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S. G. Bradley

Abstract

The design of an instrument that sorts raindrops according to fall speed is described. The apparatus consists of two rotating disks, the upper one allowing rain to fall through a slit into collectors on the lower disk. Drops are collected in a manner allowing chemical concentrations and pH to be determined as a function of drop size. Laboratory and field tests validate the detailed design theory for conditions of low windspeed.

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S. G. Bradley

Abstract

Raindrop size distributions are obtained from the Doppler frequency spectrum of an acoustic radar. Number concentrations of 12 drop diameters with a minimum diameter 0.14 cm are obtained and averaged over 3–15 min at 20-m range gates from 20 to 220 m. The last three range gates are used to estimate rain intensity–dependent background noise, which is dynamically subtracted from the signals. Multifrequency sounding is also used.

Intercomparisons with the vertical rain intensity profile from an X-band radar and with drop size distributions from an impact disdrometer show general agreement between instruments and demonstrate the usefulness of the acoustic profiler in giving vertical continuity below the range of electromagnetic radars. Temporal variations in raindrop size distributions are found to have an essentially flat spectrum for periodicities shorter than 12 min, although the step response to a sudden change in rainfall rate is a function of drop size. Principal component analysis applied to a time series of drop spectra shows that nearly all the variation is at the large-drop end. The utility of the acoustic radar is demonstrated for examining the microphysics of rain through time-dependent changes.

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S. G. Bradley

Abstract

An analytic model of three-dimensional radiative transfer is modified to include cloud-cloud interactions and finite surface albedo. The spectrally integrated output is used to derive extinction coefficients for cumulus clouds from aircraft observations of cloud albedo and directional reflectance. Four independent estimates of extinction coefficient are compared for various actual and postulated boundary conditions. Surface albedo is found to be an important parameter. Results suggest it is possible to remotely detect microphysical differences between clouds growing in different air masses using broad-band measurements.

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S. G. Bradley and R. G. Gibson

Abstract

Laboratory and airborne observations show that the protective hemispheres of aircraft pyrgeometers are partly covered by water when used in cloud. This cover can reduce the incident longwave flux by as much as 60%. Improved agreement between observations and theory is obtained when a parameterization of water cover in terms of cloud liquid water content is used to correct flux divergence and cooling rate data. It is suggested that all previous in-cloud pyrgeometer measurements may suffer cloud-water contamination.

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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 W. D. King

Abstract

A recent treatment of the dynamic behavior of anemometer hot wires is simplified and used to show that the response of the CSIRO liquid-water hot wire can be described by a second-order differential equation. This is confirmed by experiment, and it is shown that the inherent time scale is fully calculable in terms of the thermal properties of the wire and its support and the amplifier constants. For most clouds a response time of 0.005 s can be readily achieved.

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S. G. Bradley and C. D. Stow

Abstract

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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

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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