Variability in the Diffusional Growth Rate of a Riming Graupel Surface and Its Implications for Graupel Electrification

The suggestion that the faster growing surface obtains the positive charge, during an ice crystal to graupel interaction/collision, is called the Relative Diffusional Growth Rate (RDGR) electrification mechanism and may be the most accepted thunderstorm charging mechanism. RDGR takes the source contribution of vapor from rime droplets freezing on the graupel surface, called the near field contribution, under consideration. RDGR was used to explain the observed trends of some extensive laboratory charging experiments. However, explaining all laboratory experiments and thunderstorm electrification by RDGR remains an open discussion among interested researchers.

Here, we contribute to that discussion by using a variety of numerical techniques to carry forward the simple equation of Baker et al. (1987) into a more sophisticated model. First, we perform three-dimensional Computational Fluid Dynamic (CFD) simulations of the flow around the graupel to estimate droplet trajectories and collision probabilities along the graupel surface. Then, we conduct simulations of diffusion including the advection of vapor from the flow solution. These solutions quantify the spatial variability in the diffusional growth rate of a riming graupel surface. The specific cases include graupel sizes of 160 and 250 µm, and various droplet size spectra.

Past calculations applied ventilation effects with no spatial variability. We explicitly calculate the ventilation effect including its spatial variability. For the far field, the ventilation enhances the diffusional growth where ice crystals are more likely to collide. For the near field, the ventilation, compared to the average, reduces the effect of freezing droplets as sources of vapor by advecting some to the environment. The consequences for the fraction of the graupel surface in a positive charging state are discussed as the droplet size spectrum varies.