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Theodore W. Cannon

Abstract

In situ photographs are taken of atmospheric particles with radius 4 μm and larger with a special particle camera installed on a research sailplane. Ice particle shapes, sizes and concentrations, raindrop sizes and concentrations, and microscopic cloud particle concentrations and size estimates are obtained from the photographs. Liquid particles can be distinguished from ice particles if their radii exceed about 50 μm.

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Theodore W. Cannon

Abstract

Atmospheric particles ≳10μ in diameter have been successfully photographed in situ using cameras with microsecond-duration flash lamps. Several such cameras developed and used in the past are discussed in this article. A bright-field camera for photographing precipitation particles, and a light-weight portable dark-field camera for photographing cloud particles have been developed at this laboratory. Several representative photographs are presented to demonstrate these cameras' imaging properties and to illustrate their use. The cameras have been used successfully to photograph particles at ground level. Mechanical motion compensation will be required at aircraft speeds if nonstreaked images are to be produced.

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Theodore W. Cannon

Abstract

A technique is described for photographing airborne particles using small forward-scattering angles. The sample volume is defined as the region where collimated light beams of two different primary colors overlap. Images of particles within that volume are identified as those having the additive color of the two beams. The technique has the advantage of providing significantly more scattered light toward the camera lens than with 90° scattering commonly used in cloud chamber photography.

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Theodore W. Cannon and Walter W. Grotewold

Abstract

Drop generators have been developed at NCAR based on the “wire egression” principle. They are outstanding in their flexibility and simplicity. Drops ranging in diameter from 6 μm to 1 mm have been generated with one model. Application of the generators to calibration of NCAR particle cameras and the PMS Forward Scattering Spectrometer Probe (FSSP) as well as applications to cloud physics laboratory research are described.

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J. Doyne Sartor and Theodore W. Cannon

Abstract

The modification of thunderstorms to suppress hail requires a knowledge of where, when and how much to seed. We show that growth by accretion by precipitating particles (hail, rain and graupel) in a summer convective storm depends on the path that the particles take with respect to the cloud air circulation in which the cloud droplets are embedded, as well as on the ambient atmospheric parameters of temperature, moisture, stability and larger scale circulations. For this purpose we used a two-dimensional simulation of the circulation in which the most important features of one-dimensional time-dependent microphysics simulation can be incorporated into the calculations at each time step.

The effect of changes in the altitude of ice particle initiation is calculated using simulations of the clouds and their environment on two days during this period when the total amount of hail differed by more than an order of magnitude. The simulated hail size and amount varied in the same sense as the observed.

The simulation is applied to the case used by Browning and Foote (1976) to develop a conceptual model of a severe hailstorm. The results show that in order to obtain the tilt of the updraft shown by Browning and Foote, the presence of a squall line or gust front would be required—a situation that they say is possible from their observed meteorological data.

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J. Doyne Sartor and Theodore W. Cannon

Abstract

The observational results from sailplane flights into the updrafts of developing cumulus clouds in north-eastern Colorado show some important variations in the microstructure of the cloud droplet and ice particle distributions. Some of these variations are apparently caused by the combined interactions of cloud droplets and precipitation particles with the horizontal and vertical components of the updraft and its horizontal and vertical structure.

Data from these observations are introduced into a circulation framework in an attempt to understand how the microphysics and the circulation can interact to give the features observed. The results cast doubt on the validity of the often made assumption that the microphysical properties of a cloud are distributed randomly with respect to each other on the smaller scales, and that this condition exists uniformly throughout the cloud.

The observed precipitation shafts with bimodal size distributions in the middle and lower parts of a cloud can be recreated in a two-dimensional simulation of the observed cloud air circulation with embedded microphysics. The observed and calculated frozen water content can increase by one to two orders of magnitude over the liquid water content when moving from cloudy air into a precipitation shaft. The observed change in concentration with height of the ice particles exceeds (by over two orders of magnitude) the expected ice nuclei concentration usually found in the atmosphere at comparable temperatures. The average concentrations of ice particles observed occasionally exceed 400 l −1.

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Theodore W. Cannon, James E. Dye, and Vim Toutenhoofd

Abstract

Some cloud microphysical measurements made from the sailplane The Explorer are presented. Drop sizes and concentrations and ice particle sizes, concentrations, shapes, and, in some cases, internal structures are determined from in situ photographs taken with a recently developed particle camera. The data from photographs taken in northeastern Colorado during the early stages of the development of spring and summer cumulus clouds and in their precipitation suggest that the first precipitation forms primarily by the ice process rather than by condensation–coalescence. Ice particles were frequently photographed in these clouds, but above the freezing altitude, water drops > 50 μm radius were very rare.

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Cleon J. Biter, Theodore W. Cannon, Edwin L. Crow, Charles A. Knight, and Philip M. Roskowski

Abstract

An airborne photographic system, in which the cameras are coupled with an inertial navigation system, was developed and used in a 1978 convective cloud study, Photogrammetric analysis from such a system is enhanced: cloud-feature positions can be determined without external references such as the earth's horizon or cloud base in the photographs, and the data reduction process can be considerably automated.

This paper describes the instrumentation, the photogrammetric theory, and the procedures for obtaining cloud measurements from the photographs. An empirical error analysis based on photographs of terrestrial targets is also presented. Cloud top heights determined without any reference height in the photographs are considered to be accurate to within 440 m at a range of 60 km. The largest source of error in determining cloud top height using the 1978 measurements is the uncertainty in determining the aircraft-to-cloud distance rather than inaccuracy in the photographic system. This error can be reduced in future programs by flying as closely in altitude as possible to the cloud features of interest.

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James E. Dye, Charles A. Knight, Vim Toutenhoofd, and Theodore W. Cannon

Abstract

Much of the previous work which has led to the conclusion that coalescence is the dominant precipitation forming mechanism in cumulus clouds is reviewed. Observations in northeastern Colorado from several independent methods of investigation are summarized to show that in northeastern Colorado the ice (Bergeron–Findelsen) process is in all probability the dominant mechanism in spring and summer cumuli in their early and intermediate stages of development.

Results of microphysical observations coordinated with simultaneous radar observations are presented. The microphysical observations in clouds with observed effective reflectivities of up to 40 dBZ show that the observed reflectivities can be accounted for by measured ice particle sizes and concentrations. Liquid precipitation elements are not necessary and have been observed only rarely in these clouds except below the melting level.

Possible explanations of the differences between clouds in northeastern Colorado and those in other areas are discussed. The rarity of liquid precipitation particles coupled with the general inefficiency of the ice process at temperatures warmer than −10C suggests that there is potential for rainfall enhancement in the clouds in northeastern Colorado.

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