Charles A Knight

• NSF National Center for Atmospheric Research

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 turbu...

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 degrees 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 temperatu...

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 Brisban...

During the Queensland Cloud Seeding Research Program, the "CP2" polarimetric radar parameter differential radar reflectivity Z(dr) 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...

It is widely accepted, and we agree, that the lowering of the temperature at which ice can grow in a water solution of one of the biological antifreezes is a result of adsorption of the antifreeze molecules at the ice surface. However, how this can produce a well-defined "freezing point" that varies with the solution concentration has remained prob...

The aim of this study is to investigate the significance of nonpolar functional groups in the surface orientation-dependent adsorption of natural and synthetic antifreeze proteins (AFPs) on ice. We investigate using nanoliter osmometry, ice etching, and circular dichroism spectroscopy a series of nine designed α-helical peptides to probe the nature...

Early radar echo development in trade wind cumulus clouds is studied using the equivalent reflectivity factor Z(e) combined with the differential reflectivity Z(dr). The clouds studied are among the largest of trade wind cumulus, developing significantly positive values of Z(dr) and attaining at least about a 30-dBZ equivalent reflectivity factor....

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 tw...

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 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 studen...

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 app...

Without Abstract

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...

It is well known that ice-cube spikes form by the extrusion of water at the surface of a freezing ice cube, driven by the expansion accompanying freezing. The growing spikes are water-filled ice tubes, growing at their tips as the water is expelled. This paper represents an exploration of their formation and the principles behind whether a freezing...

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) under...

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....

During May-July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) was conducted in the High Plains, near the Colorado-Kansas border, in order to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included: 1) un...

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 prin...

Extracellular macromolecules associated with Antarctic sea ice diatoms were previously shown to have ice-binding activities. The function of these ice-active substances (IASs) has not been identified. Here we show that two of the IASs have a strong ability to inhibit the recrystallization of ice, possibly signifying a cryoprotectant function. To te...

Thin plates, slender needle-like crystals, skeletal plates and skeletal linear forms can be produced in the free growth of cubic tetrahydrofuran hydrate from the melt, with growth rates differing by orders of magnitude on adjacent, crystallographically equivalent {111} faces. These anisotropic growth modes are distinctly different from each other a...

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 clou...

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 rada...

Different fish, insects, and plants have evolved a range of peptides and glycopeptides that adsorb to ice from water solution and dramatically influence ice growth in ways that benefit the organisms. Understanding the adsorption itself is fundamental but difficult to study experimentally. One fundamental attribute of the adsorption is the orientati...

Different fish, insects, and plants have evolved a range of peptides and glycopeptides that adsorb to ice from water solution and dramatically influence ice growth in ways that benefit the organisms. These “antifreeze” biomolecules are fairly large, the smallest so far discovered having a MW of 1600, and they prevent ice crystal growth from water a...

The purposes in this chapter are to provide a critical summary of current thinking about the distinction between hailstorms and other severe storms, and to discuss the outstanding problems of hailstorms and approaches toward their solution. The last reviews of this subject were by Browning (1977), Macklin (1977), and other authors in the AMS monogr...

Antifreeze proteins allow certain organisms with no thermal control to survive cold conditions, the best known examples to date coming from fish. Papers elsewhere in this issue1, 2, 3 now describe the characterization of such proteins from grass and two species of insect. They provide insights into the mysterious ways that antifreeze proteins work.

Recently antifreeze proteins (AFP) have been the subject of many structure-function relationship studies regarding their antifreeze activity. Attempts have been made to elucidate the structure-function relationship by various amino acid substitutions, but to our knowledge there has been no successful from first principles design of a polypeptide th...

Growth and inhibition of single crystals of ethylene oxide clathrate hydrate provide experimental accessibility to similar phenomena in methane clathrate hydrates. Ethylene oxide hydrate single crystals were grown in a simple experimental setup. The crystals grew from the subcooled melt as rhombic dodecahedra, macroscopically exhibiting {110} as th...

A recent report claims that the absence of prism facets on ice growing from slightly supercooled water at atmospheric pressure is due to the presence of air. We have reproduced the experiment and found the result to be incorrect: no prism facets are seen in an air-free environment.

Growth and inhibition of single crystals of ethylene oxide clathrate hydrate provide experimental accessibility to similar phenomena in methane clathrate hydrates. Ethylene oxide hydrate single crystals were grown from the supercooled melt as rhombic dodecahedra, exhibiting {110} as the slowest growing face. The addition of poly(N-vinylcaprolactam)...

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. W...

Single crystals of structure II (sII) and structure I (sI) clathrate hydrates were grown in aqueous tetrahydrofuran (THF) and ethylene oxide (EO) solutions. Normal growth habits from the melt are predominantly {111} crystallographic planes for sII, and {110} for sI. Addition of polymeric so-called kinetic inhibitors in very small amounts changed th...

Three mutant polypeptides of the type I 37-residue winter flounder 'antifreeze' protein have been synthesized. All four threonine residues in the native peptide were been mutated to serine, valine and glycine respectively and two additional salt bridges were incorporated into the sequences in order to improve aqueous solubility. The peptides were a...

Critical supersaturations have been measured for the vapor growth of ice crystals on both the basal and prism faces between 16 and 0.4C. The values are low: approximately constant at 0.4% for the prism face, less for the basal face between 3 and 9C, but greater at higher and lower temperatures. The transitions between tabular (platelike) and column...

We designed a series of experiments to verify the value of the diffusion coefficient of HNO3 in ice at -15°C using a serial sectioning technique. Our first series of experiments with diffusion times of 70 to 100 h gave an average value of about 1.7×10-11cm2s-1 (StD 0.8×10-11) and a second set of experiments at 1000 h resulted in an apparent diffusi...

Tetrahydrofuran clathrate hydrate single crystals grow from the supercooled melt as octahedra, showing that {1 1 1} is the slowest-growing face. The addition of poly(vinylpyrrolidone), poly(vinylcaprolactam) or a random terpolymer of the previous two with dimethylaminoethylmethacrylate causes the crystals to grow as hexagonal plates also bounded by...

Single crystals of structure II (sII) and structure I (sI) hydrates were grown in aqueous tetrahydrofuran (THF) and ethylene oxide (EO) solutions Normal growth habits from the melt are {111} crystallographic planes for su, and {110} for si. Addition of polymeric inhibitors in very small amounts changed the growth habit of sII to thin, 2-dimensional...

A new method for growing single crystals with sensitive control of supersaturation and minimal influence from foreign surfaces is described.

In this paper we report the results of our studies on the stereospecific binding of shorthorn sculpin antifreeze protein (AFP) to (2 -1 0) secondary prism faces of ice. Using ice crystal growth and etching techniques together with molecular modeling, molecular dynamics, and energy minimization, we explain the nature of preferential binding of short...

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 become...

There is a widespread opinion that ice surfaces are covered by a liquid layer at equilibrium, and several treatments of the ice surface describe it as a homogeneous layer with a definable thickness. Arguments are presented that the ice surface cannot have a homogeneous surface layer and that representations of it that use such a layer are not usefu...

Large, single ice crystals containing no air bubbles and free of both small-angle grain boundaries and visible stress birefringence can be grown using a very simple growth chamber within a temperature-controlled, outer enclosure. The method relies upon the spontaneous formation of an ice crystal with its c axis accurately normal to a free, slightly...

Large, single ice crystals containing nо air bubbles and free of both small-angle grain boundaries and visible stress birefringence can be grown using a very simple growth chamber within a temperature-controlled, outer enclosure. The method relies upon the spontaneous formation of an ice crystal with its с axis accurately normal to a free, slightly...

Evidence is presented that the nonequilibrium antifreeze peptide (AFP) from winter flounder has a special ability to inhibit recrystallization in ice only when an appreciable amount of liquid is present, as is the case when the system contains salts and the temperature is not too low. In this circumstance the AFP binds to the ice surface at the ice...

Antifreeze glycopeptides (AFGPs) that adsorb to ice from liquid water solution and prevent ice crystal growth over a range of supercooling are found in the blood of some Antarctic fish. They are polymers with from four to about 52 monomer units. The shortest, with four and five of the repeating units, adsorb at prism-face orientations, (100), align...

Natural snow dendrites sometimes display symmetry of side branches along each arm, whereas on other occasions such symmetry does not occur. Laboratory studies under simulated atmospheric conditions show that a fluctuation of local airflow, equivalent to changing supersaturation, for a period of only a few seconds leads to symmetrical initiation of...

Studies were conducted to investigate the adaptations of the wood centipede, Lithobius forficatus, to winter conditions. During winter centipedes were freeze tolerant. However, inoculative freezing initiated from external ice was required for survival of freezing. Because the lower lethal temperature (after 24 hr exposure) was only −6°C, it appears...

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 hyd...

Experimental results show that fish antifreeze glycopeptides (AFGPs) 8 and 7 (with 4 and 5 repeats respectively of the Ala-Ala-Thr backbone sequence) bond onto ice prism planes aligned along a-axes, and inhibit crystal growth on prism planes and on surfaces close to that orientation. The 9.31-A repeat spacing of the AFGP in the polyproline II helix...

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 refle...

Proteins which produce a thermal hysteresis (i.e. lower the freezing point of water below the melting point) are common antifreezes in cold adapted poikilothermic animals, especially fishes from ice-laden seas and terrestrial arthropods. However, these proteins have not been previously identified in plants. 16 species of plants collected from north...

The noncolligative peptide and glycopeptide antifreezes found in some cold-water fish act by binding to the ice surface and preventing crystal growth, not by altering the equilibrium freezing point of the water. A simple crystal growth and etching technique allows determination of the crystallographic planes where the binding occurs. In the case of...

This is a preliminary exploration of using hailstones as natural probes of cloud droplet chemistry Big hailstones grow primarily in the strongest updraft regions of the severest convective storms, so that if their chemical contents are representative of the cloud droplets from which they grow, they could be useful probes of chemical transport from...

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 proce...

The melting of pure ice single crystals can be inhibited by the presence of an antifreeze glycopeptide isolated from an Antarctic fish. This inhibition effect exhibits crystallographic dependence and can result in superheating of the crystal by heat conduction across the ice-solution interface. The antifreeze molecules inhibit melting in a manner m...

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 excepti...

Slush layers often form after snowfall on floating ice sheets, producing after they freeze, the upper layer of white, bubbly ice that is often found. The stability conditions for slush layers and snow on top of floating ice are examined in some detail, the patterns of flooding and the reasons for them are discussed, and it is pointed out that it ma...

Air and particle trajectory calculations using internal motions from Doppler radar observations are used to identify kinematic features and hail growth processes operating in a supercell storm. 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 w...

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...

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...

Nucleation rates on cylindrical nuclei are calculated using the classical theory. For very good but very small nuclei, the minimum cylinder radius that causes nucleation is about half that for spherical nuclei, other things being equal. The ice-supercooled water system is used for illustration.

It has long been recognized that lake ice is composed of two layers: a lower, relatively clear layer of “black” ice that forms by downward freezing, and an upper, bubbly layer of “white” ice that forms when snow falls onto an ice sheet and depresses it to the point where water floods the surface, making slush that later freezes solid [1, 2, 3]. The...

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.

Thermal hysteresis antifreeze proteins, first discovered in polar marine fishes, are fairly common in overwintering insects where they also help prevent freezing. However, a few species of insects that tolerate freezing also contain these proteins, and in these species the function of the proteins is uncertain. The studies outlined here demonstrate...

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 crit...

A variety of frost-crystal forms found growing from the vapor in ice caves and tunnels in the Antarctic are described and illustrated. Complex layered structures found within large, skeletal crystals are ascribed to the action of the temperature gradient. Some novel c -axis growth forms and a rare type of bicrystal growth – accelerated growth in a...

A variety of frost-crystal forms found growing from the vapor in ice caves and tunnels in the Antarctic are described and illustrated. Complex layered structures found within large, skeletal crystals are ascribed to the action of the temperature gradient. Some novel c-axis growth forms and a rare type of bicrystal growth – accelerated growth in a p...

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...

Aircraft measurements in a vigorous, highly turbulent continental cumulus show predominantly bimodal and multiple peaked cloud droplet spectral shapes. The data are 100m (l s) averages. Three factors involved in the development of the cloud droplet size spectrum are discussed.-from Authors

Antifreeze glycopeptide and peptides from the blood of polar fishes prevent the growth of ice crystals in water at temperatures down to approximately 1 degree C below freezing point, but do not appreciably influence the equilibrium freezing point. This freezing point hysteresis must be a disequilibrium effect, or it would violate Gibbs' phase rule,...

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 photo...

Takahashi's calculation [J. Crystal Growth 59 (1982) 441] that hexagonal ice may nucleate as cubic ice applies for homogeneous nucleation. The special bicrystal orientations that he discusses have only been observed from heterogeneous nucleation, and may be explainable in another way.

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...

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, an...

An airborne photographic system, in which the cameras are coupled with an intertial 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, a...

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 Reclam...