A Model of Hygroscopic Seeding in Cumulus Clouds

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  • 1 Bureau of Reclamation, Denver, CO 80225
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Abstract

A systematic modeling exploration has been conducted to map the growth and trajectory of hygroscopically initiated precipitation particles. The model used is a one-dimensional, steady-state, condensation-coalescence model with adiabatic cloud water content. Drop breakup and freezing were simulated but competition among precipitation particles was not considered. Sizes of initial hygroscopic seeds varied from 5 to 400 μm in diameter, updraft speed ranged from 1 to 25 m s−1, and cloud base temperature varied from 0 to 20°C. The 23 July 1970 salt seeding case reported by Biswas and Dennis was also analyzed using the model.

The numerical simulations reveal several complex interactions: 1) For slow updrafts, the larger hygroscopic seeds travel through a lower trajectory and sweep out less water than small, hygroscopic seeds which are also more apt to grow large enough to break up and create additional large precipitation particles. 2) For fast updrafts, the larger hygroscopic seeds grow into precipitation and stand a better chance of breaking up and initiating a Langmuir chain reaction, while the small hygroscopic particles are carried up to the cirrus level and are lost before they reach precipitation size. 3) For very strong updrafts only large hygroscopic seeds will have a chance to convert to precipitation, and in this situation hail is produced. 4) Hygroscopic seeding produces a greater water yield from warmer based clouds.

Abstract

A systematic modeling exploration has been conducted to map the growth and trajectory of hygroscopically initiated precipitation particles. The model used is a one-dimensional, steady-state, condensation-coalescence model with adiabatic cloud water content. Drop breakup and freezing were simulated but competition among precipitation particles was not considered. Sizes of initial hygroscopic seeds varied from 5 to 400 μm in diameter, updraft speed ranged from 1 to 25 m s−1, and cloud base temperature varied from 0 to 20°C. The 23 July 1970 salt seeding case reported by Biswas and Dennis was also analyzed using the model.

The numerical simulations reveal several complex interactions: 1) For slow updrafts, the larger hygroscopic seeds travel through a lower trajectory and sweep out less water than small, hygroscopic seeds which are also more apt to grow large enough to break up and create additional large precipitation particles. 2) For fast updrafts, the larger hygroscopic seeds grow into precipitation and stand a better chance of breaking up and initiating a Langmuir chain reaction, while the small hygroscopic particles are carried up to the cirrus level and are lost before they reach precipitation size. 3) For very strong updrafts only large hygroscopic seeds will have a chance to convert to precipitation, and in this situation hail is produced. 4) Hygroscopic seeding produces a greater water yield from warmer based clouds.

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