Clear-air Seeding: Opportunities and Strategies

A Detwiler, R. Pratt


Clear-air seeding is the artificial creation or enhancement of cloud cover in clear air which is between saturation with respect to ice and saturation with respect to liquid water. Previous work has demonstrated that clear-air seeding is possible, and may be economically worthwhile through its effects on the radiation balance near the ground. Order-of-magnitude arguments, contrail observations and direct measurements are used to assess the frequency and horizontal extent of opportunities for clear-air seeding in the upper troposphere. Clear, ice-supersaturated air is a common occurrence in the mid-latitude upper troposphere at all times of year, typically appearing over 100 km-wide regions downstream of cyclonic or convective storm systems. The frequency of opportunities was also estimated for a particular application of clear-air seeding-cloud cover enhancement during winter nights to reduce space heating costs. Opportunities were studied for the middle and lower troposphere through a careful analysis of radiosonde humidity data, and for the upper troposphere through observalions of natural cirrus. Under suitable cloud cover and wind conditions, five to ten opportunities each season are estimated for locations in the northeastern United States, a frequency sufficient for economic advanlage based on a previous analysis. Two methods are discussed for seeding clear, ice-superssLurated air to create clouds. Seeding with ice crystals is studied using a numerical simulation of ice crystal cloud growth, which includes both ice physics and radiative transfer. Ice crystal seeding is most effective when the seeding rate is greatest and the moist layer seeded is deepest. Aircraft contrails generally utilize only a small fraction of the available water vapor in excess of ice saturation, before they settle out of the moist layer in which they were created. The development of clouds seeded with ice-nucleating aerosols, rather than ice crystals, depends greatly on the rate at which the seed material nucleates ice crystals. This type of seeding would produce a longer lasting, lower density cloud than would seeding with ice crystals.

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