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James E. Dye

Abstract

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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
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
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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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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Andrew J. Heymsfield
,
James E. Dye
, and
Cleon J. Biter

Abstract

Temperature measurements from aircraft, taken with a reverse flow sensor and Rosemount probe, were compared in a penetration through a cloud to determine the effect of sensor wetting on the deduced entrainment. The temperature measured using the Rosemount probe was depressed relative to that measured with the reverse flow probe, leading to an underestimate of the equivalent potential temperature and an overestimate of the deduced entrainment. These results were used to identify possible errors in recent entrainment studies that used temperature as an indicator of entrainment.

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Edwin L. Crow
,
Alexis B. Long
,
James E. Dye
, and
Carlton W. Ulbrich

Abstract

The hailstone size (diameter) distributions measured by hailpads during the 1972-74 randomized seeding experiment of the National Hail Research Experiment are analyzed statistically for evidence of seeding effects and differences from year to year. Two approaches are taken, one comparing the entire empirical size distributions on seed days and on control days and the other comparing the mean diameters. The latter is based on the consistency with the exponential distribution (truncated at a prescribed minimum diameter), since the exponential distribution can be characterized completely by the difference between the mean diameter and the minimum diameter. Both approaches yield statistically significant results (10% level) only for 1974, when the hailstones were larger on seed days than on control days on the average. This may have resulted from the addition of seeding by rockets in 1974 or from differences in the hailpads used in that year. However, the physical hypothesis for the experiment predicted smaller stones on seed days; that tendency did appear in 1973 (though not significantly) and the difference was negligible in 1972.

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Edwin L. Crow
,
Alexis B. Long
,
James E. Dye
,
Andrew J. Heymsfield
, and
Paul W. Mielke Jr.

Abstract

An extensive statistical analysis is made of the precipitation data collected during the randomized seeding experiment conducted by the National Hail Research Experiment during 1972-74, aimed at testing the feasibility of diminishing hail by seeding with silver iodide. The major conclusion is that no effect of seeding is detected at the 10% significance level. This is true regardless of whether hail or rainfall response variables are considered, which of two methods of obtaining daily values for the response variables over the target area is used, or what distribution, if any, is assumed for the variables. Even though the ratios of hailfall or rainfall on seed days to those on control days are generally greater than 1, the confidence intervals attached to these ratios are so large, because of the large natural variance in each response variable and the small sample sizes, that the true underlying seeding effects could in every case have ranged from substantial decreases to large increases. The large confidence intervals emphasize the necessity of large sample sizes, large experimental areas or effective covariates for obtaining definitive results in precipitation modification experiments.

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Francis J. Merceret
,
Jennifer G. Ward
,
Douglas M. Mach
,
Monte G. Bateman
, and
James E. Dye

Abstract

Electric-field measurements made in and near clouds during two airborne field programs are presented. Aircraft equipped with multiple electric-field mills and cloud physics sensors were flown near active convection and into thunderstorm anvil and debris clouds. The magnitude of the electric field was measured as a function of position with respect to the cloud edge to provide an observational basis for modifications to the lightning launch commit criteria (LLCC) used by the U.S. space program. These LLCC are used to reduce the risk that an ascending launch vehicle will trigger a lightning strike that could cause the loss of the mission or vehicle. Even with fields of tens of kV m−1 inside electrically active convective clouds, the fields external to these clouds decay to less than 3 kV m−1 within 15 km of cloud edge. Fields that exceed 3 kV m−1 were not found external to anvil and debris clouds.

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Jeffrey L. Stith
,
James E. Dye
,
Aaron Bansemer
,
Andrew J. Heymsfield
,
Cedric A. Grainger
,
Walter A. Petersen
, and
Robert Cifelli

Abstract

The results from airborne in situ sampling of convective tropical storms in the Amazon and Kwajalein are presented. Three cases from the Amazon and two from Kwajalein are compared and provide examples of the much larger dataset that was obtained from field campaigns in these two regions during 1999. The strength of the updraft was a major factor in determining the microphysical characteristics of hydrometeors. Weak updrafts exhibited a well-developed warm rain process by the time droplets had reached the freezing level. Stronger updrafts (>5 m s−1) contained smaller droplets or ice particles at cloud midlevels than regions with the weaker updrafts. Significant supercooled liquid water was found only at temperatures warmer than −12°C, although traces of liquid water were observed at temperatures as cold as −18°C. In deep stratiform anvil regions, aggregation was observed to be a major growth mechanism. These clouds did not contain appreciable amounts of supercooled water. Clouds with similar updrafts in the Amazon and Kwajalein exhibited similar particle types and concentrations. The implications of these results for current Tropical Rainfall Measuring Mission (TRMM) investigations are discussed.

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