A numerical study on the riming process in the transition from a pristine crystal to a graupel particle

In this paper the flow field and riming efficiency of ice particles evolving from non-rimed to heavily rimed state is investigated by means of computational fluid dynamics. Riming process is one of the most efficient pathways to evolve the ice particles into precipitation in the atmosphere. For instance, the riming efficiency specified in cloud resolving models can modulate the spatial distribution of surface precipitation as well as cloud dynamics significantly by changing the mass of ice particles, therefore the terminal velocity. The flow fields around crystals depend on the geometry, so the riming efficiency of crystals depends on the geometry as well. The flow field calculation from the pristine crystal to the graupel particle remain challenging and not well understood because the flow field interacts with the crystal whose geometry is evolving due to the flow-dependent riming.

For this work, we first determine the flow field around the falling hexagonal ice plate. From the trajectory analysis of the super cooled droplets around the ice crystal we determine the flux density of droplets impacting the hexagonal plate ice crystal. These super-cooled droplets are freezing on contact with the surface of the hexagonal plate and are forming a porous media. To consider the influence of the rimmed portion of the crystal we change the geometry of the crystal by adding a porous media in accordance with the super-cooled water droplets flux density. The proprieties of the porous media are correlated with the size distribution of the super-cooled water droplets. This process is carried out iteratively as the rimming process is allowed to continue. The riming efficiency as well as the flow fields will be discussed in the presentation.