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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
,
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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Andrew J. Heymsfield
,
Charles A. Knight
, and
James E. Dye

Abstract

Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.

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Andrew J. Heymsfield
,
Peter N. Johnson
, and
James E. Dye

Abstract

The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements.

The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.

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Theodore W. Cannon
,
James E. Dye
, and
Vim Toutenhoofd

Abstract

Some cloud microphysical measurements made from the sailplane The Explorer are presented. Drop sizes and concentrations and ice particle sizes, concentrations, shapes, and, in some cases, internal structures are determined from in situ photographs taken with a recently developed particle camera. The data from photographs taken in northeastern Colorado during the early stages of the development of spring and summer cumulus clouds and in their precipitation suggest that the first precipitation forms primarily by the ice process rather than by condensation–coalescence. Ice particles were frequently photographed in these clouds, but above the freezing altitude, water drops > 50 μm radius were very rare.

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Charles A. Knight
,
Nancy C. Knight
,
James E. Dye
, and
Vim Toutenhoofd

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.

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James E. Dye
,
Charles A. Knight
,
Vim Toutenhoofd
, and
Theodore W. Cannon

Abstract

Much of the previous work which has led to the conclusion that coalescence is the dominant precipitation forming mechanism in cumulus clouds is reviewed. Observations in northeastern Colorado from several independent methods of investigation are summarized to show that in northeastern Colorado the ice (Bergeron–Findelsen) process is in all probability the dominant mechanism in spring and summer cumuli in their early and intermediate stages of development.

Results of microphysical observations coordinated with simultaneous radar observations are presented. The microphysical observations in clouds with observed effective reflectivities of up to 40 dBZ show that the observed reflectivities can be accounted for by measured ice particle sizes and concentrations. Liquid precipitation elements are not necessary and have been observed only rarely in these clouds except below the melting level.

Possible explanations of the differences between clouds in northeastern Colorado and those in other areas are discussed. The rarity of liquid precipitation particles coupled with the general inefficiency of the ice process at temperatures warmer than −10C suggests that there is potential for rainfall enhancement in the clouds in northeastern Colorado.

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Andrew J. Heymsfield
,
Aaron Bansemer
,
Paul R. Field
,
Stephen L. Durden
,
Jeffrey L. Stith
,
James E. Dye
,
William Hall
, and
Cedric A. Grainger

Abstract

This study reports on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled.

Most of the spirals began at cloud top, with temperatures (T) as low as −50°C, and ended at cloud base or below the melting layer (ML). The PSDs broadened from cloud top toward cloud base, with the largest particles increasing in size from several millimeters at cloud top, to 1 cm or larger toward cloud base. Some continued growth was noted in the upper part of the ML. Concentrations of particles less than 1 mm in size decreased with decreasing height. The result was a consistent change in the PSDs in the vertical. Similarly, systematic changes in the size dependence of the particle cross-sectional area was noted with decreasing height. Aggregation—as ascertained from both the changes in the PSDs and evolution of particle habits as observed in high detail with the cloud particle imager (CPI) probe—was responsible for these trends.

The PSDs were generally well-represented by gamma distributions of the form N = N D μ e λ Γ D that were fitted to the PSDs over 1-km horizontal intervals throughout the spirals. The intercept (N ), slope (λ Γ), and dispersion (μ) values were derived for each PSD. Exponential curves (N = N 0 e λD ; μ = 0) were also fitted to the distributions. The λ Γ values for given spirals varied systematically with temperature as did the values of λ (exponential), and the data generally conformed to values found in previous studies involving exponential fits to size distributions in midlatitude frontal and cirrus layers. Considerable variability often noted in the PSD properties during the loops of individual spirals was manifested primarily in large changes in N and N 0, but μ, λ Γ, and λ remained fairly stable. Temperature is not found to be the sole factor controlling λ Γ or λ, but is a primary one. Direct relationships were found between λ Γ and N , or λ Γ and μ, for the gamma distributions, and λ and N 0 for the exponential. The latter relationship was not found as distinctly in earlier studies; observed PSDs in this study had better fidelity with less scatter. The μ values changed monotonically with T over the range of temperatures and were directly related to N or λ Γ, thereby reducing the number of variables in the PSD functional equation to two. In the upper part of the ML, N 0, and λ continued to decrease, and in the lower part these values began to increase as the largest particles melted.

General expressions relating various bulk microphysical, radar, and radiative-transfer-related variables to N and λ Γ were developed; they are useful for both tropical and midlatitude clouds. These relationships facilitate the specification of a number of bulk properties in cloud and climate models. The results presented in this paper apply best to temperatures between 0° and −40°C, for which the measured radar reflectivities fall in the range of 0 to 25 dBZ e .

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