The enhancement of droplet collision by electric charges and atmospheric electric fields

Guo, Shian; Xue, Huiwen

The effects of electric charges and fields on droplet collision–coalescence and the evolution of cloud droplet size distribution are studied numerically. Collision efficiencies for droplet pairs with radii from 2 to 1024 inline-formulaµm and charges from inline-formula−32inline-formular2 to inline-formula+32inline-formular2 (in units of elementary charge; droplet radius inline-formular in units of inline-formulaµm) in different strengths of downward electric fields (0, 200, and 400 V cminline-formula−1) are computed by solving the equations of motion for the droplets. It is seen that the collision efficiency is increased by electric charges and fields, especially for pairs of small droplets. These can be considered as being electrostatic effects.

The evolution of the cloud droplet size distribution with the electrostatic effects is simulated using the stochastic collection equation. Results show that the electrostatic effect is not notable for clouds with the initial mean droplet radius of inline-formula M9inlinescrollmathml true r normal ¯ = normal 15 32pt10ptsvg-formulamathimg81c05f1310b0c1b11feee2c0aa7d267b acp-21-69-2021-ie00001.svg32pt10ptacp-21-69-2021-ie00001.png inline-formulaµm or larger. For clouds with the initial inline-formula M11inlinescrollmathml true r normal ¯ = normal 9 26pt10ptsvg-formulamathimge491318909a55f3eb4118b8087de356f acp-21-69-2021-ie00002.svg26pt10ptacp-21-69-2021-ie00002.png inline-formulaµm, the electric charge without a field could evidently accelerate raindrop formation compared to the uncharged condition, and the existence of electric fields further accelerates it. For clouds with the initial inline-formula M13inlinescrollmathml true r normal ¯ = normal 6.5 35pt10ptsvg-formulamathimg65a224c4bae5b3eb08d8668abd884e98 acp-21-69-2021-ie00003.svg35pt10ptacp-21-69-2021-ie00003.png inline-formulaµm, it is difficult for gravitational collision to occur, and the electric field could significantly enhance the collision process. The results of this study indicate that electrostatic effects can accelerate raindrop formation in natural conditions, particularly for polluted clouds. It is seen that the aerosol effect on the suppression of raindrop formation is significant in polluted clouds, when comparing the three cases with inline-formula M15inlinescrollmathml true r normal ¯ = normal 15 32pt10ptsvg-formulamathimge053dc4187a428a13d18f92ffa11a63c acp-21-69-2021-ie00004.svg32pt10ptacp-21-69-2021-ie00004.png , 9, and 6.5 inline-formulaµm. However, the electrostatic effects can accelerate raindrop formation in polluted clouds and mitigate the aerosol effect to some extent.

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Guo, Shian / Xue, Huiwen: The enhancement of droplet collision by electric charges and atmospheric electric fields. 2021. Copernicus Publications.

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