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Terry Deshler

Abstract

Freezing nuclei were used as tracers in experiments to determine whether ice particles are accreted by rime during dry growth. Experiments were conducted in a wind tunnel and in natural clouds at a mountaintop observatory. The results in both cases indicated that a few percent of the ice particles in the cloud were accreted by the rime.

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Terry Deshler

Abstract

Particle probes from Particle Measuring Systems are routinely designed to be mounted on an aircraft or other movable platform. There are attempts, however, to apply this technology to stationary surface measurements by inducing airflow through the sample area. Use of these measurements quantitatively requires an understanding of how the measurements are distorted by the aftificial airflow. In an application to surface snowfall measurements, a 2D-C probe with a horn-shaped aspirator has been used for six winter seasons in the Sierra Nevada of California. To correct measurements from this instrument, three other methods to measure ice particle concentrations and size distributions at the ground are presented and compared to simultaneous measurements by the aspirated 2D-C. The aspirated 2D-C was found to routinely overestimate ice particle concentration by factors of 2.4 to 3.2. In addition, the amount of overestimation was found to be a function of particle size and surface wind speed.

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Terry Deshler
and
Gabor Vali

Abstract

Atmospheric concentrations of contact-freezing nuclei were measured using a technique primarily sensitive to submicron aerosol particles. Diffusion and phoretic forces were relied on for the capture of nuclei by supercooled drops of distilled water exposed to the sample air. Nucleus concentrations were deduced from the rate at which the drops were observed to freeze, interpreting that rate on the basis of a theoretical prediction of aerosol capture rate for different assumed sizes of the nuclei. Measurements at Laramie, Wyoming, yielded average concentrations of contact-freezing nuclei of 1.7 L−1 at −15°C and 3.1 L−1 at −18°C for an assumed radius of 0.01 μm for the nucleating particles.

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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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Terry Deshler
and
David W. Reynolds

Abstract

No abstract available.

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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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Daniel Breed
,
Roy Rasmussen
,
Courtney Weeks
,
Bruce Boe
, and
Terry Deshler

Abstract

An overview of the Wyoming Weather Modification Pilot Project (WWMPP) is presented. This project, funded by the State of Wyoming, is designed to evaluate the effectiveness of cloud seeding with silver iodide in the Medicine Bow and Sierra Madre Ranges of south-central Wyoming. The statistical evaluation is based on a randomized crossover design for the two barriers. The description of the experimental design includes the rationale behind the design choice, the criteria for case selection, facilities for operations and evaluation, and the statistical analysis approach. Initial estimates of the number of cases needed for statistical significance used historical Snow Telemetry (SNOTEL) data (1987–2006), prior to the beginning of the randomized seeding experiment. Refined estimates were calculated using high-resolution precipitation data collected during the initial seasons of the project (2007–10). Comparing the sample size estimates from these two data sources, the initial estimates are reduced to 236 (110) for detecting a 10% (15%) change. The sample size estimates are highly dependent on the assumed effect of seeding, on the correlations between the two target barriers and between the target and control sites, and on the variance of the response variable, namely precipitation. In addition to the statistical experiment, a wide range of physical studies and ancillary analyses are being planned and conducted.

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Jaclyn M. Ritzman
,
Terry Deshler
,
Kyoko Ikeda
, and
Roy Rasmussen

Abstract

Annual precipitation increases of 10% or more are often quoted for the impact of winter orographic cloud seeding; however, establishing the basis for such values is problematic for two reasons. First, the impact of glaciogenic seeding of candidate orographic storms has not been firmly established. Second, not all winter precipitation is produced by candidate “seedable” storms. Addressing the first question motivated the Wyoming state legislature to fund a multiyear, crossover, randomized cloud-seeding experiment in southeastern Wyoming to quantify the impact of glaciogenic seeding of wintertime orographic clouds. The crossover design requires two barriers, one randomly selected for seeding, for comparisons of seeded and nonseeded precipitation under relatively homogeneous atmospheric conditions. Addressing the second question motivated the work here. The seeding criteria—700-hPa temperatures ≤−8°C, 700-hPa winds between 210° and 315°, and the presence of supercooled liquid water—were applied to eight winters to determine the percent of winter precipitation that may fall under the seeding criteria. Since no observational datasets provide precipitation and all of the atmospheric variables required for this study, a regional climate model dynamical downscaling of historical data over 8 years was used. The accuracy of the model was tested against several measurements, and the small model biases were removed. On average, ~26% of the time between 15 November and 15 April atmospheric conditions were seedable over the barriers in southeastern Wyoming. These seedable conditions were accompanied by precipitation ~12%–14% of the time, indicating that ~27%–30% of the winter precipitation resulted from seedable clouds.

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Roy M. Rasmussen
,
Sarah A. Tessendorf
,
Lulin Xue
,
Courtney Weeks
,
Kyoko Ikeda
,
Scott Landolt
,
Dan Breed
,
Terry Deshler
, and
Barry Lawrence

Abstract

The Wyoming Weather Modification Pilot Project randomized cloud seeding experiment was a crossover statistical experiment conducted over two mountain ranges in eastern Wyoming and lasted for 6 years (2008–13). The goal of the experiment was to determine if cloud seeding of orographic barriers could increase snowfall and snowpack. The experimental design included triply redundant snow gauges deployed in a target–control configuration, covariate snow gauges to account for precipitation variability, and ground-based seeding with silver iodide (AgI). The outcomes of this experiment are evaluated with the statistical–physical experiment design and with ensemble modeling. The root regression ratio (RRR) applied to 118 experimental units provided insufficient statistical evidence (p value of 0.28) to reject the null hypothesis that there was no effect from ground-based cloud seeding. Ensemble modeling estimates of the impact of ground-based seeding provide an alternate evaluation of the 6-yr experiment. The results of the model ensemble approach with and without seeding estimated a mean enhancement of precipitation of 5%, with an inner-quartile range of 3%–7%. Estimating the impact on annual precipitation over these mountain ranges requires results from another study that indicated that approximately 30% of the annual precipitation results from clouds identified as seedable within the seeding experiment. Thus the seeding impact is on the order of 1.5% of the annual precipitation, compared to 1% for the statistical–physical experiment, which was not sufficient to reject the null hypothesis. These results provide an estimate of the impact of ground-based cloud seeding in the Sierra Madre and Medicine Bow Mountains in Wyoming that accounts for uncertainties in both initial conditions and model physics.

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