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G. Brant Foote

Abstract

Hail growth is studied in the Westplains, Colorado, storm for which a variety of observations were collected. Measurements from a triple-Doppler radar network and a penetrating aircraft are used to synthesize fields of liquid water content and three-dimensional, evolving air motion from which detailed hail growth trajectories are computed. Explicit treatment of the growth of hail embryos is sidestepped. Rather, for the purpose of the study a wide range of embryo sizes is assumed to exist over a broad region of the storm and the analysis examines which particles are predicted to grow and what their growth trajectories are. Sensitivity of individual trajectories to many parameter such as initial position, embryo size, and particle drag law, is found. However, the general results, including the following, are insensitive: the storm flow field is organized such that an efficient mechanism exists for transporting particles into the updraft; following growth of the embryo outside the large updraft, a single pass though the updraft is sufficient to account for hail of the observed maximum size (1.5- 2.0 cm); fall-out positions and growth times are consistent with the observations; growth takes place primarily between -10 and -25°C. A wide variety of interweaving trajectories is predicted. In general they conform to the supercell model of Browning and Foote, though several differences are noted. Hail apparently can grow in the storm from a wide range of initial positions, but the favorable initial region for the smaller and more numerous embryos that are expected to dominate the hail production is more restricted. The width of the updraft is found to be an important factor in limiting hail growth in the storm. Melting during fall-out is also significant. The present results are shown to be consistent with a generalized hailstorm model termed an open-cell simple-trajectory model. By this it is meant that (a) the embryo stage of growth occurs outside the large updraft in which (b) the hail itself grows by making a single traverse, as opposed to following several looping "trajectories. Evidence supporting the general importance of this mode of behavior of hailstorms is presented. Alternative theories using them concepts are also discussed.

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G. Brant Foote

Abstract

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G. Brant Foote

Abstract

No abstract available.

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G. Brant Foote

Abstract

A continuous record of precipitation falling through a fixed volume in space was obtained by means of a vertically-pointing pulsed-Doppler radar during a period of continuous precipitation. The range gate was held fixed at an altitude of 910 m above the ground. The output of the radar gave the power returned in several adjacent velocity intervals, from which the drop size distribution and liquid water content were calculated.

Variance spectra for power in selected velocity channels, total liquid water, and reflectivity were computed. The results revealed that despite the uniformity of radar reflectivity at elevations much below the melting layer, a good deal of structure was still evident in the time (space) variations in number of drops of given size. The variance spectra for concentration of drops falling at 7 m sec−1 (about 2 mm drop diameter) revealed significant modes at wavelengths of 2–3 km, and about 350–800 m, which support other evidence of organized atmospheric activity at these scales.

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G. Brant Foote
and
Carl G. Mohr

Abstract

The surface hail and rain data collected during the randomized seeding experiment of the National Hail Research Experiment (NHRE) are stratified according to storm intensity in a search for seeding effects. The problems encountered in attempting to stratify the data according to whether the storm cells exhibited radar vaults are discussed, but no stratification is attempted on this basis. The stratification by storm intensity is accomplished using a one-dimensional steady-state cloud model in conjunction with measured radar echo top heights to estimate the maximum updraft speed of the storm. It is shown that the technique is quite successful in classifying the storm days according to hailfall intensity. Analysis of hail and rain data on seed and control days in the various strata do not show an effect of seeding as assessed by 90% confidence intervals. The confidence intervals are quite wide, however, and a variety of possible effects of seeding are also not inconsistent with the data.

The frequency with which vaulted cells were observed during the experiment is documented, and they are shown to constitute only ∼1% of the total. However, the hailfall from vaulted cells, which tend to be very large and long-lived, can be extreme. Three vaulted cells were seeded during the experiment. Analysis of the radar reflectivity structure of these cells did not reveal any obvious effects of seeding.

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G. Brant Foote
and
Charles G. Wade

Abstract

A detailed description is given of the morphology and evolution of a moderate hailstorm in terms primarily of quantitative S-band reflectivity factor measurements. During the early phase of the storm's life its movement was strongly influenced by the propagation of new cells on its right flank in a manner typical of “organized” multicell norms. During its later phase, when it was being intensively observed by research aircraft and Doppler radar, the new cells tended to form on the front flank of the storm in a manner similar to that analysed for the multicellular Raymer storm that has been discussed extensively in the literature. In the present case, however, emphasis is given not to the discrete nature of the cellular propagation, but rather to the quasi-steady overall structure that is comparable in certain ways to previous descriptions of supercell storms, with transient weak-echo vaults and a pronounced forward overhang in the echo structure. The present storm was smaller and less intense than the archetypal supercell, and inferred pulsations in updraft intensity indicate a degree of unsteadiness not generally acknowledged for supercell storms. It is suggested that the present regime represents the extension of a steady airflow pattern to environmental conditions with higher instability or weaker shear.

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John D. Tuttle
and
G. Brant Foote

Abstract

In the warm season the optically clear boundary layer often contains scatterers that can be detected by sensitive radars to distances of 50–100 km. Inhomogeneities in the field of reflectivity lead to echo patterns that have some persistence over periods as long as 5–10 minutes. These echo patterns translate with the local wind and can, for example, be easily followed by eye on PPI playback loops reviewing 15–20 minutes of data. In this paper a technique is discussed that determines the wind field by objectively identifying and tracking local echo. patterns. The technique, called TREC (Tracking Radar Echoes by Correlation), involves the cross-correlation of the echo features measured at two times a few minutes apart. The translation of a local feature during the measurement interval then determines the local wind. Use of this technique in the clear boundary layer means that problems associated with noisy data (reflectivities just above the minimum detectable signal) and ground clutter will be common. A number of methods for dealing, with these matters are presented. The use of TREC in clear air rather than in storms carries the advantage that misleading results associated with the sedimentation of hydrometeors in a sheared flow are avoided.

In this study TREC is applied to data collected during the Convective Initiation and Downburst Experiment (CINDE) in northeastern Colorado. The method is shown to provide horizontal winds in the boundary layer (the region of significant clear-air echo) over areas 100 to 150 km on a side with a resolution of about 10 km. TREC can be used with either conventional or Doppler radars sensitive enough to detect clear-air echo, though there are some advantages in using single Doppler measurements to improve the reliability of the technique. Applications of the present method are anticipated in a number of research and forecast areas.

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G. Brant Foote
and
Harold W. Frank

Abstract

Triple-Doppler measurements are presented for a hailstorm of moderate intensity that occurred over the High plains of northeastern Colorado, and an airflow model is synthesized. The storm had a number of persistent features similar to previous descriptions of supercells. However, its evolution is shown to be intermediate in a certain sense between classical multicell and supercell models. A new model based on the Doppler observations, termed “weak evolution,” emphasizes gradual changes in the structure of a long-lived updraft. The new airflow model explains the periodic echo intensifications noted for this storm, and seems to apply well to other storms discussed in the literature.

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G. Brant Foote
and
Charles A. Knight

Abstract

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 instruments in a fixed target area on seed and control days. The total mass of hail falling in the network was chosen as the primary measure of seeding effect. Several other hail and rain measures are also examined and a variety of post hoc analyses are presented in the later papers of this series in an attempt to expose seeding effects if they exist and to give further insight into the results of the primary analysis.

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Charles A. Knight
,
G. Brant Foote
, and
Peter W. Summers

Abstract

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 revealed a convincing seeding effect.

These results can be rationalized on the basis that the seeding method was to put extra nuclei into the strong updraft portions of the storms, as the Soviet workers do in order to seed “accumulation zones” of supercooled water. Direct and indirect evidence from the research component of NHRE has shown that accumulation zones, if indeed they exist in northeast Colorado, rarely participate in the hailstone growth. Virtually all radar echo from above the freezing level is from ice, and hail embryos are predominantly graupel. In these circumstances, seeding in the strong updraft is unlikely to produce beneficial competition and a hail suppression effect.

In order to devise a seeding technique that has a good chance to be effective in these types of storms, and to strengthen the statistics to the point where a seeding test has a reasonable chance of detecting seeding effects, research must be done on several fundamental aspects of hailstorms. Paramount among these are the location and time of the natural ice nucleation events that lead to the formation of hail embryos, the trajectories followed by the growing hailstones, and the role of depletion of cloud water in determining hail size.

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