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- Author or Editor: Z. Levin x
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Abstract
One-dimensional time-dependent models of warm and cold clouds were constructed to test the electrical and precipitation development in the presence of a variety of charge separation mechanisms. The, models simulate charging by ion diffusion, the Wilson effect (ion conduction), polarization induction, the Workman-Reynolds effect and the thermoelectric effect. It was found that charging by ion processes does not significantly contribute to the electrical development of either warm (shallow or deep) of cold clouds. In cold continental clouds, without ice multiplication processes, charging due to ice-ice collisions did not significantly contribute to the electrical buildup because of the low concentrations of ice particles and their low electrical conductivities at low temperatures.
The most significant charging resulted from collisions of ice particles and water droplets. These collisions produced strong charging by both the inductive and non-inductive processes. It also was found that when both polarization and the Workman-Reynolds effect operate together, strong Acids develop. The resulting locations of the main charge centers and the corresponding electric fields in the model are in agreement with many recent observations, pointing to the presence of a negative charge center around the −10°C level. The effect of corona discharge from the ground on the ion concentration and ion charging near and below cloud base, and on the electrical conductivities within the cloud, also were found to agree well with many recent observations.
Abstract
One-dimensional time-dependent models of warm and cold clouds were constructed to test the electrical and precipitation development in the presence of a variety of charge separation mechanisms. The, models simulate charging by ion diffusion, the Wilson effect (ion conduction), polarization induction, the Workman-Reynolds effect and the thermoelectric effect. It was found that charging by ion processes does not significantly contribute to the electrical development of either warm (shallow or deep) of cold clouds. In cold continental clouds, without ice multiplication processes, charging due to ice-ice collisions did not significantly contribute to the electrical buildup because of the low concentrations of ice particles and their low electrical conductivities at low temperatures.
The most significant charging resulted from collisions of ice particles and water droplets. These collisions produced strong charging by both the inductive and non-inductive processes. It also was found that when both polarization and the Workman-Reynolds effect operate together, strong Acids develop. The resulting locations of the main charge centers and the corresponding electric fields in the model are in agreement with many recent observations, pointing to the presence of a negative charge center around the −10°C level. The effect of corona discharge from the ground on the ion concentration and ion charging near and below cloud base, and on the electrical conductivities within the cloud, also were found to agree well with many recent observations.
Abstract
Trends in the orographic rainfall ratio R 0 over Israel are reevaluated. It is shown that the rainfall has not changed significantly over most of the mountainous stations, with some significant increases over the central mountains. The overall evaluation of R 0 for all potential station pairs, calculating the ratio of each mountain station separately over each coastal or seashore station, indicates that about 50% of all pairs show a positive trend in R 0. The high spatial variability, especially over the mountains, allows for finding orographic rainfall ratio trends that are significant in both the positive and negative directions. The correct definition of R 0 in the Israeli case requires the use of a seashore cluster of stations. If some of the seashore stations are replaced by inland stations, and in particular stations that are right over the region of maximum positive rainfall urban enhancement due to the thermal heat island or other urban effects, a seemingly decreasing “orographic ratio” is unavoidable. In such a case, urban dynamical positive effects on coastal rainfall can be erroneously interpreted as pollution suppression of orographic rainfall. When seashore stations are selected as required by a proper definition of the orographic ratio, increasing R 0 is obtained over central Israel and an insignificant trend over the north is found. Furthermore, evaluation of the ratio of rainfall for the upwind in comparison with the downwind side of the Galilee Mountains exhibits an increasing trend, opposite to the recent findings of Givati and Rosenfeld. The rainfall analysis shows no evidence of any suppression of rainfall over the mountains due to pollution, and at least in Israel other factors besides aerosols are predominant in defining the trends in the orographic rainfall ratio.
Abstract
Trends in the orographic rainfall ratio R 0 over Israel are reevaluated. It is shown that the rainfall has not changed significantly over most of the mountainous stations, with some significant increases over the central mountains. The overall evaluation of R 0 for all potential station pairs, calculating the ratio of each mountain station separately over each coastal or seashore station, indicates that about 50% of all pairs show a positive trend in R 0. The high spatial variability, especially over the mountains, allows for finding orographic rainfall ratio trends that are significant in both the positive and negative directions. The correct definition of R 0 in the Israeli case requires the use of a seashore cluster of stations. If some of the seashore stations are replaced by inland stations, and in particular stations that are right over the region of maximum positive rainfall urban enhancement due to the thermal heat island or other urban effects, a seemingly decreasing “orographic ratio” is unavoidable. In such a case, urban dynamical positive effects on coastal rainfall can be erroneously interpreted as pollution suppression of orographic rainfall. When seashore stations are selected as required by a proper definition of the orographic ratio, increasing R 0 is obtained over central Israel and an insignificant trend over the north is found. Furthermore, evaluation of the ratio of rainfall for the upwind in comparison with the downwind side of the Galilee Mountains exhibits an increasing trend, opposite to the recent findings of Givati and Rosenfeld. The rainfall analysis shows no evidence of any suppression of rainfall over the mountains due to pollution, and at least in Israel other factors besides aerosols are predominant in defining the trends in the orographic rainfall ratio.
Abstract
A general formulation of the collection process is derived and the relative merits of the Berry and the Kovetz-Olund schemes assessed.
Abstract
A general formulation of the collection process is derived and the relative merits of the Berry and the Kovetz-Olund schemes assessed.
Abstract
Both gram-positive and gram-negative bacteria become excellent condensation nuclei when lyophilized to dryness. The same freeze-dry procedure does not inactivate the highly effective freezing nuclei produced by ice nucleation active bacteria. Therefore, irrespective of their contact nucleation potential, ice nucleation-active bacteria ought to effect condensation freezing at −10°C of warmer in cloud systems. Output from a numerical cloud model suggests that the condensation freezing capability of ice nucleation-active bacteria at warmer temperatures could be exploited to produce rainfall from clouds too warm to respond positively to inorganic nucleants like silver iodide.
Abstract
Both gram-positive and gram-negative bacteria become excellent condensation nuclei when lyophilized to dryness. The same freeze-dry procedure does not inactivate the highly effective freezing nuclei produced by ice nucleation active bacteria. Therefore, irrespective of their contact nucleation potential, ice nucleation-active bacteria ought to effect condensation freezing at −10°C of warmer in cloud systems. Output from a numerical cloud model suggests that the condensation freezing capability of ice nucleation-active bacteria at warmer temperatures could be exploited to produce rainfall from clouds too warm to respond positively to inorganic nucleants like silver iodide.
Abstract
Freezing spectra of INA bacteria from different parts of the world were compared. A slight increase in efficiency of freezing nuclei produced by strains from warmer climates was observed. Whole cells of the most efficient strain produced nuclei active at temperatures ranging from −2 to −10°C, whereas fragments from these cells exhibited activity only at −8 to −10°C. In all cases, the frequency of active cells in a population proved low. Thus, activity at −8 to −10°C was evidenced by 1 cell in about 300, the corresponding ratio being 1 in 104 at −2 to −4°C. It was shown in several ways that the variety of “freezer” individuals was not due to a need for multicell aggregation or any other cooperative process. Also, the time at which a given individual in a cell population expressed its latent freezing potential was shown to vary with time and cell physiological state.
Abstract
Freezing spectra of INA bacteria from different parts of the world were compared. A slight increase in efficiency of freezing nuclei produced by strains from warmer climates was observed. Whole cells of the most efficient strain produced nuclei active at temperatures ranging from −2 to −10°C, whereas fragments from these cells exhibited activity only at −8 to −10°C. In all cases, the frequency of active cells in a population proved low. Thus, activity at −8 to −10°C was evidenced by 1 cell in about 300, the corresponding ratio being 1 in 104 at −2 to −4°C. It was shown in several ways that the variety of “freezer” individuals was not due to a need for multicell aggregation or any other cooperative process. Also, the time at which a given individual in a cell population expressed its latent freezing potential was shown to vary with time and cell physiological state.
Abstract
No Abstract available.
Abstract
No Abstract available.
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.
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.
Aircraft inlets connect airborne instruments for particle microphysical and chemical measurements with the ambient atmosphere. These inlets may bias the measurements due to their potential to enhance or remove certain particle size fractions in the sample. The aircraft body itself may disturb the ambient air streamlines and, hence, the particle sampling. Also, anisokinetic sampling and transmission losses within the sampling lines may cause the sampled aerosol to differ from the ambient aerosol. In addition, inlets may change the particle composition and size through the evaporation of water and other volatile materials due to compressibility effects or heat transfer. These problems have been discussed at an international workshop that was held at the Leibniz-Institute for Tropospheric Research (IfT) in Leipzig, Germany, on 12–13 April 2002. The discussions, conclusions, and recommendations from this workshop are summarized here.
Aircraft inlets connect airborne instruments for particle microphysical and chemical measurements with the ambient atmosphere. These inlets may bias the measurements due to their potential to enhance or remove certain particle size fractions in the sample. The aircraft body itself may disturb the ambient air streamlines and, hence, the particle sampling. Also, anisokinetic sampling and transmission losses within the sampling lines may cause the sampled aerosol to differ from the ambient aerosol. In addition, inlets may change the particle composition and size through the evaporation of water and other volatile materials due to compressibility effects or heat transfer. These problems have been discussed at an international workshop that was held at the Leibniz-Institute for Tropospheric Research (IfT) in Leipzig, Germany, on 12–13 April 2002. The discussions, conclusions, and recommendations from this workshop are summarized here.
Aircraft inlets connect airborne instruments for particle microphysical and chemical measurements with the ambient atmosphere. These inlets may bias the measurements due to their potential to enhance or remove certain particle size fractions in the sample. The aircraft body itself may disturb the ambient air streamlines and, hence, the particle sampling. Also, anisokinetic sampling and transmission losses within the sampling lines may cause the sampled aerosol to differ from the ambient aerosol. In addition, inlets may change the particle composition and size through the evaporation of water and other volatile materials due to compressibility effects or heat transfer. These problems have been discussed at an international workshop that was held at the Leibniz-Institute for Tropospheric Research (IfT) in Leipzig, Germany, on 12–13 April 2002. The discussions, conclusions, and recommendations from this workshop are summarized here.
Aircraft inlets connect airborne instruments for particle microphysical and chemical measurements with the ambient atmosphere. These inlets may bias the measurements due to their potential to enhance or remove certain particle size fractions in the sample. The aircraft body itself may disturb the ambient air streamlines and, hence, the particle sampling. Also, anisokinetic sampling and transmission losses within the sampling lines may cause the sampled aerosol to differ from the ambient aerosol. In addition, inlets may change the particle composition and size through the evaporation of water and other volatile materials due to compressibility effects or heat transfer. These problems have been discussed at an international workshop that was held at the Leibniz-Institute for Tropospheric Research (IfT) in Leipzig, Germany, on 12–13 April 2002. The discussions, conclusions, and recommendations from this workshop are summarized here.
