Collision efficiency of charged cloud droplets in electric fields

Abstract

Collision efficiencies are shown for 5‐ and 10‐micron droplets colliding with 30‐, 40‐, and 50‐micron drops. The charge on the drop is always positive, but the sign of the charge on the droplet is alternately positive and negative. The magnitude of the charge on the droplet extends from zero to 10−14 coulomb. The calculations are also extended to include electric field intensities of 0, 900, 2100, and 3600 v/cm oriented at angles of 0°, 90°, and 180° with respect to the direction of fall of the drop. The electrical effects were found to influence collision efficiencies only when the droplet charges were greater than about 10−16 coulomb or when the electric field intensity exceeded 900 v/cm. The pertinence of the calculations to the physics of electrified clouds is briefly discussed.

The 15–25 μm barrier to drop growth in warm rain

“…An assembly of charged drops of mixed sign might be expected to have an increased collection efficiency due to electrostatic attraction. An experimental study by Woods (1965) and theoretical calculations by Semonin and Plumlee (1966) confirm that charged droplets enhance collision efficiencies linearly with increasing charge. However, the effect only commences after a certain threshold charge of about 10~1 6 C (about 800 elementary charges) per drop is reached.…”

“…Electric fields may enhance the accretion process either by inducing polarization charges which increase the efficiency of accretion, or by causing slightly charged drops to travel in straight lines due to the electric force, and thus not be deflected by aerodynamic forces from an impending collision with another drop. The effects of electric fields on accretion have been studied theoretically by Lindblad and Semonin (1963), Plumlee and Semonin (1966) and Hocking and Jonas (1970), and experimentally by Woods (1965) and Latham (1969). The results of the various workers do not vary significantly.…”

Modification of Precipitation from Warm Clouds—A Review

“…Estimated summer rainfall increases downwind of St. Louis amounted to 25%, and even greater percentage increases in thunderstorms and hail (Changnon et al, 1976). Observations of greater droplet concentrations and narrower droplet distributions in urban and downwind fair weather cumulus, rather than in upwind clouds, led Braham (1974) and Semonin and Changnon (1974) to hypothesize the existence of giant CCN to explain lower first-echo heights in urban areas, rather than rural areas. In the absence of the hypothesized giant CCN, the observed "more continental" urban clouds would be expected to be more colloidally stable, and thus have higher first-echo heights.…”

“…Moreover, owing to the very small total collection kernels of droplets r < 20 /um, the influence of turbulence on collision and coalescence must be very great in order to account for the initiation of precipitation embryos. The effects of drop charge and electric fields on collision and coalescence have been examined both theoretically and experimentally by numerous investigators over the years (Davis, 1964a,b;Sartor, 1960;Davis, 1965;Krasnogorskaya, 1965;Lindblad and Semonin, 1963;Plumlee and Semonin, 1965;and Semonin and Plumlee, 1966). The results of these studies suggest that the collision efficiencies for small, strongly charged drops can be considerably enhanced in field strengths characteristic of thunderstorms.…”

Capture of Particles on Spheres by Inertial and Electrical Forces

“…There have also been a number of studies (e.g., Sartor, 1960;Plumlee and Semonin, 1965;Semonin and Plumlee, 1966) of the collection of free-falling water droplets, considering both inertial and electrical forces. The results of those studies are difficult to compare to the present one, since they usually involve the collision of droplets whose sizes are the same order of magnitude (and for which hydrodynamic interactions should be considered) and also consider the influence of gravity.…”