The impact of heterogeneous freezing on the microphysical properties and the radiative budget of orographic cirrus clouds

The influence of heterogeneous freezing on the microphysical and optical properties of orographic cirrus clouds has been simulated with the cloud resolving model EULAG. Idealized simulations with different concentrations of IN in a dynamically dominated regime with high vertical velocities have been performed. Our studies show, that the onset of homogeneous freezing is shifted in time depending on the IN concentration as the available water vapor is depleted before the critical threshold for homogeneous freezing is reached. Although the high vertical velocities in an orographic gravity wave lead to a strong adiabatic cooling followed by high ice supersaturations, a small number concentration of IN in the order of 5 l-1 is already able to change the cloud properties significantly. Furthermore, the cloud extend increases. Heterogeneous freezing already starts upstream of the mountain as the streamlines are slightly lifted. This small lifting provides enough supersaturation for initiating the heterogeneous freezing. As these effects strongly modulate the optical properties of the clouds and the IN concentration due to anthropogenic activities might increase in future, it is of great importance to investigate the effect of heterogeneous IN on cirrus clouds in detail.6-2010-->