Numerical simulations of the diffusional growth rate pattern on graupel and implications for graupel electrification

The Relative Diffusional Growth Rate (RDGR) theory states that the ice particle growing faster from vapor diffusion at the point of collision obtains the positive charge. The application of this theory in modeling studies has assumed the ice particle grows at a specific rate uniformly across the surface, but laboratory studies agree that accretion affects the ice surface unevenly, consequently impacting the charge transfer. Our goal is to numerically model the diffusional growth rate of a graupel particle at a refined spatial scale to understand how the growth pattern varies depending on ventilation and droplet accretion in an idealized setting. We perform Computational Fluid Dynamic (CFD) simulations of the flow around graupel to estimate droplet collision probabilities along the surface. Then, we conduct simulations of the vapor transport in the domain from the flow solution with and without droplets modifying the original graupel shape. These results are used to calculate the total radius diffusional growth rate from the perspective of the graupel surface. For the far field, the ventilation enhances the diffusional growth in the particle’s lower hemisphere. For the local field, the flow advects a fraction of the vapor released by freezing droplets to the environment, reducing its use in graupel's diffusional growth. This decrease is dependent on the droplet size and flow speed. In conditions of positive charging by the RDGR theory, the area prone to positive charging is more extensive with greater variability for a drop size distribution (DSD) of larger droplets than smaller ones, but requires a greater rime accretion mass. We found differences between ventilated and stationary solutions, by varying the DSD and whether using a constant or variable droplet collision rate. The overlapping of the droplets on the graupel surface revealed competing effects in expanding regions prone to positive charging or locally intensifying the growth rate.