This report is based on my PhD thesis: Integrated modeling of aerosol indirect effects. Some modifications have been made mainly in the model description section in order to bring the report up to date with respect to recent model developments.

The atmosphere is heavily polluted with accumulation mode aerosols which affect cloud formation and precipitation development. Consequently, continental clouds generally include two orders of magnitude more cloud droplets than marine clouds.

An increased number of small droplets, in warm clouds, leads to an increase in cloud reflectance and affects cloud lifetime. Complex cloud and aerosol -microphysical and cloud dynamical feedbacks shape the response to an increased number of smaller cloud droplets and may feed back on the aerosol and trace gas distributions of the atmosphere on short time scales. The importance of such feedbacks is unknown and in this study an online coupled chemical weather model is developed, tested and employed in a case study, investigating the importance of such feedbacks on trace gas distributions.

Enviro-HIRLAM is developed as an extension of the short-range weather forecast model HIRLAM and includes emission, advection, turbulent diffusion, convection and deposition of trace gases and aerosols as well as gas-phase chemistry, aerosol dynamics, gas-aerosol equilibration and aerosol activation. The activated aerosols are coupled to the cloud scheme leading to reflectance enhancement and suppression of precipitation in warm convective and stratiform clouds.

In a particular case study considering convective summertime conditions it was shown that the aerosol mass concentration was satisfactory predicted and two-meter temperature predictions improved slightly when including the aerosol effects. The distribution of NO2 near the surface was affected by the feedbacks over the 24 hour period. The feedbacks induced changes in cloud cover, temperature and in local circulations by inducing convective activity which lead to a dynamical redistribution of the species rather than to changes in chemical reactions.

The suppression of rain was of greater importance than reflectance enhancement and nonlinear effects acted to damp the influence of the feedbacks. Hence, in this case study the feedbacks were of great importance in determining the trace gas distributions.