The Nature of Winter Clouds and Precipitation in the Cascade Mountains and their Modification by Artificial Seeding. Part I: Natural Conditions

Abstract

This is the first of three papers describing field investigations, carried out from 1969 to 1974, of winter clouds and precipitation in the Cascade Mountains of Washington State, and physical evaluations of their modification by artificial seeding. The present paper describes airborne and ground observations of the natural clouds and precipitation.It has been observed that (i) in pre-frontal conditions ice particles dominate over water droplets above the ?10°C level, but the ratio of ice to water is lower in post-frontal conditions; (ii) the passage of an occluded or warm front causes a sharp lowering of the diffusional growth layers of the ice crystals, so that particles reaching the ground change from unrimed crystals which grow at low temperatures to rimed crystals which form at higher temperatures; (iii) the maximum ice particle concentrations in the clouds are often several orders of magnitude greater than measurements of ice nuclei would suggest; (iv) the growth of precipitation particles by riming and aggregation is particularly rapid in the last kilometer of fall; (v) snow particles reaching the ground originate 10 to 100 km upwind; and (vi) on the western slopes of the Cascades the degree of riming and precipitation rates increase with increasing wind speed and water vapor content at 3 km, but this is not the case on the eastern slopes of the Cascades.