphere Monitoring Service (CAMS). Preliminary results of single-column and full model experiments in cases of dust intrusion and very clean air illustrate the interactions, sensitivities and uncertainties of the forecast, related to the choices of aerosol source data, parametrization schemes and their settings.

Currently, the HARMONIE-AROME model within ACCORD uses monthly climatologies of total aerosol optical depth at 550 nm of four aerosol types (land, sea, urban and desert) for radiation parametrizations only. These data come in coarse global grid and represent the average conditions. Radiation impacts of dust and carbon aerosols, that can be significant especially during dust intrusions and wildfires, cannot really be taken into account. On the other hand, currently the aerosol impact on liquid and ice cloud formation and evolution are not taken into account at all. The most important aerosol impacts on cloud microphysics are presumably related to the water-soluble aerosol like sulfates and sea salt, that are abundant but regionally variable in the atmosphere.

The possibility to use near-real-time aerosol as an external source in the high-resolution NWP system requires that the model is adapted to use these data in an optimal way. For radiation, this means better definition of the aerosol optical properties depending on the species and wavelengths as well as on the real-time atmospheric humidity at different elevations. For the single-band radiation schemes, aerosol optical depths of the aerosol mixture for short-wave and long-wave bands are obtained as weighted averages, based on the data of aerosol mass mixing ratios and the prescribed aerosol inherent optical properties. Considering cloud-precipitation microphysics, droplet number concentration and specific cloud ice content depend on aerosol. These influence the cloud, precipitation and fog evolution and also, via the cloud particle size, the radiation transfer in clouds.