A Modeling Study on the Sensitivities of Atmospheric Charge Separation According to the Relative Diffusional Growth Rate Theory to Nonspherical Hydrometeors and Cloud Microphysics

Abstract

Collisional charge transfer between graupel and ice crystals in the presence of cloud droplets is considered the dominant mechanism for charge separation in thunderclouds. According to the relative diffusional growth rate (RDGR) theory, the hydrometeor with the faster diffusional radius growth is charged positively in such collisions. We explore sensitivities of the RDGR theory to nonspherical hydrometeors and six parameters (pressure, temperature, liquid water content, sizes of ice crystals, graupel, and cloud droplets). Idealized simulations of a thundercloud with two-moment cloud microphysics provide a realistic sampling of the parameter space. Nonsphericity and anisotropic diffusional growth strongly control the extent of positive graupel charging. We suggest a tuning parameter to account for anisotropic effects not represented in bulk microphysics schemes. In a susceptibility analysis that uses automated differentiation, we identify ice crystal size as most important RDGR parameter, followed by graupel size. Simulated average ice crystal size varies with temperature due to ice multiplication and heterogeneous freezing of droplets. Cloud microphysics and ice crystal size thus indirectly determine the structure of charge reversal lines in the traditional temperature-water-content representation. Accounting for the variability of ice crystal size and potentially habit with temperature may help to explain laboratory results and seems crucial for RDGR parameterizations in numerical models. We find that the contribution of local water vapor from evaporating rime droplets to diffusional graupel growth is only important for high effective water content. In this regime, droplet size and pressure are the dominant RDGR parameters. Otherwise, the effect of local graupel growth is masked by small ice crystal sizes that result from ice multiplication.

Microphysical and Transportive Contributions to Normal and Anomalous Polarity Subregions in the 29–30 May 2012 Kingfisher Storm

“…Is there simply evidence for less SLW at low temperatures overall? Can observations such as these be used to evaluate the differences of specific NI electrification studies on storm scales (e.g., Glassmeier et al., 2018; Saunders & Peck, 1998; Takahashi, 1978)? Extensions of this methodology to other cases could begin to answer some of these questions and further inform our theoretical model of storm electrification.…”

“…The aforementioned individual flashes, which initiated at heights that would not be expected to have large electric field magnitudes based on the overall inferred charge regions, suggest a more complicated charge distribution than is revealed by the flash and therefore time-integrated analysis described above. This could be from some small variations in particle electrification due to local variations in environment or particle types (e.g., Avila et al, 2005;Glassmeier et al, 2018;Tsenova & Mitzeva, 2009) or even from likely charge deposited by flashes on ice particles within this region of high flash rates as hypothesized by MacGorman and Burgess (1994) and as simulated by Helsdon et al (1992). As this region did also contain small events that could not be analyzed as previously mentioned, it is likely that there was at least rapid charge replenishment and perhaps small-scale heterogeneities in electrification regimes.…”

“…They do generally agree that relatively warm, moist environments support positive graupel electrification, so that will be the focus of discussion in this manuscript, with the acknowledgement that the precise temperature and moisture characteristics that result in either positive or negative graupel electrification are debated (e.g., Saunders & Peck, 1998; Takahashi, 1978). There are many additional factors that can also influence electrification: Particle roughness can affect the relative growth rate of hydrometeors and by extension the polarity of charge transferred in collisions (e.g., Avila et al., 2005; Glassmeier et al., 2018; Tsenova & Mitzeva, 2009); impact velocity may also be a factor, as included in rime accretion rate‐controlled laboratory studies (e.g., Brooks et al., 1997; Saunders & Peck, 1998); and electrification at low water content is under active investigation (e.g., Dye & Bansemer, 2019; Luque et al., 2016).…”