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James E. Dye

Abstract

Detailed comparisons of measurements from the electrostatic disdrometer and from soot-coated impactor slides show consistent differences in droplet size distributions determined by the two techniques. The disdrometer-derived distribution almost always shows highest concentrations in the first size interval and decreasing concentrations in successively larger size intervals, even in cases when the slides have recorded very few droplets in the smallest sizes. A comparison of the mean radii determined from the two sources for 36 different cases shows that the radii determined from the slides vary between 5.5 and 10 μm, while those determined from the disdrometer vary only between 5.5 and 6.5 μm. Also, as the mean droplet radius increases, the disdrometer measures increasingly higher droplet concentrations than the slides.

Uncertainties and possible errors associated with both the impaction slide and disdrometer measurements are examined. From additional laboratory experiments it is concluded that the disdrometer does not properly size droplets which enter the orifice off center or at an appreciable angle relative to the axis of the orifice. A method for overcoming this problem is suggested.

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James E. Dye

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James E. Dye and Darrel Baumgardner

Abstract

Laboratory studies of the Forward Scattering Spectrometer Probe (FSSP), were conducted to better understand the operation, to determine limitations and to define the measurement accuracy of the instrument for airborne cloud physics research. The studies included electronic cheeks of the instrument sensitivity to simulated particles of different sizes, airspeeds and arrival rates; measurement of important aspects of the optical configuration; and intercomparisons of six different FSSPs in a small wind tunnel with a droplet spray. The tests demonstrated measurement differences between various probes in several areas as well as areas in which there was reasonable agreement. Part of the differences can be attributed to different feature and design specifications of different probes as changes were made to improve the FSSP. Areas in which care needs to be taken in the calibration and processing of data from the FSSP are identified.

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James E. Dye and Brooks E. Martner

Abstract

Data from the hailpad network of the National Hail Research Experiment were examined in relation to the equivalent radar reflectivity factors recorded in the lowest level sweeps of the radar beam over the pads during hailstorms in 1972 and 1976. The relationship between hail detected at the ground and reflectivity factor was examined for both areal coverage and on a point-by-point basis for each hailpad. The comparisons show that reflectivity factors of 55 dBZ are often measured when no hail is observed at the ground. Rain alone can give rise to reflectivities of this magnitude. The results of the study show that in northeastern Colorado low-level equivalent radar reflectivity factors alone cannot be used to determine the region of hailfall at the ground, nor are they likely to augment quantitative measurements by a ground network of hail sensors. The results found in northeastern Colorado are compared to results from other geographical regions.

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James E. Dye and John C. Willett

Abstract

A study of two long-lived Florida anvils showed that reflectivity >20 dBZ increased in area, thickness, and sometimes magnitude at the midlevel well downstream of the convective cores. In these same regions electric fields maintained strengths >10 kV m−1 for many tens of minutes and became quite uniform over tens of kilometers. Millimetric aggregates persisted at 9–10 km for extended times and distances. Aggregation of ice particles enhanced by the strong electric fields might have contributed to reflectivity growth in the early anvil, but is unlikely to explain observations farther out in the anvil. The enhanced reflectivity and existence of small, medium, and large ice particles far out into the anvil suggest that an updraft was acting, perhaps in weak convective cells formed by instability generated from the evaporation and melting of falling ice particles. It is concluded that charge separation must have occurred in these anvils, perhaps at the melting level but also at higher altitudes, in order to maintain fields >10 kV m−1 at 9–10 km for extended periods of time over large distances. The authors speculate that charge separation occurred as a result of ice–ice particle collisions (without supercooled water being present) via either a noninductive or perhaps even an inductive mechanism, given the observed broad ice particle spectra, the strong preexisting electric fields, and the many tens of minutes available for particle interactions. The observations, particularly in the early anvil, show that the charge structure in these anvils was quite complex.

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James E. Dye and Daniel W. Breed

Abstract

During June 1977 ground-based measurements of the CCN spectrum were made twice daily near Kericho, Kenya, and two flights were flown in that region to investigate the microstructure of the clouds. Both the CCN and the cloud-droplet distribution measurements show that the cloud microstructure is continental.

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Darrel Baumgardner, Walter Strapp, and James E. Dye

Abstract

Cloud particle concentrations measured by the Forward Scattering Spectrometer Probe (FSSP) can be underestimated when particles are either coincident or pass through the sensing area of the probe during the electronic dead-time. In the absence of any corrections, the differences between actual and measured concentrations can typically exceed 15% when aircraft mounted probes measure droplet concentrations > 500 cm−3. The sources of counting losses are described and correctional procedures derived and demonstrated.

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Charles E. Abbott, James E. Dye, and J. Doyne Sartor

Abstract

An electrostatic cloud droplet sizing device (electrostatic disdrometer) originally developed by Keily and Millen has been tested, modified extensively, and calibrated in our laboratory. The investigations have shown that soon after entry into the probe orifice, the incoming droplet is broken into many fragments. These impact and splash on an electrode raised to a 510 V potential. Measured pulses for a given droplet size give a reproducible calibration curve.

Airborne tests of the probe have shown it to operate reliably with minimal maintenance. Comparisons were made between values of the liquid water content measured by the electrostatic disdrometer and by the Johnson-Williams hot-wire, liquid-water-content meter and between the droplet size distributions measured by the disdrometer and by impaction slide replicas. The comparisons were satisfactory within the limits of instrument measuring and sampling errors and actual variations in the droplets spectra resulting from the separation of the instruments on the aircraft during the tests.

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L. Jay Miller, James E. Dye, and Brooks E. Martner

Abstract

Measurements from three Doppler radars of air motion and observations of the environment and storm reflectivity structure, supplemented by aircraft measurements of precipitation and cloud particles, are used to establish the dynamical framework for precipitation development in a convective storm that grew in a weakly-sheared wind environment. The moderately intense, evolving storm consisted of a series of cells that developed in late afternoon on 25 July 1976 in southeastern Wyoming. The storm, which moved along the sub-cloud wind direction, had a persistent but unsteady updraft region on its right forward flank. This updraft region consisted of several small convective elements with two or more intense updraft cores evident at all times. Middle-level flow around the updraft region eventually resembled obstacle flow with downdrafts located on the flanks and in the wake of the updraft. This storm-wide, organized circulation apparently allowed precipitation particles to reenter an updraft and grow for periods longer than would have been possible if all their growth had occurred in a single ascent within an updraft core of 10 to 20 m s−1 speeds. Such vertical motions would have carried particles to cloud top in 5 to 10 min, a growth period too short to account for the observed millimeter-size particles in the updraft. This storm lasted for more than one hour and produced hail particles as large as 9 mm diameter that were observed at cloud base by aircraft.

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James E. Dye, Brooks E. Martner, and L. Jay Miller

Abstract

Aircraft measurements of microphysical thermodynamic, and vertical air motion properties supplemented by radar measurements of reflectivity structure are used to investigate precipitation development throughout much of the life cycle of a moderately intense convective storm in northeast Colorado. There was considerable variability of cloud properties such as updraft speed at scales of a few hundred meters early in the life of the storm. Greater organization was evident in the later, more mature stage.

The earliest radar return from a major cell came from particles larger than 1 mm diameter in concentrations less than 10 m−3. It is suggested that these particles must have already followed complicated growth trajectories even at this early stage of the storm, including more than one ascent in updraft.

In the more mature stage of the storm millimetric water drops and partially melted and refreezing ice particles were observed at 1 to 2 km above cloud base in both mixed and unmixed regions of the main updraft. Because of their size and proximity to cloud base, these particles could not have grown during a single ascent in the updraft. The observations suggest that penetrative downdrafts, sedimentation, growth in weak updrafts, and recycling, among others, acted singly or in concert to increase particle growth times in this cloud, and that individual particles may have spent an appreciable length of time outside of favorable growth regions.

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