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Abstract
Calculations have been made of the radar backscattering and attenuation cross sections of dry and spongy ice spheres. One set of calculations was the cross sections of spheres with diameters exponentially distributed. As expected, attenuation cross sections are greater at a wavelength of 3.21 cm than at 5.05 and 10.0 cm. Calculations were also made of attenuation by monodisperse distributions of spheres composed of spongy ice and having diameters as large as about 8 cm. Attenuation of 3-cm radiation by dry ice and spongy ice spheres can be very large. At most diameters and water volume fractions, the one-way attenuation of 10-cm radiation by monodisperse spheres, in concentrations giving a radar reflectivity of 60 dBZ, is negligibly small (i.e., <0.1 dB km−1), but at a few diameters and water fractions, attenuation can be substantially larger. Although in most circumstances attenuation increases as wavelength decreases, there are exceptions at some diameters and water volume fractions. These calculations may explain observations that C-band attenuation in hailstorms is not always larger than S-band attenuation.
Abstract
Calculations have been made of the radar backscattering and attenuation cross sections of dry and spongy ice spheres. One set of calculations was the cross sections of spheres with diameters exponentially distributed. As expected, attenuation cross sections are greater at a wavelength of 3.21 cm than at 5.05 and 10.0 cm. Calculations were also made of attenuation by monodisperse distributions of spheres composed of spongy ice and having diameters as large as about 8 cm. Attenuation of 3-cm radiation by dry ice and spongy ice spheres can be very large. At most diameters and water volume fractions, the one-way attenuation of 10-cm radiation by monodisperse spheres, in concentrations giving a radar reflectivity of 60 dBZ, is negligibly small (i.e., <0.1 dB km−1), but at a few diameters and water fractions, attenuation can be substantially larger. Although in most circumstances attenuation increases as wavelength decreases, there are exceptions at some diameters and water volume fractions. These calculations may explain observations that C-band attenuation in hailstorms is not always larger than S-band attenuation.
Abstract
From studies of the properties of convective clouds over the mountains of southeastern Arizona, it was found that many failed to precipitate even though they were supercooled and were quite deep. A carefully controlled cloud-seeding program was designed in 1957 and has been in progress for three years. This article discusses the experimental design and the observational techniques and gives some preliminary results.
Since the research has two more years to run, statistical evaluations are not given in this article.
Abstract
From studies of the properties of convective clouds over the mountains of southeastern Arizona, it was found that many failed to precipitate even though they were supercooled and were quite deep. A carefully controlled cloud-seeding program was designed in 1957 and has been in progress for three years. This article discusses the experimental design and the observational techniques and gives some preliminary results.
Since the research has two more years to run, statistical evaluations are not given in this article.
Abstract
Observations of precipitation and cloud-top height in the central United States and in the Caribbean area, obtained from radar-equipped airplanes, have been analyzed in terms of the fraction of clouds of a given height which contains precipitation. These data are compared with observations of a similar type taken by Braham, Reynolds and Harrell in New Mexico. It is concluded that the condensation-coalescence process can account for the formation of precipitation in convective clouds in all three regions but in varying proportions of clouds.
Abstract
Observations of precipitation and cloud-top height in the central United States and in the Caribbean area, obtained from radar-equipped airplanes, have been analyzed in terms of the fraction of clouds of a given height which contains precipitation. These data are compared with observations of a similar type taken by Braham, Reynolds and Harrell in New Mexico. It is concluded that the condensation-coalescence process can account for the formation of precipitation in convective clouds in all three regions but in varying proportions of clouds.
Abstract
Observations of Doppler spectra from particles 610 m above the ground were obtained by means of a vertically pointing X-band radar during a period when large hail was falling at the ground. It was found that the variance of the Doppler spectrum was a fairly good indicator of maximum hailstone sizes. Calculations of ice-particle size spectra were made on the basis of assumptions of particle shape, composition and fallspeed. The many assumptions needed to bring calculated ice-sphere spectra into approximate conformity with observations indicates the complexity of this procedure and the need for at least another independent observation of a property of the hailstones. Measurements of depolarization might yield an independent estimate of particle shape. It is concluded that the pronounced minima in the backscattering curves of large ice spheres should serve as an independent indicator of the speed of vertical air motions.
Abstract
Observations of Doppler spectra from particles 610 m above the ground were obtained by means of a vertically pointing X-band radar during a period when large hail was falling at the ground. It was found that the variance of the Doppler spectrum was a fairly good indicator of maximum hailstone sizes. Calculations of ice-particle size spectra were made on the basis of assumptions of particle shape, composition and fallspeed. The many assumptions needed to bring calculated ice-sphere spectra into approximate conformity with observations indicates the complexity of this procedure and the need for at least another independent observation of a property of the hailstones. Measurements of depolarization might yield an independent estimate of particle shape. It is concluded that the pronounced minima in the backscattering curves of large ice spheres should serve as an independent indicator of the speed of vertical air motions.
Abstract
By means of a vertically pointing pulsed-doppler radar, observations were made of a growing thunderstorm moving overhead. An updraft having a maximum speed exceeding 19 m sec−1 was observed in the upper part of the cloud. The character of the vertical velocity distribution supports earlier speculations about the pattern of vertical velocity near the top of an updraft. Inferences are made of the characteristics of precipitation in this cloud. The analyses lead to the conclusion that there were hail particles in the cloud. Although maximum echo intensity occurred in regions where hail particles with diameters of about 1 cm were present, maximum backscatter of radar power came from particles about half this size.
Abstract
By means of a vertically pointing pulsed-doppler radar, observations were made of a growing thunderstorm moving overhead. An updraft having a maximum speed exceeding 19 m sec−1 was observed in the upper part of the cloud. The character of the vertical velocity distribution supports earlier speculations about the pattern of vertical velocity near the top of an updraft. Inferences are made of the characteristics of precipitation in this cloud. The analyses lead to the conclusion that there were hail particles in the cloud. Although maximum echo intensity occurred in regions where hail particles with diameters of about 1 cm were present, maximum backscatter of radar power came from particles about half this size.
Abstract
An X-band pulsed-Doppler radar having its beam fixed at an elevation angle of 3°, has supplied new data on radial velocity spectra in two showers. It was found that the mean Doppler velocity, variance of the Doppler spectrum, and radar reflectivity varied markedly over distances of less than a kilometer. The observed variances were less than about 1 m2 sec−2 in more than 80% of the observations and exceeded 3 m2 sec−2 in about 4% of the cases. An analysis of ΔV̄/Δr, the radial gradient of the mean Doppler velocity, yielded a nearly Gaussian curve having a mean of 0.2 × 10−2 sec−1 and a standard deviation of 5.8 × 10−3 sec−1. The effects of the radial gradient of the radial wind apparently can explain less than about 15% of the observed variance of the Doppler spectrum. It appears that a major part of the variance is attributable to scales of motion smaller than the dimensions of the sampling volume.
Abstract
An X-band pulsed-Doppler radar having its beam fixed at an elevation angle of 3°, has supplied new data on radial velocity spectra in two showers. It was found that the mean Doppler velocity, variance of the Doppler spectrum, and radar reflectivity varied markedly over distances of less than a kilometer. The observed variances were less than about 1 m2 sec−2 in more than 80% of the observations and exceeded 3 m2 sec−2 in about 4% of the cases. An analysis of ΔV̄/Δr, the radial gradient of the mean Doppler velocity, yielded a nearly Gaussian curve having a mean of 0.2 × 10−2 sec−1 and a standard deviation of 5.8 × 10−3 sec−1. The effects of the radial gradient of the radial wind apparently can explain less than about 15% of the observed variance of the Doppler spectrum. It appears that a major part of the variance is attributable to scales of motion smaller than the dimensions of the sampling volume.
Abstract
The radar backscattering cross section of a spongy ice hailstone—a mixture of ice and liquid water—depends on its size, shape and dielectric function. There are two types of theories of the effective dielectric function of two-component mixtures: Maxwell-Garnet and Bruggeman theories. In the latter, the two components are treated symmetrically, whereas in the former they are not. We have generalized the Maxwell-Garnet expression, originally derived for spherical inclusions in an otherwise homogeneous matrix, to ellipsoidal inclusions. When this expression, with all ellipsoidal shapes equally probable, is used in calculations of radar backscattering by ice spheres coated with spongy ice, the results are in generally good agreement with measured cross sections. Agreement is better if the inclusions are ellipsoidal rather than spherical, but only slightly so. Shape considerations are less important than taking ice to be the inclusions and liquid water to be the matrix.
Abstract
The radar backscattering cross section of a spongy ice hailstone—a mixture of ice and liquid water—depends on its size, shape and dielectric function. There are two types of theories of the effective dielectric function of two-component mixtures: Maxwell-Garnet and Bruggeman theories. In the latter, the two components are treated symmetrically, whereas in the former they are not. We have generalized the Maxwell-Garnet expression, originally derived for spherical inclusions in an otherwise homogeneous matrix, to ellipsoidal inclusions. When this expression, with all ellipsoidal shapes equally probable, is used in calculations of radar backscattering by ice spheres coated with spongy ice, the results are in generally good agreement with measured cross sections. Agreement is better if the inclusions are ellipsoidal rather than spherical, but only slightly so. Shape considerations are less important than taking ice to be the inclusions and liquid water to be the matrix.
Abstract
No abstract available
Abstract
No abstract available
Abstract
Observations of a thunderstorm by means of a vertically pointing pulsed-Doppler radar were used to compare two presently used schemes for estimating vertical air velocity in convective clouds. It was found that the air velocity given by the positive bound of the Doppler velocity spectrum and the air velocity obtained by the use of a procedure proposed by R. R. Rogers in 1964 were well correlated. In regions where raindrops would be expected, the standard deviation of the difference of the two velocity estimates was 1.2 m sec−1. Part of the differences can be attributed to uncertainties in the size distribution of the scatterers and to difficulties in establishing the correct upper bound of the Doppler spectrum. This analysis concludes, in the rain-filled region of this cloud, that the effects of turbulence on the Doppler spectrum were small. The observations show that the vertical velocity and radar reflectivity fields in some convective clouds are much more variable than they are usually assumed to be by cloud physicists and dynamicists. Large differences of cloud properties were observed over distances of 500–1000 m.
Abstract
Observations of a thunderstorm by means of a vertically pointing pulsed-Doppler radar were used to compare two presently used schemes for estimating vertical air velocity in convective clouds. It was found that the air velocity given by the positive bound of the Doppler velocity spectrum and the air velocity obtained by the use of a procedure proposed by R. R. Rogers in 1964 were well correlated. In regions where raindrops would be expected, the standard deviation of the difference of the two velocity estimates was 1.2 m sec−1. Part of the differences can be attributed to uncertainties in the size distribution of the scatterers and to difficulties in establishing the correct upper bound of the Doppler spectrum. This analysis concludes, in the rain-filled region of this cloud, that the effects of turbulence on the Doppler spectrum were small. The observations show that the vertical velocity and radar reflectivity fields in some convective clouds are much more variable than they are usually assumed to be by cloud physicists and dynamicists. Large differences of cloud properties were observed over distances of 500–1000 m.
Abstract
The radar reflectivity factors, the reflectivity-weighted mean terminal velocities (
Abstract
The radar reflectivity factors, the reflectivity-weighted mean terminal velocities (
