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Edmond W. Holroyd III

Abstract

A study of the influence of mountain-generated cumulonimbus systems on the rainfall of the northern Great Plains, using daily rainfalls and satellite images, has shown the seasonal variations and areal extent of the rain swaths. Rainfall from these systems is greatest during June and July. The greatest rainfalls occur ∼400 km northeast from the eastern boundaries of the mountains; the rain then decreases to beyond 1000 km.

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Edmond W. Holroyd III

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Edmond W. Holroyd III

Abstract

A technique has been designed that uses observable properties of images from a 2D-C optical array probe (size, linearity, area, perimeter, and image density) to classify unsymmetrical ice particles into nine habit classes. Concentrations are calculated by requiring that the center of each accepted particle appear to be within the field of view of the probe. Once the size and habit are estimated, a generic mass and terminal velocity can be assigned to each particle to calculate its contribution to ice water content and to precipitation rate. Examples are given to indicate the value of a habit classifier in analyzing the structure of storms, showers, orographic clouds, and seeded clouds. Though the techniques work well for most natural snowfalls, some examples of imperfections are given to remind the analyst to look at the images and to understand how the classifer will treat them.

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Edmond W. Holroyd III

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A 2D-C probe was mounted on the front of a truck for operation in a horizontal orientation, as on an aircraft, and also in a vertical orientation using an aspirator. A snowstorm with calm conditions was sampled by alternatively driving the truck through the snow, using a calibrated anemometer to control the strobe rate, and then sampling with an aspirator while parked. The concentration measurements using an aspirator were about 2.4 times larger than those using the standard aircraft orientation mode. There was little size dependence in this factor for the dendrite/aggregate snow sampled. It is not yet known what causes the aspirator concentration measurements to be larger than those from other techniques.

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Edmond W. Holroyd III

Abstract

Satellite photographs of the TIROS and ESSA series were examined for the presence and dimensions of lake-effect clouds over the Great Lakes and Gulf of St. Lawrence. It was found that nearly all lake-effect clouds occurred when the 850-mb temperature was more than 13C colder than the lake surface temperature. The clouds were organized into parallel bands resembling but having larger dimensions than cloud streets. Enlarged cloud bands were found which were 2.5 times larger than normal lake-effect bands. These enlarged lake storms had preferred origins and appear to be generated by frictional differences between land and water, by the geometry of the body of warm water with respect to the prevailing wind, and by certain urban influences.

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Edmond W. Holroyd III and James E. Jiusto

Abstract

Few documented cases exist to demonstrate that highly convective supercooled clouds can be completely glaciated or overseeded. By “overseeding” we imply a sufficient concentration of ice nuclei to accommodate all the water generated in the updraft and to consume rapidly the existing cloud liquid water. One such case is herein presented that describes the ground variations in snow crystal type, size and concentration as a seeded cloud passed by. During this period, snow crystal concentrations increased by approximately two orders of magnitude, and, within the limits of accuracy of the experiment, showed a one-to-one correspondence with the concentration of silver iodide released. Snowflake aggregates were dominant and individual crystals comprising the aggregates averaged only 200 μ, in general agreement with model predictions. Riming of crystals was significantly reduced, with thick plates and solid columns indicative of a “dry” environment replacing the original rimed dendrites. It was evident that heavy seeding, while limiting the riming and size of individual crystals, amplified the snowflake aggregation mechanism.

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Edmond W. Holroyd III, Arlin B. Super, and Bernard A. Silverman

Abstract

Dry ice is shown to be an attractive agent for on-top seeding of convective clouds. A modest payload of small dry ice pellets can effectively seed dozens of clouds, depending on cloud volumes encountered and crystal concentrations desired. A dry ice pellet size of about 7 mm diameter is suggested for efficient use of seeding agent when dropped from the −10°C level.

Supercooled convective clouds that were seeded on-top with dry ice were investigated to determine empirical nucleation effectiveness values. The clouds were repeatedly penetrated to measure the resulting ice crystal concentrations. The experiments gave conservative effectiveness values of 2 to 5 × 1011 crystals per gram of dry ice, but with possible error bars extending an order of magnitude to each side of those values. A well-documented experiment giving effectiveness values twice as large is discussed in detail.

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Edmond W. Holroyd III, Jack T. McPartland, and Arlin B. Super

Abstract

A series of wintertime airborne tracing experiments was examined to determine some characteristics of the plumes of silver iodide smoke released either from the ground or from an aircraft over the Grand Mesa of Colorado. The plumes were identified in nearly every experiment by detecting the airborne AgI particles and often also by observing resulting ice particle plumes in essentially the same airspace. The lateral and vertical plume positions of Wound-released AgI from eight sites were determined for several wind, cloudiness and stability conditions. The instantaneous ground-released plume had a median spreading angle of 15° and meandered within a median angle of 38°. The median plume height above the crest exceeded 500 m. The lateral spreading rates of aircraft-released AgI were estimated at over 2 m s−1 for cloudy conditions and less in clear conditions. The implications for future cloud seeding strategies are discussed.

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B. F. Ryan, E. R. Wishart, and Edmond W. Holroyd III

Abstract

The mass of columnar ice crystals between −5C and −9C has been measured as a function of time. It is shown that the measured growth rates are not markedly different from the empirical formula proposed by Hindman and Johnson. However, during the first 3 min of growth the parametric form of the axial dimensions can be adequately described by a linear function of time rather than a power law.

Over the same temperature range the bulk density/temperature curve deduced by Fukuta is valid for time periods at least as long as 3 min and for a wide variety of ice crystal concentrations.

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Edmond W. Holroyd III, Arlin B. Super, and Bernard A. Silverman

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