A Numerical Study on the Impact of Hygroscopic Seeding on the Development of Cloud Particle Spectra

Abstract

In a detailed microphysical model embedded into a parcel dynamics, a number of sensitivity studies were effected to test hygroscopic seeding of warm and mixed-phase clouds, varying the size distribution and total number of salt particles. For warm clouds in general, earlier results in the literature could be confirmed. More specifically the following conclusions were reached: Certainly, from practical considerations (weight, easy handling), hygroscopic flares have advantages with respect to earlier hygroscopic seeding techniques. The modeling study supports this choice. Smaller seeding particles have the advantage of increasing the number of drizzle-size drops, which increases the chance of the seeding material staying in the cloud and affecting the entire cloud. Larger salt crystals, though increasing precipitation production in the model, risk premature precipitation fallout. The competition between natural and seeding particles was studied. Concerning the number concentration of the seeding agents, it seems that increasingly higher number concentrations result in a kind of saturation of the seeding effect, not further improving rain formation. Mean radii between 0.5 and 6 ?m seem to be the optimum to get a maximum seeding effect. Concerning the mixed-phase clouds, the same kinds of sensitivity tests concerning the seeding agents, their size distributions, and their number concentrations were performed. As in warm clouds, hygroscopic seeding in mixed-phase clouds modifies the collision and coalescence process. The formed drops freeze, leading to the formation of numerous graupel particles between 1- and 9-mm radius, while depleting the liquid phase. The small ice crystals remained almost unaffected.