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Arlen W. Huggins and Alfred R. Rodi

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Arlen W. Huggins and Alfred R. Rodi

Abstract

The effect of seeding convective clouds with dry ice was studied using simultaneous aircraft and radar observations. Clouds that were initially ice-free with supercooled liquid water contents of 0.5 g m−3 when the tops reached the −10°C level had similar responses to seeding, although significant natural variability existed. Aircraft particle probes detected sharp increases of small crystals (<100 μm) in 3–6 min followed by > 1 mm aggregates about 10 min after seeding. Observations supported the expectation that riming growth should not be important at these liquid water contents. Initial radar echoes formed in 7 win with distinctive time-height profiles of reflectivity.

Most radar echoes forming downwind of the seeding line were small and relatively weak compared with the natural echoes forming further downwind over the mountains. The impact of the seeding was shown to be observable but relatively small. It was found that unseeded clouds formed radar echoes later, and produced reflectivity time-height profiles that were significantly different from the seeded ones. The difference are considered in part to be due to variability in the initial cloud properties as well as the obvious and well-documented effects of injection of the seeding material early in the cloud lifetime. While the meteorological impact was small, documentation of the evolution of the seeding effect from cloud to ground is a prerequisite to further experimentation.

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Jeffrey C. Weil, R. Paul Lawson, and Alfred R. Rodi

Abstract

Relative dispersion of ice crystals was measured in 30 seeded cumulus clouds. A quasi-instantaneous, vertical area source of ice was generated by releasing dry-ice pellets from an airplane. The ice concentration distribution and relative dispersion were measured normal to the source and were complemented by cloud turbulence measurements, namely, velocity variances and the energy dissipation rate ε. The clouds were selected based on an objective set of criteria and were treated as members of the same ensemble.

The observed mean relative dispersion σrx agreed well with predictions from a Lagrangian stochastic two-particle model, which reproduces Batchelor's theoretical results for σrx. For short times t after the seeding time ts, the predictions and observations suggested a growth like σrxtts rather than Batchelor's “intermediate” time prediction, σrx ∝ ε1/2 (tts)3/2. This difference was attributed to the rather large initial dispersion σ0 of ice crystals, 27–53 m, inferred from the measurements; Batchelor's result is only valid for σ0 ≪ σva 3/ε, where σva 2 is the average velocity variance. At long times, the predictions and observations approached the same asymptotic limit, σrx ∝ (tts)1/2.

In addition to the mean dispersion, probability density functions (pdfs) of the individual dispersion observations were constructed and showed an evolution from a highly skewed pdf at small times to a more symmetrical one at large times. This is one of the first reports of the σrx pdf, which is important for determining the variance and pdf of the randomly varying concentration in a small ice cloud or plume of material.

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