Aerosol can affect clouds in various ways. Beside the micro-physical
impact of aerosol particles on cloud formation, the interference of
aerosol with atmospheric radiation leads to changes in local heating,
surface fluxes and thus meso-scale circulations all of which may also
modify clouds. Rather little is known about these so-called semi-direct
effects in realistic settings-a reason, why this study investigates the
impact of absorbing aerosol particles on cloud and radiation fields over
Germany. Using advanced high-resolution simulations with grid spacings
of 312 and 625 m, numerical experiments with different aerosol optical
properties are contrasted using purely-scattering aerosol as control
case and realistic absorbing aerosol as perturbation. The combined
effect of surface dimming and atmospheric heating induces positive
temperature and negative moisture anomalies between 800 and 900 hPa
impacting low-level cloud formation. Decreased relative humidity as well
as increased atmospheric stability below clouds lead to a reduction of
low-level cloud cover, liquid water path and precipitation. It is
further found that direct and semi-direct effects of absorbing aerosol
forcing have similar magnitudes and equally contribute to a reduction of
net radiation at the top of the atmosphere .