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R. E. Ruskin

Abstract

During the 1965 Project Stormfury experiments a combination of six cloud physics aircraft instruments were used to measure the changes in the water-ice budget in a large tropical cumulus which had been seeded. Pyrotechnic “Alectos” were used to produce an estimated 180 AgI nuclei per liter effective at −5C in the seeded area. Measurements showed an 80 per cent conversion to ice at the −5C level (17,000-ft pressure altitude) in a 1 km region of the cloud 5 min after seeding, together with a 1.5C rise in temperature. At the same time the adjacent unseeded region of the same cloud decreased in percentage ice at that level, but increased about the same amount in temperature, probably because of increased updraft induced by the heat of fusion energy released in the seeded region.

On two passes 10 min apart 300-m length of “wisp” visible outside the main cloud produced many 10–20 μ replicas of ice particles in air which had a measured 75 to 90 per cent relative humidity, including the moisture from the cloud particles which were vaporized in an instrument which measures the cloud total water content.

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R. E. Ruskin

Abstract

Pollution effects on cloud condensation nucleus (CCN) concentration vary widely, sometimes even decreasing the CCN. The interpretation of these pollution effects may be invalid if the nuclei are measured by expansion-type Aitken nucleus counters operated at low expansion ratios to provide a readout (with natural nuclei) equal to that of a thermal-gradient diffusion cloud chamber (TGDCC) instrument operated at 1% supersaturation. Calculations indicate that the expansion chamber operated at a ΔP of 1.4 inch Hg produces a supersaturation of 26%, allowing for vapor depletion effects during the first few hundredths of a second after expansion. The maximum supersaturation in clouds is typically 0.1–1%; therefore, 26% causes a, large positive error. Further, vapor depletion in the instrument during droplet growth limits the growth rate and size of droplets at supersaturations which are low compared to those for which the instrument is designed. Therefore, use of the standard calibration curve contributes a large negative error in the readout. The combination of a large positive error from excessive supersaturation and a large negative error in calibration appears to explain the systematic errors found at the second International Workshop on Condensation and Ice Nuclei (IWCIN). With NaCl nuclei the expansion instrument undercounted 90% of the time, averaging 0.6 of the average of five TGDCC instruments. On the other hand, with the fairly nonwettable nuclei of Teflon it read 25 times the TGDCC average.

An experimental development model of a continuous mixing-jet type instrument for CCN exhibited the same tendency, reading 0.04–0.33 of the TGDCC average with NaCl and 8 times too high with Teflon. If further development can correct the uncertainties in this NCAR instrument, it promises to be convenient and continuous.

Unless proven differently by future comparison tests which are more definitive than those of the IWCIN, interpretations of data on CCN in pollution should be evaluated in terms of the supersaturation at which the data are taken as compared to that in clouds. Future decisions on instrumentation for measurements of CCN should take into account the results of the IWCIN, particularly for measurements in polluted air where most Aitken nuclei do not act as CCN.

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J. M. Averitt
and
R. E. Ruskin

Abstract

An aircraft instrument which provides continuous replication of water droplets and ice particles as an aircraft traverses a cloud is described. The instrument makes use of a continuously moving 16-mm film on which a Formvar varnish is applied just before exposure to the cloud particles at a sampling port. The film then passes through an appropriate drying process, thus producing replicas of the cloud particles in the hardened Formvar coating.

The considerations which enter into the interpretation of the replicas are discussed. Two of the problems which greatly limit the amount of quantitative data which can be obtained are 1) shattering of cloud particles (ice or water) either at the film or at the edges of the entry port, and 2) coalescence of water droplets after impaction of the film.

Examples are shown of the kind of replicas which were obtained in field use of the instrument.

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W. A. Hoppel
,
J. E. Dinger
, and
R. E. Ruskin

Abstract

Measurements of the number density, volatility and supersaturation spectra of CCN during aircraft soundings to about 15,000 ft MSL over Arizona, Central Pacific, Alaska, and in a convective region over Florida, and on five flights to an altitude of 35,000 ft off the cast and west coasts of the United States are presented. The results show that generally over continental areas away from strong anthropogenic sources the number of CCN is about ten times as great as in marine air masses and as found over snow-packed polar regions. Over continental areas the count decreases with altitude; whereas in the oceanic environments and polar regions the count sometimes increases above the inversion such that at higher altitudes there are no systematic differences between oceanic and continental environments. On several occasions, over the Pacific, anomalously high counts of several thousand CCN cm−3 have been encountered. Five soundings to 35,000 ft indicate no systematic increase or decrease in CCN with altitude above 20,000 ft. Above the inversion about 60% of the total aerosol population are CCN active at a supersaturation of 0.7%, the nuclei having an average radius <2.6×10−6 cm.

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S. C. Mossop
,
R. E. Ruskin
, and
K. J. Heffernan

Abstract

A long-lived cumulus cloud off the southern coast of Australia was found to contain ice crystals, even though the coldest temperature in it was about −4C. Two instrumented aircraft made successive passes through the cloud and continuous samplers were used to replicate cloud particles. It was found that columnar crystals were present in concentrations up to 100 liter−1. This was at least three orders of magnitude greater than the expected concentration of active ice nuclei as determined by cloud chamber measurements made under the base of the same cloud. It appears that some ice crystal multiplication mechanism was at work, though the exact nature of the process cannot be decided on available evidence.

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