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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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Terry Deshler and David W. Reynolds

Abstract

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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Terry Deshler and David W. Reynolds

Abstract

No abstract available.

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K. L. Coulson and David W. Reynolds

Abstract

The amount of solar energy reflected from various soils and types of vegetation has been measured as a function of sun elevation in six different wavelength ranges in the ultraviolet, visible and near-infrared regions of the spectrum. It is shown that there is a significant dependence of reflectance on both wavelength and elevation of the sun for all surfaces for which measurements were made.

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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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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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Terry Deshler, David W. Reynolds, and Arlen W. Huggins

Abstract

Cloud seeding experiments devoted to physical measurements of the effects of seeding shallow stable winter orographic clouds have been conducted in the central Sierra Nevada of California from 1984 to 1986. Seeding was done by aircraft using either dry ice or silver iodide at temperatures between −6° and −14°C. Aircraft, radar, and surface instruments were used to measure the effects. A trajectory model was used to target seeded precipitation to the ground where the surface instruments were deployed. Results from these experiments are presented in two case studies and a summary analysis of all 36 experiments. Observations from the various measurement platforms conformed with results expected from seeding in 35 percent of the seedlines sampled with a research aircraft, 4 percent of those observed with radar, and 17 percent of these which passed over the surface instrumentation; however, the complete seeding chain was believed to be documented in only 2 of 36 experiments. The failures result from difficult technical and logistical problems, and from the variability of even simple cloud systems, particularly in the spatial and temporal distributions of liquid water and in the natural fluctuations in ice crystal concentrations. Based on the difficulty of these experiments and the magnitude of seeding effects observed, a statistical experiment would be a formidable undertaking.

During the two experiments when seeding effects were detected by all measurement platforms the following effects were observed. A high concentration, 50–100 L−1, of small compact ice crystals formed quickly along the seedline. Although aggregation was seldom observed, riming often began 5–10 min after seeding. The seeded ice crystals dispersed at 1 m s−1 and cloud liquid-water evaporated in regions corresponding to the seedlines. Seeding in a non-echoing region occasionally produced echoes of 3–10 dBZ in portions of the seedlines. At the surface seeding effects arrived 35 to 60 min after seeding, 20–30 km downwind. Snow crystal concentrations increased, snow crystal habits changed to small rimed particles, and precipitation rates increased by 0.1–1.0 mm h −1. The duration of these effects was short, <10 min per seedline. Changes in ice particle development induced by seeding were similar when seeding with either dry ice or silver iodide. This was found to be the case even at temperatures as warm as −6°C using AgI NH4I NH4ClO4 burned in an acetone solution.

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David W. Reynolds, Thomas H. Vonder Haar, and Stephen K. Cox

Abstract

Data from an experiment to measure the upward and downward components of solar radiation from aircraft during the Barbados Oceanographic and Meteorological Experiment (BOMEX) have been analyzed in the present study. Two groups of results were found.

In the cloud-free tropical troposphere: 1) Absorption of solar radiation in the entire troposphere can be twice as large as previous estimates of 13% given by Manabe and Strickler. Comparison of observed heating rates to calculations shows that the increase in attenuation may be due to non-gaseous constituents in the atmosphere. 2) The vertical profile of solar radiative heating was particularly variable in the lowest layers while in the mean, the profile suggests a slight maximum near 700 mb. 3) In using the solar radiation observations of this study in an energy budget of the tropics a hypothesis regarding a nighttime maximum of precipitation in the tropical regions was formed.

Findings from a radiative study of certain cloud type cases show (i) selective vertical absorption in stratocumulus acts to destabilize the clouds' environment; (ii) large cumulus or cumulonimbus clouds occasionally decrease the solar insolation reaching the surface to only 3% of that incoming at cloud top; and (iii) cirrus clouds are possible stabilizing mechanisms in the tropical environment since they act to warm the local environment at high altitudes while suppressing solar warming from cloud base to surface.

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Robert M. Rauber, Robert D. Elliott, J. Owen Rhea, Arlen W. Huggins, and David W. Reynolds

Abstract

A diagnostic technique for targeting during airborne seeding experiments has been developed for the Sierra Cooperative Pilot Project (SCPP). This technique was used operationally during SCPP for real-time guidance to aircraft, providing 1) the location and orientation of the seeding line required to target ice particles created by seeding to a specified ground location and 2) an estimate of the areal coverage of the seeding effect on the ground. Procedures to use this technique as a real-time guidance tool during seeding operations in Sierra wintertime storms are discussed.

Three evaluation studies of the targeting method are presented. These include 1) comparisons of diagnosed wind fields with those measured by aircraft; 2) comparisons of ice particle growth rates and habits within seeded cloud regions with those used in the targeting computations; and 3) comparison of radar echo evolution within seeded cloud regions with calculated particle trajectories.

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