A Numerical Investigation of the Effect of Electric Charges and Vertical External Electric Fields an the Collision Efficiency of Cloud Drops: Part II

R. J. Schlamp Department of Atmospheric Science, University of California at Los Angeles, 90024

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S. N. Grover Department of Atmospheric Science, University of California at Los Angeles, 90024

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H. R. Pruppacher Department of Atmospheric Science, University of California at Los Angeles, 90024

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A. E. Hamielec Department of Chemical Engineering, McMaster University, Hamilton, Ontario Canada L8S-4K1

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Abstract

The numerical model of Schlamp et al. (1976) for determining the collision efficiency of electrically charged or unchanged cloud drops in the presence or absence of a vertical electric field has been extended to study the two following cases, both of which include the presence of a vertical field due to a net positive charge in the upper part of the cloud and a net negative charge in the lower part of the cloud: (i) the larger drop is negatively charged and is initially above the smaller drop, which is positively charged; (ii) the larger drop is negatively charged and it is initially below the smaller drop, which is again positively charged. Also, for the purpose of resolving more accurately the critical electric charge on the drops and the critical electric field necessary to significantly affect the collision efficiency, additional computations have been carried out for charged drops in the absence of an electric field and for unchanged drops in the presence of a vertical electric field.

The sizes of drops considered range from 1–118 μm in radius. The magnitude of the electric charges on the drops range from 0–2.8×10−4 esu, and the electric fields range in strength from 0–3429 V cm−1, which include the charges and fields typically observed in thunderstorms.

It is found that electric fields and charges even of relatively modest values have a profound effect upon the collision efficiency. The results of case (i) show that the electrostatic forces are responsible for determining the shape of the collision efficiency curves with the hydrodynamic forces being of secondary importance. These results are significantly different from either those of case (ii) or those of Schlamp et al. (1976).

Abstract

The numerical model of Schlamp et al. (1976) for determining the collision efficiency of electrically charged or unchanged cloud drops in the presence or absence of a vertical electric field has been extended to study the two following cases, both of which include the presence of a vertical field due to a net positive charge in the upper part of the cloud and a net negative charge in the lower part of the cloud: (i) the larger drop is negatively charged and is initially above the smaller drop, which is positively charged; (ii) the larger drop is negatively charged and it is initially below the smaller drop, which is again positively charged. Also, for the purpose of resolving more accurately the critical electric charge on the drops and the critical electric field necessary to significantly affect the collision efficiency, additional computations have been carried out for charged drops in the absence of an electric field and for unchanged drops in the presence of a vertical electric field.

The sizes of drops considered range from 1–118 μm in radius. The magnitude of the electric charges on the drops range from 0–2.8×10−4 esu, and the electric fields range in strength from 0–3429 V cm−1, which include the charges and fields typically observed in thunderstorms.

It is found that electric fields and charges even of relatively modest values have a profound effect upon the collision efficiency. The results of case (i) show that the electrostatic forces are responsible for determining the shape of the collision efficiency curves with the hydrodynamic forces being of secondary importance. These results are significantly different from either those of case (ii) or those of Schlamp et al. (1976).

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