Implementation of a CAMS-based aerosol climatology in the IFS

This document describes an aerosol climatology based on a set of reanalysis of atmospheric composition developed by Copernicus Atmosphere Monitoring Service (CAMS) and its implementation in the ECMWF Integrated Forecasting System (IFS).We discuss the technical aspects of the implementation and the impact of the new climatology on the model climate and on the medium-range weather forecasts.

CAMS provides operational daily analysis and forecast of aerosol optical depth (AOD) for five aerosol species using an on-line integrated module for aerosol and chemistry coupled to IFS (CIFS). A fully prognostic aerosol model has a large impact on the weather forecasts in case of large aerosol concentrations as found during dust storms or strong pollution events. Due to its computational burden, prognostic aerosols are not yet feasible in the ECMWF operational weather forecasts though, and monthly-mean climatological fields are used instead.

We revised the aerosol climatology used in the operational ECMWF forecast model with one derived combining a set of short range forecasts and reanalysis for the period 2003-2014 with the CAMS system. The new climatology differs from the previous one both in the spatial distribution of the total AOD and the optical properties of each aerosol species. These changes affect the model mean biases at various spatial and temporal scales.

We report that the change to the new aerosol climatology has small a impact on the forecast skill of large-scale weather patterns but details of the regional distribution of aerosol radiative forcing can have a large local impact. This is the case of the area of the Arabian peninsula and the northern Indian Ocean where changes in the radiative forcing of the mineral dust significantly affect the summer monsoon circulation.