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Joseph A. Warburton

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Joseph A. Warburton

Abstract

The oxygen 18/oxygen 16 (18O/16O) and deuterium/hydrogen (D/H) ratios of snowmelt have been used for estimating the weighted mean temperatures in clouds where ice-phase water capture has occurred during the precipitation-forming process. The isotopic measurements were combined with ice crystal replication and microphotographic observations of primary ice crystal habits and degrees of riming. Measurements from two complete winter seasons have enabled the development of climatological databases of these ice-phase water capture temperatures for the central Sierra Nevada and the Snowy Mountains of Australia. The results are based on the linear relationships between the temperature of formation in the clouds of ice crystals grown by vapor deposition and the departures (δ18O, δD) of the oxygen and hydrogen isotopic ratios in these crystals from the standard mean ocean water values. It was found that precipitation falling from orographic winter storms collects most of its water substance in the lower 1–2 km of the supercooled clouds. In the Sierra Nevada, average ice-phase water capture occurred around −10°C. In the Snowy Mountains the primary capture region appears to have an average temperature of −5°C with a secondary region centered on −12°C. Such databases may be useful when designing cloud-seeding projects in regions where snow is the principal form of precipitation.

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Joseph A. Warburton, Richard H. Stone III, and Byron L. Marler

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Trace chemical measurements of the silver content of snow have been used to investigate the transport and dispersion of silver iodide cloud seeding aerosols into and around two large target areas in the central Sierra Nevada between 1978 and 1992. The background concentration of silver in snow samples in this region is extremely low [B Ag = 2.0 parts per trillion (ppt); standard deviation, σ = 1.0 ppt], and the silver from the seeding activities is readily detectable. The studies, in winter snowstorm conditions, show that targeting of the seeding aerosols was modest to poor with large variability both spatially and temporally. Analysis of several thousand snow samples over a period of several years has demonstrated that only 20% (average) of the precipitation that fell within the intended target area during seeding activities contained silver from the silver iodide seeding aerosol above the assigned “threshold” concentration of B Ag + 2σ = 4.0 ppt. Targeting of one of the catchment areas under southerly flow storm conditions was particularly poor (less than 8% of the sampled snow containing detectable silver above the “threshold” value).

Evidence is also presented of the transport of silver iodide in directions and into areas other than those intended including upwind control areas used for estimating seeding effects in the target. In one period of the study between 1987 and 1990, emphasis was placed on southerly flow storms. It was found that contamination of two control areas in the Lake Almanor region generally occurred in the early phases of these southerly flow storm periods when winds at the lower levels were from northeast to southeast prior to frontal passage. The method used for estimating the layer-averaged wind prior to storm classification for seeding purposes in this project placed equal emphasis on winds at all levels from 2000 m, the approximate elevation of the ground generators, to the −10°C level. Hence, aerosols released at ground level were often transported inadvertently toward the control areas by these low-level winds. The problems that these results raise in regard to the traditional use of precipitation statistics for assessing seeding effects are discussed. The results of the studies indicate that it would be necessary to produce very substantial precipitation changes in the limited areas where the seeding silver is present in the snowfall to yield a statistically acceptable change over an entire target area. The trace chemistry results presented may explain why one of the randomized cloud seeding experiments did not achieve statistically significant seeding effects and why another statistically significant result may have been misinterpreted.

New seeding program designs and assessment methodologies need to be developed that not only will produce better targeting, but also positively identify the precipitation that has been impacted by the seeding process, so that seeding effects may be quantified with a higher degree of confidence.

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Joseph A. Warburton, Steven K. Chai, and Lawrence G. Young

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Belay B. Demoz, Arlen W. Huggins, Joseph A. Warburton, and Richard L. Smith

Abstract

In the winter of 1986, two microwave radiometers were operated side by side at a high-altitude weather observation site in the central Sierra Nevada for the purpose of comparing measurements in a variety of ambient weather conditions. The instruments continuously recorded measurements of vertically integrated water vapor and liquid water during storms affecting the area. One radiometer was designed with a spinning reflector to shed precipitation particles while the other radiometer's reflector was fixed. Temporal records of the data show periods of wet weather contamination for the fixed reflector radiometer. The absence (presence) of these contaminated periods is mainly explained by the difference in the design of the radiometers. These contaminated periods led to larger standard deviation in the data from the fixed-reflector radiometer and lower correlation coefficients between the two instruments. Correlation coefficients of 0.83 for the liquid and 0.68 for the vapor values were found for the radiometer-radiometer comparisons. When some of the points suspected of contamination were removed, the correlation coefficients improved to 0.87 and 0.71 for the liquid and vapor channels, respectively. The standard deviations were 0.1 mm and 0.12 cm for the liquid and vapor channels, respectively, of the spinning reflector radiometer. For the fixed-reflector design radiometer, a standard deviation of 0.1 mm for the liquid and 0.26 cm for the vapor was found. Comparison of radiometer vapor and rawinsonde precipitable water resulted in a correlation coefficient of 0.97 for the spinning-reflector radiometer and 0.8 for the fixed-reflector radiometer.

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Vern N. Smiley, Bruce M. Whitcomb, Bruce M. Morley, and Joseph A. Warburton

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Results of lidar measurements of atmospheric ice crystal layers during 36 clear-sky precipitation events at South Pole (2850 m MSL) during the winter over the period March-November 1975 are presented and correlated with ice crystal replicator, radiosonde and visual data. Low-lying “precipitation” and higher “formation” layers were frequently discerned in the lidar returns. In 25% of the events a single surface layer was observed with intermittent or no higher layers present. It was determined from the backscatter profiles that the crystals were growing in the precipitation layer in more than 50% of the events. The average altitudes of the precipitation layer tops during austral night and day were 420 ± 130 m and 650 ± 170 m, respectively, while the average altitudes of the temperature inversion tops for the same two periods were 580 ± 200 m and 500 ± 150 m, respectively.

The lidar returns for a few days before and a few days after sunrise (21 September) were unique; high, geometrically thick layers up to 8000 m were observed which were often much more strongly scattering than the precipitation layer, yet were still optically thin.

Wind data for the 36 events showed that the direction at the 650 and 600 mb levels was predominately upslope. The principal ice crystal habits observed were plates, prisms, bullets and clusters, ranging in size from 80 to 225 μm.

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