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B. B. PHILLIPS

Abstract

The electrical conductivity existing in electrified clouds is computed under the assumption that a primary positive dipole charge distribution exists within the cloud and that the small ion concentrations are controlled by the rate of ion production from cosmic ray ionization, and by the rates of ion loss from the separate processes of ion recombination, diffusion, and conduction. The results show that the cloud is nearly non-conducting by the small ion conduction mechanism when electric fields are present. The charge transfer mechanisms in the cloud boundary regions and in the cloud updraft are examined, and the thickness of the sheathing layer charge distribution in quasi-static cloud boundaries is estimated.

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B. B. PHILLIPS

Abstract

The charge transport within thunderclouds by the convective motions of the cloud is examined. In the presence of the primary positive dipole, shielding charge distributions are formed within, the lower and upper cloud boundaries as the result of ion conduction from the free air and ion capture by droplets and precipitation in peripheral cloud layers. Cloud droplets positively charged by the conduction current to the cloud base are lifted by the updraft into appear cloud volumes where they contribute to the positive pole of the primary dipole. In turn, precipitation developing in the cloud top carries negative charge to lower cloud levels during fall under gravity forces. The role of water accumulation in the upper cloud is shown to be an important factor in establishing the non-neutralizing current paths in upper cloud layers. The charge transport by the convection mechanism is believed to be a major current flow of storms. The role of other thunderstorm electrification mechanisms is only briefly considered.

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B. B. PHILLIPS

Abstract

An analysis is made of the charge distribution in a quasi-static thundercloud system with the assumptions that, (1) the cloud has a charge separation mechanism at its midpoint and (2) the conductivity within the cloud is reduced by a given fraction from the free-air conductivity remote from the storm. The charge separation mechanism produces the primary positive dipole of the storm. The analysis shows that the growth of the charge in the central cloud primary dipole is accompanied by the development of shielding layers of opposing sign charge at and about the cloud-air interface. A positive shielding charge distribution is established at the cloud base while a negative charge shielding distribution occurs about the upper cloud. The analysis permits an evaluation of the limiting charge magnitudes that exist outside the cloud boundary as a result of the unbalance of the positive and negative ion concentrations, and those that exist inside the cloud boundary principally as the result of deposition of charge on cloud particulars. The lower positive shielding charge completely accounts for the positive charge center often observed in the base of storms. The growth rates and total charge of the shielding charge distributions approximate the growth rate and total charge of the primary dipole. Following the lightning discharge, the shielding layer charge readjustment occurs at a more rapid rate, determined principally by the free-air conductivity.

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B. B. Phillips and Ross Gunn

Abstract

The electrical charge transferred to highly insulated spheres by the diffusion of ions from ionized moving air is directly measured. The equilibrium charge is proportional to the logarithm of the ratio of the positive and negative ion conductivities, and depends upon the velocity of the sphere relative to its ionized environment. The measurements agree with theory.

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Ross Gunn and B. B. Phillips

Abstract

Measurements in a giant expansion chamber show that the size of newly formed cloud droplets depends critically upon the cleanliness of the processed air. Droplets formed in ordinary air are small, but droplets large enough to precipitate are immediately formed by condensation whenever the condensation-nuclei density is sufficiently reduced. The product of mean drop mass and the activated nuclei density approximates the available free water per unit volume. Cloud droplets formed from polluted surface air are usually too small to precipitate, but large droplets formed overhead by condensation in sufficiently clean air may fall through the polluted cloud and initiate rain through association processes. Since the activated nuclei density is normally observed to decrease with increasing altitude, the probability of generating droplets sufficiently large to initiate rain increases as the vertical development of a cloud system increases. The population densities of large cloud droplets normally observed near the tops of precipitating clouds may be explained in terms of an overlying parcel of cooled air that is initially relatively free of nuclei.

Since pollution is swept out of the atmosphere by diffusion onto cloud droplets, and by droplet movement, it is suggested that periods of general cloudiness and precipitation reduce the original nuclei density and permit the subsequently formed droplets to grow still larger, thus increasing the probability of appreciable precipitation. The rain producing cycle is, therefore, equipped with a feed-back or regenerative mechanism which normally proceeds, in a given mass of air, until the air is appreciably desiccated.

Condensation nuclei as well as water vapor normally accumulate simultaneously in fair weather. The presence of nuclei may delay the initiation of precipitation until sufficient vertical instability can be established to lift or cool the relatively clean overlying layers. The precipitation cycle may then be re-established. The investigation shows that rain may be formed directly from the vapor in clean air, without the production of clouds.

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Byron B. Phillips

Abstract

A dense 450-station network of hail and rain gages in 2025 km2 of northeastern Colorado has provided surface precipitation data from a moderate intensity thunderstorm that crossed the network on 22 July 1972. The storm, which existed in an environmental vertical wind shear of 6 × 10−2 sec−1 for over 4 hr, exhibited characteristics of the supercell-type storm including a well-defined and continuing weak echo region, a deviate motion to the right of the local winds, a precipitation wall advancing toward the region of strongest inflow, and a forward overhang of high reflectivity as far as 10 km and more ahead of the precipitation wall. The surface precipitation network results from the storm are analyzed and correlated with other measurements obtained from precision radar, instrumented aircraft, multiple high-altitude released dropsondes, rawinsondes, and surface observations. Hail recorded at 12 network stations was associated with two inflow regions that existed simultaneously and moved in divergent directions. Hailfalls correlated with regions of high radar reflectivity gradient. Hailfalls that comprised a principal hailswath were precisely located with respect to the weak echo region of the primary updraft. Four distinct rainfall maxima occurred within the network, each regularly spaced 13 km apart at 15-min intervals. A description of the storm processes is given.

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Phillip B. Chilson

Abstract

Range imaging (RIM) is used to describe a constrained optimization signal-processing method that can be applied to wind profilers capable of operating over a small set of distinct transmitter frequencies. The results of the signal-processing method are typically high-resolution maps of the backscattered power as a function of range. In this paper it is discussed how RIM processing can be implemented in order to additionally obtain high-resolution estimates of the Doppler velocity. The method has been demonstrated using data from a 915-MHz tropospheric profiler located in Platteville, Colorado. Examples of data collected during an experiment conducted on 10 April 2001 are presented. In this experiment the radar was operated alternately in two different modes. The cycle time for the two modes was about 50 s. The particular operation of the radar allowed comparison of radar reflectivity (η) and vertical velocity (V) measurements collected using the two modes. In the first mode, 2-μs pulses were transmitted and RIM processing was used to produce estimates of η and V on a grid with a separation of only 15 m. Without RIM processing the range resolution of the data would have been 300 m. In the second mode, 0.5-μs pulses were used, corresponding to a range resolution of 75 m. Estimates of η and V were then obtained from these data through conventional Doppler spectral processing. A conditional averaging method was used to process the reflectivity and vertical velocity data from the two modes. It is shown that the RIM-processed data can be used to resolve structures in the height profiles of η and V on scales less than those of the conventional range resolution of the radar as dictated by the pulse width.

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B. B. Phillips and R. H. Woessner

Abstract

No Abstract Available

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B. B. Phillips and Gilbert D. Kinzer

Abstract

The size and the free electric charge of more than 6000 individual natural cloud droplets have been measured. The measurements were made at a mountain laboratory in southeastern United States in strato-cumulus clouds and in thunderstorm-associated clouds. Stratocumulus clouds with fair-weather electric fields were found to have approximate Gaussian-charge distributions symmetric about zero charge. The magnitude and distribution of charge observed on these non-storm clouds approach values described recently by R. Gunn in his theory of aerosol electrification by the diffusion of environmental light ions. Thundercloud droplets were highly electrified and within given cloud volumes the droplets were charged entirely positive, entirely negative, or fractionally positive and negative.

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W. A. Hoppel and B. B. Phillips

Abstract

No abstract available.

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