Development of ice particles in convective clouds observed over the Black Forest mountains during COPS

A combination of modelling studies and ground-based and aircraft measurements is used to examine the development of ice particles in convective clouds observed over the Black Forest mountains during the Convective and Orographically-induced Precipitation Study. High concentrations of relatively small ice particles were observed in the weaker northern cell that developed on convergence lines over the mountains in the much-studied 15 July 2007 case. The conditions in the cloud were not conducive for the Hallett-Mossop process. Instead, the explanation for such high concentrations of ice is likely associated with the type of ice nuclei ingested into the cloud. Biological nuclei, oxidised organic aerosol particles in the polluted air vented from the Murg Valley into the cloud base, and desert dust are all possible candidates. A model sensitivity test with biological nuclei produced similar concentrations of ice particles to the observations. In contrast, the high concentration of ice particles measured in clouds that advected over the Black Forest mountains on 11 July 2007 were likely due to the Hallett-Mossop process.

The deep convective cell at the southern end of the COPS domain on 15 July 2007 developed in less polluted air than the shallower northern cloud. A model sensitivity test with lower aerosol loading produced a more vigorous cloud with a higher top, more precipitation, and greater reflectivity, more similar to the radar observations. The results suggest that aerosol particles vented out of the valleys could have a significant impact on orographically-induced precipitation.