A Numerical Model of Precipitation from Seeded and Unseeded Cold Orographic Clouds

Abstract

This paper describes a numerical model of ice-phase precipitation from orographic clouds which includes the effects of seeding with artificial ice nuclei. The model describes the events which take place when a layer of moist air of near-neutral stability, overlain by a more stable dry layer, flows over a mountain ridge. A two-dimensional, steady-state model of the flow in the vertical plane normal to the ridge furnishes a field of the flow streamlines along which microphysical processes are followed. The cloud physics model describes the formation of the supercooled cloud, the formation of ice particles from both natural and artificial ice nuclei, and the growth and precipitation of ice particles. Growth by both vapor deposition and riming are included. Artificial ice nuclei are released from a localized source at ground level. A simple Fickian diffusion process is used to describe the vertical transport of nuclei to the cold upper region of the cloud.The model calculates the rate of precipitation at ground level as a function of horizontal position. The dependence of precipitation efficiency on cloud top temperature is in good agreement with field observations from the Climax experiment. Substantial precipitation increases are produced by seeding clouds whose natural precipitation efficiencies are low. However, it is shown that the seeding rate required to produce a given increase in precipitation rate is a strong function of cloud temperature. The model also shows that precipitation rates depend upon the activity of the ice nuclei as a function of temperature.