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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.

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.

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.

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...

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.

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....

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 ...

Collocation of a 5.5-cm weather radar and a 33-cm wind profiler during the Hawaiian Rainband Project of 1990 presented several opportunities to compare vertical profiles of reflectivity in the atmosphere at the two wavelengths. On three occasions the beams of both systems were pointed vertically as light rain drifted overhead. The atmospheric reflectivity structure was then dominated by rain at low altitudes but included an elevated layer at the height of the trade wind inversion. Allowing for minor uncertainties in radar sensitivity and range, we found close agreement in the patterns of reflectivity factor Z in the rain as measured by the two systems. The reflectivity factor of the inversion echo at 33 cm then exceeded that at 5.5 cm by approximately 29 dB on average, an amount predicted by the theory of scattering by spatial fluctuations in the clear-air refractive index. These observations are consistent with the assumptions that (1) the inversion echo is a clear-air echo, uncontaminated by raindrops or other Rayleigh scatterers and (2) the reflectivity of the inversion at both wavelengths is explained by Bragg scattering from refractive index fluctuations induced by mixing in the inertial subrange of homogeneous turbulence.

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.

8The new Health Care Agreements for 1998?2003 are currently being negotiatedbetween the Commonwealth and State and Territory governments. TheCommonwealth?s offer aims to address the major problems associated with the currentAgreements, including cost-shifting incentives and funding rigidities.It has proposed a funding model whereby admitted and non-admitted patient servicesare funded on an output basis. Other aspects of the proposal include funding forquality enhancement and structural change, and specific funds for mental health andpalliative care. The proposal also aims to achieve a more equal sharing of risks andbenefits between the Commonwealth and the States and Territories.The views expressed are those of the authors and should not be taken as an officialview of the Department or the Minister. While the authors have had primaryresponsibility for the development of the Commonwealth model and the preparationof this paper, they wish to thank other members of the Health Care Agreements Branch(Heather Cocks, Ian Macdonald, Elaine Pringle and Shane Wright) for theirassistance.

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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...

Reliable assessment of the effect of a solute upon ice recrystallization is accomplished with "splat cooling," the impaction of a small solution droplet onto a very cold metal plate. The ice disc has extremely small crystals, and recrystallization can be followed without confusing effects caused by grain nucleation. This method confirms the exceptionally strong recrystallization inhibition effect of antifreeze protein from Antarctic fish and shows that grain growth rate is a sensitive function of both grain size and solute concentration.

Icicles grow by dissipating the heat of fusion through the exterior liquid film. Their growth is therefore partly analogous to hailstone growth, and it is also partly analogous to stalactite growth. Like hailstones, they may grow as virtually solid ice or as a mixture of ice and water. Some aspects of the crystallinity, shape, and air bubble structure of icicles are described and discussed. Icicles are an interesting and instructive phenomenon for observation or for organized study.

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...

A three-dimensional viscous cascade code has been developed for linear cascades with flat, parallel endwalls. It employs scalar implicit approximate factorization, a finite-volume formulation, second-order upwind differencing, and a two-equation q-& turbulence model based on integration to the wall. A special form of the thinlayer approximation for the compressible Navier-Stokes equations is used which gives accurate skin-friction predictions on highly skewed meshes, now based on sheared H-grids. This code has been validated by considering three-dimensional viscous flows in Langston's large-scale turbine blade geometry. This validation includes threedimensional heat-transfer predictions. Agreement with experiment is quite good.

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 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...

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 spectrum of a petroleum fluid inclusion, obtained from a producing oil well in the Gulf of Mexico, was compared with spectra obtained on three bulk oil samples produced from the same well. Comparison of the spectra of bulk and included petroleum samples shows several major differences. These differences are consistent with the loss of the more volatile components from the bulk oil samples.

Transverse acoustic waves in pulsed lasers result from nonuniform pumping transverse to the flow direction. This theoretical study is based on linear acoustics and disregards hot/cold gas interfaces. First, transverse waves in an infinite parallel hard-walled duct are considered, in which case cavity medium inhomogeneity typically decays as t ~ ' / 2 . An idealized treatment is next given to show that exponential decay can be expected with an anode absorber. There is an optimum absorber resistivity and improved attenuation rate results with increased backing volume. Tilted sidewalls are the final topic. Decay is again exponential, and rapid attenuation requires fairly large tilt angles. Examples are chosen to apply to CO2 electric discharge, chemical, and excimer lasers.

Thermal homogeneity in a single-pulse excimer laser cavity is of interest because it directly impacts medium optical quality. This study is primarily oriented toward an XeF* laser in which the cavity is hot (500 K) and is pumped by an e-beam from a cold cathode structure in a vacuum chamber next to the cavity. The foil and support structure are on a vertical wall between these hot and cold zones. In consequence, due to radiative heat transfer, the wall assumes a nonuniform termperature and induces natural convection and thermal inhomogeneity within the cavity. Studies of radiative, conductive, and natural convective heat-transfer processes are summarized which lead to a plausible engineering approach to achieving medium homogeneity of (6p/p)rms - 5 X10 ~ 5 in the closed XeF* laser cavity.

Sidewall mufflers are used in pulsed exciplex lasers to rapidly attenuate pressure waves and restore cavity medium homogeneity at successive pulses. The technical basis for designing these is reviewed in the paper with primary focus on the near-cavity region and emphasis on physical understanding. The approach combines rudimentary theory and flow visualization, starting with simplest situation and building toward a more complete picture. Important results are as follows: longitudinal wave attenuation in a finite capacity muffler involves a balance between precursor pulse decay and recompression wave strength which defines an optimum configuration. Acoustic absorber is required in the backing volume, and a basis for choosing its resistivity is indicated. The contribution of a close-in muffler to transverse wave decay within the laser cavity is also characterized. Further, an entropy wave is shown to be generated within a muffler which is confined to the vicinity of the sidewalls. Other entropy sources in a closed loop flow system are noted.