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The National Center for Atmospheric Research (NCAR) developed an artificial snow-generation system designed to operate in a laboratory cold chamber for testing aircraft anti-icing fluids under controlled conditions. Flakes of ice are produced by shaving an ice cylinder with a rotating carbide bit; the resulting artificial snow is dispersed by turbulent airflows and falls approximately 2.5 m to the bottom of the device. The resulting fine ice shavings mimic snow in size, distribution, fall velocity, density, and liquid water equivalent (LWE) snowfall rate. The LWE snowfall rate can be controlled using either a mass balance or a precipitation gauge, which measures the snowfall accumulation over time, from which the computer derives the LWE rate. LWE snowfall rates are calculated every 6 s, and the rate the ice cylinder is fed into the carbide bit is continually adjusted to ensure that the LWE snowfall rate matches a user-selected value. The system has been used to generate LWE snowfall rates ranging from 0 to 10 mm h−1 at temperatures from −2 to −30°C and densities of approximately 0.1–0.5 g cm−3. Comparisons of the snow-machine fluid tests with the outdoor fluid tests have shown that the snow machine can mimic natural outdoor rates under a broad range of conditions.

Results are summarized and illustrated from a long series of experiments on ice growth from the vapor, nearly all in a very small range of conditions: −5°C, slightly below liquid water saturation, with minimal environmental gradients and no imposed ventilation. The temperature was chosen because c-axis ice needles grow in a narrow temperature interval there, which coincides with the temperature at which the Hallett–Mossop ice multiplication process operates most effectively, and one may suspect that this coincidence is likely to be meaningful. The ice growth habit is poorly reproducible in these conditions, dictating many runs with little change. Growth as plates can persist for hours, and two distinct types of needle growth occur, called sheath needles and sharp needles. Both are distinct from thin columns in that they taper to a point, with no discernible basal face. Both deviate slightly from parallel to the c axis. Sharp needles have been reported before, but only as occurring with an applied high DC voltage. New crystal orientations nucleate occasionally at the tips of the sharp needles; this also has been seen before in the presence of strong electric fields. There appears to be an ice multiplication mechanism in these conditions that does not involve riming.

As a response to extreme water shortages in southeast Queensland, Australia, brought about by reduced rainfall and increasing population, the Queensland government decided to explore the potential for cloud seeding to enhance rainfall. The Queensland Cloud Seeding Research Program (QCSRP) was conducted in the southeast Queensland region near Brisbane during the 2008/09 wet seasons. In addition to conducting an initial exploratory, randomized (statistical) cloud seeding study, multiparameter radar measurements and in situ aircraft microphysical data were collected. This comprehensive set of observational platforms was designed to improve the physical understanding of the effects of both ambient aerosols and seeding material on precipitation formation in southeast Queensland clouds. This focus on gaining physical understanding, along with the unique combination of modern observational platforms utilized in the program, set it apart from previous cloud seeding research programs. The overarching goals of the ...

During the Queensland Cloud Seeding Research Program, the “CP2” polarimetric radar parameter differential radar reflectivity Zdr was used to examine the raindrop size evolution in both maritime and continental clouds. The focus of this paper is to examine the natural variability of the drop size distribution. The primary finding is that there are two basic raindrop size evolutions, one associated with continental air masses characterized by relatively high aerosol concentrations and long air trajectories over land and the other associated with maritime air masses with lower aerosol concentrations. The size evolution difference is during the growth stage of the radar echoes. The differential radar reflectivity in the growing continental clouds is dominated by large raindrops, whereas in the maritime clouds differential reflectivity is dominated by small raindrops and drizzle. The drop size evolution in many of the maritime air masses was very similar to those observed in the maritime air of the Caribbean Sea observed with the NCAR S-band polarimetric radar (S-Pol) during the Rain in Cumulus over the Ocean (RICO) experiment. Because the tops of the Queensland continental clouds ascended almost 2 times as fast as the maritime ones in their growth stage, both dynamical and aerosol factors may be important for the systematic difference in drop size evolution. Recommendations are advanced for future field programs to understand better the causes for the observed variability in drop size evolution. Also, considering the natural variability in drop size evolution, comments are provided on conducting and evaluating cloud seeding experiments.

The 24 June 2006 Boulder hailstorm produced very heavy precipitation including disklike hailstones that grew with low density. These disklike hailstones, 4–5 cm in diameter, are unusual, and some of them appear to have accumulated graupel while aloft. A large amount of very fine-grained slush was left on the ground along with the hail. The hail and the great amount of slush suggest that most of the hydrometeor growth in the cloud was by low- or very-low-density riming. Consistent with that, the radar data suggest that the storm updraft had substantially depleted liquid water content. There is evidence that low-density hydrometeor growth within storms may be considerably more frequent than is commonly suspected.

An unusual, isolated hailstorm descended on Boulder, Colorado, on the evening of 24 June 2006. Starting with scattered large, flattened, disk-shaped hailstones and ending with a deluge of slushy hail that was over 4 cm deep on the ground, the storm lasted no more than 20 min and did surprisingly little damage except to vegetation. Part I of this two-part paper examines the meteorological conditions preceding the storm and the signatures it exhibited on Weather Surveillance Radar-1988 Doppler (WSR-88D) displays. There was no obvious upper-tropospheric forcing for this storm, vertical shear of the low-level wind was minimal, the boundary layer air feeding the storm was not very moist (maximum dewpoint 8.5°C), and convective available potential energy calculated from a modified air parcel was at most 1550 J kg−1. Despite these handicaps, the hail-producing storm had low-level reflectivity exceeding 70 dBZ, produced copious low-density hail, exhibited strong rotation, and generated three extensive bounded weak-echo regions (BWERs) in succession. The earliest of these filled with high reflectivities as the second one to the south poked up through precipitation-filled air. This has implications for low-density hail growth, as discussed in Part II.

Early radar echo development in trade wind cumulus clouds is studied using the equivalent reflectivity factor Ze combined with the differential reflectivity Zdr. The clouds studied are among the largest of trade wind cumulus, developing significantly positive values of Zdr and attaining at least about a 30-dBZ equivalent reflectivity factor. The measures used for analysis are values calculated for entire constant–elevation angle sweeps through the clouds and entire volume scans—not maximum single-pulse-volume values. The radar echo evolution follows fairly closely the Marshall–Palmer distribution with scatter toward higher values of Zdr especially in the earliest stages of echo intensification, where some of the scatter in the whole-sweep values is caused by size sorting. The data provide no evidence for an important role of ultragiant aerosols (UGA) in initiating coalescence. They are in strong contrast with similar data from a cloud over northern Alabama that do suggest a major role for UGA in producing several-mm-diameter raindrops that dominate its weak, early radar echo.

Examination of the early radar echo histories of several vigorous, cumulus clouds in northeast Colorado and northwest Kansas, with sensitive, dual-polarization radar, reveals the formation of millimeter-sized water drops at about the same time that the conventional, first precipitation echo (from ice) forms aloft. The early, positive ZDR values appear in the vicinity of the 0°C level (the radar data do not specify height accurately) and soon extend both above and below it. Positive ZDR is found within and to the upwind side of the updraft, separate from the conventional first precipitation echoes, which appear first at higher altitude, generally downwind of the updraft core, and have no significantly positive ZDR. Big, liquid drops were not expected this early in the formation of continental cumulus. The early presence of supercooled water drops larger than cloud droplets may be a significant factor in the glaciation of these clouds. The kind of early radar coverage illustrated here would be a priceless adjunct to aircraft studies of precipitation formation in cumulus. Microphysical data from aircraft must be interpreted with numerical models in order to deduce (or verify) the processes, and such models require the kind of early data illustrated here, both for initialization and verification.

The Aurora, Nebraska, hailstorm of 22 June 2003 produced some exceptionally large hailstones, and was widely publicized. Nineteen hailstones obtained from local people have been sectioned and photographed and eight are illustrated here, recording their interior layering and external appearance. They exhibit great variability, with features that are common to large hail as well as one unusual growth form: a roughly prolate external shape indicating an approximately constant falling orientation during final growth, forming large, icicle-like projections at one end. Much of the growth is wet but not appreciably spongy, as appears to be common for large hail. While a hailstone from this storm has been called the largest recorded in the United States on the basis of its longest dimension, we suggest that the most meaningful measure of hailstone size is weight. Weight is unambiguous and easily measured, and is an excellent indicator of volume for large hail. People generally think of hail as spherical ...

During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

The development of convective cells within anvil precipitation, in a region of moderate convective activity that might be called a small mesoscale convective system, is described and discussed. The presence of precipitation-sized hydrometeors in the air as the convection develops makes early stages visible to radar that might not otherwise be seen. Two kinds of convective initiation are illustrated. In one, a vigorous cell is initiated over an outflow boundary, but within light precipitation. In the other, the initiation is evidently by an instability created by the melting layer, perhaps by a mechanism first discussed by Findeisen. In this latter type, the new convective elements are not severe but they generate supercooled cloud within the anvil, extend entirely through the anvil to altitudes above 12 km MSL, and produce graupel showers with rain at the ground exceeding 50 dBZ. The instability itself may be generated in large part by moistening and cooling the sounding by the falling precipitation.

Observations of the formation of the first radar echoes in small cumulus clouds are compared with results of a stochastic coalescence model run in the framework of a closed parcel. The observations were made with an instrumented aircraft and a high-powered dual-wavelength radar during the Small Cumulus Microphysics Study (SCMS) in Florida. The principal conclusion is that coalescence growth on giant and ultragiant nuclei may be sufficient to explain observations. The concentration of cloud droplets varied from under 300 cm−3 when surface winds were from the ocean, to over 1000 cm−3 when the wind direction was from the mainland. Although there is a slight tendency for the altitude of the first 0-dBZ echo to be lower on average in maritime than in continental clouds there were several cases where it was higher. The model results suggest that the lack of correlation is consistent with drops forming on giant and ultragiant nuclei. The first 0-dBZ echo was observed to form at higher altitudes in cloud...

The early histories of radar echo and polarization differential reflectivity (ZDR) from growing cumulus clouds observed in Florida with a 10-cm-wavelength radar are reported in detail. Raindrops 1 to several millimeters in diameter are present at about cloud-base level in most cases as soon as any identifiable precipitation echo is seen within cloud (distinct from Bragg scattering and echo from cloud droplets). This is in most cases by the time of the first 10-dBZ radar echo aloft. The very early occurrence of large drops is consistent with origination directly from coalescence on ultragiant aerosol. However, they appear to exist so early and so low in the clouds as to be unexpected if the cumulus were single, vigorous thermals. The explanation may lie in the presence of a more gradual, very early cloud stage that is generally not observed in any detail. Simultaneous Ze and ZDR measurements in the early stages of developing, warm cumulus will provide a powerful test of understanding the onset of ...

The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.

No Abstract available.

Studies of small cumulus clouds in Florida using X- and S-band radar (3- and 10-cm wavelengths) reveal both hydrometeor and Bragg scattering signals. Turbulent mixing between cloudy and drier environmental air can produce centimeter-scale variations in refractive index that can lead to strong mantle echoes around the sides and tops of the clouds. When the environmental air is exceptionally dry, the S-band Bragg scattering signals are as strong as 10 dBZ at cloud boundaries, with weaker echoes in the cloud cores where hydrometeor scattering is also present. The Bragg signal at S-band is typically about 19 dB stronger than that at X-band, as expected from theory. However, there is in many cases an unexplained, Bragg-like return from the clouds at S-band that correlates with the X-band echo but is only about 10 dB stronger. The X-band echo is often dominated by backscattering from the cloud droplets, and shows adiabatic ascent within the cloud cores fairly often up to at least 1 km above cloud base....

Critical supersaturations have been measured for the vapor growth of ice crystals on both the basal and prism faces between −16° and −0.4°C. The values are low: approximately constant at 0.4% for the prism face, less for the basal face between −3° and −9°C, but greater at higher and lower temperatures. The transitions between tabular (platelike) and columnar growth habits that occur near −3° and −9°C are thus directly understandable in terms of layer nucleation as the growth mechanism, without explicitly considering the surface migration of water molecules or spiral steps. These low values of critical supersaturation are consistent with a disordered ice surface, but not with a surface melt layer, even at −1°C.

Measurements of cloud droplet size spectra from a forward scattering spectrometer probe (FSSP) in developing cumulus show an excellent correlation between the calculated radar reflectivity factor and cloud liquid water content at constant altitude, if the data are restricted to the early stage of cumulus development before coalescence growth becomes significant. The correlation is examined using droplet data from the FSSP both with and without the correction for the nonuniformities in the laser beam field of the FSSP. The close correlation between liquid water content and reflectivity factor suggests that sensitive, relatively low wavelength radar can be useful for mapping liquid water content in the early stages of cumulus development. The analysis reveals several ambiguities that require further study, including the altitude-dependent parameter in the empirical correlation, the value of which appears anomalous. The relation between liquid water content and reflectivity factor provides indirect ...

In attempting to use centimeter-wavelength radars to investigate the early stage of precipitation formation in clouds, “mantle echoes” are rediscovered and shown to come mostly from scattering by small-scale variations in refractive index, a Bragg kind of scattering mechanism. This limits the usefulness of single-wavelength radar for studies of hydrometeor growth, according to data on summer cumulus clouds in North Dakota, Hawaii, and Florida, to values of reflectivity factor above about 10 dBZe with 10-cm radar, 0 dBZe with 5-cm radar, and −10 dBZe with 3-cm radar. These are limits at or above which the backscattered radar signal from the kinds of clouds observed can be assumed to be almost entirely from hydrometeors or (rarely) other particulate material such as insects. Dual-wave-length radar data can provide the desired information about hydrometeors at very low reflectivity levels if assumptions can be made about the inhomogeneities responsible for the Bragg scattering. The Bragg scattering ...

A Lagrangian, trajectory-tracing scheme for modeling precipitation formation by the ice process is used for extensive sensitivity testing and is applied to a CCOPE (Cooperative Convective Precipitation Experiment) first-echo case. One major purpose is to try to gain a judgement of the degree of model simplification of the microphysical growth processes that is jusfiable in light of present uncertainties regarding both particle growth rates and cloud water content. Considerable microphysical simplification appears justified in that the time interval between nucleation and the onset of efficient accretional growth is far more important than any other factor in the growth equations. The model reproduces semiquantitatively the observed, first precipitation formation in the modeled cloud; it does not show a need for any novel ice nucleation schemes or ice multiplication processes.

The Rain in Cumulus over the Ocean (RICO) field campaign carried out a wide array of educational activities, including a major first in a field project—a complete mission, including research flights, planned and executed entirely by students. This article describes the educational opportunities provided to the 24 graduate and 9 undergraduate students who participated in RICO.

Much of the previous work which has led to the conclusion that coalescence is the dominant precipitation forming mechanism in cumulus clouds is reviewed. Observations in northeastern Colorado from several independent methods of investigation are summarized to show that in northeastern Colorado the ice (Bergeron–Findelsen) process is in all probability the dominant mechanism in spring and summer cumuli in their early and intermediate stages of development. Results of microphysical observations coordinated with simultaneous radar observations are presented. The microphysical observations in clouds with observed effective reflectivities of up to 40 dBZ show that the observed reflectivities can be accounted for by measured ice particle sizes and concentrations. Liquid precipitation elements are not necessary and have been observed only rarely in these clouds except below the melting level. Possible explanations of the differences between clouds in northeastern Colorado and those in other areas are di...

The growth trajectories of precipitation particles that attain diameters from 0.5 to 2.0 cm are modeled within the wind field of a small, relatively steady-state, southeastern Montana thunderstorm. The trajectories are calculated backwards, from systematic arrays of particles of specified sizes at a level near cloud base. Using a simple set of criteria for rejecting the obviously impossible trajectories, the patterns of accepted trajectory end-points are compared with the radar echo patterns. Good agreement lends credence to the qualitative aspects of the trajectories. For a given size of precipitation particle, the method helps one to assign different trajectory types to specific regions within the horizontal plant on which the calculations were started. The relative importance of the different types of trajectories can thus be estimated. Particle origin mechanisms are discussed in terms of the regions in which the trajectories are found to start. The variety of successful trajectories leading t...

Crystalline deposits grown by accreting supercooled droplets onto rotating rods in conditions simulating natural hailstone growth are described and illustrated. While the air bubble features conform to observations made by previous in investigators, the crystal textures are much more complex than has been indicated before. Fresh accretions growth in most conditions with the mean deposit temperature below 0°C show crystals with such highly developed substructure that it can be difficult to determine crystal size. Examples of similar substructure in freshly quenched hailstones are shown. Annealing at 0°C for times as short as 2 min causes gross changes in the crystal textures of the artificial accretions with substructure. This appears to explain why most natural hail does not have such substructure, since it is expected usually to have been annealed at 0°C longer than that. Such annealing probably occurs during growth of large stones, during fall below the melting level or on the ground before col...

The Cooperative Convective Precipitation Experiment, CCOPE, was an outgrowth of the perceived need for more comprehensive data sets on convective clouds. It was planned and executed by a large group of participants, with the leadership of the Convective Storms Division of NCAR and the Office of Atmospheric Resources Research of the Bureau of Reclamation. The field program ran from 18 May through 7 August 1981, involving networks of eight radars—seven Dappler, two dual-wavelength—123 mesonet stations, seven upper-air sounding sites, and 14 research aircraft (of which as many as eight were flown in coordinated missions on single storms). The field program was an operational success, with a lot of convective activity within the densely instrumented area, permitting many relatively complete data sets to be obtained. The data are now becoming available, and the analysis effort is commencing.

Spongy hailstones have been collected by the quenching method, and their structures examined. The most common type of spongy hail in Colorado is small hail that formed as loose rime (graupel) and became soaked with liquid water during fall. Hail that grew spongily has also been observed, and the structures are illustrated. The interpretations are confirmed by experiment.

A systematic examination of cloud droplet size spectra from the Cooperative Convective Precipitation Experiment (CCOPE) reveals no tendency for an increase in the maximum droplet size with increasing dilution or cloud age.

The results of a three-year randomized seeding experiment carried out as part of the National Hail Research Experiment are described in a nine-part series. In this first part the design of the statistical experiment is presented, including the physical hypothesis for hail suppression by seeding, the seeding procedures, the randomization scheme and the method of evaluation. The experiment was of the singlearea type and the randomization was by day. The criterion for selecting sample days involved the magnitude of radar returns from cumulonimbus clouds over and near the target area. Storms on approximately half the days were seeded according to a random selection procedure. The seeding method involved dispersing silver iodide in the storm updraft from aircraft maneuvering just below cloud base. An air-borne rocket system designed to deliver the nucleant at about the −5°C level in the storm was also employed in one season. The evaluation was based on the measurement of hailfall over a network of ins...

Results of a previous study have been confirmed by additional measurements on many hailstones from several different areas. They indicate that relatively clear hailstone embryos, interpreted as frozen drops, are associated with large soil or organic particles. Opaque (bubbly) embryos do not contain foreign particles larger than 40 μm diameter. The difference is observed even when the two hailstone embryo types occur in a single hailfall. The results suggest that different embryo types originate in different parts of a storm and are carried by the thunderstorm airflow into a common final growth and fallout region.

Impacts of hydrometeors on aluminum foil over a grooved backing, as in standard airborne foil impactors, have been produced in the laboratory using a modified crossbow to achieve aircraft speeds. The purpose of the work was to test the ability to distinguish the phase of the hydrometeors from the nature of the imprints. Solid, spherical ice pellets (frozen drops) and drops of slush leave impressions that are often indistinguishable from those left by liquid drops, at sizes below about 3 mm diameter.

Using quantitative analysis of time-lapse motion pictures from aircraft and a sensitive meteorological radar, the cloud top history is related to the early radar echo development in 12 vigorous, summer, convective cloud turrets in northeastern Colorado. At a threshold of about 5 dB(Z), the first echoes appear typically 5–10 min after the cloud top passes the -−20°C level. The first echo either appears at cloud top or reaches the top very quickly. It sometimes appears at a well-defined height, but sometimes nearly simultaneously over an altitude range of 3 km or more. Radar echo at 5 dB(Z) typically fills the visual cloud 5–10 min after first echo. In terms of overall cloud lifetime there is plenty of time for the particles responsible for the first echo to form by the ice process. A detailed model of the rates of ice particle formation by vapor growth followed by riming gives a 5 dB(Z) radar echo within 7–10 min at concentrations as low as 1 m−3, at most temperatures between −10 and −20°C and in ...

Detailed radar echo structures and histories of two storms are presented. Both advanced into cloudless skies and had prominent, bounded weak echo regions. The storms had comparable size and intensity, and their environments provided similar amounts of shear and potential instability. One was an organized, multicellular storm, and had detailed photographic coverage from an aircraft. The combination of visual and radar data suggests the possibility of seeding of turrets in the “flanking line” by ice particles falling from the anvil. The other storm was a supercell. It had a rather steady echo configuration with a radar echo vault for about 40 min, and produced an exceptionally heavy hailswath, with hail up to 10 cm deep. However, the heavy hailfall at the ground started before vault formation and ended well before vault dissipation. The hailfall relates best to the onset of the strong updraft that (presumably) produced the vault, but does not relate to the mere fact of the existence of the bounded ...

The deuterium content (δ) of the water vapor in the air entering thunderstorms has been measured, along with that of the growth layers of hailstones from the same storms. δ may vary importantly with position as well as time in the inflow of large, multicell storms, and there is some evidence for important variation with time in a simpler storm. It appears that the potential for drawing firm, quantitative deductions from hailstone deuterium or 018 profiles is quite limited. Nevertheless, the present results and previous deuterium results agree in placing most hailstone growth in the −10 to −25°C temperature range, and in indicating surprisingly narrow altitude ranges of growth for most individual hailstones. It therefore seems probable that liquid water is usually substantially depleted above the −25°C level in hailstorms and that hailstones usually fall out of the updraft above the −10°C level. Hailstone embryo formation is usually at ii lower level than the growth of the bulk of the hailstones.

The deuterium, crystal and air bubble structures of 11 large hailstones from three severe storms have been examined. It is emphasized that there are a number of assumptions underlying the interpretation of such data and these are discussed. In seven of the hailstones the ambient temperatures at which they grew were inferred from the crystal size. The deuterium concentrations and ambient temperatures generally show similar variations and the crystal data thereby provide a useful way of placing an absolute temperature scale against the deuterium values. Throughout most of their growth, the hailstones grew in the updraft between about the ambient temperature levels of –17 to –30°C. The air bubble analyses showed that the hailstones grew near the wet growth limit or slightly wet and heat balance considerations give values of 2–3 g m−3 for the effective liquid water concentrations. On the assumption that the median volume radius of the cloud droplets is 10 µm, the actual liquid water concentrations ar...

Hailstones collected within or near the National Hail Research Experiment (NHRE) target area on 23 days of the randomized seeding experiment were sectioned and classified as to embryo type. No significant correlations were found between embryo type and hail size, hail amount or cloud base temperature, but a suggestive relation between seeding and embryo type does exist. The seeded storms had a substantially greater tendency to produce hail with “frozen drop” embryos than did non-seeded storms. Two simple tests give probabilities of obtaining the results by chance of 0.13 and 0.22. The result is suggestive enough to be worth investigating in a future experiment.

Crystal size parameters are studied, using cylindrical accretions formed from supercooled cloud in dry growth in a wind tunnel as a function of deposit temperature and cloud temperature. Effects of recrystallization, of interpretation of crystal boundaries and of the technique of making thin sections are explicitly examined. The crystal features are in general influenced in important ways by all of these variables. A shape factor is defined–a measure of grain elongation–and found to reflect the degree of recrystallization. Similarities to and differences from previous experimental results are discussed and the applicability of these results to interpreting natural hailstones is examined. Relatively crude but useful deductions of temperature (or altitude) of growth can be made with confidence, but detailed deductions would have to be verified in some independent way at the present state of knowledge.

A large number of photographs of thin sections of quenched hailstones have been studied to determine the most common patterns of water distribution within hailstones. The way in which water is distributed throughout the hailstone determines which of the mixing rules should be used to calculate an effective refractive index of water-ice mixture. We have concluded that the Maxwell-Garnett mixing rule for ice grains embedded in a water matrix is suitable for most graupel soaked with water. Some of the large hailstones with spongy growth suggest that a proper generalization of the Bruggeman mixing rule should be used to determine their effective index of refraction at centimeter wavelengths.

The three-year, randomized hail suppression test of the National Hail Research Experiment (NHRE) used seeding methods patterned after the Soviet hail suppression activities, applied over a specified target area on a randomly selected portion of days identified as having a high potential for hailfall by a radar criterion. Statistical tests for seeding effects were performed on data from ground networks of hail measuring instruments within the target area using total hail mass within the target area as the primary response variable. The statistical results indicate a range of possible mean seeding effects from a reduction of 60% to an increase of as much as 500% in total hail mass, depending on the test used, within the 90% confidence limits. The range is so wide that no conclusion about a seeding effect can be drawn. Analysis of hail size, embryo type and various radar parameters, and stratification either in terms of estimates of “hail potential” or operational seeding efficiency have also not re...

A new explanation is proposed for the rare Scheiner's halo, observed in the sky at an angle of 28° from the sun or moon. The existing explanation invokes the presence in the atmosphere of the cubic form of ice, ice Ic. However, extensive laboratory work has not demonstrated that ice Ic can form under conditions found in the atmosphere. We point out an alternative, that polycrystals of ice Ih (the ordinary hexagonal polymorph), in which specific orientation relations exist between adjacent crystals, are another possible cause of Scheiner's halo. Polycrystals with the appropriate orientation relation are not uncommon in the atmosphere, but concentrations sufficient to produce optical effects are expected to be rare. There appears to be no decisive evidence to rule out either of these explanations.

Several observations illustrate the ease with which particles can contaminate hailstones collected from the ground. There are two modes of penetration: in large hailstones, particles enter through the lobe boundaries, and in smaller ones, they enter directly into porous portions of the stones. Washing or melting the exterior layer of a hailstone will not assure that the remainder is uncontaminated.

Air and particle trajectory calculations using internal motions from Doppler radar observations are used to identify kinematic feature and hail growth processes operating in a supercell storm that occurred on 2 August 1981 in southeastern Montana. As the Rock Springs storm moved rapidly east-southeastward across the Cooperative Convective Precipitation Experiment (CCOPE) observational network, it produced a significant hailfall composed mostly of 2 to 3 cm diameter hailstones. Some diameters were as large as 6 to 10 cm. At least one funnel cloud was sighted, and there was extensive crop and property damage. In the hail growth model 1 to 5 cm hailstones were readily produced from frozen drops in the size range 50 μm to about 1 mm. Most of the hail apparently grew from frozen droplets that originated within either the upwind stagnation zone southwest of the main updraft core or the overhanging radar echo ahead of the updraft. Potential hailstone embryos entered the stagnation zone from a flanking c...

A simple and computationally efficient method is described for estimating hydrometeor size distributions within a convective storm. The method requires air motion measurements (from Doppler radar in this case, but it could be used with a dynamic model), and specification of the cloud water field and the mechanism by which the hydrometeors originate. The cloud water field that corresponds to the wind field used is estimated by calculating condensation and depletion rates along air parcel trajectories. It is assumed that the storm is in steady state and that hydrometeors grow only by accretion. The technique is applied to one of the storms documented in the Cooperative Convective Precipitation Experiment (CCOPE), assuming that hydrometeors originate by primary ice nucleation alone. The distribution of hydrometeor sizes that is obtained is very unrealistic, in such a way that one or more other sources must have dominated hydrometeor formation. Since the trajectory analysis indicated that the source ...

The forms of snow crystals and flakes in various stages of melting can be observed with substantial fidelity after collecting the particles in subfreezing hexane. These forms are largely determined by minimum surface area considerations, which can lead to the breakup of some forms. The collection method might be useful in detailed studies of the radar bright band.

Aircraft measurements in a vigorous, highly turbulent continental cumulus show predominantly bimodal and multiple peaked cloud droplet spectral shapes. The data are 100 m (1 s) averages. Three factors involved in the development of the cloud droplet size spectrum are discussed. First, evidence is given for local activation, far above cloud base, of significant concentration of new cloud droplets. This evidence is that strong peaks in the concentration of very small droplets are found in this data set and are in one-to-one correspondence with the simultaneous occurrence of a low concentration of the larger cloud droplets and a vigorous updraft. Second, a simple model of droplet sedimentation and evaporation at boundaries between clear, unsaturated air and cloud air, as occurs during entrainment, shows the development of broad and bimodal droplet spectra in boundary zones in periods of a few tens of seconds. Third, an analysis of the relation between the droplet size and concentration at the large-...

An airborne photographic system, in which the cameras are coupled with an inertial navigation system, was developed and used in a 1978 convective cloud study, Photogrammetric analysis from such a system is enhanced: cloud-feature positions can be determined without external references such as the earth's horizon or cloud base in the photographs, and the data reduction process can be considerably automated. This paper describes the instrumentation, the photogrammetric theory, and the procedures for obtaining cloud measurements from the photographs. An empirical error analysis based on photographs of terrestrial targets is also presented. Cloud top heights determined without any reference height in the photographs are considered to be accurate to within 440 m at a range of 60 km. The largest source of error in determining cloud top height using the 1978 measurements is the uncertainty in determining the aircraft-to-cloud distance rather than inaccuracy in the photographic system. This error can be ...

Artificial accretions with cylindrical symmetry have been grown using a simple wind tunnel. The main features of the apparatus are described. Factors concerning the validity of the simulation of natural hail are discussed. Thin sections of the samples are used to study the crystal fabric of the accretions. This study is devoted mainly to the “dry” regime of growth, and the effects on the fabric of sample rotation during growth and of mixed clouds are reported. In dry growth no effects are observed for rotational speed up to 40 Hz. Marked asperities form on wet and spongy deposits formed at 30–40 Hz; their structure strongly resembles that of icicle lobes in large hailstones. Ice crystals in the airstream are readily captured by and modify the structure of wet and spongy growths. Although ice crystals do not appear to be captured by dry growths in quantities sufficient to influence the growth rates, their presence in the airstream does modify the crystal structure. In following parts of this serie...