Scavenging of Particles by Cloud Droplets with Varying Particle Density and Relative Humidity
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Results of a Monte Carlo trajectory model calculating collision rate coefficients between electrically charged 3.0μm radius cloud droplets and aerosol particles are presented in this work. For particles with radius less than 0.2μm the collision rate coefficients are mainly determined by diffusion and electric forces, and if the electric force is a purely inverse square Coulomb force there exists an analytic solution, however, the image electric force induced by a particle charge can significantly enhance the collision rate coefficient. As particle radius increases the results become complicated, the effect of flow around particle (‘fap’) on the droplet’s movement becomes significant, and the intercept effect and the weight effect also become important. These can increase or decrease the collision rate coefficient. Especially, if the particle density is greater than the droplet density, as the particle radius increases its fall speed will approach to the fall speed of the droplet, and then collisions occur at the rear-side of the droplet due to the effect of the region of low flow speed around the droplet, which will also influence other effects. When the droplet and particle charges are same-sign, for large particles with density less than that of the droplet the predominant effect is electro-scavenging due to the short-range image electric force induced by droplet charge, but for particles with density greater than that of droplet the effect of a stagnation region below the droplet becomes significant and can reduce the effect of short-range image electric forces. The effect of relative humidity reaches maximum for about 1.0μm particle radius and it will dominate over other effects if the relative humidity departs far from 100%; however, for smaller particles the effect of relative humidity will diminish and disappear. The solar wind modulates current flow in the global electric circuit (GEC) through which the solar activity can affect weather and climate, in this link the electrification of the cloud boundaries by the GEC can enhance the microphysical processes in clouds by charging droplets and particles, which in turn affects the microphysical collision rates as simulated in this work, and ultimately affects cloud development. However, this mechanism has still not been incorporated in current climate and cloud models due to the complexity of the microphysics. Our current efforts have been focused on the parameterization of collision rate coefficients for varying droplet radius, droplet charges, particle radius, particle charges, particle density and relative humidity. Our aim is to make the use of collision rate coefficients in cloud models simple, less time consuming and user friendly, by providing a code applying the parameterization. Thus it will be possible to incorporate the role of the global electric circuit into global climate models.