Supercooled liquid water and secondary ice production in Kelvin–Helmholtz instability as revealed by radar Doppler spectra observations

Abstract. Mixed-phase clouds are globally omnipresent and play a major role in the Earth's radiation budget and precipitation formation. The existence of liquid droplets in the presence of ice particles is microphysically unstable and depends on a delicate balance of several competing processes. Understanding mechanisms that govern ice initiation and moisture supply are important to understand the life cycle of such clouds. This study presents observations that reveal the onset of drizzle inside a ∼ 600 m deep mixed-phase layer embedded in a stratiform precipitation system. Using Doppler spectral analysis, we show how large supercooled liquid droplets are generated in Kelvin–Helmholtz (K–H) instability despite ice particles falling from upper cloud layers. The spectral width of the supercooled liquid water mode in the radar Doppler spectrum is used to identify a region of increased turbulence. The observations show that large liquid droplets, characterized by reflectivity values larger than −20 dBZ, are generated in this region. In addition to cloud droplets, Doppler spectral analysis reveals the production of columnar ice crystals in the K–H billows. The modeling study estimates that the concentration of these ice crystals is 3–8 L−1, which is at least 1 order of magnitude higher than that of primary ice-nucleating particles. Given the detail of the observations, we show that multiple populations of secondary ice particles are generated in regions where larger cloud droplets are produced and not at some constant level within the cloud. It is, therefore, hypothesized that K–H instability provides conditions favorable for enhanced droplet growth and formation of secondary ice particles.

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Supercooled liquid water and secondary ice production in Kelvin–Helmholtz instability as revealed by radar Doppler spectra observations

10.5194/acp-2021-338 ◽

2021 ◽

Author(s):

Haoran Li ◽

Alexei Korolev ◽

Dmitri Moisseev

Keyword(s):

Liquid Water ◽

Supercooled Liquid ◽

Mixed Phase ◽

Ice Crystals ◽

Radiation Budget ◽

Doppler Spectrum ◽

Droplet Growth ◽

Liquid Droplets ◽

Cloud Droplets ◽

Ice Particles

Abstract. Mixed-phase clouds are globally omnipresent and play a major role in the Earth's radiation budget and precipitation formation. The existence of liquid droplets in presence of ice particles is microphysically unstable and depends on a delicate balance of several competing processes. Understanding mechanisms that govern ice initiation and moisture supply are important to understand the life-cycle of such clouds. This study presents observations that reveal the onset of drizzle inside a ∼600 m deep mixed-phase layer embedded in a stratiform precipitation system. Using Doppler spectra analysis, we show how large supercooled liquid droplets are generated in Kelvin-Helmholtz (K-H) instability despite ice particles falling from upper cloud layers. The spectral width of supercooled liquid water mode in radar Doppler spectrum is used to identify a region of increased turbulence. The observations show that large liquid droplets, characterized by reflectivity values larger than −20 dBZ, are generated in this region. In addition to cloud droplets, Doppler spectral analysis reveals the production of the columnar ice crystals in the K-H billows. The modelling study estimates that the concentration of these ice crystals is 3 ∼ 8 L−1, which is at least one order of magnitude higher than that of primary ice nucleating particles. Given the detail of the observations, we show that multiple populations of secondary ice particles are generated in regions where larger cloud droplets are produced and not at some constant level within the cloud. It is therefore hypothesized that K-H instability provides conditions favorable for enhanced droplet growth and formation of secondary ice particles.

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Photomineralization mechanism changes the ability of dissolved organic matter to activate cloud droplets and to nucleate ice crystals

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-12397-2019 ◽

2019 ◽

Vol 19 (19) ◽

pp. 12397-12412 ◽

Cited By ~ 7

Author(s):

Nadine Borduas-Dedekind ◽

Rachele Ossola ◽

Robert O. David ◽

Lin S. Boynton ◽

Vera Weichlinger ◽

...

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Dissolved Organic Matter ◽

Organic Acids ◽

Liquid Water ◽

Supercooled Liquid ◽

Ice Nucleation ◽

Ice Crystals ◽

Cloud Droplets ◽

Photochemical Processing

Abstract. An organic aerosol particle has a lifetime of approximately 1 week in the atmosphere during which it will be exposed to sunlight. However, the effect of photochemistry on the propensity of organic matter to participate in the initial cloud-forming steps is difficult to predict. In this study, we quantify on a molecular scale the effect of photochemical exposure of naturally occurring dissolved organic matter (DOM) and of a fulvic acid standard on its cloud condensation nuclei (CCN) and ice nucleation (IN) activity. We find that photochemical processing, equivalent to 4.6 d in the atmosphere, of DOM increases its ability to form cloud droplets by up to a factor of 2.5 but decreases its ability to form ice crystals at a loss rate of −0.04 ∘CT50 h−1 of sunlight at ground level. In other words, the ice nucleation activity of photooxidized DOM can require up to 4 ∘C colder temperatures for 50 % of the droplets to activate as ice crystals under immersion freezing conditions. This temperature change could impact the ratio of ice to water droplets within a mixed-phase cloud by delaying the onset of glaciation and by increasing the supercooled liquid fraction of the cloud, thereby modifying the radiative properties and the lifetime of the cloud. Concurrently, a photomineralization mechanism was quantified by monitoring the loss of organic carbon and the simultaneous production of organic acids, such as formic, acetic, oxalic and pyruvic acids, CO and CO2. This mechanism explains and predicts the observed increase in CCN and decrease in IN efficiencies. Indeed, we show that photochemical processing can be a dominant atmospheric ageing process, impacting CCN and IN efficiencies and concentrations. Photomineralization can thus alter the aerosol–cloud radiative effects of organic matter by modifying the supercooled-liquid-water-to-ice-crystal ratio in mixed-phase clouds with implications for cloud lifetime, precipitation patterns and the hydrological cycle.Highlights. During atmospheric transport, dissolved organic matter (DOM) within aqueous aerosols undergoes photochemistry. We find that photochemical processing of DOM increases its ability to form cloud droplets but decreases its ability to form ice crystals over a simulated 4.6 d in the atmosphere. A photomineralization mechanism involving the loss of organic carbon and the production of organic acids, CO and CO2 explains the observed changes and affects the liquid-water-to-ice ratio in clouds.

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Observing ice particle growth along fall streaks in mixed-phase clouds using spectral polarimetric radar data

10.5194/acp-2017-1032 ◽

2018 ◽

Author(s):

Lukas Pfitzenmaier ◽

Christine M. H. Unal ◽

Yann Dufournet ◽

Herman J. W. Russchenberg

Keyword(s):

Particle Growth ◽

Supercooled Liquid ◽

Radar Data ◽

Detection Algorithm ◽

Mixed Phase ◽

Ice Crystals ◽

Liquid Droplets ◽

Growth Processes ◽

Retrieval Technique ◽

Mixed Phase Clouds

Abstract. The growth of ice crystals in presence of super-cooled liquid droplets represents the most important process for precipitation formation in the mid-latitudes. Such mixed-phase interaction processes remain however pretty much unknown, as capturing the complexity in cloud dynamics and microphysical variabilities turns to be a real observational challenge. Ground-based radar systems equipped with fully polarimetric and Doppler capabilities in high temporal and spatial resolutions 5 such as the S-band Transportable Atmospheric Radar (TARA) are best suited to observe mixed-phase growth processes. In this paper, measurements are taken with the TARA radar during the ACCEPT campaign (Analysis of the Composition of Clouds with Extended Polarization Techniques). Besides the common radar observables, the 3D wind field is also retrieved due to TARA unique three beam configuration. The novelty of this paper is to combine all these observations with a particle evolution detection algorithm based on a new fall streak retrieval technique in order to study ice particle growth within complex 10 precipitating mixed-phased cloud systems. In the presented cases, three different growth processes of ice crystals, plate-like crystals, and needles, are detected and related to the presence of supercooled liquid water. Moreover, TARA observed signatures are assessed with co-located measurements obtained from a cloud radar and radiosondes. This paper shows that it is possible to observe ice particle growth processes within complex systems taking advantage of adequate technology and state of the art retrieval algorithms. A significant improvement is made towards a conclusive interpretation of ice particle growth processes 15 and their contribution to rain production using fall streak rearranged radar data.

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In-situ Observation of Riming in Mixed-Phase Clouds using the PHIPS probe

10.5194/acp-2021-833 ◽

2021 ◽

Author(s):

Fritz Waitz ◽

Martin Schnaiter ◽

Thomas Leisner ◽

Emma Järvinen

Keyword(s):

Liquid Water ◽

Prediction Models ◽

Supercooled Liquid ◽

Liquid Water Content ◽

Mixed Phase ◽

Ice Crystals ◽

The Arctic ◽

In Situ Observation ◽

Cloud Particle ◽

Mixed Phase Clouds

Abstract. Mixed-phase clouds consist of both supercooled liquid water droplets and solid ice crystals. Despite having a significant impact on Earth‘s climate, mixed-phase clouds are poorly understood and not well represented in climate prediction models. One piece of the puzzle is understanding and parameterizing riming of mixed-phase cloud ice crystals, which is one of the main growth mechanisms of ice crystals via the accretion of small, supercooled droplets. Especially the extent of riming on ice crystals smaller than 500 μm is often overlooked in studies – mainly because observations are scarce. Here, we investigated riming in mixed-phase clouds during three airborne campaigns in the Arctic, the Southern Ocean and US east coast. Riming was observed from stereo-microscopic cloud particle images recorded with the Particle Habit Imaging and Polar Scattering (PHIPS) probe. We show that riming is most prevalent at temperatures around −7 °C, where, on average, 43 % of the investigated particles in a size range from 100 ≤ D ≤ 700 μm showed evidence of riming. We discuss the occurrence and properties of rimed ice particles and show correlation of the occurrence and the amount of riming with ambient meteorological parameters. We show that riming fraction increases with ice particle size (< 20 % for D ≤ 200 μm, 35–40 % for D ≥ 400 μm) and liquid water content (25 % for LWC ≤ 0.05 g m−3, up to 60 % for LWC = 0.5 g m−3). We investigate the ageing of rimed particles and the difference between "normal" and "epitaxial" riming based on a case study.

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Theoretical Analysis of Liquid–Ice Interaction in the Unsaturated Environment with Application to the Problem of Homogeneous Mixing

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0228.1 ◽

2018 ◽

Vol 75 (4) ◽

pp. 1045-1062 ◽

Cited By ~ 4

Author(s):

M. Pinsky ◽

A. Khain ◽

A. Korolev

Keyword(s):

Relaxation Time ◽

Liquid Water ◽

Mixed Phase ◽

Complete Disappearance ◽

Liquid Droplets ◽

Phase Relaxation ◽

Mixing Ratios ◽

Water Interaction ◽

Ice Particles ◽

Phase Relaxation Time

Abstract The process of ice–liquid water interaction in the unsaturated environment is explored both analytically and with the help of a numerical simulation. Ice–liquid water interaction via the condensation–evaporation mechanism is considered in relation to the problem of homogeneous mixing in an unmovable air volume. The process is separated into three stages: the homogenization stage, during which the rapid alignment of thermodynamic and microphysical parameters in the mixing volume takes place; the glaciation stage, during which the liquid droplets evaporate; and the ice stage, which leads to attaining a thermodynamic equilibrium. Depending on the initial temperature, humidity, and mixing ratios of liquid water and of ice water, the third stage may result in two outcomes: existence of ice particles under zero supersaturation with respect to ice or a complete disappearance of ice particles. Three characteristic times are associated with the microphysical stages: the phase relaxation time associated with droplets, the glaciation time determined by the Wegener–Bergeron–Findeisen process, and the phase relaxation time associated with ice. Since the duration of the second and third microphysical stages may be of the same order as the homogenization time or even longer, the homogeneous mixing scenario is more probable in mixed-phase clouds than in liquid clouds. It is shown that mixing of a mixed-phase cloud with a dry environment accelerates cloud glaciation, leading to a decrease in the glaciation time by more than 2 times. The conditions of fast ice particles’ disappearance due to sublimation are analyzed as well.

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Photomineralization mechanism changes the ability of dissolved organic matter to activate cloud droplets and to nucleate ice crystals

10.5194/acp-2019-427 ◽

2019 ◽

Author(s):

Nadine Borduas-Dedekind ◽

Rachele Ossola ◽

Robert O. David ◽

Lin S. Boynton ◽

Vera Weichlinger ◽

...

Keyword(s):

Organic Matter ◽

Dissolved Organic Matter ◽

Hydrological Cycle ◽

Supercooled Liquid ◽

Mixed Phase ◽

Ice Crystals ◽

Radiative Properties ◽

Cloud Droplets ◽

Photochemical Processing ◽

Mixed Phase Clouds

Abstract. An organic aerosol particle has a lifetime of approximately one week in the atmosphere during which it will be exposed to sunlight. Yet, the effect of photochemistry on the propensity of organic matter to participate in the initial cloud-forming steps is difficult to predict. In this study, we quantify on a molecular scale the effect of photochemical exposure of naturally occurring dissolved organic matter (DOM) and of a fulvic acid standard on its ability to form mixed-phase clouds, by acting as cloud condensation nuclei (CCN) and by acting as ice nucleating particles (INPs). We find that photochemical processing, equivalent to 4.6 days in the atmosphere, of DOM increases its ability to form cloud droplets by up to a factor of 2.5 but decreases its ability to form ice crystals at a loss rate of −0.04°CT50 h−1 of sunlight at ground level. In other words, the ice nucleation activity of photooxidized DOM can require up to 4 degrees colder temperatures for 50 % of the droplets to activate as ice crystals under immersion freezing conditions. This temperature change could impact the ratio of ice to water droplets within a mixed phase cloud by delaying the onset of glaciation and by increasing the supercooled liquid fraction of the cloud, thereby modifying the radiative properties and the lifetime of the cloud. Concurrently, a photomineralization mechanism was quantified by monitoring the loss of organic carbon and the simultaneous production of organic acids, such as formic, acetic, oxalic and pyruvic acids, CO and CO2. This mechanism explains and predicts the observed increase in CCN and decrease in INP efficiencies. Indeed, we show that photochemical processing can be a dominant atmospheric aging process, impacting CCN and INP efficiencies and concentrations. Photomineralization can thus alter the aerosol-cloud radiative effects of organic matter by modifying the supercooled liquid water-to-ice crystal ratio in mixed-phase clouds with implications for cloud lifetime, precipitation patterns and the hydrological cycle.

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Scavenging of black carbon in mixed phase clouds at the high alpine site Jungfraujoch

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-1797-2007 ◽

2007 ◽

Vol 7 (7) ◽

pp. 1797-1807 ◽

Cited By ~ 100

Author(s):

J. Cozic ◽

B. Verheggen ◽

S. Mertes ◽

P. Connolly ◽

K. Bower ◽

...

Keyword(s):

Black Carbon ◽

Cloud Condensation Nuclei ◽

Aerosol Particles ◽

Liquid Water Content ◽

Mixed Phase ◽

Ice Crystals ◽

Research Station ◽

Liquid Droplets ◽

Cloud Droplets ◽

Mixed Phase Clouds

Abstract. The scavenging of black carbon (BC) in liquid and mixed phase clouds was investigated during intensive experiments in winter 2004, summer 2004 and winter 2005 at the high alpine research station Jungfraujoch (3580 m a.s.l., Switzerland). Aerosol residuals were sampled behind two well characterized inlets; a total inlet which collected cloud particles (droplets and ice particles) as well as interstitial (unactivated) aerosol particles; an interstitial inlet which collected only interstitial aerosol particles. BC concentrations were measured behind each of these inlets along with the submicrometer aerosol number size distribution, from which a volume concentration was derived. These measurements were complemented by in-situ measurements of cloud microphysical parameters. BC was found to be scavenged into the condensed phase to the same extent as the bulk aerosol, which suggests that BC was covered with soluble material through aging processes, rendering it more hygroscopic. The scavenged fraction of BC (FScav,BC), defined as the fraction of BC that is incorporated into cloud droplets and ice crystals, decreases with increasing cloud ice mass fraction (IMF) from FScav,BC=60% in liquid phase clouds to FScav,BC~5–10% in mixed-phase clouds with IMF>0.2. This can be explained by the evaporation of liquid droplets in the presence of ice crystals (Wegener-Bergeron-Findeisen process), releasing BC containing cloud condensation nuclei back into the interstitial phase. In liquid clouds, the scavenged BC fraction is found to decrease with decreasing cloud liquid water content. The scavenged BC fraction is also found to decrease with increasing BC mass concentration since there is an increased competition for the available water vapour.

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Observing ice particle growth along fall streaks in mixed-phase clouds using spectral polarimetric radar data

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-7843-2018 ◽

2018 ◽

Vol 18 (11) ◽

pp. 7843-7862 ◽

Cited By ~ 5

Author(s):

Lukas Pfitzenmaier ◽

Christine M. H. Unal ◽

Yann Dufournet ◽

Herman W. J. Russchenberg

Keyword(s):

Particle Growth ◽

Supercooled Liquid ◽

Radar Data ◽

Detection Algorithm ◽

Mixed Phase ◽

Ice Crystals ◽

Liquid Droplets ◽

Growth Processes ◽

Retrieval Technique ◽

Mixed Phase Clouds

Abstract. The growth of ice crystals in presence of supercooled liquid droplets represents the most important process for precipitation formation in the mid-latitudes. However, such mixed-phase interaction processes remain relatively unknown, as capturing the complexity in cloud dynamics and microphysical variabilities turns to be a real observational challenge. Ground-based radar systems equipped with fully polarimetric and Doppler capabilities in high temporal and spatial resolutions such as the S-band transportable atmospheric radar (TARA) are best suited to observe mixed-phase growth processes. In this paper, measurements are taken with the TARA radar during the ACCEPT campaign (analysis of the composition of clouds with extended polarization techniques). Besides the common radar observables, the 3-D wind field is also retrieved due to TARA unique three beam configuration. The novelty of this paper is to combine all these observations with a particle evolution detection algorithm based on a new fall streak retrieval technique in order to study ice particle growth within complex precipitating mixed-phased cloud systems. In the presented cases, three different growth processes of ice crystals, plate-like crystals, and needles are detected and related to the presence of supercooled liquid water. Moreover, TARA observed signatures are assessed with co-located measurements obtained from a cloud radar and radiosondes. This paper shows that it is possible to observe ice particle growth processes within complex systems taking advantage of adequate technology and state of the art retrieval algorithms. A significant improvement is made towards a conclusive interpretation of ice particle growth processes and their contribution to rain production using fall streak rearranged radar data.

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Evaluating the ability of a microwave radiometer and wrf to detect and simulate in-cloud icing conditions

10.5194/egusphere-egu2020-14316 ◽

2020 ◽

Author(s):

Jose Luis Sanchez ◽

Pablo Melcon ◽

Guillermo Merida ◽

Andres Merino ◽

Eduardo Garcia-Ortega ◽

...

Keyword(s):

Liquid Water ◽

Weather Forecasting ◽

Freezing Point ◽

Microwave Radiometer ◽

Supercooled Liquid ◽

Liquid Water Content ◽

Indirect Detection ◽

Liquid Droplets ◽

Aircraft Icing ◽

Cloud Droplets

Icing occurs when an unheated solid structure is exposed to liquid cloud droplets at temperatures below the freezing point. Supercooled liquid water (SLW) in the atmosphere can persist in a physically metastable state until coming into contact with a solid object &#8220;In-cloud icing&#8221; occurs when super cooled liquid droplets (SLD) like clouds collide with a structure or object and freezes.Atmospheric icing prediction has gain attention in the last years. Despite the progress made in meteorology, both weather forecasting modelling and atmospheric observations through advanced experimental technologies, there are still limitations in the accurate forecast and detection of icing conditions. The GFA&#8208;ULE group has carried out some NWPs. In a previous work, we investigated the capability of the Weather Research and Forecasting model to detect regions containing supercooled cloud drops, proposing a multiphysics ensemble approach. Four microphysics and two planetary boundary layer schemes were used. Morrison and Goddard parameterizations with the YSU scheme, yielded superior results in evaluating the presence of liquid water content.Concerning the remote detection of icing conditions, some European research centres (i.e. DLR, CIRA, ONERA, INCAS) as well as University of Leon (GFA-ULE) already have nowcasting or forecasting activities for detection of clouds and icing conditions. In this work a multichannel, microwave radiometer (MMWR) was used to detect the appearance of SLW. Consequently, we present both comparison between indirect detection of SLW and the output obtained by WRF with the two combination of parametrizations selected.In our work we have taken into account:<ol><li>The comparison has been made at different levels, from the ground up to 5000 meters high</li> <li>We have taken different thresholds of the SLW: 0.05, 0.1, 0.15, 0.20, 0.25 and 0.30 g m-3 because of the flight campaigns carried out previously, which revealed that the presence of low concentrations of SLW could lead to the appearance of aircraft icing.</li> </ol>The results show a good concordance between the number of events found by the MMWR and the result of the two numerical modeling performed. Therefore, everything seems to indicate that indirect detection by MMWR can be an accurate technology to detect the appearance of SLW and that the models can be qualitatively validated.Acknowledgments: Data support came from the Atmospheric Physics Group, IMA, University of Le&#243;n, Spain, and the National Institute of &#160;Aerospace Technology (INTA). This research was carried out in the framework of the SAFEFLIGHT project, financed by MINECO (CGL2016&#8208;78702) and LE240P18 project (Junta de Castilla y Le&#243;n). We also thank R. Weigand for computer support.

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A Binned Approach to Cloud-Droplet Riming Implemented in a Bulk Microphysics Model

Journal of Applied Meteorology and Climatology ◽

10.1175/2007jamc1664.1 ◽

2008 ◽

Vol 47 (2) ◽

pp. 694-703 ◽

Cited By ~ 71

Author(s):

Stephen M. Saleeby ◽

William R. Cotton

Keyword(s):

Liquid Water ◽

Collection Efficiency ◽

Cloud Droplet ◽

Supercooled Liquid ◽

Sharp Contrast ◽

Cloud Droplets ◽

Ice Particles ◽

Realistic Representation ◽

Orographic Cloud ◽

Individual Size

Abstract This paper presents the development and application of a binned approach to cloud-droplet riming within a bulk microphysics model. This approach provides a more realistic representation of collision–coalescence that occurs between ice and cloud particles of various sizes. The binned approach allows the application of specific collection efficiencies, within the stochastic collection equation, for individual size bins of droplets and ice particles; this is in sharp contrast to the bulk approach that uses a single collection efficiency to describe the growth of a distribution of an ice species by collecting cloud droplets. Simulations of a winter orographic cloud event reveal a reduction in riming when using the binned riming approach and, subsequently, larger amounts of supercooled liquid water within the orographic cloud.

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