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Andrew Heymsfield
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Andrew Heymsfield

Abstract

Terminal velocities of different ice crystal forms were calculated using the most recent ice crystal drag coefficients, aspect ratios and densities. The equations derived were primarily for use in calculating precipitation rates by sampling particles with an aircraft in cirrus clouds, and determining particle size in cirrus clouds by Doppler radar. However, the equations are sufficiently general for determining particle terminal velocity at any altitude, and most any crystal type. Two sets of equations were derived. The “general” equations provide a good estimate of terminal velocities at any altitude. The “specific” equations are a set of equations for ice crystal terminal velocities at 1000 mb. The calculations are in good agreement with terminal velocity measurements. The results from the present study were also compared to prior calculations by others and seem to give more reasonable results, particularly at higher altitudes.

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Andrew Heymsfield

Abstract

The growth of the ice phase in cirrus uncinus and cirrostratus clouds was studied through aircraft measurement of cloud particle spectra at different altitudes. Five different cirrus uncinus clouds were studied; one of the cirrus uncinus evolved into cirrostratus. The temperature range of sampling was −19 to −58°C. In cirrus uncinus heads, crystals were determined to be nucleated and grown in the upshear region, before being carried into the trail region of the head downshear as a result of wind shear. The updraft region is upshear, and the downdraft region downshear. A “hole” was found to separate the up-and downshear regions of the head, with a horizontal extent of about 150 m. The concentrations of crystals in the head region were on the order of 0.5 cm −3, with 0.025-0.05 cm −3 longer than 100 µm. Accumulation of particles in the updraft region was noted. The mean length of crystals longer than 100 µm (precipitation size particles) ranged between 0.5 and 1.0 mm, and crystals as long as 2 mm were found at temperatures as low as −56°C. The average ice water content was found to be 0.15–0.3 gm −3 in the head. The cirrostratus clouds sampled had their nucleation regions near the top of the clouds; crystals sedimented and grew from this source region near the top to near the base, and then evaporated to the base. The crystal concentrations were about 0.2 cm−3, WITH 0.01 −3 longer than 100 µm. The mean length of crystals larger than 100 µm ranged between 0.2–0.5mm. The ice water content ranged between 0.01–0.16 g m−3.

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Andrew Heymsfield

Abstract

The structure and circulations of the cirrus uncinus generating head were determined from aircraft measurements of the temperatures, horizontal wind velocities and particle spectra at different altitudes. Stable layers were found to exist directly above and below the head. The head was found to exist in a region with a dry adiabatic lapse rate. Waves were observed in the stable layer below the head. The head was found to he divided into two regions in active cirrus uncinus. The upshear part of the head is the updraft region, and the downshear part the downdraft region. A region containing almost no crystals was found to separate the up- and downdraft regions. This “hole” was typically 150 m across.

The vertical velocities in cirrus uncinus were determined from aircraft and Doppler radar measurements. Typical vertical velocities were estimated to range from 100–200 cm s−1 from aircraft particle measurements, and determined from Doppler radar measurements to range from 120–180 cm s s−1 Typical downdraft velocities of 50 cm s s−1 were determined from the aircraft measurements and from the Doppler measurements to be a maximum of 80 cm s−1, with 20–40 cm s−1 typical velocity.

Two mechanisms are suggested for the formation of cirrus uncinus clouds. For cirrus uncinus oriented in lines almost perpendicular to the wind direction, it is suggested that there is layer lifting and that convective cells develop along the lifting line. In the case of isolated cirrus uncinus, it is suggested that a wave in the stable layer below the head region causes a perturbation in the head region which results in convection in the layer. Two mechanisms are suggested for the formation of new generating cells upwind or downwind of the original cell, which significantly increases the lifetime of the cloud. Evaporative cooling in the trail region may induce the formation of new turrets above the trail of an original cell. A second possible mechanism is the formation of a convergent and divergent region at the stable layer below the head region induced by the downdraft in the trail region of the head.

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Andrew Heymsfield

Abstract

Equations were developed to calculate the growth of the ice phase in cirrus clouds. Calculations indicated that nucleation of ice crystals in cirrus uncinus heads forming at temperatures lower than −35°C generally should occur near the upwind base of the head, and in cirrostratus clouds at the top of the cloud.

The growth of ice crystals and the resulting shape of cirrus uncinus clouds with an updraft velocity of 100 cm s−1 were calculated. With an initial crystal concentration of 0.025 cm −2 and a nucleation temperature of −40°C, crystals of 0.45 mm length, and a maximum ice water content of 0.3 g m −3 were predicted.

Latent beat release due to the ice crystal growth increased the initial updraft velocity only slightly. A downdraft velocity comparable in magnitude to the original updraft velocity was calculated to occur in the downshear part of the cirrus uncinus head.

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Andrew Heymsfield
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Andrew J. Heymsfield

Abstract

A technique is described for simulating the development of particles in a storm when data on the internal composition and wind-field structures are available. Simulations such as these can be used to investigate particle growth mechanisms.

The method is used to simulate particle development in a hailstorm that occurred in northeastern Colorado on 22 July 1976. Wind fields derived from triple-Doppler radar scans during six periods are used for the calculations and the temporal evolution of the winds during a measurement period is considered. Particles of different types and sizes are initialized at positions throughout the storm at the beginning of the analysis period from radar data using correlations from the in-situ measurements. Particles are “nucleated” within updrafts at later times. Particle growth and sedimentation are calculated according to the habits of particles as they are carried through the storm until they fall to the ground. The liquid water content and drop-size distribution at positions along particle trajectories are calculated from the vertical air velocities. Input parameters of the calculations are varied for the purpose of sensitivity analyses. A data set was compiled of information on positions, sizes, terminal velocities, and other parameters during the development of each of more than 130 000 initialized particles. Information from this data set was compared with available radar, surface and in-situ observations to verify the model inputs and evaluate the simulations.

The calculated manner of particle development in the storm compares favorably with the observed radar, surface and in-situ observations. Several discrepancies between the calculations and the observations are attributed primarily to inadequacies of the wind-field data. Underestimates in the liquid water content could also have accounted for some discrepancies.

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Andrew J. Heymsfield

Abstract

The growth of ice particles through aggregation is investigated for seeded clouds using currently available field data and a numerical particle-growth model. Observations indicate that the aggregation process is fairly common, even when moderate liquid water contents, ~0.5 g m–3, are available for particle growth through accretion. The modeling study suggests that certain temperature ranges are especially conducive to aggregate formation.

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Andrew Heymsfield
and
Paul Willis

Abstract

Progress in understanding the formation of ice in lower-tropospheric clouds is slowed by the difficulties in characterizing the many complex interactions that lead to ice initiation and to the dynamic, non-steady-state nature of the clouds. The present study characterizes the conditions where secondary ice particles, specifically identified as needle or thin columnar types, are observed in tropical maritime convection with modest liquid water contents during the Ice in Clouds Experiment-Tropical (ICE-T), based out of St. Croix, U.S. Virgin Islands, and the NASA African Monsoon Multidisciplinary Analyses (NAMMA) in 2006 sampling from Cape Verde, Africa. The properties of the cloud droplet populations relevant to the secondary ice production process and the ice particle populations are characterized as a function of temperature and vertical velocity. These secondary ice particles are observed primarily in regions of low liquid water content and weak vertical velocities. Two situations are examined in detail. First, ice formation is examined by following the tops of a group of ICE-T chimney clouds as they ascend and cool from a temperature of +7° to −8°C, examining the production of the first ice. Then, using the data from a cloud system sampled during NAMMA, the authors elucidate a process that promotes ice multiplication. The intention is that this study will lead both to a better understanding of how secondary ice production proceeds in natural clouds and to more realistic laboratory studies of the processes involved.

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Andrew Heymsfield
and
Robert Wright

Abstract

This study characterizes the terminal velocities of heavily rimed ice crystals and aggregates, graupel, and hail using a combination of recent drag coefficient and particle bulk density observations. Based on a nondimensional Reynolds number (Re)–Best number (X) approach that applies to atmospheric temperatures and pressures where these particles develop and fall, the authors develop a relationship that spans a wide range of particle sizes. The Re–X relationship can be used to derive the terminal velocities of rimed particles for many applications. Earlier observations suggest that a “supercritical” Reynolds number is reached where the drag coefficient for large spherical ice—hail—drops precipitously and the terminal velocities increase rapidly. The authors draw on observations and model simulations for slightly roughened large ice particles that suggest that the critical Reynolds number is dampened and that the rapid increase in the terminal velocity of smooth spherical ice particles rarely occurs for natural hailstones.

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