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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 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 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 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

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 J. Heymsfield

Abstract

This is Part I of a study that characterizes several bulk properties of ice particle populations sampled in midlatitude and tropical cirrus and deep stratiform ice clouds, for the purpose of developing an understanding of how particles evolve in ice clouds and to derive empirical and analytical relationships that describe microphysical properties for use in cloud and climate models. The effort focuses on describing the microphysical properties of ice cloud layers in the vertical. The size distribution data and particle imagery were obtained during Lagrangian spiral descents and balloon-borne ascents through cloud layers that formed in association with synoptic-scale lifting (midlatitude) and deep convection (Tropics). Temperatures ranged between −20° and −63°C for the midlatitude clouds and between 0° and −50°C for the tropical clouds. Optical depths spanned the range 0.4–7 for the midlatitude clouds and 20–30 for the tropical clouds.

This part of the study characterizes median mass diameter (D m) and median fall velocities (V m) for the more than 2000 ensembles or particle size distributions (PSDs) examined. The D m and V m increase downward from cloud top to base, with the smallest D m and V m values found in the coldest (midlatitude) clouds and the largest values found in the warmest (tropical) clouds. The range of sizes that dominate the ice water content, and the associated range of particle fall speeds, are characterized in terms of D m and V m.

The V m are represented in terms of D m and the slopes (λ) of gamma distributions fitted to the particle size distributions. The V m values increase with D m and decrease with λ in a predictable manner. The magnitudes of the changes in V m that result from differing ambient pressures between 250 and 1000 hPa are quantified. The observations are generalized so that the results can be extended to different pressure levels and other particle size distributions.

The coefficients γ and β in the power-law relationship V t = γD β fitted to the individual spectra are found to be inversely related to D m. Many earlier studies have derived these coefficients from measurements at the surface. The wide variability noted in these coefficients may partially be attributed to variations in the D m values of the populations considered. The relationship of the γ and β coefficients found for particle ensembles at the surface to those at the pressure levels of ice clouds are derived.

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Andrew J. Heymsfield

Abstract

This is the second part of a study that characterizes several bulk properties of ice particle populations sampled in synoptically generated midlatitude and convectively generated tropical ice clouds, for the purpose of developing empirical and analytical relationships that describe microphysical properties for use in mesoscale and climate models. The purpose of this paper is to examine the interrelationships between the mass, area, and fall velocity properties of the particle size distributions, and the dependence of these properties on temperature.

Gamma distributions of the form N = N 0 D μ e λD are fitted to the measured particle size distributions (PSDs) over sizes (D) from as small as 10 μm to as large as 1.5 cm. Exponential distributions (μ = 0) are also fitted to the PSD. The intercept parameter N 0 and the slope λ are directly related, and decrease monotonically with increasing temperature. The μ values for the gamma fits tend from positive values at large λ to negative values at small λ. The maximum measured diameter D max increases with decreasing λ. The N 0 values from the midlatitude clouds are about an order of magnitude lower than those for the tropical PSDs at the same temperatures.

Bulk properties are derived from the fitted PSDs. The ice water contents (IWC) are about an order of magnitude higher for the tropical than for the midlatitude clouds. The median mass diameter (D m) and the effective diameter (D e) each increase with temperature, and are found to be related to each other.

Several aspects related to the modeling of ice particle sedimentation in general circulation models (GCMs), and the relationship of these velocities to other bulk properties, are investigated. On average, the median mass-weighted terminal velocity (V m) increases weakly with temperature. Correlations between V m and IWC are also weak. It is found that for a given particle ensemble, most of the ice mass is contained within a relatively narrow range of fall velocities, although the values of V m can be appreciable. Calculations reveal that the fallout of particles that dominate the extinction cannot be ignored, except at temperatures below −50°C. Also, the effective diameter is found to be strongly related to the ensemble mean V m, perhaps allowing the two variables to be linked in GCMs.

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