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C. D. Stow

The destructive nature of cloud-to-ground lightning strokes is well known. Loss of life and damage to buildings and other man-made structures may to a large extent be prevented by the judicial use of lightning conductors and screens but no comparable protection may be offered to expanses of agricultural crops or forests. According to Fuquay (1967) lightning is the greatest single cause of forest fires in the western United States: during the period 1946–1962, 140,000 such fires occurred causing severe losses of timber, wildlife, watershed, and recreational resources. Comparable losses occur regularly in other parts of the world. The only solution is the suppression or modification of cloud-to-ground lightning discharges. Methods of suppression are described, some of which may turn out to be practical ways of achieving this aim.

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M. C. Woodward
and
C. D. Stow

Abstract

The behavior of water drops of diameter in excess of 3 mm suspended in a vertical wind tunnel is described and the vertical-position stability quantitatively and qualitatively assessed. The contribution to vertical instability caused by measured turbulence is calculated but does not fully account for the degree of instability observed. It is suggested that an inherent instability arises from the airflow constraints required to maintain a drop in the working section of the tunnel. When a drop is retained by such constraints, the artificial conditions under which experiments are performed will limit the applicability of results obtained in the wind tunnel to the atmosphere.

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J. G. Hosking
and
C. D. Stow

Abstract

Preliminary data from ground-based measurements of rainfall using an optical disdrometer and high-resolution rain gage network are described. The recurrence of the rain period is shown to be non-Poisson with evidence of clustering, whereas a lognormal distribution is shown to be an approximate fit to the distribution of amount of rainfall in a rain period. About 40% of rain periods possess a total rainfall smaller than that resolvable by conventional recording rain gages; three-quarters of these were resolvable by the high-resolution rain gages used. Rapid fluctuations of rainfall intensity (∼1 mm h−1 s−1) are evident in individual rainfall intensity records and the distribution of duration of rainfall above a specified threshold intensity is shown to be approximately lognormal; these observations are consistent with a lognormal distribution of precipitation region sizes. An empirical fit to the average fractional duration of rainfall is given, although the curve is generally a poor fit to fractional duration data for individual rain periods; periods of intense rainfall tend to be more singular than expected from the curve, and the maximum intensity reached in a rain period is shown to be independent of the period's duration. A method for estimating spatial sizes and shapes of precipitation regions using a high-resolution rain gage array is demonstrated.

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S. G. Bradley
and
C. D. Stow

Abstract

A model for raindrop interactions incorporating satellite droplet production is used to determine drop size distributions at the ground. Assuming an exponential distribution at cloud base and using known meteorological conditions, the theoretical drop size distribution agrees well with distributions observed in the field; both predicted and observed distributions have reduced numbers of drops of radius near 0.35 mm. A time series analysis is facilitated by using a running-mean technique on each part of observed drop size distributions. A cross correlation is then performed between the time-series for large, medium and small drop concentrations. It is found that the smallest drops are associated in time with drops having a high satellite droplet production rate. On the other hand, the time delay at ground level between changes in large-drop concentration and small-drop concentration can be interpreted in terms of fallspeed differences. This allows the effective altitude of the active rain-producing region to be determined.

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S. G. Bradley
and
C. D. Stow

Abstract

An apparatus is described which enables reliable measurement of precipitation charge and size to be made. Design criteria are thoroughly treated and general solutions are provided to all aspects of the problem. The theories allow the performance of the apparatus in relation to drop splashing, drop overlap, and finite sampling time to be estimated with confidence. Full constructional details and performance of the apparatus and associated data storage equipment operating under strict laboratory conditions is given.

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S. G. Bradley
and
C. D. Stow

Abstract

Measurements of the size and charge on precipitation particles obtained using the apparatus described in Part I are given. Size data are found most frequencly to fit a log-normal distribution, and charge data a normal distribution; bi-modal distributions of charge and size data are also evident. Changes in the distribution parameters during the passage of frontal systems are calculated and correlations between drop size and drop charge examined for evidence of a systematic charging mechanism. Correlations between size and charge data and atmospheric potential gradient and prevailing meteorological conditions are also discussed.

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S. G. Bradley
and
C. D. Stow

Abstract

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C. D. Stow
and
K. Jones

Abstract

A disdrometer for the measurement of the co-spectra of raindrop size and charge is described which can evaluate to a significant extent spurious data caused by unavoidable drop overlap within the charge detector volume. The sizes of individual drops in the range 0.1–2.5 mm radius and charges within the limits of magnitude 0.1–10 pC could be determined. The use of two size detectors enabled the measurement of drop velocity and the detection of drop overlap within the charge-sensitive volume to be made. Non-ideal data which arise from natural conditions, measuring sites, and from fundamental or unavoidable deficiencies in disdrometer design, could be tested and monitored automatically using a central processor under software control: TIME OUT, when a drop failed to pass through both size detectors; COINCIDENCE, caused by simultaneous occupation of both size detector volumes; SIZE MISMATCH, when the separate size measurements did not agree within limits predetermined by software; HIGH or LOW VELOCITY, when the actual drop velocity was not close to the terminal value expected from size measurement. The self-evaluating disdrometer cannot be designed to provide the minimum error content possible but offers the advantage of assessing the proportion of spurious data present; it is argued that this may be preferable to the situation in which the error content is lower but unknown. The performance of the instrument was assessed using individual drops generated in the laboratory and by exposure to natural rain falling through an aperture into a chamber shielded from wind and associated turbulence. The latter test was made at a non-ideal site in order to demonstrate the ability of the disdrometer to provide information on invalid data so that raw co-spectra may be corrected. In the preliminary tests described a substantial proportion of the drops possessed fall speeds significantly below their expected terminal velocity, in some cases as much as 30% less, and not more than 20% of the drops detected satisfied all criteria for acceptance. Further, an examination of drop arrival rates showed that not all data could be fitted to a Poisson-type distribution, either because of rapid changes in the mean arrival rate or on account of the clustering of drops. The potential seriousness of the drop overlap problem, which is fundamental to all methods of measurements, is emphasized in the trial analysis: uncertainties in the exact form of the size distribution, particularly for drops in the radius range 0.1–0.5 mm, render the design of any instruments of fixed entrance aperture size dubious; the co-spectra must be expected to show appreciable distortion unless data associated with drop overlap, particularly within the charge-sensitive volume, are excluded. Some improvements in the current disdrometer design are suggested.

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S. G. Bradley
and
C. D. Stow

Abstract

The effect of individual charge-modifying processes is tested by following the development joint distributions of raindrop size and charge through a fall depth of 1 km beneath a weakly electrified warm cloud. Evaporation, ion capture and coalescence all cause the small-drop charge distribution to become narrower and any initially skewed distribution to become more symmetrical. Ion capture does not result in substantial negative charging unless the initial charge distribution is very narrow. Ion diffusion charging is found not to be significant.

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S. G. Bradley
and
C. D. Stow

Abstract

Raindrop collisional breakup is included in a model of joint size-charge distribution development. Consistent with experimental observations the model includes a finite filament (joining separating drops) that upon rupture produces charged satellite droplets. It is shown that for positive fields, large numbers of small drops carry negative charge, leading to an increasingly negative space charge with fall depth. The model also suggests that larger separated charge exists on the major drop products than previously estimated.

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