Abstract. Aerosols affect atmospheric dynamics through their direct and
semi-direct effects as well as through their effects on cloud microphysics
(indirect effects). The present study investigates the indirect effects of
aerosols on summer precipitation in the Euro-Mediterranean region, which is
located at the crossroads of air masses carrying both natural and
anthropogenic aerosols. While it is difficult to disentangle the indirect
effects of aerosols from the direct and semi-direct effects in reality, a
numerical sensitivity experiment is carried out using the Weather Research
and Forecasting (WRF) model, which allows us to isolate indirect effects, all
other effects being equal. The Mediterranean hydrological cycle has often
been studied using regional climate model (RCM) simulations with
parameterized convection, which is the approach we adopt in the present
study. For this purpose, the Thompson aerosol-aware microphysics scheme is
used in a pair of simulations run at 50 km resolution with extremely high
and low aerosol concentrations.
An additional pair of simulations has been
performed at a convection-permitting resolution (3.3 km) to examine these
effects without the use of parameterized convection. While the reduced radiative flux due to the direct effects of the aerosols is
already known to reduce precipitation amounts, there is still no general
agreement on the sign and magnitude of the aerosol indirect
forcing effect on precipitation,
with various processes competing with each other. Although some processes
tend to enhance precipitation amounts, some others tend to reduce them. In
these simulations, increased aerosol loads lead to weaker precipitation in
the parameterized (low-resolution) configuration. The fact that a similar
result is obtained for a selected area in the convection-permitting
(high-resolution) configuration allows for physical interpretations. By
examining the key variables in the model outputs, we propose a causal chain
that links the aerosol effects on microphysics to their simulated effect on
precipitation, essentially through reduction of the radiative heating of the
surface and corresponding reductions of surface temperature, resulting in
increased atmospheric stability in the presence of high aerosol loads. Keywords. Atmospheric composition and structure (aerosols and particles)
Contribution of the North Atlantic subtropical high to regional climate model (RCM) skill in simulating southeastern United States summer precipitation