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Robert I. Sax

Abstract

In order to determine if natural glaciation proceeds rapidly or extensively enough in tropical maritime cumuli to influence attempts to modify their dynamical behavior by seeding with silver iodide, a detailed study was made of the clouds observed during the 1965 Project Stormfury experiments. From photographic coverage, notes on visual observations, and instrumentation on-board penetrating aircraft, data were compiled on cloud liquid water content, volume-median drop size, in-cloud temperature profile, and the dynamical 1ife histories of both seeded and non-seeded clouds. The validity d applying Koenig's numerical splintering model to tropical maritime cumuli, as well as an assessment of the effectiveness of silver iodide seeding, were determined by comparing the dynamical behavior of paired seeded and non-seeded clouds with glaciation times predicted by the model. Dynamical studies were initiated on two independently developed parametrized numerical cumulus models, and an excellent correlation between predicted and observed cumulas growth was found if no natural glaciation at temperatures > - 15C was assumed.

The results of this study suggest that natural glaciation does not proceed rapidly and/or extensively enough in the critical cloud updraft areas to alter the effectiveness of modifying tropical maritime cumuli by causing artificial glaciation with silver iodide.

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Robert I. Sax

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Joe Wisniewski and Robert I. Sax

Abstract

The main objective of the Florida Area Cumulus Experiments was to seed groups of supercooled tropical cumulus clouds to convert enough water to ice so that sufficient heat could be released to enhance cloud growth and augment rainfall over a specific south Florida target area. During three consecutive summers (1973–75), subprograms to monitor the rainwater silver concentrations were conducted. Results showed that rainwater silver concentrations can be detected in significantly higher concentrations in seeded vs nonseeded samples with a carefully positioned network and with precise care in the preparation, collection, handling, storage and analysis of the samples. In addition, the silver concentrations in the seeded rainwater samples are low enough to present no ecological or environmental problems whatsoever under the present seeding rates in south Florida.

Stratification of the rainwater silver concentrations, using two different trajectory techniques, showed silver concentrations collected under continental influenced regimes to be almost identical to those collected under seeding conditions. This is a crucial result since, if rainwater silver concentrations are being used to determine whether the seeding material is being targeted in the optimum areas for maximum seeding efficiency, then the results may be biased under these continental influenced regimes.

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Robert I. Sax and Vernon W. Keller

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From an in-cloud microphysical data set collected in Florida convective towers that were penetrated close to their tops near -10°C, evidence is presented to show a sequential development of cloud water, rainwater, graupel and crystalline ice as the cloud ages. Of particular interest is the very rapid onset of graupel that appears on repeat penetrations of some, but not all, towers. A separate data set shows a large scatter in the relationship between maximum value of cloud water and vertical velocity which points to the conclusion that measurements of cloud water, by itself, can be misleading as an indication of growth activity. The sequential pass data showing the evolution of ice and water are consistent with a rime-splintering, secondary ice production hypothesis.

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John B. Cunning and Robert I. Sax

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John B. Cunning and Robert I. Sax

Abstract

A better understanding of how the precipitation budget operates in tropical convective systems is a prime objective of the GATE research effort. Measurement of rainfall rate with shipboard radar is the principal method by which precipitation from tropical clouds that develop within the GATE B-scale array will be determined. Knowledge of the relationship between radar reflectivity (Z) and rainfall rate (R) is essential for an accurate interpretation of precipitation data derived through the use of radar technology. The Z-R relationship is determined through application of a least-squares linear regression to data points derived by appropriate integration of the third and sixth moments of a series of raindrop size spectra.

Drop spectra measurements were obtained during GATE by means of a foil impactor operated at cloud-base level on board the NOAA DC-6 aircraft. A total of 107 Z-R data points are available, representing showers occurring on 12 days. The best-fit Z-R relationship for the cloud-base aircraft foil data showed little variability from day to day or on the basis of stratification by rain rate. For all foil data combined, the best-fit Z-R relationship was found to have the form Z=170 R 1.52, which gives, for example, rain rates of 66, 15 and 3 mm h−1 for Z values of 50, 40 and 30 dBZ, respectively.

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Robert I. Sax and James G. Hudson

Abstract

Measurements of cloud condensation nuclei (CCN) were obtained in the south Florida region as part of NOAA's Florida Area Cumulus Experiment (FACE). During the summer of 1975, CCN measurements were obtained near the bases of cumulus clouds by means of an airborne static diffusion chamber operating at 0.75% supersaturation. Concentrations were highly variable across the peninsula. The change in concentration from low values over the water to very high concentrations over land surfaces was abrupt.

A continuous surface-based CCN monitoring program was carried out during the summer of 1976 with two continuous flow diffusion chambers, operating at 0.4 and 1.0% supersaturation. These confirmed the continental (high CCN) characteristics of the surface aerosol (overall mean 718 cm−3 at 0.49% and 1644 cm−3 at 1.0%). The concentration of CCN was found to vary as a function of wind speed, wind direction, and time of day. Analysis of these data suggested that the concentration of CCN was modulated by short-term localized influences superimposed on the longer term air mass characteristics. Further stratification using low-level trajectories to separate air masses of recent maritime and continental exposures resulted in a mean concentration of 543 and 1291 cm−3 at 0.4 and 1.0% supersaturation, respectively, for the maritime CCN data, and a mean concentration of 1149 and 2049 cm−3 at the respective supersaturations for the continental CCN data.

The picture that emerges is consistent with the view of local production mechanisms (probably photochemical), sometimes superimposed on larger scale advection of aged aerosol.

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Victor Wiggert, Robert I. Sax, and Ronald L. Holle

Abstract

The potential for enhancing rain output over central Illinois through modification of summertime convective clouds was investigated by use of model predictions of cumulus growth, as well as direct, internal measurements of cloud physical characteristics. No clouds were seeded in Illinois. Comparisons were made with results gathered during seeding experiments on single convective clouds in Florida and also during the Florida Area Cumulus Experiment (FACE).

Predictions of the top heights of unseeded and seeded Illinois clouds (and the predicted, post-seeding height increase, the “seedability”) were made using a one-dimensional cumulus model and 10 summers of regularly-gathered radiosonde data from Rantoul and Peoria. Seedability (which in Florida is correlated with enhanced rain volume) was small (<2 km) or zero in Illinois much more frequently than in Florida, and was largest, in the mean, in July and smallest in June; it varied markedly from morning to evening, from month to month within a summer, and from summer to summer.

Measurements made during two July days in 1977, in cumuli over central Illinois, described the natural evolution of the ice-water budget and the life history of the updraft. In-cloud microphysical characteristics, near the −10°C level (∼6 km) in Illinois, in convective clouds that were developing in moist air in advance of a weak cold front, were equivalent to those characteristics encountered at the same penetration level of Florida cumuli of similar size and depth. Clouds that were penetrated in the dry air behind the cold front had microphysical structures that differed greatly from those in clouds developing in tropical maritime air mass conditions.

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Joe Wisniewski, William R. Cotton, and Robert I. Sax

Abstract

A subprogram of NOAA's 1973 Florida Area Cumulus Experiment (FACE) was undertaken to determine the silver content of precipitation associated with convective clouds massively seeded with silver iodide nucleant over southern Florida. An atomic absorption analysis of 127 rainwater samples collected just below cloud base by a polypropylene-lined scoop mounted on the fuselage of the NOAA DC-6 aircraft indicated that the mean concentration of silver obtained on seed days (69 samples) was no greater (and, in fact, appreciably less) than that obtained on no-seed days (58 samples). In both sets of samples, the median concentration of silver was more than two orders of magnitude lower than the U. S. Public Health safety limit of 5 × 10−8 g ml−1. Of the 69 aircraft samples collected on seed days, only two contained a concentration of silver in excess of 1 × 10−9 g ml−1. Of the 58 aircraft samples collected on no-seed days, eight contained silver in concentrations exceeding 1 × 10−9 g ml−1. The samples collected from the aircraft showed higher mean concentrations of silver than did those collected on the ground. An atomic absorption analysis of 79 rainwater samples collected at both fixed and mobile sites on the surface showed that the mean concentration of silver on seed days was three orders of magnitude less than 5 × 10−8 g ml−1; the maximum concentration of silver found in any sample did not exceed 1 × 10−9 g ml−1. Statistical results from the nonparametric Mann-Whitney Wilcoxon test suggest that the surface seeded (34 samples) and surface nonseeded (45 samples) data come from the same population (i.e., no significant differences between the two data sets). There is some evidence (from a separate set of surface rainwater samples collected upwind of the target area) to suggest a persistently higher (by about a factor of 2 or 3) mean concentration of silver during the course of the experiment than either before or after the experiment.

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Robert I. Sax, Dennis M. Garvey, and Farn P. Parungo

Abstract

Isothermal cold chamber tests indicate that the NEI TB-1 pyrotechnic used in FACE after July 1975 emits a silver iodide smoke which has a much greater nucleating effectiveness in the temperature range −6 to −12°C than that emitted from the Olin pyrotechnic in predominant use in FACE prior to August 1975. The NEI TB-1 pyrotechnic was also found at “warm” temperatures to have a higher nucleating effectiveness than the U. S. Navy WMU-9 flare which also contains the TB-1 formulation. From transmission electron microscopic analyses, the size distributions of the particles produced from all three types of flares were found to be similar. Because of the small sizes of the particles (mean diameter ∼0.03 μm) a diffusive contact mechanism probably is playing an important role in the nucleation of water droplets. It is postulated that soluble potassium iodide, present in a complexed form, is detrimental at warm temperatures to the nucleating effectiveness of particles produced from the Olin pyrotechnic. Differences in nucleating effectiveness in the two TB-1 mixtures are attributed to the presence of small amounts of chlorine in the binding material of the NEI pyrotechnic.

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