Search Results

You are looking at 1 - 10 of 11 items for

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

Full access
Sergey Y. Matrosov
,
Alexei V. Korolev
, and
Andrew J. Heymsfield

Abstract

A remote sensing method is proposed for the retrievals of vertical profiles of ice cloud microphysical parameters from ground-based measurements of radar reflectivity and Doppler velocity with a vertically pointed cloud radar. This method relates time-averaged Doppler velocities (which are used as a proxy for the reflectivity-weighted particle fall velocities) to particle characteristic sizes such as median or mean. With estimated profiles of particle characteristic size, profiles of cloud ice water content (IWC) are then calculated using reflectivity measurements. The method accounts for the intrinsic correlation between particle sizes and parameters of the fall velocity–size relations. It also accounts for changes of particle bulk density with size. The range of applicability of this method encompasses ice-phase clouds and also mixed-phase clouds that contain liquid drops, which are small compared to ice particles, so the radar signals are dominated by these larger particles. It is, however, limited to the observational situations without strong up- and downdrafts, so the residual of mean vertical air motions is small enough compared to the reflectivity-weighted cloud particle fall velocities. The Doppler-velocity reflectivity method was applied to the data obtained with an 8.6-mm wavelength radar when observing Arctic clouds. Typical retrieval uncertainties are about 35%–40% for particle characteristic size and 60%–70% for IWC, though in some cases IWC uncertainties can be as high as factor of 2 (i.e., −50%, +100%). Comparisons with in situ data for one observational case yielded 25% and 55% differences in retrieved and in situ estimates of characteristic size and IWC, respectively. The results of the microphysical retrievals obtained from the remote sensing method developed here were compared with data obtained from the multisensor technique that utilizes combined radar–IR radiometer measurements. For pure ice-phase layers unobstructed by liquid clouds (i.e., conditions where the multisensor approach is applicable), the relative standard deviations between the results of both remote sensing approaches were about 27% for mean particle size and 38% for IWC, with relative biases of only 5% and 20%, respectively.

Full access
Stewart G. Cober
,
George A. Isaac
, and
Alexei V. Korolev

Abstract

In situ measurements of microphysics conditions, obtained during 38 research flights into winter storms, have been used to characterize the performance of a Rosemount Icing Detector (RID). Characteristics of the RID were determined under a wide range of cloud environments, which included icing conditions within mixed phase, freezing rain, and freezing drizzle environments. Cloud conditions observed included temperatures between 0° and −29°C and liquid water contents (LWCs) up to 0.7 g m−3. The detection threshold for LWC was found to be 0.007 ± 0.010 g m−3 for the RID operated at an air speed of 97 ± 10 m s−1, which agrees well with theoretical predictions. A signal level of 0 ± 2 mV s−1 accounted for 99.6% of the measurements in clear air and 98.5% of the measurements in glaciated clouds, when the data were averaged over 30-s intervals. No significant response to glaciated clouds was found during any of the research flights, implying that the instrument can be used to segregate glaciated and mixed phase clouds. There was no change in the RID response between liquid and mixed phase conditions, suggesting that ice crystals neither eroded ice accumulation nor accreted to the RID surface under the range of conditions experienced. During sustained icing conditions, a linear relationship between the RID signal and LWC was observed after the RID signal exceeded 400 mV above the clear-air signal level. The LWC derived from the RID was found to agree with LWC measurements from Nevzorov probes within ±50% for 92% of the data. The relationship between the RID signal and LWC was unchanged for freezing precipitation environments with drop median volume diameters >100 μm. The Ludlam limit was estimated for low LWC values and was found to agree well with theoretical calculations. The analysis provides considerable insight into the strengths and weaknesses of the instrument for operations in natural icing conditions.

Full access
Sergey Y. Matrosov
,
Alexei V. Korolev
, and
Andrew J. Heymsfield
Full access
Peter Brechner
,
Greg M. McFarquhar
,
Alfons Schwarzenboeck
, and
Alexei V. Korolev

Abstract

Total ice water content (IWC) derived from an isokinetic evaporator probe and ice crystal particle size distributions (PSDs) measured by a two-dimensional stereo probe and precipitation imaging probe installed on an aircraft during the 2014 European High Altitude Ice Crystals–North American High IWC field campaign (HAIC/HIWC) were used to characterize regions of high IWC consisting mainly of small ice crystals (HIWC_S) with IWC ≥ 1.0 g m−3 and median mass diameter (MMD) < 0.5 mm. A novel fitting routine developed to automatically determine whether a unimodal, bimodal, or trimodal gamma distribution best fits a PSD was used to compare characteristics of HIWC_S and other PSDs (e.g., multimodality, gamma fit parameters) for HIWC_S simulations. The variation of these characteristics and bulk properties (MMD, IWC) was regressed with temperature, IWC, and vertical velocity. HIWC_S regions were most pronounced in updraft cores. The three modes of the PSD reveal different dominant processes contributing to ice growth: nucleation for maximum dimension D < 0.15 mm, diffusion for 0.15 < D < 1.0 mm, and aggregation for D > 1.0 mm. The frequency of trimodal distributions increased with temperature. The volumes of equally plausible parameters derived in the phase space of gamma fit parameters increased with temperature for unimodal distributions and, for temperatures less than −27°C, for multimodal distributions. Bimodal distributions with 0.4 mm in the larger mode were most common in updraft cores and HIWC_S regions; bimodal distributions with 0.4 mm in the smaller mode were least common in convective cores.

Restricted access
Stewart G. Cober
,
George A. Isaac
,
Alexei V. Korolev
, and
J. Walter Strapp

Abstract

In situ microphysics measurements made during the First and Third Canadian Freezing Drizzle Experiments (CFDE I and III, respectively) have been used to assess the relative responses to ice and liquid hydrometeors for several common instruments. These included the Rosemount icing detector, 2D-C monoscale and 2D-C grayscale probes, forward-scattering spectrometer probes (FSSP) on three measurement ranges, Nevzorov liquid water content (LWC) and total water content probes, and King LWC probes. The Nevzorov LWC and King LWC probes responded to between 5% and 30% of the ice water content, with an average response of approximately 20%. The average FSSP measurements of droplet spectra were dominated by ice particles for sizes greater than 35 μm, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 L−1. In contrast, the FSSP measurements of the droplet spectra less than 30 μm appeared free of ice-crystal contamination, independent of the ice-crystal concentrations observed. Glaciated cloud conditions always had FSSP-measured median volume diameters greater than 30 μm and particle concentrations less than 15 cm−3, whereas similar measurements in entirely liquid-phase clouds were observed less than 4% of the time. Images of drops greater than or equal to 125 μm in diameter, which were collected in warm clouds greater than 0°C, were used to calibrate geometric criteria, which were, in turn, used to segregate 2D images into circular and noncircular categories. It is shown that, on average, between 5% and 40% of ice crystals greater than or equal to 125 μm in diameter will be classified as circular, depending on the particle size, with the percentage decreasing with increasing particle size. In liquid-phase clouds, between 85% and 95% of the 2D images will be correctly classified as circular for all particle sizes. At temperatures less than −4°C, a Rosemount icing-detector threshold of 2 mV s−1, corresponding to a maximum LWC of 0.002 g m−3, was used to help to identify glaciated and nonglaciated conditions. A methodology for segregating liquid, mixed, and glaciated cloud regions, based on instrument responses to liquid and ice hydrometeors, was developed and applied to the CFDE dataset. The results were used to determine the relative frequency of liquid, mixed, and glaciated cloud conditions for the data collected during the two field projects. Approximately 40% of the in-cloud observations at temperatures less than 0°C were assessed as liquid phase. The fractions of mixed-phase and glaciated-phase conditions were 26% and 34% for CFDE I and 46% and 14% for CFDE III, respectively. Because the ice-crystal responses for each instrument depend on the aircraft sampling speed and the ice-crystal sizes and concentrations, the results may be limited to conditions similar to those in clouds in midlatitude winter storms. Regardless, the results may have application to several fields, including development of parameterizations for numerical modeling, precipitation formation, remote sensing, ice multiplication, radiative transfer, and aircraft icing investigations. Important implications for aircraft icing investigations are discussed.

Full access
Alexei V. Korolev
,
Matthew P. Bailey
,
John Hallett
, and
George A. Isaac

Abstract

The water vapor deposition growth of frozen drops with diameter greater than 100 μm has been studied in a thermal diffusion chamber. For varying periods of time, it was found that frozen drops experience spherical growth. The characteristic time of spherical growth depends on supersaturation, temperature, and drop size, and it varies from minutes to tens of minutes. The average rate of frozen drop growth agrees well with that predicted by the Maxwellian growth equation for ice spheres. Observations in natural clouds conducted with a cloud particle imager probe has yielded evidence that frozen drops may retain spheroidal shapes for at least 15–20 min under conditions close to saturation over water. These observations are in agreement with the laboratory experiments. The observation of frozen drops in natural clouds may be correlated to freezing drizzle generated by overlying cloud layers that may lead to hazardous in-flight icing.

Full access
David E. Kingsmill
,
Sandra E. Yuter
,
Andrew J. Heymsfield
,
Peter V. Hobbs
,
Alexei V. Korolev
,
Stith Jeffrey L
,
Aaron Bansemer
,
Julie A. Haggerty
, and
Arthur L. Rangno

Abstract

A customized product for analysis of microphysics data collected from aircraft during field campaigns in support of the Tropical Rainfall Measuring Mission (TRMM) program is described. These “common microphysics products” (CMPs) are designed to aid in evaluation of TRMM spaceborne precipitation retrieval algorithms. Information needed for this purpose (e.g., particle size spectra and habit, liquid and ice water content) was derived by using a common processing strategy on the wide variety of microphysical instruments and raw native data formats employed in the field campaigns. The CMPs are organized into an American Standard Code for Information Interchange (ASCII) structure to allow easy access to the data for those less familiar with microphysical data processing and without the tools to accomplish it. Detailed examples of the CMP show its potential and some of its limitations. This approach may be a first step toward developing a generalized microphysics format and an associated community-oriented, nonproprietary software package for microphysics data processing—initiatives that would likely broaden community access to, and use of, microphysics datasets.

Full access