Aerosol Radiative Forcing and Weather Forecasts in the ECMWF Model

Abstract:

Aerosols play an important role in the energy balance of the Earth
system via direct scattering and absorpiton of short-wave and long-wave
radiation and indirect interaction with clouds. Diabatic heating or
cooling by aerosols can also modify the vertical stability of the
atmosphere and influence weather pattern with potential impact on the
skill of global weather prediction models.

The Copernicus Atmosphere Monitoring Service (CAMS) provides operational
daily analysis and forecast of aerosol optical depth (AOD) for five
aerosol species using a prognostic model which is part of the
Integrated Forecasting System of the European Centre for Medium-Range
Weather Forecasts (ECMWF-IFS). The aerosol component was developed
during the research project Monitoring Atmospheric Composition and
Climate (MACC).

Aerosols can have a large impact on the weather
forecasts in case of large aerosol concentrations as found during dust storms
or strong pollution events. However, due to its computational burden,
prognostic aerosols are not yet feasible in the ECMWF operational
weather forecasts, and monthly-mean climatological fields are used instead.

We revised the aerosol climatology used in the operational ECMWF IFS
with one derived from the MACC reanalysis. We analyse
the impact of changes in the aerosol radiative effect on the mean model
climate and in medium-range weather forecasts, also in comparison with prognostic aerosol fields.

The new climatology differs from the previous one by Tegen et al
1997, both in the spatial distribution of the total AOD and the optical
properties of each aerosol species. The radiative impact of these changes affects the model mean
bias at various spatial and temporal scales. On one hand we report small
impacts on measures of large-scale forecast skill but on the other hand
details of the regional distribution of aerosol concentration have a
large local impact. This is the case for the northern Indian Ocean where
the radiative impact of the mineral dust on the local temperature gradients
affects the summer monsoon circulation reducing forecast errors.