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  • Author or Editor: R. List x
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D. R. Hudak and R. List

Abstract

The development of precipitation was studied in southern Africa in 23 clouds, 12 unseeded and 11 seeded, from 11 days during the Bethlehem Precipitation Research Project. Surface and upper air data were used to describe the environmental conditions while aircraft and radar data were used to determine the ice water budget in the clouds. Two relatively simple cloud models were used to help identify seedable situations. They were a one-dimensional steady state model with bulk microphysical parameterization and a one-dimensional time-dependent model with detailed microphysics.

The data were divided into three sets based on the main airmasses affecting the area: maritime tropical (mT), continental tropical (cT), and maritime polar (mP). The smaller clouds on the mT days, with tops warmer than − 20°C, were the most likely candidate for precipitation enhancement from both the microphysical and dynamic seeding viewpoints. There the time-dependent model calculated a precipitation efficiency increase from 2% to 15% due to seeding. For clouds in the cT air the rapid natural onset of ice suggested that they were not seedable microphysically. Clouds in the mP air were determined to be not seedable because they were either very efficient microphysically or their lifetimes were too short.

The developed procedures give insight into the chances for rain enhancement in a given meteorological situation. To treat these results in a statistically significant manner would require a much larger sample of cases.

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R. List, K. R. Gabriel, B. A. Silverman, Z. Levin, and T. Karacostas

Abstract

A randomized rain enhancement experiment was carried out during 1988–94 in the area of Bari and Canosa, Italy, on the Adriatic coast. It was commissioned by the Italian Department of Agriculture and Forestry and the region of Puglia, with TECNAGRO, a nonprofit Italian company, as overall manager, and with EMS, an Israeli company, as field operator. The original purpose was to study rain-producing weather systems in southern Italy, establish similarities with Israel, and transfer Israeli technology. The experiment was a cross-over design with two alternating target areas, a buffer in between, and two additional control areas. Seeding was by injection of silver iodide into clouds by aircraft flying near the bases of clouds along predetermined tracks upwind of the target area. The experimental units were rainy days. Based on historical rain gauge data, it was estimated that 303 rainy days were required to establish a 15% rain increase at a significance level of 0.05 and 90% power.

In 1995, TECNAGRO asked the Scientific Committee for a statistical evaluation to investigate if a seeding effect could be established before the original goal of 303 seeding days was reached. The results of the analysis of the 260 available rainy days were that no discernable seeding effect could be found. This was evident from the root double ratio (RDR) and root regression ratio (RRR), which yielded RDR − 1 = −0.083 ± 0.089 and RRR − 1 = −0.004 ± 0.057, respectively (the ± sign represents the standard error of the estimate). Based on that result, it was decided to terminate the Puglia seeding experiment.

Preliminary exploratory studies suggest that the two target areas might have been affected differently by seeding and that an apparent substantial seeding effect occurred in the Bari area under conditions of moderate precipitable water between 700 and 850 mb. If these findings are confirmed by the recommended meteorological analyses and airflow studies, a new experiment with an appropriate design might be justified.

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Greg M. McFarquhar, Roland List, David R. Hudak, Robert P. Nissen, J. S. Dobbie, N. P. Tung, and T. S. Kang

Abstract

During the Joint Tropical Rain Experiment of the Malaysian Meteorological Service and the University of Toronto, pulsating raindrop ensembles, hereafter pulses, were observed in and around Penang Island. Using a Doppler radar on 25 October 1990, a periodic variation of precipitation aloft 30 km from the radar site, with an approximate 8-min period, was established and seemed to be caused by the evolution and motion of horizontal inhomogeneities existing within the same cell. On 30 October 1990, using a new volume scanning strategy with a repetition cycle of 3.5 min, pulsations of the same frequency were observed up to 3 km above the radar and at the ground by a disdrometer. High concentrations of large drops were followed by high concentrations of successively smaller drops at the ground. This provides observational evidence to support the recent argument for using a time-varying release of precipitation-sized particles to model observed pulsating rainfall.

Many cases of nonsteady rain from convective clouds displayed repetition periods of between 8 and 25 min.

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