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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 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
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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 Detwiler and Andrew J. Heymsfield

Abstract

An analysis of aircraft-measured data obtained in the lower portion of a High Plains thunderstorm anvil is presented. A “wind shadow” is still evident 5 to 7 core diameters downstream of the storm core. The wind fluctuations are predominantly horizontal on large scales and isotropic on small scales. Little evidence for gravity waves is found in this convectively neutral region of the anvil. Small-scale turbulence is encountered sporadically along cross-anvil penetrations. Weak zones of smooth cloud-edge downdraft are found along the lateral boundaries. The power spectra of the wind components is shallower than the −5/3 value predicted for an inertial subrange turbulent cascade at the smallest scales resolved (<2 km). Lightning is encountered/triggered by the aircraft twice in different relatively turbulent regions far from the storm core where the temperature is −35°C and small graupel is present.

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Andrew J. Heymsfield

Abstract

Measurements from the National Center for Atmospheric Research Sabreliner aircraft are combined with a multiple Doppler radar synthesis of the wind field to investigate particle growth processes in the anvil region of a severe thunderstorm. The aircraft measurements, obtained in mid- to lower anvil levels at temperatures from −25° to −36°C, show than the size spectra broaden with decreasing altitude, yet ice water content values and other measurements indicate this to be a zone of evaporation. Aggregation—allowing particles to develop to sizes as 1 cm—accounts for this observed growth. Intensification of the storm over the observational period results in the development of increasingly large aggregates.

Growth histories and trajectories of anvil particles are calculated using the wind field measurements and are compared to the hydrometeor measurements. Calculations show that particles forming the outer flanks of the anvil initiate in the upwind (relative to the midlevel wind) portions of the updraft and grow larger than those forming the inner anvil region which initiate in the updraft core and further downwind. Thus, the location and size of particles relative to the anvil axis are strongly influenced by the initial particle position. Calculations show that crystal collisions can account for the significant growth through aggregation observed in the anvil.

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Andrew J. Heymsfield

Abstract

The physical characteristics of graupel particles were investigated from in in situ collections of ice particles sampled in first-echo summertime cumulus congestus clouds in northeastern Colorado with the NCAR/NOAA sailplane operating during the National Hail Research Experiment. Ice particles were collected in vials containing silicone oil. Each particle was photographed at several different orientations and then melted to determine its equivalent diameter. This permitted the mass and axial dimensions to be determined and the density, axial ratios and terminal velocity to be estimated for each of 125 particles.

The mass and terminal velocity of graupel particles were found to be considerably lower than those of equivalent diameter ice spheres. Best-fit equations to the mass-diameter and terminal velocity-diameter data were computed. Graupel densities were typically lower than 0.5 g cm−3 systematic differences were noted between the densities of lump and conical graupel. Calculations indicated that the results reported in this paper may not he applicable to thunderstorms with warm cloud-base temperatures.

Measurements of the microstructure of the penetrated cumulus congestus clouds permitted several generalizations to he made on the growth of conical graupel and enabled previous theories of conical graupel formation to be examined. Conical graupel formation through riming of small single crystals appears to be the dominant mechanism operating in the clouds investigated.

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Andrew J. Heymsfield

Abstract

A laboratory experiment was initiated to grow the banded columnar crystals found in cirrus clouds and to determine if they nucleated through a freezing nucleus. Banded columnar crystals were collected in cirrus clouds and also grown in a laboratory cold box over a temperature range of −20 to −46C. They were observed to grow from a frozen droplet through several distinct crystalline transitions. The first distinct crystalline form observed following the growth of a frozen droplet was a polyhedral crystal. With further growth, the polyhedral crystal developed end plates, and it appeared to be a “double plate.” Then the separation between the plates nearly filled in to leave the characteristic band. Certain columns (−6 to −10C) and plates (−10 to −20C) were also observed to grow from a frozen droplet and showed similar crystalline transitions.

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