Chaff Seeding Effects in a Dynamical-Electrical Cloud Model

Abstract

A two-dimensional, slab-symmetric, time-dependent cloud model has been devised to simulate deep convection in the atmosphere. The dynamics and thermodynamics of deep convection are prescribed and the microphysics of the liquid phase is parameterized. Within this framework, the electrical nature of the atmosphere has been added. Small positive and negative ions as well as the charge associated with rain and cloud particles are included. The electrical properties are allowed full interaction with the hydrodynamic properties of the model. Charge is transported by conduction, convection and turbulent diffusion, and free ions interact with hydrometeors through conduction, diffusion and evaporation. A separation probability is specified for cloud and raindrops colliding in the electric field allowing for charge transfer by the polarization mechanism. Similarly, the charge on cloud droplets is transferred to raindrops during coalescence. The formulation for the simulation of the chaff seeding process is then developed. An initial distribution of chaff fibers is devised allowing for the introduction of a prescribed amount of chaff at any time and altitude. Transport of the chaff fibers in the wind field is accounted for and ion production by chaff in the electric field is parameterized. Five cases, a control case and four seeding cases, are examined to explore the effect of the chaff ions on the electrical properties of the cloud. The control case (case 1) is reviewed showing its basic dynamical, microphysical and electrical nature. The chaff seeding experiments are then discussed with the seeding altitudes, times and the amount of chaff dispensed differentiating the four cases. It is found that chaff seeding at a rate of 4 kg km?1, which yields an initial maximum concentration of 8.66 ? 10?8 kg m?3, attains a reduction in the electric field strength within the cloud in two ways. In a direct manner, the chaff ions reduce the charge on the hydrometeors by conduction and diffusion. In an indirect manner, the reduction of charge on the hydrometeors in one region is seen to reduce the electric field in other regions, thereby reducing the efficiency of the polarization mechanism there, and subsequently reducing the amount of charge separated. Comparison of an early and late seeding case reveals that the initial effect of the chaff ions is different, but that after a short time the effects become comparable and the final result of the two cases is nearly identical. This suggests that as long as the chaff fibers penetrate the active portions of the cloud, the exact seeding time is of little consequence. Finally, it is suggested that chaff seeding may be useful in discriminating between inductive and non-inductive charge separation mechanisms.