15th Conference on Planned and Inadvertent Weather Modification


Calculations of the effect of seeding material from hygroscopic flares on the collision-coalescence process and the development of rainfall in clouds

Vidal Salazar, NCAR, Boulder, CO; and N. Hernández-Carrillo and R. T. Bruintjes

This paper will focus on the microphysical conditions that are necessary to support effective coalescence growth via a cloud seeding technique using hygroscopic materials from flares. More specifically, this paper will examine the development of rain in warm convective clouds, suggesting a quantitative threshold for the onset of effective coalescence growth due to the seeding material. In addition, the differences between flare compositions and size distributions, was used to simulate simultaneous condensation and coalescence processes, in order to explore how hygroscopic seeding below cloud base might affect the initiation of coalescence and the production of rain. It was found that if the seeding particles are smaller than natural cloud condensation nuclei (CCN), they will not be activated and will have no effect in the rain processes. However if they are much larger than the natural CCN, they may provide embryos on which raindrops can form. For cases where the CCN size distribution is comparable to, or slightly larger than natural CCN, the consequences are less certain. If the added particles simply modify the natural CCN population to one with similar sizes but higher concentrations, smaller and more numerous droplets would be expected and coalescence would be slower. The calculated effects of seeding with hygroscopic flares suggest that the formation of precipitation via coalescence might be accelerated by such seeding. This calculations support preliminary results from cloud seeding programs using hygroscopic flares.

Session 5, Physical evidence on the effects of weather modification programs including area and hydrology aspects, pollution effects on cloud microphysical parameters and processes
Thursday, 18 January 2001, 8:30 AM-10:00 AM

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