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Charles A. Knight and L. Jay Miller

Abstract

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. In these cases, the radar echo profiles can be used to estimate the adiabatic droplet concentration, given rough knowledge of the cloud-base height and temperature. The first precipitation shafts often occur before the cloud tops reach the 0°C level, are narrow, and probably consist of low concentrations of drops several millimeters in diameter.

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Neil F. Laird, L. Jay Miller, and David A. R. Kristovich

Abstract

This article presents a detailed examination of the kinematic structure and evolution of the 5 December 1997 winter mesoscale vortex in the vicinity of Lake Michigan using the synthetic dual-Doppler (SDD) technique. When such a mesoscale event propagates a distance large enough that the viewing angle from a single-Doppler radar changes by about 30° and the circulation is sufficiently steady during this time period, then the SDD method can reveal reliable details about the circulation. One such detail of the observed vortex was a pattern of convergence and divergence associated with radial bands, where heavier snowfall was located. Another was the steadiness and vertical coherence of the derived vorticity and convergence patterns within the cyclonic circulation.

On 5 December 1997, the observed reflectivity field remained remarkably steady for nearly 2.5 h as the vortex moved southeastward allowing for the application of the SDD technique. The reflectivity field exhibited a pronounced asymmetric convective structure with at least three well-defined, inward-spiraling radial snowbands, and a distinct weak-reflectivity region or “eye” near the center of cyclonic circulation. The SDD results showed the vortex circulation was composed of a combination of rotation on the meso-β scale and convergence on the meso-γ scale associated with the embedded radial snowbands. Vertical profiles of derived meso-β-scale, area-mean convergence and vorticity suggest that this winter vortex was likely a warm-core system, similar to both tropical cyclones and polar lows.

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L. Jay Miller, James E. Dye, and Brooks E. Martner

Abstract

Measurements from three Doppler radars of air motion and observations of the environment and storm reflectivity structure, supplemented by aircraft measurements of precipitation and cloud particles, are used to establish the dynamical framework for precipitation development in a convective storm that grew in a weakly-sheared wind environment. The moderately intense, evolving storm consisted of a series of cells that developed in late afternoon on 25 July 1976 in southeastern Wyoming. The storm, which moved along the sub-cloud wind direction, had a persistent but unsteady updraft region on its right forward flank. This updraft region consisted of several small convective elements with two or more intense updraft cores evident at all times. Middle-level flow around the updraft region eventually resembled obstacle flow with downdrafts located on the flanks and in the wake of the updraft. This storm-wide, organized circulation apparently allowed precipitation particles to reenter an updraft and grow for periods longer than would have been possible if all their growth had occurred in a single ascent within an updraft core of 10 to 20 m s−1 speeds. Such vertical motions would have carried particles to cloud top in 5 to 10 min, a growth period too short to account for the observed millimeter-size particles in the updraft. This storm lasted for more than one hour and produced hail particles as large as 9 mm diameter that were observed at cloud base by aircraft.

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James E. Dye, Brooks E. Martner, and L. Jay Miller

Abstract

Aircraft measurements of microphysical thermodynamic, and vertical air motion properties supplemented by radar measurements of reflectivity structure are used to investigate precipitation development throughout much of the life cycle of a moderately intense convective storm in northeast Colorado. There was considerable variability of cloud properties such as updraft speed at scales of a few hundred meters early in the life of the storm. Greater organization was evident in the later, more mature stage.

The earliest radar return from a major cell came from particles larger than 1 mm diameter in concentrations less than 10 m−3. It is suggested that these particles must have already followed complicated growth trajectories even at this early stage of the storm, including more than one ascent in updraft.

In the more mature stage of the storm millimetric water drops and partially melted and refreezing ice particles were observed at 1 to 2 km above cloud base in both mixed and unmixed regions of the main updraft. Because of their size and proximity to cloud base, these particles could not have grown during a single ascent in the updraft. The observations suggest that penetrative downdrafts, sedimentation, growth in weak updrafts, and recycling, among others, acted singly or in concert to increase particle growth times in this cloud, and that individual particles may have spent an appreciable length of time outside of favorable growth regions.

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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.

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Charles A. Knight, L. Jay Miller, and Robert A. Rilling

Abstract

Early radar echo development in trade wind cumulus clouds is studied using the equivalent reflectivity factor Ze 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. 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 Z dr 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.

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L. Jay Miller, John D. Tuttle, and Charles A. Knight

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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 cloud line associated with the surface gust front. Graupel particles grown in this weak updraft region were carried by the southwesterly airstream either into the hook echo or into the forward overhang. Hailstones and graupel particles in both branches were found to shed water drops as they traveled northward and descended below the melting level along the shoulders of the main updraft. This resulted not only in a substantial rainfall, but also in a source of droplet embryos especially within the forward overhang. Considerable numbers of hailstones were produced, since many of these shed drops passed westward through the main updraft.

The route through the forward overhang or embryo curtain is the familiar particle recycling path which was proposed as the one that enabled large hailstones to grow in the Wokingham storm in England and the Fleming storm in northeastern Colorado. Indeed most of the hailstones ranging in diameter from 1 to 2 cm were produced in the Pock Springs storm along a path similar to this one. However, some of the largest hailstones originated in a narrow band behind (west of) the updraft axis near the center of rotation of the middle level mesocyclone. Trajectories such as these were instrumental in the development of the hook echo and several produced 3 to 5 cm hail that fell out close to where the embryos were initialized. Recycling did not happen in the model; all initialized particles followed simple up-and-down trajectories through the supercooled region.

Hail from this large, severe supercell was produced along several paths, some similar to trajectories proposed for other storms and some different. Overall, the hail growth is more complicated in this well-documented storm than in any of the simple idealizations previously proposed.

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Charles A. Knight, L. Jay Miller, and William D. Hall

Abstract

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. Two kinds of convective initiation are illustrated. In one, a vigorous cell is initiated over an outflow boundary, but within light precipitation. In the other, the initiation is evidently by an instability created by the melting layer, perhaps by a mechanism first discussed by Findeisen. In this latter type, the new convective elements are not severe but they generate supercooled cloud within the anvil, extend entirely through the anvil to altitudes above 12 km MSL, and produce graupel showers with rain at the ground exceeding 50 dBZ. The instability itself may be generated in large part by moistening and cooling the sounding by the falling precipitation.

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L. Jay Miller, Carl G. Mohr, and Andrew J. Weinheimer

Abstract

Periodic sampling of the Doppler radar return signal at the pulse repetition frequency causes measured velocities to be ambiguous (folded) when true meteorological velocities along the radial direction exceed the Nyquist or folding value. Furthermore, mean radial velocity estimates become more uncertain as the spatial variability of velocity increases or the returned signal strength decreases. These data have conventionally been prepared for such uses as multiple-Doppler radar wind synthesis by unfolding and editing them in the sampling domain (range-azimuth-elevation spherical coordinates).

An alternative method of locally (to the output grid point) unfolding the unedited radial velocities during their linear interpolation to a regular Cartesian grid is presented. The method preserves the spatial discontinuities of order twice the Nyquist value associated with velocity folding. A nondimensional velocity quality parameter is also computed which serves to identify interpolated values that contain too much variance to be reliable. Editing of radar data is thereby postponed until all radar data are mapped to the analysis coordinate system. This allows for iterative global unfolding and multiple-Doppler synthesis of complicated convective storm flow patterns. The resolution of folding in such flow fields may require more information than is usually available from single radar radial velocity fields in spherical coordinates. Further, the amount of data that must be subsequently manipulated is reduced about ten-fold in the process of interpolation.

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William Blumen, Nimal Gamage, Robert L. Grossman, Margaret A. LeMone, and L. Jay Miller

Abstract

This investigation examines the meso- and microscale aspects of the 9 March 1992 cold front that passed through Kansas during the daylight hours. The principal feature of this front is the relatively rapid frontogenesis that occurred. The total change in the cross-frontal temperature is about 6 K, with most of the change occurring between about 0820 and 1400 local time and over a relatively small subsection of the total frontal width. The surface data are able to resolve a sharp horizontal transition zone of 1–2 km. The principal physical processes that produce this frontogenesis are shown to be the cross-frontal differential sensible heating, associated with differential cloud cover, and the convergence of warm and cold air toward the front. The former process is responsible for an increase in the magnitude of the differential temperature change across the front; the latter process concentrates the existing temperature differential across an ever-decreasing transitional zone until a near discontinuity in the horizontal temperature distribution is essentially established during the period of a few hours. Two approaches are taken to demonstrate that these processes control the observed frontogenesis. First, surface data from an enhanced array, set up during the Storm-scale Operational and Research Meteorology Fronts Experiment System Test, are used to evaluate the terms that contribute to the time rate of change of the gradient of potential temperature, d|∇θ| / dt, following the motion of the front. Then, the processes of differential sensible heating and convergence are incorporated into a simple two-dimensional nonlinear model that serves to provide a forecast of the surface temperature and velocity fields from given initial conditions that are appropriate at the onset of the surface heating. Verification of the model predictions by observed data confirms that both processes contribute to the observed daytime frontogenesis on 9 March 1992. A critique of the model does. however, suggest that the accuracy of some quantitative evaluations could be improved.

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