Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: G. Brant Foote x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
G. Brant Foote

Abstract

No abstract available.

Full access
G. Brant Foote

Abstract

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access
L. Jay Miller, John D. Tuttle, and G. Brant Foote

Abstract

Computations of air motion and precipitation growth using winds derived from Doppler radar measurements were analyzed to reveal important flow features that influenced the production of precipitation during the nearly steady phase of a well-observed severe storm in Montana that produced hail as large as 5 cm in diameter. The storm had many features commonly associated with supercells, though it exhibited a gently sloping overhang on its low-level inflow side, rather than the more classical vaulted structure. Formed initially as the right member of a splitting storm pair, it moved slowly eastward while embedded in moderately sheared environmental winds.

Characteristic hail growth trajectories and precipitation fallout positions are considered in conjunction with the deduced embryo sources and formation regions. Based on particle growth calculations, measurements by radar and research aircraft, cloud photography and direct hailstone examination, four general sources of hail embryos were apparent: 1) graupel grown along the updraft fringes, 2) a derivative of the former consisting of drops produced by melted graupel, 3) water drops shed from melting hail, and 4) shedding from hailstones that were in wet growth conditions. The graupel embryos were deduced to originate primarily in two columnar regions on the flanks of the updraft core. One column was within the stagnation zone on the west side (upwind with respect to the midlevel flow), and the other one was at the center of the midlevel, mesocyclonic circulation on the cast (downwind) side. The cyclonically streaming branch of the flow transported some graupel from the west flank to the southwest and south sides of the storm where a fraction of them melted completely before entering the strong updraft. The occasional merger of drifting, isolated cumulus congestus clouds with the storm and the ingestion of graupel from them was also documented.

Following the embryo growth stage, three types of hail growth trajectories were found: 1) those passing into the southern (cyclonic) branch of the middle-to-upper level airflow, 2) those passing into the northern (anti-cyclonic) branch of this flow, and 3) those passing in a nearly straight line through the updraft core in midlevels (preferentially the northeastern side of the core). Of these the straight-line trajectory produced the largest hail. Growth trajectories were mostly of a simple up-and-down nature, without multiple passes or loops through the main updraft. Particles that followed cyclonic trajectories produced a broad maximum in reflectivity cast of the updraft, while those that followed straight-line trajectories produced a similar broad maximum west of the updraft. Large hail from straight-line trajectories and some graupel and hail from the cyclonic branch passed through the region that otherwise would have been the echo-weak vault. A subset of the cyclonic trajectories appeared able to loop back on themselves in such a way that low-level melting and breakup of graupel following this circuit could have led to a self-sustaining mode of precipitation growth.

Precipitation from graupel grown in the western updraft fringes, from drops produced by transport and melting of these graupel, and from graupel grown in the northern updraft fringes was necessary to explain the observed patterns of radar reflectivity, dual-wavelength ratio and specific attenuation. Further, only embryos from the west and south flanks led to large diameter hail near where stones of similar sizes were observed from aircraft and at the ground. Precipitation from the remaining source east of the updraft maximum duplicated only the central portion of fallout from the other sources. Though embryo transport from the upwind side of the updraft core and around its south side was a necessary deduction from aircraft and radar observations, details of the early particle formation within the upwind region were not well documented. This region was characterized as a vigorous cluster of cumuliform cloud without dominant or discrete turrets. For reasonable particle concentrations and liquid water contents near-millimeter particle sizes had to be present to explain the low reflectivities measured there. Presumably graupel growth was initiated in the upper parts of this peripheral weak-echo region with turbulent diffusion mixing particles throughout the region.

Full access