CALIFORNIA STORM CHARACTERISTICS AND WEATHER MODIFICATION

Abstract

In connection with cloud seeding projects in Pacific Coast states, detailed analyses have been prepared of many storms. From this material, average values of significant parameters have been computed and are employed to construct a model of a typical cold-front-type occlusion moving into the California coast from the west, a type of storm responsible for a substantial fraction of California precipitation. The model shows some deviations from the classical cold-front-type occlusion, such as the absence of a discernible elevated warm-front surface within 150 mi of the surface cold front. The system is discussed as a steady-state mechanism wherein the generation of liquid moisture in vertical currents is balanced by its removal through precipitation, primarily in the instability zone near the surface front, and by evaporation, primarily in the altostratus zone in advance of the instability zone. The system being treated as an engine for the production of precipitation reaching the ground, its efficiency is discussed on the basis of simplified calculations of growth of precipitation in relation to ice-forming nuclei supply. It is shown that in this storm model efficiencies are normally well below 100 per cent, and that the introduction of artificial ice-forming nuclei can raise this efficiency markedly.