Electric Charge Effects on In-Cloud Scavenging: A Comprehensive New and Simplified Parameterization for Comparison with Observations

New simulations of collision rate coefficients for aerosol particles with droplets (in-cloud scavenging) extend to particle radii 2.5 microns, particle density 2000 kg m^-3, and droplet radii up to 15 microns, with varying particle and droplet electrical charges and varying relative humidity. The simulations are made with a Monte-Carlo trajectory model and the results are fitted with a new and simplified parameterization that treats short-range effects as equivalent to an increment on droplet radius, in the form of a modified analytic solution (MAS; Zhang et al., 2018). This extends the simple analytic solution for thermophoresis, diffusiophoresis, and Coulomb electric forces to short-range effects of the image electric forces and weight, intercept, and flow around the particle. The results can be extended for scavenging in ice clouds.

The macroscopic consequences for clouds and atmospheric dynamics can be complex, and extend over several cycles of condensation and evaporation. For smaller particles (< 0.1 microns radius) in space charge (excess of same-sign charges) regions the predominant electrical effect is a reduction in scavenging rates (electro-anti-scavenging) and a cumulative increase in interstitial aerosol concentration over values for symmetric charging. For larger particles in the symmetrically charged interior of clouds, the predominant effect is an increase in collection rates (electro-scavenging). This could be significant in moderate to strong updrafts, where thermophoresis around condensing droplets tends to reduce the collision rates for interstitial particles to near zero (Young, 1993). So the electro-scavenging increases these low collision rates with ice nuclei for contact ice nucleation if the droplets are supercooled, and increases the collection of immersion nuclei below the freezing level, for droplet freezing when carried to higher altitudes. Overall these can increase the rate of primary ice production.

Space charge regions are produced by the downward current density J_z flowing between the ionosphere and the Earth’s surface and passing through gradients in conductivity in clouds due to gradients in droplet concentration. These are obviously present at upper and lower cloud boundaries, but can also occur within clouds due to mixing. Thus the electro-anti-scavenging rates in these regions depend on J_z. There is strong observational evidence relating small changes in cloud opacity, radiative forcing, and surface pressure changes in polar regions to J_z, consistent with electro-anti-scavenging in thin stratus-like ice clouds. (Zhou et al., 2017; Frederick, 2016, 2017, Frederick and Tinsley, 2018).

Effects of electrically-induced scavenging on precipitation appear to be complex. Electro-anti-scavenging affects mainly small condensation and ice nuclei and is likely to decrease precipitation from layer clouds with low updraft speeds. Electro-scavenging applies mainly to larger cloud condensation nuclei and ice nuclei. The reduction in their concentration in clouds with moderate to strong updrafts can initially decrease precipitation by narrowing the droplet size distribution and inhibiting rain production, but if, as a consequence, more liquid water is carried above the freezing level, the resulting latent heat release can invigorate the storm by increasing ice production and latent heat release, and ultimately increase precipitation. The dynamical consequences of the latent heat transfer and radiative forcing by changes in high-level cloud are even more complex, and require exploration with cloud and storm modeling.

The equilibrium charge on aerosol particles depends on their size, and for diffusive charging it varies as the square root of the particle radius (Beard and Ochs, 1986). However, the residues after evaporation of droplets, which have much higher charges, retain the droplet charge for ten minutes or so, and so these residues, with their relatively large (for an aerosol particle) charge, can lead also to enhanced ice nucleation. Evaporation residues often have heterogeneous surface coatings from the evaporation process, increasing their ice-nucleation effectiveness. There is some evidence relating storm invigoration and storm vorticity in sub-polar regions to J_z, consistent with ice production via electro-scavenging (Tinsley, 2012; Zhou et al., 2014).

References:

Beard, K. V. K., and H. T. Ochs (1986). Charging mechanisms in clouds and thunderstorms, in The Earth’s Electrical Environment, National Academy Press, Washington, DC. 1986.

Frederick, J. (2016). Solar irradiance observed at Summit, Greenland: Possible links to magnetic activity on short timescales, J. Atmos. Solar Terr. Phys., 147, 59-70. doi.org/10.1016/j.jastp.2016.07.001

Frederick, J. (2017). An analysis of couplings between solar activity and atmospheric opacity at the South Pole, J. Atmos. Solar Terr. Phys., 164, 97-104. doi.org/10.1016/j.jastp.2017.08.011

Frederick, J. A., and B. A. Tinsley (2018). The response of longwave radiation to electrical and magnetic variations at the South Pole: correlations with inputs from meteorological generators and the solar wind, J. Atmos. Solar Terr. Phys., under revision.

Tinsley, B. A. (2012). A working hypothesis for connections between electrically-induced changes in cloud microphysics and storm vorticity, with possible effects on circulation, J. Adv. Space Res., 50, 791-805. doi.org/10.1016/j.asr.2012.04.008.

Young, K. C. (1993). Microphysical Processes in Clouds, Oxford University Press, Oxford, 1993.

Zhang et al. (2018). Parameterization of aerosol scavenging due to atmospheric ionization: Part 3. Effects of varying droplet radius, J. Geophys. Res. Atmos., in press.

Zhou, L., B. A. Tinsley and J. Huang (2014). Effects on winter circulation of short and long-term solar wind changes, Adv. Space Phys., 54, 2478-2490.

Zhou, L., B. A. Tinsley, L. Wang, and G. Burns (2018). The zonal-mean and regional tropospheric pressure responses to changes in ionospheric potential, J. Atmos. Solar-Terr. Phys., 171, 111-118. doi.org/10.1016/j.jastp.2017.07.010