Shallow convective clouds are important players in Earth’s energy budget and hydrological cycle, ... more Shallow convective clouds are important players in Earth’s energy budget and hydrological cycle, and are abundant in the tropical and subtropical belts. They greatly contribute to the uncertainty in climate predictions due to their unresolved, complex processes that include coupling between the dynamics and microphysics. Analysis of cloud structure can be simplified by considering cloud motions as a combination of moist adiabatic motions like adiabatic updrafts and turbulent motions leading to deviation from adiabaticity. In this work, we study the sizes and occurrence of adiabatic regions in shallow cumulus clouds during their growth and mature stages, and use the adiabatic fraction (AF) as a continuous metric to describe cloud processes and properties from the core to the edge. To do so, we simulate isolated trade wind cumulus clouds of different sizes using the System of Atmospheric Modeling (SAM) model in high resolution (10 m) with the Hebrew University spectral bin microphysic...
The process of mixing in warm convective clouds and its effects on microphysics are crucial for a... more The process of mixing in warm convective clouds and its effects on microphysics are crucial for an accurate description of cloud fields, weather, and climate. Still, they remain open questions in the field of cloud physics. Adiabatic regions in the cloud could be considered non-mixed areas and therefore serve as an important reference to mixing. For this reason, the adiabatic fraction (AF) is an important parameter that estimates the mixing level in the cloud in a simple way. Here, we test different methods of AF calculations using high-resolution (10 m) simulations of isolated warm cumulus clouds. The calculated AFs are compared with a normalized concentration of a passive tracer, which is a measure of dilution by mixing. This comparison enables the examination of how well the AF parameter can determine mixing effects and the estimation of the accuracy of different approaches used to calculate it. Comparison of three different methods to derive AF, with the passive tracer, shows that one method is much more robust than the others. Moreover, this method's equation structure also allows for the isolation of different assumptions that are often practiced when calculating AF such as vertical profiles, cloud-base height, and the linearity of AF with height. The use of a detailed spectral bin microphysics scheme allows an accurate description of the supersaturation field and demonstrates that the accuracy of the saturation adjustment assumption depends on aerosol concentration, leading to an underestimation of AF in pristine environments.
The mechanism of drizzle formation in shallow stratocumulus clouds and the effect of turbulent mi... more The mechanism of drizzle formation in shallow stratocumulus clouds and the effect of turbulent mixing on this process are investigated. A Lagrangian-Eularian model of the cloud-topped boundary layer is used to simulate the cloud measured during flight RF07 of the DYCOMS-II field experiment. The model contains ∼ 2000 air parcels that are advected in a turbulence-like velocity field. In the model all microphysical processes are described for each Lagrangian air volume, and turbulent mixing between the parcels is also taken into account. It was found that the first large drops form in air volumes that are closest to adiabatic and characterized by high humidity, extended residence near cloud top, and maximum values of liquid water content, allowing the formation of drops as a result of efficient collisions. The first large drops form near cloud top and initiate drizzle formation in the cloud. Drizzle is developed only when turbulent mixing of parcels is included in the model. Without mixing, the cloud structure is extremely inhomogeneous and the few large drops that do form in the cloud evaporate during their sedimentation. It was found that turbulent mixing can delay the process of drizzle initiation but is essential for the further development of drizzle in the cloud.
Quarterly Journal of the Royal Meteorological Society, 2002
Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model... more Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any arti cial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base. An increase of the collision kernel in turbulent surroundings is found to be an important factor in the acceleration of large droplet and raindrop formation. Droplet spectrum formation was found to be affected by three stages of in-cloud droplets' nucleation: (a) nucleation near the cloud base, forming the primary mode of the droplet spectrum; (b) nucleation within a parcel, where supersaturation exceeds its maximum at the cloud base, this type of nucleation forming the secondary spectral mode; and (c) nucleation within the zone of intensive collisions, when a rapid decrease in drop concentration leads to an increase in supersaturation. It is shown that the secondary mode in the droplet spectrum contributes signi cantly to raindrop formation, therefore the absence of the secondary mode (the single-mode spectrum) can reduce or even inhibit formation of raindrops. The contributions of diffusion and collision growth to drop spectrum formation are compared. Effective collisions are found to start when the effective radius attains about 15 ¹m. The level where the effective radius attains 15 ¹m can be considered as the level of the rst radar echo. This height is shown to crucially depend on cloud dynamics (in particular, on the vertical velocity at the cloud base) and on the concentration of aerosol particles.
During the last decade numerous studies considered collisions of inertial particles in turbulent ... more During the last decade numerous studies considered collisions of inertial particles in turbulent flows. A magnitude of the turbulence-induced collision rate enhancement factor reported in these studies ranges from a few percent to several hundred. The authors of the majority of the studies apply their results to explanation of rain formation in atmospheric clouds. At the same time many of these investigations were performed under the conditions quite different from those encountered in real clouds. For instance, in most analytical and direct numerical simulations (DNS) the effect of gravity-induced differential drop sedimentation was neglected. Using the collision enhancement factors obtained in these studies for cloud modeling may lead to unrealistic cloud evolution and impair research in cloud physics. In this study we present an analysis of the applicability of the results obtained in different recent studies (mainly DNS simulations) to actual clouds. We discuss the progress reached in the topic as well as unsolved problems.
Retrieval of Cloud Liquid Water Content Profiles with Radar and Lidar: Application to Multi-annual Data Sets and Comparison with Microphysical Cloud Simulations
The utilization of polarimetric weather radars for optimizing cloud models is a next frontier of ... more The utilization of polarimetric weather radars for optimizing cloud models is a next frontier of research. It is widely understood that inadequacies in microphysical parameterization schemes in numerical weather prediction (NWP) models is a primary cause of forecast uncertainties. Due to its ability to distinguish between hydrometeors with different microphysical habits and to identify “polarimetric fingerprints” of various microphysical processes, polarimetric radar emerges as a primary source of needed information. There are two approaches to leverage this information for NWP models: (1) radar microphysical and thermodynamic retrievals and (2) forward radar operators for converting the model outputs into the fields of polarimetric radar variables. In this paper, we will provide an overview of both. Polarimetric measurements can be combined with cloud models of varying complexity, including ones with bulk and spectral bin microphysics, as well as simplified Lagrangian models focuse...
Evolution of droplet size distribution (DSD) due to mixing between cloudy and dry volumes is inve... more Evolution of droplet size distribution (DSD) due to mixing between cloudy and dry volumes is investigated for different values of the cloud fraction and different initial DSD shapes. The analysis is performed using a diffusion-evaporation model which describes time-dependent processes of turbulent diffusion and droplet evaporation within a mixing volume. Time evolution of the DSD characteristics such as droplet concentration, liquid water content, mean volume and the effective radii is analyzed. The mixing diagrams are plotted for the final mixing stages. It is shown that the difference between the mixing diagrams for homogeneous and inhomogeneous mixing is insignificant and decreases with an increase in the DSD width. The dependencies of normalized cube of the effective radius on the cloud fraction were compared with those on normalized droplet concentration and found to be quite different. In case the normalized droplet concentration is used, mixing diagrams do not show any signif...
What is Adiabatic Fraction in Cumulus Clouds: High-Resolution Simulations with Passive Tracer
. The process of mixing in warm convective clouds and its effects on microphysics, is crucial for... more . The process of mixing in warm convective clouds and its effects on microphysics, is crucial for an accurate description of cloud fields, weather, and climate. Still, it remains an open question in the field of cloud physics. Adiabatic regions in the cloud could be considered as non-mixed areas and therefore serve as an important reference to mixing. Therefore, the adiabatic fraction (AF) is an important parameter that estimates the mixing level in the cloud in a simple way. Here, we test different methods of AF calculations using high-resolution (10 m) simulations of isolated warm Cumulus clouds. The calculated AFs are compared with a normalized concentration of a passive tracer, which is a measure of dilution by mixing. This comparison enables us to examine how well the AF parameter can determine mixing effects, and to estimate the accuracy of different approaches used to calculate it. The sensitivity of the calculated AF to the choice of different equations, vertical profiles, cloud base height, and its linearity with height are all tested. Moreover, the use of a detailed spectral bin microphysics scheme demonstrates that the accuracy of the saturation adjustment assumption depends on aerosol concentration, and leads to an underestimation of AF in pristine environments.
Theoretical Analysis of the Entrainment–Mixing Process at Cloud Boundaries. Part I: Droplet Size Distributions and Humidity within the Interface Zone
Journal of the Atmospheric Sciences
The problem of a complex entrainment–mixing process is analyzed by solving a diffusion–evaporatio... more The problem of a complex entrainment–mixing process is analyzed by solving a diffusion–evaporation equation for an open region in the vicinity of the cloud–dry air interface. Upon normalization the problem is reduced to a one-parametric one, the governing parameter being the potential evaporation parameter R proportional to the ratio of saturation deficit in the dry air to the available liquid water content in the cloud air. As distinct from previous multiple studies analyzing mixing within closed adiabatic volumes, we consider a principally nonstationary problem that never leads to a homogeneous equilibrium state. It is shown that at R < −1 the cloud edge shifts toward the cloud; that is, the cloud dissipates due to mixing with dry air, and the cloud volume decreases. If R > −1, the cloud edge shifts outside; that is, the mixing leads to an increase in the cloud volume. The time evolution of droplet size distribution and its moments, as well as the relative humidity within th...
This is the author manuscript accepted for publication and has undergone full peer review but has... more This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as
Glaciation in mixed-phase adiabatic cloudy parcels is investigated analytically using two new equ... more Glaciation in mixed-phase adiabatic cloudy parcels is investigated analytically using two new equations: the equation for coexistence of liquid water and ice and the mass balance equation. The analysis of glaciation time is performed for a vertically moving adiabatic mixed-phase cloud parcel. The effects of vertical velocity, liquid water content, and concentrations of ice particles, liquid droplets, temperature, and other parameters on the glaciation process are discussed. It is shown analytically that, for a certain envelope of vertical velocities, the glaciation time depends only on the vertical displacement of the parcel and does not depend on the trajectory along which the cloud parcel travels toward the glaciation point. Analytical dependencies of the glaciation time and of the altitude of glaciation on vertical velocity are presented. The results demonstrate a good agreement with those obtained using the corresponding parcel model. The limitations of the newly proposed approa...
The present study considers final stages of in-cloud mixing in the framework of classical concept... more The present study considers final stages of in-cloud mixing in the framework of classical concept of homogeneous and extreme inhomogeneous mixing. Simple analytical relationships between basic microphysical parameters were obtained for homogeneous and extreme inhomogeneous mixing based on the adiabatic consideration. It was demonstrated that during homogeneous mixing the functional relationships between the moments of the droplets size distribution hold only during the primary stage of mixing. Subsequent random mixing between already mixed parcels and undiluted cloud parcels breaks these relationships. However, during extreme inhomogeneous mixing the functional relationships between the microphysical parameters hold both for primary and subsequent mixing. The obtained relationships can be used to identify the type of mixing from in situ observations. The effectiveness of the developed method was demonstrated using in situ data collected in convective clouds. It was found that for th...
Corrigendum to “The orientation dynamics of small prolate and oblate spheroids in linear shear flows” [International Journal of Multiphase Flow 83 (2016) 103–114]
Continuous symmetry maps and shape classification. The case of six-coordinated metal compoundsElectronic supplementary information (ESI) available: tables of CSD refcodes, structural parameters and symmetry measures for the studied compounds. See http://www.rsc.org/suppdata/nj/b2/b202096n/
New J Chem, 2002
ABSTRACT A continuous symmetry study of the structures of transition metal six-vertex polyhedra i... more ABSTRACT A continuous symmetry study of the structures of transition metal six-vertex polyhedra is presented, considering both molecular models and experimental structural data. The concept of symmetry map is introduced, consisting of a scatterplot of the symmetry measures relative to two alternative ideal polyhedra. In the case of hexacoordinated complexes, we take as reference shapes the octahedron and the equilateral trigonal prism and study different distortions from these two extremes, including the Bailar twist that interconverts one into another. Such a symmetry map allows us to establish trends in the structural chemistry of the coordination sphere of hexacoordinated transition metal atoms, including the effects of several factors, such as the electron configuration or the presence of bidentate, terdentate or encapsulating ligands. Also introduced is the concept of a symmetry constant, which identifies a distortive route that preserves the minimum distance to two reference symmetries. A wide variety of model distortions are analyzed, and the models are tested against experimental structural data of a wide variety of six-coordinated complexes.
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