Search Results

You are looking at 1 - 10 of 33 items for

  • Author or Editor: Alexei Korolev x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access
Alexei Korolev
and
George A. Isaac

Abstract

The results of in situ observations of the relative humidity in liquid, mixed, and ice clouds typically stratiform in nature and associated with mesoscale frontal systems at temperatures −45°C < Ta < −5°C are presented. The data were collected with the help of instrumentation deployed on the National Research Council (NRC) Convair-580. The length of sampled in-cloud space is approximately 23 × 103 km. The liquid sensor was calibrated in liquid clouds with the assumption that the air in liquid clouds is saturated with respect to water. It was found that the relative humidity in mixed-phase clouds is close to saturation over water in the temperature range from −5° to −35°C for an averaging scale of 100 m. In ice clouds the relative humidity over ice is not necessarily equal to 100%, and it may be either lower or higher than saturation over ice, but it is always lower than saturation over water. On average the relative humidity in ice clouds increases with a decrease of temperature. At −40°C the relative humidity over ice is midway between saturation over ice and liquid. A parameterization for the relative humidity in ice clouds is suggested. A large fraction of ice clouds was found to be undersaturated with respect to ice. The fraction of ice clouds undersaturated with respect to ice increases toward warmer temperatures.

Full access
Alexei Korolev
and
George A. Isaac

Abstract

The data on cloud particle sizes and concentrations collected with the help of aircraft imaging probes [optical array probes OAP-2DC, OAP-2DP, and the High Volume Precipitation Spectrometer (HVPS)] are widely used for cloud parameterization and validation of remote sensing. The goal of the present work is to study the effect of shattering of ice particles during sampling. The shattering of ice particles may occur due to 1) mechanical impact with the probe arms prior to their entering the sample volume, and 2) fragmentation due to interaction with turbulence and wind shear generated by the probe housing. The effect of shattering is characterized by the shattering efficiency that is equal to the ratio of counts of disintegrated particles, to all counts. The shattering efficiency depends on the habit, size, and density of ice particles, probe inlet design, and airspeed. For the case of aggregates, the shattering efficiency may reach 10% or even more. The shattering of ice particles results in an overcounting of small particles and an undercounting of large ones. The number of fragments in the images of shattered particles may reach several hundreds. It was found that particles as small as 600 μm may shatter after impact with the probe arms. The effect of particle shattering should be taken into account during data analysis and carefully considered in future designs of airborne cloud particle size spectrometers.

Full access
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.

Full access
Alexei V. Korolev
and
Ilia P. Mazin

Abstract

A theoretical framework is developed to estimate the supersaturation in liquid, ice, and mixed-phase clouds. An equation describing supersaturation in mixed-phase clouds in general form is considered here. The solution for this equation is obtained for the case of quasi-steady approximation, that is, when particle sizes stay constant. It is shown that the supersaturation asymptotically approaches the quasi-steady supersaturation over time. This creates a basis for the estimation of the supersaturation in clouds from the quasi-steady supersaturation calculations. The quasi-steady supersaturation is a function of the vertical velocity and size distributions of liquid and ice particles, which can be obtained from in situ measurements. It is shown that, in mixed-phase clouds, the evaporating droplets maintain the water vapor pressure close to saturation over water, which enables the analytical estimation of the time of glaciation of mixed-phase clouds. The limitations of the quasi-steady approximation in clouds with different phase composition are considered here. The role of phase relaxation time, as well as the effect of the characteristic time and spatial scales of turbulent fluctuations, are also discussed.

Full access
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.

Full access