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David W. Reynolds

Abstract

A combination of rawinsonde balloon ascent rates, low-elevation aircraft, and ground-based tracer sampling measurements are presented. These data indicate that mountain-induced gravity waves have a significant impact on the transport of ice crystals produced by the release of liquid propane from high-altitude dispensers along the crest of the northern Sierra Nevada in California. Special rawinsonde launches were made just downwind of the main Sierra Nevada crest. Balloon ascent rates show a very well defined mountain lee wave present during most precipitation events. Strong descent to the lee of the Sierra will thus have a detrimental effect on the growth of particles generated on the crest. The tracer SF6 (sulfur hexaflouride) is used to simulate the transport and dispersion of propane-generated ice crystals. Sulfur hexaflouride was released from two propane dispenser sites as a proxy for seeded ice crystals. Aircraft measurements of SF6 indicated that at the normal flight altitudes of 2500 m over the downwind valley and 2800 m over the downwind ridge the aircraft was flying near the top of the plumes. When the aircraft was able to fly below cloud base, near the release altitude of 2200 m, substantial SF6 was observed. The lower portion of the plume was also observed to descend into the valley some 700 m below the release altitude. A simple two-dimensional model is used to determine the impact that these gravity waves have on particle trajectories. Model output is presented for one well-documented seeding case to determine how well such models might be used operationally to predict particle trajectories downwind of the Sierra.

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David W. Reynolds

Abstract

During the months of May, June, July and August, 1978, a record number of damaging hailstorms, causing losses upward of $100 million, struck along the High Plains and Front Range regions of New Mexico, Colorado, Wyoming and Montana. Nine of these storms were observed from the GOES-E geostationary satellite with the digital visible and infrared data recorded at the CSU Direct Readout Satellite Groundstation. The digital, navigated imagery were processed on an interactive image processing system for detection of hail signatures.

In all but one case of reported hail, the coldest cloud-top temperature of the storm system located nearest the hailfall was from 1 to 8°C colder than the environmental tropopause temperature during at least a portion of its lifetime. In most cases this occurred coincident with the best estimate of the onset of hail. Also, the imagery showed each of these storm complexes having long lifetimes (2–5 h), with some exhibiting temperatures colder than the tropopause temperature for this length of time. Through analysis of the 22 June 1976 NHRE storm complex, it was determined that hailfall occurred at close to the maximum growth rate of the storm. This paper thus begins to identify a potential technique for identifying damaging hailstorms through proper enhancement of digital GOES infrared imagery.

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David W. Reynolds

Cloud seeding to increase winter snowpacks over mountainous regions of the western United States have been in existence for almost 40 years. However, our understanding of the physical processes taking place in the clouds in response to this seeding and the expected precipitation increases are still subjects of great scientific interest and investigation. Recent field observations that have emphasized direct physical observations of winter clouds, their structure and liquid water content, as well as their response to the injection of glaciogenic seeding agents have added to our knowledge. These physical observations are helping to provide some insight into the mechanisms of precipitation increases, inferred from statistical analyses, that have been reported in certain winter orographic cloud seeding programs. This paper attempts to compare physical and statistical results, to show consistency, and to help provide limits to what one might expect when winter snowpack augmentation is applied within suitable cloud systems.

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Terry Deshler and David W. Reynolds

Abstract

No abstract available.

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Terry Deshler and David W. Reynolds

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No abstract available.

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Terry Deshler and David W. Reynolds

Abstract

A single case-study of a winter orographic cloud over the central Sierra Nevada is presented in which the effects of aerial seeding with silver iodide, an AgI NH4C1O4 mixture burned in acetone, were observed to persist for over 90 min after seeding and 100 km downwind of the seedline. A research aircraft was able to locate and track the line source of AgI using an ice nucleus counter. High ice crystal concentrations due to seeding were not apparent until more than one hour after seeding. This may have been partially due to the high natural concentrations of ice, but post-mission analysis revealed that most sampling passes during the first hour following seeding were made below the AgI seeded volume. Ice nucleus measurements confirmed sampling of the seedline from 1–1.5 h after seeding, with associated increases in ice crystal concentrations. The effectiveness of the seeding material in the field was higher than laboratory measurements would suggest.

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David W. Reynolds and Thomas H. Vonder Haar

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No abstract available.

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Mark F. Heggli and David W. Reynolds

Abstract

A storm bearing close structural resemblance to a katafront was observed from the ground with microwave radiometry and a vertically pointing Ka-band radar over the Sierra Nevada of California. The onset and duration of supercooled liquid water was determined and matched to a split front model used to describe the synoptic features of a katafront. Results indicate that prior to the passage of the upper front no supercooled liquid water was observed. This portion of the storm provided the deepest cloud and coldest cloud tops. Supercooled liquid water was most prevalent after the upper front passage, and persisted until the suspected surface front passage. The duration of measured supercooled water was 16 hours.

This information broadens the knowledge regarding the presence of supercooled liquid water, and thus possible seeding potential, within winter storms so that treatment can be confined to the period of storms amenable to cloud seeding. Future studies may well confirm the ease with which these periods can be predicted on an operational basis in the Sierra Nevada.

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Robert A. Maddox and David W. Reynolds

Abstract

Geostationary Operational Environmental Satellite (GOES) enhanced infrared (IR) imagery depicted very cold temperatures over Colorado on the morning of 8 December 1978. The situation was unusual because skies were clear and the cold temperatures were not associated with high cloud tops. Instead, satellite data mapped large areas that were experiencing extremely cold surface temperatures. The GOES data were also examined using the Colorado State University interactive data processing system and it was found that the cold IR readings corresponded well with early morning low temperatures over the state. GOES data can be of use in monitoring surface temperatures and can, in certain situations, provide detailed spatial and temporal information over regions experiencing extreme temperatures.

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David W. Reynolds and Arunas P. Kuciauskas

Abstract

A small subset of midlatitude, midwinter precipitation events affecting the central Sierra Nevada are analyzed. The examples given are representative of 60% of the storm types documented during the past 4 yr of the Sierra Cooperative Pilot Project (SCPP). The structure of thee frontal systems is consistent with those observed in the United States Pacific Northwest and the British Isles.

Combining information from a vertically pointing microwave radiometer, conventional radar, satellite imagery, and detailed time cross sections of rawinsonde data, relationships are developed between these remote sensing devices and the onset of supercooled liquid water (SLW). For the storms described. the highest concentration of SLW occurs after passage of an upper jet with accompanying upper-level front or surface cold ana- and/or katafront. Thee frontal passages lead to decreasing cloud thickness, warming cloud tops, decreasing precipitation rate, and shallow embedded convection over the Sierra.

Discontinuities in cloud top temperature, rainbands, and decreasing echo height, associated with the passage of the upper jet and accompanying front, can be identified with satellite and radar several hours before affecting the Sierra Nevada. thus providing a prediction for the onset or increase in SLW. These relationships have application to wintertime cloud modification programs over the central Sierra.

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