Further Results on Numerical Cloud Seeding Simulations of Stratiform-Type Clouds

Abstract

Numerical simulations of ice-phase seeding of stratiform-type clouds have revealed the possibility of producing embedded convective cells in a stably stratified cloudy atmosphere. The requirements for this to occur are a high production rate of ice crystals from the seeding agent and a relatively low diffusion rate of the agent. The physical effect of the seeding is to glaciate a region of the cloud and cause the cloudy atmosphere to change from a saturated state with respect to liquid water to one saturated with respect to ice. Such a transformation releases heat of deposition, causing a few tenths of a degree increase in temperature and a few ms^-1 increase in vertical velocity. The transformation also produces one to two tenths of a gram of cloud ice per kilogram of air. These results have implications for convective clouds also. The temperature changes may help explain the often quoted positive impact of cloud seeding on moderate size convective clouds. Convective clouds in the range from 2 to almost 6 km depth show the largest potential for dynamic growth. These depths normally cover the -10°C to -25°C levels in the atmosphere, the temperature range of maximum effects for the transformation from saturation with respect to water (liquid) to one with respect to ice.