Data for Simulating the seeder-feeder impacts on cloud ice and precipitation over the Alps

The ice phase impacts many important cloud properties and the lifetime of the clouds. Ice particles that sediment into a lower cloud from an upper cloud (external seeder-feeder process) or into the mixed-phase region of a deep cloud from cirrus levels (internal seeder-feeder) can amplify cloud glaciation and enhance surface precipitation. Recently, numerical weather prediction modeling studies have aimed at representing the ice crystal number concentration in mixed-phase clouds more accurately by including secondary ice formation processes. The increase in the ice crystal number concentration can impact the number of ice particles that sediment into the lower cloud and alter its composition and precipitation formation. In the Swiss Alps, the orography permits the formation of orographic clouds, making it ideal for studying the occurrence of multi-layered clouds and the seeder-feeder process. We present results from a case study on May 18, 2016, showing the occurrence frequency of multi-layered clouds and the seeder-feeder process. We included ice-graupel breakup to enhance the ice crystal number concentration and investigate the precipitation formation processes. 47.6\% of all observed clouds were categorized as multi-layered, in which the external seeder-feeder process occurred in 10.3\% of these clouds which is similar to what was found in several studies. In between cloud layers, 58.4\% of the ice particle mass was lost due to sublimation or melting. The external seeder-feeder process was found to be more important, with regard to the impact on precipitation, than the internal seeder-feeder process in this case study. In the case where the external seeder-feeder process was blocked, the average surface precipitation and riming rate over the domain were both reduced by 8.5\% and 3.9\%, respectively. When ice-graupel collisions were allowed, further large reductions were seen in the liquid water fraction and riming rate. Blocking of the internal seeder-feeder process enhanced the liquid water fraction of 6\% compared to a reduction of 5.8\% in the cloud condensate and, therefore, pointing towards the deamplification in cloud glaciation and a reduction in surface precipitation.