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Alexei Korolev

Abstract

Imaging optical array probes (OAPs) have become conventional instruments in studies of cloud microphysics. Previous works have shown that the error particle sizing in OAPs may reach 100%. Correcting the particle size measurements is not a trivial task, since the error depends on its size and distance from the object plane. A new technique for the size reconstruction of spherical particles from its measured image is introduced here. This technique also enables the retrieval of the particle position along the depth of field in the sample volume. The essence of the algorithm consists in the deduction of size and position from the relationships between the size of the Poisson spots and the geometrical dimensions of the image. The retrieval technique has been tested on the simulated discrete binary diffraction images of spherical particles, similar to those produced by OAPs. The images were modeled using the Fresnel diffraction approximation. It is demonstrated that the new algorithm can be applied to discrete binary images of spherical particles consisting of more than three pixels in size. An important feature of the retrieval technique is that it does not depend on the pixel resolution, and it can be applied for any type of OAPs that use a monochromatic coherent source of illumination.

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Alexei Korolev

Abstract

Phase transformation and precipitation formation in mixed-phase clouds are usually associated with the Wegener–Bergeron–Findeisen (WBF) process in which ice crystals grow at the expense of liquid droplets. The evolution of mixed-phase clouds, however, is closely related to local thermodynamical conditions, and the WBF process is just one of three possible scenarios. The other two scenarios involve simultaneous growth or evaporation of liquid droplets and ice particles. Particle evolution in the other two scenarios differs significantly from that associated with the WBF process. Thus, during simultaneous growth, liquid droplets compete for the water vapor with the ice particle, which slows down the depositional growth of ice particles instead of promoting their growth at the expense of the liquid as in the WBF process. It is shown that the WBF process is expected to occur under a limited range of conditions and that ice particles and liquid droplets in mixed-phase clouds are not always processed in accordance with the WBF mechanism.

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Alexei V. Korolev

Abstract

The concept of droplet spectrum local broadening and narrowing is introduced. It is shown that a cloud droplet spectrum may be narrowed at one size interval and broadened at another simultaneously. Numerical simulations indicate that the salinity and surface curvature terms may produce absolute and relative broadening of droplet spectra in stratiform clouds in several tens of minutes, with variations of the supersaturation arising from typical turbulent vertical velocity fluctuations. The changes in shape of the droplet size spectrum are not reversible in these processes.

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Alexei Korolev
and
George Isaac

Abstract

The frequency of occurrence of the aspect ratio and roundness of particles in ice clouds from aircraft observations have been examined. Images of cloud particles were measured by a cloud particle imager (CPI) at 2.3-μm resolution, installed on the National Research Council (NRC) of Canada Convair-580. Data were collected in winter midlatitude and polar stratiform clouds associated with frontal systems during three field projects in the Canadian and U.S. Arctic and over the Great Lakes. Approximately 106 images of particles measured in ice clouds were included in the statistics. The frequency of occurrence of the aspect ratio and roundness were calculated in eight 5° temperature intervals from −40°C to 0°C. In each temperature interval, the distributions were calculated for 12 size intervals in the range from 20 μm to 1 mm. It was found that the roundness is a function of particle size and within each size interval it does not depend significantly on temperature. However, the aspect ratio of particles with 60 μm < D < 1000 μm is mainly a function of temperature and does not depend on size. The fraction of spherical particles in ice clouds rapidly decreases with particle size. The fraction of spherical particles in the size range 20 μm < D max < 30 μm on average does not exceed 50%. Ice clouds do not contain significant numbers of spherical particles larger than 60 μm. The information on the habits of small ice particles obtained here gives an insight on the mechanisms of ice formation in clouds. The results can be used for parameterization of particle habits in radiation transfer, weather and climate models, and in remote sensing retrievals. It may also be of interest for calculations of collision efficiency in modeling of interaction of cloud particles with moving platforms related to in-flight icing.

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Darrel Baumgardner
and
Alexei Korolev

Abstract

The Particle Measuring System’s optical array probes have a sample volume that depends upon the diameter of the particle measured. The sample volume also depends upon the velocity of particles that pass through the probe because of the electronic response time of these instruments. This note discusses an algorithm that has been derived to calculate sample volume as a function of size and velocity, and demonstrates the need for such an algorithm by comparison of measurements from several types of optical array probes and a forward-scattering spectrometer probe. These comparisons show that the optical array probes greatly underestimate droplet concentrations of particles less than 100 μm in diameter at typical aircraft research speeds unless sample volumes are adjusted for electronic response time limitations.

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Alexei Korolev
,
Edward Emery
, and
Kirk Creelman

Abstract

Ice particle shattering may significantly contaminate measurements taken by airborne particle probes in ice clouds. Environment Canada and the NASA Glenn Research Center (GRC) undertook efforts to modify and test probe tips in order to mitigate the effect of shattering on measurements. This work presents an overview of the results obtained during the design work on the particle probe arm tips. Even though this work was focused on the modifications of three of the probes—Particle Measuring Systems Inc. (PMS) Forward Scattering Spectrometer Probe and optical array probe, and Droplet Measurement Technologies (DMT) Cloud Imaging Probe—the outcomes of this work bear a general character and are applicable to other similar instruments. The results of the airflow analysis around the probe’s housing and the simulations of particle bouncing from the probe tips are discussed here. The originally designed and modified tips were tested in a high-speed wind tunnel in ice and liquid sprays. The ice particle bouncing processes as well as patterns of water shedding over the surface of the probes arms were studied with the help of a high-speed video camera. It was found that at aircraft speed, after bouncing from a solid surface, ice particles may travel several centimeters across the airflow and bounce forward up to 1 cm. For the first time it has been directly documented with high-speed video recording that the sample volumes of particle probes with the originally designed tips are contaminated by shattered and bounced particles. A set of recommendations on the existing modification and the design of future particle probe housings is presented.

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Alexei Korolev
,
Alex Shashkov
, and
Howard Barker

Abstract

A new airborne instrument that measures extinction coefficient β in clouds and precipitation has been designed by Environment Canada. The cloud extinction probe (CEP) utilizes the transmissometric method, which is based on direct measurement of light attenuation between the transmitter and receiver. Transmissometers are known to be susceptible to forward scattering, which becomes increasingly significant as the particle size increases. A new technique for calibrating transmissometers was developed here in order to determine the response function of the probe. Laboratory calibrations show that CEP-derived β may be underestimated by a factor of 2 for circular particles with diameters greater than 100 μm. Results for spherical particles are in good agreement with theoretical predictions. For nonspherical particles, however, estimates of β can deviate significantly from those derived for spheres that have the same projected area. For in situ observations of ice particles, CEP measurements often deviate significantly from theoretical calculations, whereas for small cloud droplets agreement is good. It is hypothesized that CEP-derived estimates of β for ice clouds depend much on variations in the scattering phase function that arise from details in ice crystal surface roughness and fine crystal structure. This would complicate greatly the estimation of β from transmissometers for ice-bearing clouds.

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Alexei V. Korolev
and
George A. Isaac

Abstract

A new conceptual model is proposed for enhanced cloud droplet growth during condensation. Rapid droplet growth may occur in zones of high supersaturation resulting from isobaric mixing of saturated volumes with different temperatures. Cloud volumes having a temperature different from the general cloud environment may form due to turbulent vertical motions in a temperature lapse rate that is not pseudoadiabatic. This mechanism is most effective in the vicinity of cloud-top inversions. It is also shown that the isobaric mixing of saturated and dry volumes with different temperatures may also lead to high supersaturations. The high supersaturations are associated with zones of molecular mixing, and they have a characteristic size of the order of millimeters with a characteristic lifetime near tenths of a second. Some small proportion of cloud droplets, over many supersaturation events, may grow large enough to grow effectively through collision–coalescence. This hypothesis of isobaric mixing may help explain freezing and warm drizzle formation.

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Alexei Korolev
and
Paul R. Field

Abstract

A theoretical framework has been developed describing nonequilibrium formation and maintenance of mixed-phase clouds. The necessary and sufficient conditions required to activate liquid water within a preexisting ice cloud, and thus convert it to mixed phase, are considered for three scenarios: (i) uniform ascent, (ii) harmonic vertical oscillations, and (iii) turbulent fluctuations. The general conditions are the following:

  1. First necessary condition: The vertical velocity of an ice cloud parcel must exceed a threshold velocity to activate liquid water.

  2. Second necessary condition: The activation of liquid water within an ice cloud parcel, below water saturation, requires a vertical ascent above some threshold altitude to bring the vapor pressure of the parcel to water saturation.

Only when the first and second conditions are true do these conditions become sufficient for the activation of liquid water in ice clouds. These required conditions for the generation of mixed-phase cloud are supported by parcel modeling results and analogous conditions for a harmonic oscillation concerning the amplitude and tangential velocity of the parcel motion are proposed. The authors do not assume steady-state conditions, but demonstrate that nonequilibrium evolution of cloud parcels can lead to long-term steady existence of mixed-phase cloud.

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Alexei Korolev
,
Mark Pinsky
, and
Alex Khain

Abstract

A new mechanism has been developed for size distribution broadening toward large droplet sizes. This mechanism may explain the rapid formation of large cloud droplets, which may subsequently trigger precipitation formation through the collision–coalescence process. The essence of the new mechanism consists of a sequence of mixing events between ascending and descending parcels. When adiabatically ascending and descending parcels having the same initial conditions at the cloud base arrive at the same level, they will have different droplet sizes and temperatures, as well as different supersaturations. Isobaric mixing between such parcels followed by further ascents and descents enables the enhanced growth of large droplets. The numerical simulation of this process suggests that the formation of large 30–40-μm droplets may occur within 20–30 min inside a shallow adiabatic stratiform layer. The dependencies of the rate of the droplet size distribution broadening on the intensity of the vertical fluctuations, their spatial amplitude, rate of mixing, droplet concentration, and other parameters are considered here. The effectiveness of this mechanism in different types of clouds is discussed.

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