NUMERICAL EXPERIMENTS PERTAINING TO WARM-FOG CLEARING

Abstract

An attempt has been made quantitatively to assess the prospects for modifying warm fogs by seeding them with condensation nuclei. This has been done by calculating the time-dependent changes in the sizes and concentrations of fog droplets that are predicted by the ordinary equations of diffusion of water vapor to and from the surface of droplets. Their size, molality, and ambient water-vapor density are taken into account. Initial conditions consist of a homogeneous volume of air of specified height and aerosol content. An external cooling rate and seed dosage are specified. The effects of various combinations of cloud height and seed properties (such as size, mass density, and rate of injection) on the metamorphosis of the fog-droplet population are examined. Usually, the cloud-forming process (in which temperature decreases with time) was allowed to continue after the completion of seeding. It is tentatively concluded that the optimum-size seeding material is a function of the fog thickness; material smaller than about 4 µm in diameter should be excluded, and particles 20 to 50 µm in diameter are most suitable for seeding moderately thick (?100 m) xclouds.