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- Author or Editor: Louis J. Battan x
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Abstract
Calculations of the normalized back-scattering cross-section, σb, of ice spheres surrounded by shells of liquid water have been made from an extension of the Mie theory to a two-layer model. Curves of σb as a function of the thickness of the liquid-water shell are presented for various-sized spheres for 3.21, 4.67 and 10.0 cm radiation. It is shown that, depending upon the size of the sphere and the wavelength of the incident radiation, the back-scattering may either increase or decrease as the ice acquires a liquid-water shell. For certain-sized spheres, interference phenomena, which in some instances may lower the value of σb by several orders of magnitude, are in evidence during the course of melting.Comparisons are made between the theoretical results presented here and experimental measurements of σb for melting ice spheres performed by Atlas et al (1960).
Abstract
Calculations of the normalized back-scattering cross-section, σb, of ice spheres surrounded by shells of liquid water have been made from an extension of the Mie theory to a two-layer model. Curves of σb as a function of the thickness of the liquid-water shell are presented for various-sized spheres for 3.21, 4.67 and 10.0 cm radiation. It is shown that, depending upon the size of the sphere and the wavelength of the incident radiation, the back-scattering may either increase or decrease as the ice acquires a liquid-water shell. For certain-sized spheres, interference phenomena, which in some instances may lower the value of σb by several orders of magnitude, are in evidence during the course of melting.Comparisons are made between the theoretical results presented here and experimental measurements of σb for melting ice spheres performed by Atlas et al (1960).
Abstract
Visual observations on several mountain peaks in the Santa Catalina Mountains in southeastern Arizona reveal that solid ice particles commonly fall from cumulonimbus clouds in the summer. Hailstones having diameters >2 cm occurred on only one day out of a total of 76 days over a four-summer period. On the other hand, on the top of a 2800-m peak, ice or snow pellets or hail fell on 18 out of 30 observational days during a two-summer period. It seems reasonable to speculate that virtually all thunderstorms in this area contain such particles at some stage in their development.
Abstract
Visual observations on several mountain peaks in the Santa Catalina Mountains in southeastern Arizona reveal that solid ice particles commonly fall from cumulonimbus clouds in the summer. Hailstones having diameters >2 cm occurred on only one day out of a total of 76 days over a four-summer period. On the other hand, on the top of a 2800-m peak, ice or snow pellets or hail fell on 18 out of 30 observational days during a two-summer period. It seems reasonable to speculate that virtually all thunderstorms in this area contain such particles at some stage in their development.
Abstract
Calculations of radar backscattering by inhomogeneous precipitation particles require values of the dielectric function of two-component mixtures. Four such dielectric functions are critically examined and their relative merits are weighed. Although apparently different, two are shown to be equivalent: the effective-medium and Polder-van Santen theories. All the dielectric functions agree when the two components are dielectrically similar. All except the Maxwell-Garnet dielectric function are symmetric with respect to interchange of the components. When compared with measurements on ice-air mixtures, the effective-medium and Maxwell-Garnet dielectric functions are marginally better than the Debye function, which has previously been used in backscattering calculations. When the fraction of water is high, the effective-medium function gives calculated values of radar backscattering that are in good agreement with measurements on ice spheres coated with a mixture of ice and water. The Maxwell-Garnet theory, with ice inclusions embedded in a water matrix, is also in good agreement with these measurements, and is so over a wider range of water-volume fractions than the effective-medium theory. Although there are no compelling reasons for preferring one above the other, on the basis of the evidence presented, we would be inclined to use the Maxwell-Garnet dielectric function in radar backscattering calculations.
Abstract
Calculations of radar backscattering by inhomogeneous precipitation particles require values of the dielectric function of two-component mixtures. Four such dielectric functions are critically examined and their relative merits are weighed. Although apparently different, two are shown to be equivalent: the effective-medium and Polder-van Santen theories. All the dielectric functions agree when the two components are dielectrically similar. All except the Maxwell-Garnet dielectric function are symmetric with respect to interchange of the components. When compared with measurements on ice-air mixtures, the effective-medium and Maxwell-Garnet dielectric functions are marginally better than the Debye function, which has previously been used in backscattering calculations. When the fraction of water is high, the effective-medium function gives calculated values of radar backscattering that are in good agreement with measurements on ice spheres coated with a mixture of ice and water. The Maxwell-Garnet theory, with ice inclusions embedded in a water matrix, is also in good agreement with these measurements, and is so over a wider range of water-volume fractions than the effective-medium theory. Although there are no compelling reasons for preferring one above the other, on the basis of the evidence presented, we would be inclined to use the Maxwell-Garnet dielectric function in radar backscattering calculations.
Abstract
Observations were made of the depolarization of 3-cm radar signals by hydrometeors in a thunderstorm. Observations of backscattered power and Doppler spectra were made at the same time. It was found that depolarizations ranged from about −12 to −24 db and were highly variable over distances of 500–1000 m. It is speculated that the large depolarizations were produced by wet ice particles resembling prolate spheroids.
Abstract
Observations were made of the depolarization of 3-cm radar signals by hydrometeors in a thunderstorm. Observations of backscattered power and Doppler spectra were made at the same time. It was found that depolarizations ranged from about −12 to −24 db and were highly variable over distances of 500–1000 m. It is speculated that the large depolarizations were produced by wet ice particles resembling prolate spheroids.
Observations of thunderclouds obtained with a 3-cm height-finding radar set are used to obtain a description of the vertical shear of thunderclouds. Several photographs are given which show the shearing of the radar clouds. A scattergram of wind shear plotted against echo shear is presented and shows that the two variables are related, with the former exceeding the latter in almost all cases. Scatter-diagrams are given which verify that strong vertical wind shear tends to restrict the growth of thunderstorms. A series of radar cross sections illustrates the displacement of the upper part of a thundercloud which is subjected to wind shear, and the growth of another cloud column from the lower part of the thundercloud.
Observations of thunderclouds obtained with a 3-cm height-finding radar set are used to obtain a description of the vertical shear of thunderclouds. Several photographs are given which show the shearing of the radar clouds. A scattergram of wind shear plotted against echo shear is presented and shows that the two variables are related, with the former exceeding the latter in almost all cases. Scatter-diagrams are given which verify that strong vertical wind shear tends to restrict the growth of thunderstorms. A series of radar cross sections illustrates the displacement of the upper part of a thundercloud which is subjected to wind shear, and the growth of another cloud column from the lower part of the thundercloud.
Abstract
Calculations have been made of the radar backscattering, differential reflectivity, and circular depolarization ratio of large oblate spheroids composed of spongy ice. Results are compatible with laboratory measurements by earlier investigators. As expected, scattering of 10-cm radiation depends to an important extent on the size, water content, and axial ratios of the spheroids. Observations of differential reflectivities close to zero in hailstorms can be explained, as was done by V. N. Bringi and his associates, as resulting from the irregular shapes and tumbling of hailstones. But such observations could also be explained by size-distributed oblate spheroids with equivalent diameters greater than about 3.5 cm, falling with vertical symmetry axes.
Abstract
Calculations have been made of the radar backscattering, differential reflectivity, and circular depolarization ratio of large oblate spheroids composed of spongy ice. Results are compatible with laboratory measurements by earlier investigators. As expected, scattering of 10-cm radiation depends to an important extent on the size, water content, and axial ratios of the spheroids. Observations of differential reflectivities close to zero in hailstorms can be explained, as was done by V. N. Bringi and his associates, as resulting from the irregular shapes and tumbling of hailstones. But such observations could also be explained by size-distributed oblate spheroids with equivalent diameters greater than about 3.5 cm, falling with vertical symmetry axes.
