Aerosol-cloud interactions and the effects on orographic precipitation

Anthropogenic and natural aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. An increase of the aerosol load leads to an increase of the cloud droplet number concentration and, for a given liquid water content, to a decrease of the average cloud droplet size. Since the collision efficiency is small for small droplets, the increased aerosol load induces a deceleration of the cloud drop coalescense process in warm phase clouds. Furthermore, the rain drop development through the (auto-)conversion process is prolongated. This prolongation effect extends the cloud lifetime and leads to a modification of precipitation formation.

In the case of shallow orographic clouds the aerosol-cloud interactions are suspected to reduce the amount of precipitation on the upslope side of the mountain and to enhance the precipitation on the downslope side of the mountain. The net effect may lead to a shift of the precipitation towards the leeward side of mountain ranges which affects the hydrological cycle on a local scale and, hence, is an important aspect of climate change.

The main purpose of this study is to investigate aerosol-cloud interactions in warm phase clouds and to quantify the aerosol indirect effect on the hydrological cycle. Herefore, simulations of moist orographic flow past topography are conducted and the influence of aerosol particles on the orographic precipitation formation is analyzed by comparing a polluted case against a clean reference case. The degree of aerosol pollution is simulated by prescribing different number concentrations of CCN which are then available for the cloud drop nucleation.

The simulations are performed with the Local Model (LM) which is operationally used as numerical weather prediction model with a horizontal resolution of about 7km times 7km at the German weather service (DWD).

Throughout this study the focus is put on warm phase clouds. The considered microphysical processes are the nucleation of cloud droplets, the selfcollection, the accreation and the autoconversion of cloud droplets into rain. These warm phase processes are treated within the framework of a two-moment microphysics scheme.