Nancy C. Knight

Hail Growth in an Oklahoma Multicell Storm

Abstract Hail growth in an Oklahoma multicellular storm is studied using a numerical model of hailstone growth and decay and dual-Doppler derived wind fields. Hail was collected at the time of the Doppler radar data collection which provided input for computation of the modeled trajectories. A unique feature of this investigation is the subsequent comparison of model hail trajectories with deuterium derived trajectories obtained from the hail samples. Formation of large hail is found to be almost entirely due to injection of embryos into the major storm updraft from the upwind side, with subsequent growth occurring during repeated vertical excursions through the prime growth layer between 7 and 8 km. Primary embryo source regions are a feeder cell and the precipitation debris region between the feeder and main cells. Qualitative comparisons between observed and modeled hailstones falling near the collection site reveal strong similarities, particularly with respect to ambient temperature during ice format...

Measurement and Interpretation of Hailstone Density and Terminal Velocity

Abstract The mass, density, fall velocity and internal structure have been determined for low-density hailstones from an truly spring storm in Boulder, Colorado, and drag coefficients were calculated. The density of the permeable hail is determined from a new method of volume measurement. The results are compared to those of particles grown in a one-dimensional parcel model using the appropriate sounding. The implications of the results are discussed with reference to hail growth in convective storms.

The Falling Behavior of Hailstones

Abstract A detailed examination and interpretation of hailstone structure and symmetry leads to the conclusion that the normal falling behavior of moderate-to-large hailstones is rapid, symmetrical tumbling. The tumbling behavior is evidently critically sensitive to one or more of the important parameters, such as surface roughness, density distribution, etc., because similar hailstones grow with different symmetries. The concept of aerodynamic molding is found to be inapplicable to the growth and shape of natural hailstones, with the possible exception of small stones and icicle lobe structures. If, as is the conclusion, hailstones tumble while falling, previous deductions of terminal velocities and heat transfer rates may be substantially incorrect.

Effects of ice-crystal structure on halo formation: cirrus cloud experimental and ray-tracing modeling studies

During the 1986 Project FIRE (First International Satellite Cloud Climatology Project Regional Experiment) field campaign, four 22° halo-producing cirrus clouds were studied jointly from a groundbased polarization lidar and an instrumented aircraft. The lidar data show the vertical cloud structure and the relative position of the aircraft, which collected a total of 84 slides by impaction, preserving the ice crystals for later microscopic examination. Although many particles were too fragile to survive impaction intact, a large fraction of the identifiable crystals were columns and radial bullet rosettes, with both displaying internal cavitations, and radial plate-column combinations. Particles that were solid or displayed only a slight amount of internal structure were relatively rare, which shows that the usual model postulated by halo theorists, i.e., the randomly oriented, solid hexagonal crystal, is inappropriate for typical cirrus clouds. With the aid of new ray-tracing simulations for hexagonal hollow ended column and bullet-rosette models, we evaluate the effects of more realistic ice-crystal structures on halo formation and lidar depolarization and consider why the common halo is not more common in cirrus clouds.

Hailstone Shape Factor and Its Relation to Radar Interpretation of Hail

Abstract The shape factor of hailstones, defined as the ratio of their long and short axes (m′/m), has been measured for hailstones from three geographical areas: northeastern Colorado, central Oklahoma and central Alberta. The results show a general tendency toward decreasing sphericity with increasing size and are different for different areas. The results are relevant to remote hail sensing by radar techniques utilizing polarization.

The Mechanism of Precipitation Formation in Northeastern Colorado Cumulus I. Observations of the Precipitation Itself

Abstract The structure of ice precipitation in northeastern Colorado, collected within summer cumulus clouds and at the ground beneath thunderstorms, shows that the dominant precipitation formation mechanism is the riming of small ice particles, not liquid coalescence. While direct evidence of a stage involving the vapor growth of ice crystals is rare, such a stage is probably present in most cases.

Radial and tangential variation of deuterium in hailstones

Abstract Radial variations of deuterium content are measured in nine large hailstones from three storms; of tritium content in one of the nine; and tangential variations of deuterium content in a wet-growth layer in five of the nine. Crystal size and air bubble content are noted as well. It is concluded 1) that probably neither air bubble content nor crystal size is as good a relative height indicator as deuterium content; 2) that the trajectories of large hailstones can be very simple, not involving recirculations at larger-than-embryo sizes though two of the nine stones analyzed do show recirculation; 3) that the maximum altitude of growth of several of the large hailstones studied was at a temperature above −25°C; 4) that wet-growth layers can show significant tangential variations in D-content; and 5) that studies of hailstone radial deuterium variations can yield valuable information when applied to many hailstones from single storms or when coupled with deuterium measurements on samples of liquid wa...

Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part V: Hailstone Embryo Types

Abstract 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.

Lobe Structures of Hailstones

Abstract The range of lobe structures found in natural hailstones is illustrated and classified on the basis of form and growth conditions. Lobes that form in dry growth are cusped, and probably result from a “collection efficiency” effect, lobes growing faster than their surroundings because they collect more material. These lobes probably only form to a marked extent when a hailstone is tumbling. Lobes in wet or spongy growth, icicle lobes, are not cusped and evidently form more as icicles form, by flow of liquid water over the hailstone surfaces and preferential freezing at the tips of projections. The icicle lobes are less developed in very spongy growth than in slightly spongy growth. Distinct, radial air bubble features in hailstones are associated with the cusp-lines between the dry growth lobes. Less distinct, radial air bubble trains are found along the icicle lobe axes.

Deuterium Contents of Storm Inflow and Hailstone Growth Layers

Abstract 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.