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  • Author or Editor: Lewis O. Grant x
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Robert M. Rauber
and
Lewis O. Grant

Abstract

The Physical and microphysical structure of the supercooled water fields in wintertime storms over the Park Range of the northern Colorado Rocky Mountains is examined using aircraft and ground-based measurements. Cloud top, cloud base, and zones of strong orographic lift are identified as regions in stratiform systems where supercooled water production can occur. Cloud systems over Colorado's Park Range were found to have low droplet concentrations (≪300 cm−3). In clouds with the lowest droplet concentrations (<100 cm−3), broad droplet spectra were consistently observed. Significant numbers of large (<20 μm) droplets were present in these cases.

The data presented here and in Part I are used to construct conceptual models of the structure and evolution of the liquid water fields in 1) shallow cloud systems with warm cloud tops, 2) deep stratiform clouds with cold tops, and 3) deep convective regions.

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Robert M. Rauber
and
Lewis O. Grant

Abstract

A case study of an orographic cloud system that developed over the mountains or southern Utah is presented. The storm system contained supercooled liquid water over several hours, and produced almost no precipitation. Because of the high liquid water content, low ice particle concentrations, minimal precipitation, and long duration, the storm appears to have been a good candidate for seeding to augment precipitation. A preliminary analysis of the climatological frequency of orographic cloud systems over these mountains is discussed.

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Lewis O. Grant
and
Robert E. Elliott

Abstract

The greatest potential for seeding with artificial ice nuclei to augment precipitation should occur with cloud summit temperatures in the range from about −10C to about −25C. This is the temperature region where there may be a deficiency of natural ice-forming nuclei. This cloud-top temperature range therefore constitutes a “temperature window” for seeding effectiveness. This article considers the results from a number of cloud seeding experiments reported in the literature with respect to this temperature window. The analysis of seven randomized experiments and references to four other experiments indicates that there is a window in the cloud-top temperature range for which precipitation increases are indicated. This extends from about −10C to about −24C for seeding conducted in the modes employed on these projects. At the coldest cloud-top temperatures, generally less than about −30C, decreases in precipitation are indicated. There are variations among the samples which appear to be explainable in terms of differences in the degree of convection present, the seeding methods used, or in the type of nucleant employed. No evidence is presented to show that the temperature window concept applies where there are strong dynamic effects, either natural or due to seeding, such as those in relatively large and isolated cumuli.

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Charles F. Chappell
,
Lewis O. Grant
, and
Paul W. Mielke Jr.

Abstract

The nature of precipitation changes resulting from seeding cold orographic clouds is examined by separating the observed total precipitation change into duration and intensity change components. The total precipitation change and its two components are then evaluated as functions of cloud temperature using precipitation data recorded in the primary target area during the cloud seeding experiment conducted near Climax, Colo. The results show that the total change in observed precipitation is mainly controlled by changes in precipitation duration, rather than intensity. The main effects of seeding appear to be the initiation of a precipitation release for the warmer clouds during many hours when it would not have occurred naturally, and the suppression of precipitation for the coldest clouds during some hours when it would have occurred naturally. These results are consistent with the concepts of cloud microstability and cloud over-seeding.

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Guy G. Goyer
,
Lewis O. Grant
, and
Thomas J. Henderson

Abstract

Weathereord, a 40-grain detonating fuse containing about 20 per cent of silver iodide, has been evaluated as a cloud seeding nuclei generator in the laboratory, in the field, and in aircraft cloud seeding. Comparative data on the output efficiencies of several types of silver iodide generators are presented and show that, as a dispersal system, Weathercord provides in unit time and unit volume the highest concentration of nuclei available from any known source. The tests in supercooled logs at Yellowstone National Park and two test cases of the seeding from aircraft of orographic cumuli are also described. Although of a preliminary nature these field tests suggest the effectiveness of the large concentrations of AgI nuclei, generated by Weathercord, in modifying relatively thin supercooled clouds.

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Paul W. Mielke Jr.
,
Lewis O. Grant
, and
Charles F. Chappell

Abstract

An orographic cloud seeding experiment conducted in the vicinity of Climax, Colo., has been continued for five additional wintertime periods from 1965–70. A comparison of this new independent information is made with previously discussed wintertime operations of the experiment from 1960–65. As a whole, agreement between these independent data sets is good. In particular, the agreement in temperature and wind partitions is consistent with a previously reported model which describes seeding effects under various physically defined conditions. These comparisons have been made using pooled groups of precipitation sensors having similar elevations and locations.

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Paul J. DeMott
,
William G. Finnegan
, and
Lewis O. Grant

Abstract

Chemical kinetic theory and methodology is applied to examine the ice nucleating properties of silver iodide (AgI) and silver iodide-silver chloride (AgI-AgCl) aerosols in a large cloud chamber held at water saturation. This approach uses temporal data on ice crystals formation with changes in key nucleation parameters such as temperature, water vapor concentration and droplet concentrations. The inter-relationships between ice nucleation effectiveness, nucleation mechanisms, nuclei chemical and physical properties and the rate of appearance of ice crystals can be deduced. The theory and methodology can be applied to atmospheric experimentation.

Ice nucleation effectiveness increases of up to three orders of magnitude over that of AgI aerosols can be achieved with AgI-AgCl solid solution aerosols. Both aerosols are shown to form ice crystals by predominantly contact nucleation at temperatures of −16°C and warmer. Nucleation of the ice phase following collision is identified as a very rapid process, so that the rate of appearance of ice crystals is controlled by the much slower transport rate of nuclei to cloud droplets. The higher efficiency of AgI-AgCl nuclei with respect to the standard AgI nuclei is attributed to an improvement in the relative rates of nucleation versus deactivation or solution following collision of the nuclei with cloud droplets. This increase is most probably due to epitaxy and/or surface “active site” improvements. At a temperature of −20°C, all tested aerosols formed ice crystals by a combination of contact nucleation and deposition nucleation. The percentage of ice crystals formed by deposition correlated well with a minimum particle size of 500 Å for an appreciable deposition rate.

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Paul W. Mielke Jr.
,
Lewis O. Grant
, and
Charles F. Chappell

Abstract

This study is concerned with the elevation and spatial variation effects of wintertime orographic cloud seeding over an area encompassing Fremont, Hoosier and Vail mountain passes in the central Colorado mountains during a period from 1960 to 1965. The observation network consisted of 65 precipitation stations distributed over the three passes. Depending on the grouping of precipitation stations used to represent the prime target area of the study, the average daily precipitation for all 120 seeded days was from 6 to 11% greater than the average daily precipitation for all 131 non-seeded days. There is a high probability that these differences could have occurred by chance alone.

Analyses have also been made according to physically defined stratifications based on a model which describes the seeding effects ascribed to the various strata. Statistically significant increases (decreases) were observed over much of the area for the seeded periods in comparison with the non-seeded periods when 500 mb temperatures were −20C and warmer (−27C and colder). Little or no effects were noted in the intermediate temperature range. When 500-mb wind velocities were from 22–28 m sec−1, statistically significant increases were observed during the seeded period in comparison with the non-seeded period throughout the area.

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Roger L. Steele
,
C. P. Edwards
,
Lewis O. Grant
, and
Gerhard Langer

Abstract

The NCAR acoustical ice nucleus counter was calibrated against a Bigg-Warner Weather Bureau type chamber modified as a mixing chamber. The mixing chamber was in turn calibrated against the CSU-NSF isothermal diffusion cloud chamber. This work was carried out using a 300-liter aluminized mylar bag into which known samples of silver iodide nuclei were introduced. Nuclei were transferred from the bag to the NCAR counter in a carrier gas, at a flow rate of 10 liters min−1. It was found that the NCAR counter measured from 16–52% of the count given by the mixing chamber. An NCAR unit was modified with a velvet liner to test the feasibility of eliminating the glycol system, and measurements were made as described above. The modified unit did not count reliably.

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Paul W. Mielke Jr.
,
Glenn W. Brier
,
Lewis O. Grant
,
Gerald J. Mulvey
, and
Paul N. Rosenzweig

Abstract

A reanalysis of the Climax I and II experiments is described. The concern prompting this reanalysis is a suggestion arising from Colorado State University analyses of extended area effects. Those analyses suggested a regionwide pattern of precipitation that, by chance, may have favored the randomly selected seeding days for some of the important meteorological partitions used in earlier analyses. In order to address this concern, this reanalysis employs excellent covariate relationships developed before the initiation of Climax II and which account for over half of the target variability for most meteorological partitions of major interest (e.g., warm 500 mb temperatures and southwest 700 mb wind directions). The statistical evidence of seeding-induced increases for this reanalysis is generally much stronger than in the previous analysis, which did not utilize covariates. For example, the joint one-sided Wilcoxon test statistic P-value for testing the null hypothesis that seeding did not induce a precipitation increase during warm 500 mb temperatures of the Climax I and II experiments is now 0.0013, compared to 0.0550 in the previous analysis. However, the reanalysis also indicates that previous estimates of increases attributed to seeding based strictly on ratios of seeded to non-seeded precipitation amount means are apparently too large. For example, the estimated precipitation increase of the combined Climax I and II data for the warm 500 mb temperature partition is reduced from 41 to 25% when the full set of data is employed in this reanalysis.

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