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S. C. Mossop

Abstract

Measurements of drop concentration and size distribution with the Clague droplet gun and the Forward Scattering Spectrometer Probe in 21 cumulus clouds on five days show no evidence of systematic differences between the two instruments. Though average drop concentrations and liquid water contents agree well within the estimated probable errors, values of these parameters in individual clouds occasionally show large discrepancies. Ways of reducing probable errors in the FSSP are discussed.

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S. C. Mossop

Abstract

No abstract available.

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S. C. Mossop

Abstract

Field evidence of high ice crystal concentrations in cumulus clouds at temperatures warmer than −10°C is consistent with the operation of a Hallett-Mossop “multiplication” process. This mechanism requires the generation of large drops in the cloud below the −8°C level, which is governed in turn by the cloud base temperature and cloud drop concentration. These two parameters are shown to he useful in delineating a “multiplication boundary,” separating cloud conditions in which multiplication takes place from those in which it does not.

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S. C. Mossop

Abstract

No abstract available.

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S. C. Mossop

Some cumulus clouds appear to be capable of producing ice crystals in concentrations of 10–100l −1 at temperatures approaching −5C. This has serious implications for cloud seeding with ice nuclei. We discuss various possible mechanisms by which these unexpectedly high concentrations may originate. Circumstantial evidence from field work points to their production in the riming process, but laboratory confirmation is lacking.

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S. C. Mossop

Ice crystals in supercooled clouds may form upon ice nuclei, or they may arise through secondary processes. Two of these secondary ice “multiplication” mechanisms are discussed in some detail: the rime-splintering process and the mechanical fracture of ice particles. The nature of the water-drop size distribution has an important bearing on secondary ice production. Confident predictions of ice particle concentration can only be made in a few limited cloud situations. This is a serious handicap in assessing the feasibility of artificial rainmaking through the ice crystal process.

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S. C. Mossop
and
C. Tuck-Lee

Abstract

The Warren-Nesbitt generator is widely used in Australia and other countries for producing a cloud seeding aerosol by burning a solution of silver iodide and sodium iodide in acetone. It is found that the particle size distribution follows a log probability law with median diameter 0.085 μ and standard deviation factor 1.47. Electron diffraction examination of the particles shows that they consist of beta silver iodide and sodium iodide. The available evidence indicates that each particle is probably a mixture of these two constituents.

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S. C. Mossop
and
N. S. C. Thorndike

Abstract

The membrane filter technique for detecting ice nuclei in atmospheric air has the disadvantage that the measured concentration decreases with the volume of air sampled. Various possible causes are investigated. The effect is found to be due to the presence of small numbers of ice nuclei on the filters prior to use, which give a spurious “background count,” and to the presence of other particles, sampled simultaneously with the ice nuclei, which prevent the latter from being detected.

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S. C. Mossop
and
A. Ono

Abstract

Measurements of ice crystal concentration in five clouds in northern New South Wales are reported. These confirm earlier studies in which it was found that glaciated altostratus clouds contain approximately the same concentration of ice crystals and ice nuclei. On the other hand, cumulus and stratocumulus, generally sampled at temperatures >−10C, were found to contain about 103 times as many ice crystals as expected on the basis of ice nucleus measurements.

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D. Atlas
and
S. C. Mossop

A simple method is described for calibrating a weather radar by means of a standard spherical target, thus permitting the radar to be used for quantitative measurements of storm reflectivity. The technique involves determination of that storm reflectivity which provides an echo equivalent to that from the known target. The sphere, suspended from a balloon, is tracked as it leaves the radar site. Its echo is “measured” by reducing the receiver gain control to the threshold of visibility. The threshold gain setting is thereby calibrated and subsequently provides an accurate measure of storm reflectivity. There is no need for any other test equipment such as a microwave-signal generator. Absolute accuracy is greater than that attainable with a signal generator since no reliance need be placed on the generator calibration or upon the specified antenna gain.

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