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- Author or Editor: L. RANDALL KOENIG x
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Abstract
The 1973 Rand version of the Mintz-Arakawa model of the general circulation of the atmosphere was used to study the consequences of high concentrations of hygroscopic aerosols in a limited geographic region. The experiment was designed to investigate effects of the activity of the aerosol as condensation nuclei and the consequent abnormal production of cloudiness and alteration of precipitation were of primary concern.
Values of meteorological parameters generated by the experiment were compared with values simulated by a control (in which the polluted area was absent). Statistical tests of significance were performed in order to judge the certainty that experimental changes, rather than random wandering of the simulations, actually caused changes in certain variables.
Cloudiness increased over the polluted area and the temperature at the surface and in the lower atmosphere became warmer. Rainfall, however, was not significantly altered in the polluted region in spite of changes in the parameterization of rain that would make rainfall more difficult with a given atmospheric structure.
The data suggest a possible connection between North American pollution and South American rainfall. Comparison of data from the experiment and the control indicate that in the experiment, rainfall decreases gradually and steadily within a large area of South America centered on the equator. The immediate cause is the movement of the region of maximum moisture convergence northward, toward the polluted area, while it is at the same time decreasing in value. The movement is clear and the rainfall changes were found to be statistically significant. Although possible causes are discussed, the model does not properly simulate the hydrological cycle in the tropics, and a definitive explanation of decreasing rainfall in South America is marred by an unrealistic, unstable, moisture-convergence/rain cycle that occurs in the primary control. This behavior of the model precludes a conclusion that North American pollution will cause decreased tropical rainfall, but the data are sufficiently suggestive that this possibility should be examined in future investigations.
Abstract
The 1973 Rand version of the Mintz-Arakawa model of the general circulation of the atmosphere was used to study the consequences of high concentrations of hygroscopic aerosols in a limited geographic region. The experiment was designed to investigate effects of the activity of the aerosol as condensation nuclei and the consequent abnormal production of cloudiness and alteration of precipitation were of primary concern.
Values of meteorological parameters generated by the experiment were compared with values simulated by a control (in which the polluted area was absent). Statistical tests of significance were performed in order to judge the certainty that experimental changes, rather than random wandering of the simulations, actually caused changes in certain variables.
Cloudiness increased over the polluted area and the temperature at the surface and in the lower atmosphere became warmer. Rainfall, however, was not significantly altered in the polluted region in spite of changes in the parameterization of rain that would make rainfall more difficult with a given atmospheric structure.
The data suggest a possible connection between North American pollution and South American rainfall. Comparison of data from the experiment and the control indicate that in the experiment, rainfall decreases gradually and steadily within a large area of South America centered on the equator. The immediate cause is the movement of the region of maximum moisture convergence northward, toward the polluted area, while it is at the same time decreasing in value. The movement is clear and the rainfall changes were found to be statistically significant. Although possible causes are discussed, the model does not properly simulate the hydrological cycle in the tropics, and a definitive explanation of decreasing rainfall in South America is marred by an unrealistic, unstable, moisture-convergence/rain cycle that occurs in the primary control. This behavior of the model precludes a conclusion that North American pollution will cause decreased tropical rainfall, but the data are sufficiently suggestive that this possibility should be examined in future investigations.
Abstract
The aerosols, produced by several devices, used to provide silver iodide for cloud-seeding purposes have been studied by micro-chemical techniques which permitted the characterization of individual smoke particles. The water sorptive properties, the composition, and chemical uniformity of the particles were sought. Marked differences in the generator outputs were noted. Some effort has been made to relate the findings of this work to the suitability of the different aerosols to cloud-seeding tasks.
Abstract
The aerosols, produced by several devices, used to provide silver iodide for cloud-seeding purposes have been studied by micro-chemical techniques which permitted the characterization of individual smoke particles. The water sorptive properties, the composition, and chemical uniformity of the particles were sought. Marked differences in the generator outputs were noted. Some effort has been made to relate the findings of this work to the suitability of the different aerosols to cloud-seeding tasks.
Abstract
Data on the composition of stratocumulus and nimbostratus clouds having temperatures warmer than −9C reveal, in some instances, high concentrations of ice particles (to approximately 400 liter−1 of cloud). On the basis of measurements of the concentration of ice-forming nuclei reported in the literature, it is concluded that some multiplication process was responsible for the observed concentrations of ice particles.
Since moderately large drops were present in the glaciating clouds, the data are consistent with a process whereby drops are frozen and splintered; however, in view of other evidence casting doubt on the validity of this process in the free atmosphere, alternative hypotheses are examined. Freezing of drops by a stochastic process is untenable, but a mechanical fracturing process seems possible and merits additional study.
Abstract
Data on the composition of stratocumulus and nimbostratus clouds having temperatures warmer than −9C reveal, in some instances, high concentrations of ice particles (to approximately 400 liter−1 of cloud). On the basis of measurements of the concentration of ice-forming nuclei reported in the literature, it is concluded that some multiplication process was responsible for the observed concentrations of ice particles.
Since moderately large drops were present in the glaciating clouds, the data are consistent with a process whereby drops are frozen and splintered; however, in view of other evidence casting doubt on the validity of this process in the free atmosphere, alternative hypotheses are examined. Freezing of drops by a stochastic process is untenable, but a mechanical fracturing process seems possible and merits additional study.
Abstract
The technique and results of the application of a chemical test, explicit for silver iodide, sensitive to a mass of 3 × 10−17 g, to laboratory-nucleated ice crystals and atmospheric ice crystals from cloud-seeding experiments in the field are described. In laboratory ice crystals, it was found that the silver-iodide nucleus was not always in the apparent “C” axis of the ice crystals. This deviation is discussed. Because of difficulties in the preservation of the ice crystals from the field experimentation, no evidence of the true role of silver iodide in cloud seeding was obtained.
Abstract
The technique and results of the application of a chemical test, explicit for silver iodide, sensitive to a mass of 3 × 10−17 g, to laboratory-nucleated ice crystals and atmospheric ice crystals from cloud-seeding experiments in the field are described. In laboratory ice crystals, it was found that the silver-iodide nucleus was not always in the apparent “C” axis of the ice crystals. This deviation is discussed. Because of difficulties in the preservation of the ice crystals from the field experimentation, no evidence of the true role of silver iodide in cloud seeding was obtained.
Abstract
Evidence supporting the hypothesis that the first particles of ice in small cumulonimbus clouds way form as a result of a mechanism involving the violent breakup of drops is presented. The hypothesis is supported by observations that: (1) ice nucleation occurs at relatively warm temperatures if a supercooled drop is subjected to a shock wave; (2) the bag mode of drop breakup occurs when drops are subjected to a shock wave; (3) and, the bag mode of drop breakup may occur in natural clouds even though the air is motionless except for turbulence introduced by the cloud drops themselves (i.e., shock waves or unnatural forms of turbulence are not required to promote the bag mode of drop breakup). The mechanism can only he effective in cases when clouds contain large supercooled drops. It is believed that the hypothesized mechanism may supply a critical link in the understanding of cloud glaciation by providing a means of triggering the avalanche mechanism involving ice splinter formation during drop freezing.
Abstract
Evidence supporting the hypothesis that the first particles of ice in small cumulonimbus clouds way form as a result of a mechanism involving the violent breakup of drops is presented. The hypothesis is supported by observations that: (1) ice nucleation occurs at relatively warm temperatures if a supercooled drop is subjected to a shock wave; (2) the bag mode of drop breakup occurs when drops are subjected to a shock wave; (3) and, the bag mode of drop breakup may occur in natural clouds even though the air is motionless except for turbulence introduced by the cloud drops themselves (i.e., shock waves or unnatural forms of turbulence are not required to promote the bag mode of drop breakup). The mechanism can only he effective in cases when clouds contain large supercooled drops. It is believed that the hypothesized mechanism may supply a critical link in the understanding of cloud glaciation by providing a means of triggering the avalanche mechanism involving ice splinter formation during drop freezing.
Abstract
Data gathered during consecutive traverses through summer clouds in southern Missouri have been interpreted to indicate that: (1) the glaciating characteristics of commonly occurring small cumulonimbus clouds in this region are dependent upon the size distribution of the liquid phase prior to the inception of glaciation; (2) clouds having large liquid-water drops rapidly form high concentrations of ice particles regardless of the concentrations of foreign ice-forming nuclei; (3) the ice phase within the clouds does not begin anew; it builds upon the size distribution that has been achieved by the liquid phase and consequently a continuous modification of the particle size distribution within the cloud occurs; and (4) coalescence continues to be a dominant growth mechanism and growth by sublimation essentially is by-passed.
The data appear to he compatible with a chain reaction process propagated by the formation of satellite ice particles during the solidification of water drops.
Abstract
Data gathered during consecutive traverses through summer clouds in southern Missouri have been interpreted to indicate that: (1) the glaciating characteristics of commonly occurring small cumulonimbus clouds in this region are dependent upon the size distribution of the liquid phase prior to the inception of glaciation; (2) clouds having large liquid-water drops rapidly form high concentrations of ice particles regardless of the concentrations of foreign ice-forming nuclei; (3) the ice phase within the clouds does not begin anew; it builds upon the size distribution that has been achieved by the liquid phase and consequently a continuous modification of the particle size distribution within the cloud occurs; and (4) coalescence continues to be a dominant growth mechanism and growth by sublimation essentially is by-passed.
The data appear to he compatible with a chain reaction process propagated by the formation of satellite ice particles during the solidification of water drops.
Abstract
A numerical experiment in the simulation of cloud microphysical processes is described. The purpose of the experiment was to test the hypothesis that drop-freezing/splintering is an important mechanism in cumulus cloud glaciation. Diffusion of water vapor, coalescence of water drops, coalescence of water and ice particles, breakup of unstable drops and the production of ice splinters during drop freezing were simulated in a continuous growth model of cloud-particle evolution. Computed and observed features of cloud evolution are compared. The experiment fully supports the drop-freezing/splintering hypothesis, i.e., the essential features observed in glaciating clouds are simulated if the drop-freezing/splintering mechanism is incorporated in the cloud model. But, if this mechanism is deleted from the model, the observed behavior of clouds is not duplicated, unless what are believed to be unrealistic conditions are assumed. The evidence from the experiment is not conclusive, however, and more field data are required, particularly with regard to the characteristics of small ice particles in actively glaciating clouds, before the validity of the hypothesis can be definitively stated.
Abstract
A numerical experiment in the simulation of cloud microphysical processes is described. The purpose of the experiment was to test the hypothesis that drop-freezing/splintering is an important mechanism in cumulus cloud glaciation. Diffusion of water vapor, coalescence of water drops, coalescence of water and ice particles, breakup of unstable drops and the production of ice splinters during drop freezing were simulated in a continuous growth model of cloud-particle evolution. Computed and observed features of cloud evolution are compared. The experiment fully supports the drop-freezing/splintering hypothesis, i.e., the essential features observed in glaciating clouds are simulated if the drop-freezing/splintering mechanism is incorporated in the cloud model. But, if this mechanism is deleted from the model, the observed behavior of clouds is not duplicated, unless what are believed to be unrealistic conditions are assumed. The evidence from the experiment is not conclusive, however, and more field data are required, particularly with regard to the characteristics of small ice particles in actively glaciating clouds, before the validity of the hypothesis can be definitively stated.
Abstract
An attempt has been made quantitatively to assess the prospects for modifying warm fogs by seeding them with condensation nuclei. This has been done by calculating the time-dependent changes in the sizes and concentrations of fog droplets that are predicted by the ordinary equations of diffusion of water vapor to and from the surface of droplets. Their size, molality, and ambient water-vapor density are taken into account.
Initial conditions consist of a homogeneous volume of air of specified height and aerosol content. An external cooling rate and seed dosage are specified. The effects of various combinations of cloud height and seed properties (such as size, mass density, and rate of injection) on the metamorphosis of the fog-droplet population are examined. Usually, the cloud-forming process (in which temperature decreases with time) was allowed to continue after the completion of seeding.
It is tentatively concluded that the optimum-size seeding material is a function of the fog thickness; material smaller than about 4 µm in diameter should be excluded, and particles 20 to 50 µm in diameter are most suitable for seeding moderately thick (∼100 m) xclouds.
Abstract
An attempt has been made quantitatively to assess the prospects for modifying warm fogs by seeding them with condensation nuclei. This has been done by calculating the time-dependent changes in the sizes and concentrations of fog droplets that are predicted by the ordinary equations of diffusion of water vapor to and from the surface of droplets. Their size, molality, and ambient water-vapor density are taken into account.
Initial conditions consist of a homogeneous volume of air of specified height and aerosol content. An external cooling rate and seed dosage are specified. The effects of various combinations of cloud height and seed properties (such as size, mass density, and rate of injection) on the metamorphosis of the fog-droplet population are examined. Usually, the cloud-forming process (in which temperature decreases with time) was allowed to continue after the completion of seeding.
It is tentatively concluded that the optimum-size seeding material is a function of the fog thickness; material smaller than about 4 µm in diameter should be excluded, and particles 20 to 50 µm in diameter are most suitable for seeding moderately thick (∼100 m) xclouds.
Abstract
Parametric equations describing the growth of ice particles in the range 10−11–1 g are presented. Three subranges (10−11–5 × 10−8, 5 × 10−8–10−4, and 10−4–1 g) are used. The parametric equations were developed with the aid of a complex method of calculating ice-particle growth. Two approaches to using the parametric equations in a simulation resembling a natural cloud situation are described. One method breaks the ice-particle spectrum into three components, the sizes of each being monitored. The other assumes a size distribution that can be characterized by a “growth-median” size, so that only one size need be monitored to describe the rate of growth. The parametric method is compared with more complicated methods in a simulated cloud environment, and the results are regarded as satisfactory.
Abstract
Parametric equations describing the growth of ice particles in the range 10−11–1 g are presented. Three subranges (10−11–5 × 10−8, 5 × 10−8–10−4, and 10−4–1 g) are used. The parametric equations were developed with the aid of a complex method of calculating ice-particle growth. Two approaches to using the parametric equations in a simulation resembling a natural cloud situation are described. One method breaks the ice-particle spectrum into three components, the sizes of each being monitored. The other assumes a size distribution that can be characterized by a “growth-median” size, so that only one size need be monitored to describe the rate of growth. The parametric method is compared with more complicated methods in a simulated cloud environment, and the results are regarded as satisfactory.
