Дисертації з теми "Mixed-Phase cloud"
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1
Fallas, German Vidaurre. "Characterization of mixed-phase clouds." abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3275833.
2
Farrington, Robert. "Testing mixed phase cloud parametrizations through confronting models with in-situ observations." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/testing-mixed-phase-cloud-parametrizations-through-confronting-models-with-insitu-observations(e2b7e31b-fa4a-4501-9f30-2ca2452c58fa).html.
Анотація:
Accurate representations of clouds are required in large-scale weather and climate models to make detailed and precise predictions of the Earth's weather and climate. Representations of clouds within these models are limited by the present understanding of the role of aerosols in the microphysical processes responsible for cloud formation and development. As part of a NERC funded CASE studentship with the Met Office, this thesis aims to test new aerosol-dependent mixed-phase cloud parametrizations by obtaining extensive cloud microphysical measurements in-situ and comparing and contrasting them with model simulations. Cloud particle concentrations were measured during the Ice NUcleation Process Investigation And Quantification (INUPIAQ) field campaign at Jungfraujoch in Switzerland. A new probe was used to separate droplet and small ice concentrations by using depolarisation ratio and size thresholds. Whilst the new small ice crystal and droplet number concentrations compared favourably with other instruments, the size and depolarisation ratio thresholds were found to be subjective, and suggested to vary from cloud to cloud. An upwind site was chosen to measure out-of-cloud aerosol particle concentrations during INUPIAQ. During periods where the site was out-of-cloud and upwind of Jungfraujoch, several large-scale model simulations were run using the aerosol concentrations in an aerosol-dependent ice nucleation parametrization. The inclusion of the parametrization failed to increase the simulated ice crystal number concentrations, which were several orders of magnitude below those observed in-situ at Jungfraujoch. Several possible explanations for the high observed ice crystal number concentrations at Jungfraujoch are tested using further model simulations. Further primary ice nucleation was ruled out, as the inclusion of additional ice nucleating particles in the model simulations suppressed the liquid water content, preventing the simulation of the mixed-phase clouds observed during INUPIAQ. The addition of ice crystals produced via the Hallett-Mossop process upwind of Jungfraujoch into the model only infrequently provided enough ice crystals to match the observed concentrations. The inclusion of a simple surface flux of hoar crystals into the model simulations was found to produce ice crystal number concentrations of a similar magnitude to those observed at Jungfraujoch, without depleting the simulated liquid water content. By confronting models with in-situ observations of cloud microphysical process, this thesis highlights interactions between surface ice crystals and mixed-phase clouds, and their potential impact on large-scale models.
3
Atkinson, James David. "Freezing of droplets under mixed-phase cloud conditions." Thesis, University of Leeds, 2013. http://etheses.whiterose.ac.uk/5858/.
Анотація:
Mixed-phase clouds contain both liquid and ice particles. They have important roles in weather and climate and such clouds are thought to be responsible for a large proportion of precipitation. Their lifetime and precipitation rates are sensitive to the concentration of ice. This project focuses upon the formation of ice within clouds containing liquid droplets colder than 273 K. A new bench-top instrument has been developed to study ice nucleation in liquid droplets. Pure water droplets of sizes relevant to clouds in the lower atmosphere do not freeze homogeneously until temperatures below ~237 K are reached. However, literature measurements of nucleation rates are scattered over two kelvin and there is uncertainty over the actual mechanism of ice formation in small droplets. The freezing of droplets with diameters equivalent to ~4 – 17 μm has been observed. It was found that ice nucleation rates in the smallest droplets of this size range were consistent with nucleation due to the droplet surface, but that surface nucleation does not occur at fast enough rates to be significant in the majority of tropospheric clouds. Water droplets can be frozen at higher temperatures than relevant for homogeneous freezing due to the presence of a class of aerosol particles called ice nuclei. Field observations of ice crystal residues have shown that mineral dust particles are an important group of ice nuclei, and the ice nucleating ability of seven of the most common minerals found in atmospheric dust has been described. In comparison to the other minerals, it was found that the mineral K-feldspar is much more efficient at nucleating ice. To relate this result to the atmosphere, a global chemical and aerosol transport modelling study was performed. This study concluded that dust containing feldspar emitted from desert regions reaches all locations around the globe. At temperatures below ~255 K, the modelled concentration of feldspar is sufficient to explain field observations of ice nuclei concentrations.
4
Kripchak, Kristopher J. "Cloud phase and the surface energy balance of the arctic an investigation of mixed-phase clouds." Thesis, Monterey, Calif. : Naval Postgraduate School, 2008. http://bosun.nps.edu/uhtbin/hyperion-image.exe/08Mar%5FKripchak.pdf.
Анотація:
Thesis (M.S. in Meterology)--Naval Postgraduate School, March 2008.Thesis Advisor(s): Guest, Peter. "March 2008." Description based on title screen as viewed on May 1, 2008. Includes bibliographical references (p. 59-61). Also available in print.
5
Williams, Robyn D. "Studies of Mixed-Phase Cloud Microphysics Using An In-Situ Unmanned Aerial Vehicle (UAV) Platform." Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7252.
Анотація:
Cirrus clouds cover between 20% - 50% of the globe and are an essential
component in the climate. The improved understanding of ice cloud
microphysical properties is contingent on acquiring and analyzing in-situ and
remote sensing data from cirrus clouds. In ??u observations of microphysical
properties of ice and mixed-phase clouds using the mini-Video Ice Particle Sizer
(mini-VIPS) aboard robotic unmanned aerial vehicles (UAVs) provide a promising
and powerful platform for obtaining valuable data in a cost-effective, safe, and
long-term manner.
The purpose of this study is to better understand cirrus microphysical
properties by analyzing the effectiveness of the mini-VIPS/UAV in-situ platform.
The specific goals include:
(1) To validate the mini-VIPS performance by comparing the mini-VIPS data
retrieved during an Artic UAV mission with data retrieved from the millimeterwavelength
cloud radar (MMCR) at the Barrow ARM/CART site.
(2) To analyze mini-VIPS data to survey the properties of high latitude mixedphase
clouds
The intercomparison between in-situ and remote sensing measurements
was carried out by comparing reflectivity values calculated from in-situ
measurements with observations from the MMCR facility. Good agreement
between observations and measurements is obtained during the time frame
where the sampled volume was saturated with respect to ice. We also have
1 2
shown that the degree of closure between calculated and observed reflectivity
strongly correlates with the assumption of ice crystal geometry observed in the
mini-VIPS images. The good correlation increases the confidence in mini-VIPS
and MMCR measurements. Finally, the size distribution and ice crystal geometry
obtained from the data analysis is consistent with published literature for similar
conditions of temperature and ice supersaturation.
6
Costa, Anja [Verfasser]. "Mixed-phase and ice cloud observations with NIXE-CAPS / Anja Costa." Wuppertal : Universitätsbibliothek Wuppertal, 2018. http://d-nb.info/1156625181/34.
7
Talik, Anja [Verfasser]. "Mixed-phase and ice cloud observations with NIXE-CAPS / Anja Costa." Wuppertal : Universitätsbibliothek Wuppertal, 2018. http://nbn-resolving.de/urn:nbn:de:hbz:468-20180326-082805-8.
8
Young, Gillian. "Understanding the nucleation of ice particles in polar clouds." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/understanding-the-nucleation-of-ice-particles-in-polar-clouds(4f80f81b-ed06-480a-944b-6e3594ba8471).html.
Анотація:
Arctic clouds are poorly represented in numerical models due to the complex, small-scale interactions which occur within them. Modelled cloud fractions are often significantly less than observed in this region; therefore, the radiative budget is not accurately simulated and forecasts of the melting cryosphere are fraught with uncertainty. Our ability to accurately model Arctic clouds can be improved through observational studies. Recent in situ airborne measurements from the springtime Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign are presented in this thesis to improve our understanding of the cloud microphysical interactions unique to this region. Aerosol-cloud interactions - where aerosol particles act as ice nucleating particles (INPs) or cloud condensation nuclei (CCN) - are integral to the understanding of clouds on a global scale. In the Arctic, uncertainties caused by our poor understanding of these interactions are enhanced by strong feedbacks between clouds, the boundary layer, and the sea ice. In the Arctic spring, aerosol-cloud interactions are affected by the Arctic haze, where a stable boundary layer allows aerosol particles to remain in the atmosphere for long periods of time. This leads to a heightened state of mixing in the aerosol population, which affects the ability of particles to act as INPs or CCN. Aerosol particle compositional data are presented to indicate which particles are present during the ACCACIA campaign, and infer how they may participate in aerosol-cloud interactions. Mineral dusts (known INPs) are identified in all flights considered, and the dominating particle classes in each case vary with changing air mass history. Mixed particles, and an enhanced aerosol loading, are identified in the final case. Evidence is presented which suggests these characteristics may be attributed to biomass burning activities in Siberia and Scandinavia. Additionally, in situ airborne observations are presented to investigate the relationship between the Arctic atmosphere and the mixed-phase clouds - containing both liquid cloud droplets and ice crystals - common to this region. Cloud microphysical structure responds strongly to changing surface conditions, as strong heat and moisture fluxes from the comparatively-warm ocean promote more turbulent motion in the boundary layer than the minimal heat fluxes from the frozen sea ice. Observations over the transition from sea ice to ocean show that the cloud liquid water content increases four-fold, whilst ice crystal number concentrations, N_ice, remain consistent at ~0.5/L. Following from this study, large eddy simulations are used to illustrate the sensitivity of cloud structure, evolution, and lifetime to N_ice. To accurately model mixed-phase conditions over sea ice, marginal ice, and ocean, ice nucleation must occur under water-saturated conditions. Ocean-based clouds are found to be particularly sensitive to N_ice, as small decreases in N_ice allow glaciating clouds to be sustained, with mixed-phase conditions, for longer. Modelled N_ice also influences precipitation development over the ocean, with either snow or rain depleting the liquid phase of the simulated cloud.
9
Myagkov, Alexander. "Shape-temperature relationship of ice crystals in mixed-phase clouds based on observations with polarimetric cloud radar." Doctoral thesis, Universitätsbibliothek Leipzig, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-216598.
Анотація:
This thesis is devoted to the experimental quantitative characterization of the shape and orientation distribution of ice particles in clouds. The characterization is based on measured and modeled elevation dependencies of the polarimetric parameters differential reflectivity and correlation coefficient. The polarimetric data is obtained using a newly developed 35-GHz cloud radar MIRA-35 with hybrid polarimetric configuration and scanning capabilities. The full procedure chain of the technical implementation and the realization of the setup of the hybrid-mode cloud radar for the shape determination are presented. This includes the description of phase adjustments in the transmitting paths, the introduction of the general data processing scheme, correction of the data for the differences of amplifications and electrical path lengths in the transmitting and receiving channels, the rotation of the polarization basis by 45°, the correction of antenna effects on polarimetric measurements, the determination of spectral polarimetric variables, and the formulation of a scheme to increase the signal-to-noise ratio. Modeling of the polarimetric variables is based on existing backscattering models assuming the spheroidal representation of cloud scatterers. The parameters retrieved from the model are polarizability ratio and degree of orientation, which can be assigned to certain particle orientations and shapes.
In the thesis the first quantitative estimations of ice particle shape at the top of liquid-topped clouds are presented. Analyzed ice particles were formed in the presence of supercooled water and in the temperature range from -20 °C to -3 °C. The estimation is based on polarizability ratios of ice particles measured by the MIRA-35 with hybrid polarimetric configuration, manufactured by METEK GmbH. For the study, 22 cases observed during the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign were used. Polarizability ratios retrieved for cloud layers with cloud-top temperatures of about -5, -8, -15, and -20 °C were 1.6, 0.9, 0.6, and 0.9, respectively. Such values correspond to prolate, quasi-isotropic, oblate, and quasi-isotropic particles, respectively. Data from a free-fall chamber were used for the comparison. A good agreement of detected shapes with well-known shape{temperature dependencies observed in laboratories was found.
10
Myagkov, Alexander [Verfasser], Andreas [Akademischer Betreuer] Macke, and Herman [Gutachter] Russchenberg. "Shape-temperature relationship of ice crystals in mixed-phase clouds based on observations with polarimetric cloud radar : Shape-temperature relationship of ice crystals in mixed-phase cloudsbased on observations with polarimetric cloud radar / Alexander Myagkov ; Gutachter: Herman Russchenberg ; Betreuer: Andreas Macke." Leipzig : Universitätsbibliothek Leipzig, 2017. http://d-nb.info/1240696310/34.
11
Aubry, Clémantyne. "Multiplatform radar-lidar synergistic retrieval for liquid and mixed-phase clouds." Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASJ008.
Анотація:
Les nuages jouent un rôle important dans le cycle de l'eau et le bilan radiatif de la Terre, et tendent à légèrement refroidir le climat. Cependant, de nombreuses incertitudes demeurent concernant leurs rétroactions et leur évolution dans le contexte du réchauffement climatique. Les nuages de phase mixte représentent notamment une part significative de l'effet radiatif des nuages. Ils sont constitués d'un mélange de cristaux de glace, de gouttelettes d'eau surfondues et de vapeur d'eau. Cette coexistence implique des processus complexes et la fraction de liquide et de glace affecte de manière significative leurs propriétés radiatives. Cette complexité les rend difficiles à représenter dans les modèles numériques, introduisant des biais significatifs. Il est donc crucial de mieux comprendre les processus microphysiques de ces nuages pour réduire les incertitudes des prévisions climatiques et météorologiques.Pour observer les nuages, il existe plusieurs types d'instruments, tels que les sondes in situ (directement au contact des hydrométéores) et les instruments de télédétection (observations distantes). Les radars et les lidars nous permettent d'obtenir des informations résolues en distance et peuvent être embarqués à bord d'avions ou de satellites, offrant ainsi couvertures régionale et globale. Les radars nuages travaillent à des fréquences (35 et 95GHz) auxquelles la réflectivité est sensible à la taille des particules, impliquant une réflectivité plus élevée pour les grosses particules nuageuses (les cristaux de glace) que pour les petites particules (les gouttelettes d'eau). Les lidars, quant à eux, fonctionnent habituellement entre 355 et 1064 nm et sont globalement plus sensibles à la concentration des particules. Ainsi, la rétrodiffusion lidar est plus élevée pour les particules très concentrées, telles que les gouttelettes d'eau. Leur synergie permet de tirer avantage des forces et des faiblesses de chacun pour restituer les propriétés des nuages. Cependant, ces propriétés ne sont pas directement accessibles à partir des mesures et des algorithmes de restitution sont donc utilisés pour relier les mesures aux propriétés microphysiques.Cette thèse propose une nouvelle méthode synergique radar-lidar dédiée à la restitution des propriétés des nuages d'eau surfondus, de glace et de phase mixte. Sur la base d'une méthode existante mais dédiée uniquement aux nuages de glace, une nouvelle approche permettant d'inclure à la fois l'eau surfondue et les situations de phase mixte a été développée. La première étape a été d'adapter et d'améliorer la classification servant à identifier la nature des particules observées. Ensuite, de nombreuses adaptations ont été apportées à l'algorithme afin de restituer séparément les propriétés des cristaux de glace et de l'eau surfondue. Cette approche est basée sur les sensibilités différentes du radar et du lidar vis-à-vis des deux types d'hydrométéores : les cristaux de glace dominent le signal radar tandis que l'eau surfondue domine le signal lidar.Afin d'évaluer cette nouvelle méthode, les restitutions sont comparées à des mesures in situ, provenant d'observations colocalisées et de la littérature. La première étude compare les restitutions obtenues à partir des données satellites CloudSat-CALIPSO avec des mesures in situ aéroportées colocalisées. Cette étude montre que les restitutions radar-lidar suivent les mêmes tendances que les mesures in situ et fournissent des résultats prometteurs avec un pourcentage d'erreur moyen de 49 % pour le contenu en eau liquide et 75 % pour le contenu en glace et ce malgré des échelles de mesures différentes et une colocalisation imparfaite. La méthode développée est également appliquée aux plateformes aéroportées Française et Allemande RALI et HALO. Les premiers résultats sont prometteurs et les données in situ colocalisées obtenues lors de campagnes récentes pourront être utilisées pour évaluer davantage l'algorithme et améliorer son paramétrageClouds play an important role in the Earth's water cycle and radiation balance, and tend to cool the climate slightly. However, there are still many uncertainties about their feedbacks and their evolution in the context of global warming. In particular, mixed-phase clouds account for a significant proportion of the cloud radiative effect. They are composed of a mixture of ice crystals, supercooled water droplets and water vapor. This coexistence involves complex processes and the fraction of liquid and ice significantly affects their radiative properties. This complexity makes them difficult to represent in numerical models, which introduces significant biases. For this reason, it is crucial to better understand the microphysical processes of these clouds to reduce the uncertainties in climate and weather forecasts.To observe clouds, several instrument types exist, such as in situ probes (in direct contact with the hydrometeors) and remote sensing instruments (remote observations). Radar and lidar allow us to obtain distance-resolved information. They can be deployed onboard aircraft or satellites, providing regional and global coverage. Cloud radars work at frequencies (35 and 95 GHz) at which the reflectivity is sensitive to particle size, implying higher reflectivity for large cloud particles (ice crystals) than for small particles (water droplets). Lidars, on the other hand, usually operate between 355 and 1064 nm and are generally more sensitive to particle concentration. As a result, lidar backscatter is higher for highly concentrated particles, such as water droplets. Their synergy allows us to take advantage of the strengths and weaknesses of each instrument to retrieve cloud properties. However, these properties are not directly accessible from measurements and retrieval algorithms are therefore used to relate measurements to microphysical properties.This thesis proposes a new radar-lidar synergistic method dedicated to retrieve supercooled water, ice and mixed-phase cloud properties. Based on an existing method dedicated solely to ice clouds, a new approach has been developed to include both supercooled water and mixed-phase situations. The first step was to adapt and improve the classification used to identify the nature of the observed particles. Next, numerous adaptations have been applied to the algorithm to retrieve separately ice crystals and supercooled water properties. This approach is based on the different sensitivities of radar and lidar to the two types of hydrometeors: ice crystals dominate the radar signal while supercooled water dominates the lidar signal.To assess this new method, the retrievals are compared to in situ measurements from co-located observations and the literature. The first study compares retrievals from CloudSat-CALIPSO satellite data with collocated in situ airborne measurements. This comparison shows that the radar-lidar retrievals follow the same trend as the in situ measurements and provide promising results with mean percent error of 49 % for liquid water content and 75 % for ice water content, despite the quite different measurement scales and imperfect collocation. Additionally, this has been applied to the French and German airborne platforms RALI and HALO. These first results are promising and the collocated in situ data collected during recent campaigns can be used to further assess the algorithm and improve its parameterization
12
Lauermann, Felix, Fanny Finger, André Ehrlich, and Manfred Wendisch. "Analysis of Water Content Profiles in Arctic Mixed-Phase Clouds during VERDI." Universität Leipzig, 2016. https://ul.qucosa.de/id/qucosa%3A16700.
Анотація:
Airborne measurements of liquid water content (LWC) and ice water content (IWC) were performed in mixed-phase clouds during the field campaign VERDI in Canada in April and May 2012. In single-layer and multi-layer clouds different vertical profiles of LWC and IWC could be observed. For single layer clouds the maximum LWC occurred in the upper half of the clouds while the IWC had a maximum near the cloud base. This pattern was attributed to the sedimentation of ice particles. In the lowest cloud layer of a multi-layer system both LWC and IWC reached a maximum near cloud top. Together with measured particles size distributions this suggests the presence of the seeder-feeder-process described by Fleishauer et al. (2012) for mid-level clouds.Im Rahmen der VERDI-Kampagne im April und Mai 2012 in Kanada wurden flugzeuggetragene Messungen von Flüssigwassergehalt (LWC) und Eiswassergehalt (IWC) durchgeführt. Für Einschicht- und Mehrschichtwolkensysteme konnten unterschiedliche Vertikalprofile von LWC und IWC nachgewiesen werden. In Einschichtsystemen wurden die größten Flüssigwassergehalte in der oberen Wolkenhälfte und die größten Eiswassergehalte nahe der Wolkenunterkante gemessen. Diese Verteilung wurde auf die Sedimentation von Eispartikeln zurückgeführt. In der untersten Wolkenschicht eines Mehrschichtsystems befanden sich die Maxima von LWC und IWC nahe der Wolkenoberkante. Diese Beobachtung deutet zusammen mit gemessenen Partikelgrößenverteilungen auf das Vorhandensein des Seeder-Feeder- Prozesses hin, welcher von Fleishauer et al. (2012) für mittelhohe Wolken beschrieben wurde.
13
Stachlewska, Iwona Sylwia. "Investigation of tropospheric arctic aerosol and mixed-phase clouds using airborne lidar technique." Phd thesis, Universität Potsdam, 2005. http://opus.kobv.de/ubp/volltexte/2006/698/.
Анотація:
An Airborne Mobile Aerosol Lidar (AMALi) was constructed and built at Alfred-Wegener-Institute for Polar and Marine Research (AWI) in Potsdam, Germany for the lower tropospheric aerosol and cloud research under tough arctic conditions. The system was successfully used during two AWI airborne field campaigns, ASTAR 2004 and SVALEX 2005, performed in vicinity of Spitsbergen in the Arctic. The novel evaluation schemes, the Two-Stream Inversion and the Iterative Airborne Inversion, were applied to the obtained lidar data. Thereby, calculation of the particle extinction and backscatter coefficient profiles with corresponding lidar ratio profiles characteristic for the arctic air was possible. The comparison of these lidar results with the results of other in-situ and remote instrumentation (ground based Koldewey Aerosol Raman Lidar (KARL), sunphotometer, radiosounding, satellite imagery) allowed to provided clean contra polluted (Arctic Haze) characteristics of the arctic aerosols. Moreover, the data interpretation by means of the ECMWF Operational Analyses and small-scale dispersion model EULAG allowed studying the effects of the Spitsbergens orography on the aerosol load in the Planetary Boundary Layer. With respect to the cloud studies a new methodology of alternated remote AMALi measurements with the airborne in-situ cloud optical and microphysical parameters measurements was proved feasible for the low density mixed-phase cloud studies. An example of such approach during observation of the natural cloud seeding (feeder-seeder phenomenon) with ice crystals precipitating into the lower supercooled stratocumulus deck were discussed in terms of the lidar signal intensity profiles and corresponding depolarisation ratio profiles. For parts of the cloud system characterised by almost negligible multiple scattering the calculation of the particle backscatter coefficient profiles was possible using the lidar ratio information obtained from the in-situ measurements in ice-crystal cloud and water cloud.Das Airborne Mobile Aerosol Lidar (AMALi) wurde am Alfred-Wegener-Institut für Polar- und Meeresforschung in Potsdam für die Untersuchung arktischer Aerosole und Wolken der unteren Troposphäre entwickelt und gebaut. Das AMALi wurde erfolgreich in zwei AWI Flugzeugmesskampagnen, der ASTAR 2004 und der SvalEx 2005, die in Spitzbergen in der Arktis durchgeführt wurden, eingesetzt. Zwei neue Lidar Datenauswertungsmethoden wurden implementiert: die Two-Stream Inversion und die Iterative Airborne Inversion. Damit erwies sich die Berechnung der Profile der Teilchen Rückstreu- und Extinktionskoeffizienten mit einem entsprechenden Lidar Verhältnis, das charakteristisch für arktische Luft ist, als möglich. Der Vergleich dieser Auswertungen mit den Resultaten, die mit verschiedenen Fernerkundungs- und In-situ Instrumenten gewonnen worden waren (stationäres Koldewey Aerosol Raman Lidar KARL, Sonnenphotometer, Radiosondierung und Satellitenbilder) ermöglichten die Interpretation der Lidar-Resultate und eine Charakterisierung sowohl der reinen als auch der verschmutzten Luft. Außerdem konnten die Lidardaten mit operationellen ECMWF Daten und dem kleinskaligen Dispersionsmodel EULAG verglichen werden. Dadurch konnte der Einfluss der Spitzbergener Orographie auf die Aerosolladung der Planetaren Grenzschicht untersucht werden. Für Wolkenmessungen wurde eine neue Methode der alternativen Fernerkundung mit dem AMALi und flugzeuggetragenen In-situ Messgeräten verwendet, um optische und mikrophysikalische Eigenschaften der Wolken zu bestimmen. Diese Methode wurde erfolgreich implementiert und auf Mixed-Phase Wolken geringer optischen Dicke angewendet. Ein Beispiel hier stellt das Besamen der Wolken (sogenannte Feeder-Seeder Effekt) dar, bei dem Eiskristalle in eine niedrige unterkühlte Stratokumulus fallen. Dabei konnten Lidarsignale, Intensitätsprofile und die Volumendepolarisation gemessen werden. Zusätzlich konnten in den weniger dichten Bereichen der Wolken, in denen Vielfachstreuung vernachlässigbar ist, auch Profile des Teilchen Rückstreukoeffizienten berechnet werden, wobei Lidarverhältnisse genommen wurden, die aus In-situ Messungen für Wasser- und Eiswolken ermittelt wurden.
14
Sotiropoulou, Georgia. "The Arctic Atmosphere : Interactions between clouds, boundary-layer turbulence and large-scale circulation." Doctoral thesis, Stockholms universitet, Meteorologiska institutionen (MISU), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-134525.
Анотація:
Arctic climate is changing fast, but weather forecast and climate models have serious deficiencies in representing the Arctic atmosphere, because of the special conditions that occur in this region. The cold ice surface and the advection of warm air aloft from the south result in a semi-continuous presence of a temperature inversion, known as the “Arctic inversion”, which is governed by interacting large-scale and local processes, such as surface fluxes and cloud formation. In this thesis these poorly understood interactions are investigated using observations from field campaigns on the Swedish icebreaker Oden: The Arctic Summer Cloud Ocean Study (ASCOS) in 2008 and the Arctic Clouds in Summer Experiment (ACSE) in 2014. Two numerical models are also used to explore these data: the IFS global weather forecast model from the European Center for Medium-range Weather Forecasts and the MIMICA LES from Stockholm University. Arctic clouds can persist for a long time, days to weeks, and are usually mixed-phase; a difficult to model mixture of super-cooled cloud droplets and ice crystals. Their persistence has been attributed to several mechanisms, such as large-scale advection, surface evaporation and microphysical processes. ASCOS observations indicate that these clouds are most frequently decoupled from the surface; hence, surface evaporation plays a minor role. The determining factor for cloud-surface decoupling is the altitude of the clouds. Turbulent mixing is generated in the cloud layer, forced by cloud-top radiative cooling, but with a high cloud this cannot penetrate down to the surface mixed layer, which is forced primarily by mechanical turbulence. A special category of clouds is also found: optically thin liquid-only clouds with stable stratification, hence insignificant in-cloud mixing, which occur in low-aerosol conditions. IFS model fails to reproduce the cloud-surface decoupling observed during ASCOS. A new prognostic cloud physics scheme in IFS improves simulation of mixed-phase clouds, but does not improve the warm bias in the model, mostly because IFS fails to disperse low surface-warming clouds when observations indicate cloud-free conditions. With increasing summer open-water areas in a warming Arctic, there is a growing interest in processes related to the ice marginal zones and the summer-to-autumn seasonal transition. ACSE included measurements over both open-water and sea-ice surfaces, during melt and early freeze. The seasonal transition was abrupt, not gradual as would have been expected if it was primarily driven by the gradual changes in net solar radiation. After the transition, the ocean surface remained warmer than the atmosphere, enhancing surface cooling and facilitating sea-ice formation. Observations in melt season showed distinct differences in atmospheric structure between the two surface types; during freeze-up these largely disappear. In summer, large-scale advection of warm and moist air over melting sea ice had large impacts on atmospheric stability and the surface. This is explored with an LES; results indicate that while vertical structure of the lowest atmosphere is primarily sensitive to heat advection, cloud formation, which is of great importance to the surface energy budget, is primarily sensitive to moisture advection.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.
15
Barrett, Andrew. "Why can't models simulate mixed-phase clouds correctly?" Thesis, University of Reading, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594276.
Анотація:
Motivated by the importance of clouds for weather and climate due to their radiative impact, this thesis addresses the current poor representation of thin, stratiform mixed-phase clouds by state-of-the-art numerical weather prediction (NWP) models. Due to the supercooled liquid water present at cold temperatures near the cloud top, mixed-phase clouds strongly influence the amount of solar radiation reaching the surface and have a net cooling effect on the climate. Supercooled liquid water content is underestimated, by a factor of 2 or more, in all 5 NWP models tested and ERA-Interim when compared with ground-based remote-sensing observations of mixed-phase clouds. The ice water content is better predicted, but ice cloud fraction is underestimated. A new ice cloud fraction parameterization is developed to correct this bias, based on radar observations. EMPIRE, a new high-resolution single column model is developed and used to determine the most important processes for maintaining mixed-phase clouds. It is found that altering the model specification of ice particles (size, fall speed, concentration or habit) affected the liquid water content and most also affect the ice water content. A key reason why models underestimate liquid is an overestimate of ice growth rate but parameterizing N0 as a function of ice water content based on aircraft measurements leads to a significant improvement. A strong sensitivity to the model vertical resolution is identified. At coarse resolutions EMPIRE produces less than 2% of the liquid water content of high resolution simulations. This is because the coarse resolution model does not resolve the vertical profile of temperature, liquid and ice near the cloud top. By adding a parameterization of the vertical structure of the upper part of the cloud, the resolution sensitivity is largely removed suggesting that the implementation of such a parameterization in NWP models could improve their simulation of mixed-phase clouds.
16
Ehrlich, André, Eike Bierwirth, and Manfred Wendisch. "Airborne remote sensing of Arctic boundary-layer mixed-phase clouds." Universität Leipzig, 2010. https://ul.qucosa.de/id/qucosa%3A16357.
Анотація:
This article gives an overview on the investigations on Artic boundary-layer mixed-phase clouds conducted within the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) in spring 2007. In particular the horizontal and vertical disribution of ice crystals within the clouds was determined by three independent airborne instruments (lidar, in situ and solar radiation measurements). Spectral measurements of cloud top reflectivity have been utilized to retrieve information on the ice phase by analyzing the spectral pattern of the cloud top reflectance in the wavelength range dominated by liquid water and ice absorption (1400-1700 nm). A new algorithm to derive an ice index which distinguishes pure ice, liquid water, and mixed-phase clouds was developed. The horizontal distribution of the ice index, observed during ASTAR 2007, agrees with airborne lidar and in situ measurements showing patches of glaciated clouds at an air mass transition zone within the investigated mixed-phase cloud fields. Information on the vertical distribution of ice crystals in mixed-phase clouds was derived by comparing the measured cloud top reflectivity in the wavelength band 1400-1700 nm to radiative transfer simulations. To interpret the data, the vertical weighting of the measurements was calculated. In the investigated wavelength range the weightings differ according to the spectral absorption of ice and liquid water. From the observed spectral cloud reflectivity with low values in the ice absorption maximum (1400 nm) and higher values at the liquid water absorption maximum (1700 nm) it was concluded that ice crystals were present in the otherwise liquid dominated cloud top layer. Although in situ measurements (limited due to vertical resolution and detection limits) did confirm these findings only in certain limits, the retrieved vertical structure is in agreement with published ground based remote sensing measurements.
17
Umo, Nsikanabasi Silas. "Ice nucleation by combustion products at conditions relevant to mixed-phase clouds." Thesis, University of Leeds, 2014. http://etheses.whiterose.ac.uk/7595/.
Анотація:
Quantifying the ice nucleation activity of combustion aerosols is crucial in understanding their impact on cloud properties, and consequently, on climate. This study investigates the ice nucleation abilities of soot and combustion ashes in the immersion mode at conditions relevant to mixed-phase clouds. Some physical and chemical properties of these aerosols, which aided the interpretation of their ice nucleation activities, were also explored. Soot generated from eugenol and n-decane compounds were used as proxies for atmospheric soot from biomass and hydrocarbon combustion, respectively. Combustion ashes studied were coal fly ash (CFA), wood bottom ash, domestic bottom ash, and coal bottom ash. The ice nucleation experiments were performed with droplet freezing assay instruments; the results obtained are summarised below: (1) Eugenol and n-decane soot particles in suspension were found to nucleate ice between -16.5 and -28 ºC. To describe the ice nucleation efficiency of these particles, a singular model of ice nucleation that yields the ice active nucleation sites density (ns) was applied. Both soot types showed ns in the range: 1 - ~ 107 cm-2 at a temperature range of -16 to ~ -28 °C. An estimation of potential ice nuclei (IN) number based on the ice nucleation efficiency of these soot types indicated that soot substantially contributes to primary ice formation in mixed-phase clouds from temperatures below -22 ºC. At ~ - 25 ºC, soot showed a competition with mineral dust IN. This study suggests that soot is an important IN in mixed-phase clouds especially in regions where mineral dust is not a dominant IN. (2) The freezing temperatures for combustion ashes were between -15 to -36 °C. The fraction of droplets frozen showed that the freezing temperatures were in this order of significance: CFA >> wood ash > domestic ash > coal ash. The ns values estimated for all ashes were between 10-2 and 107 cm-2 for freezing temperatures between -15 and -36 °C. The best estimate suggests that combustion ashes can account for global primary ice nuclei number up to 1 cm-3, and this could impact on primary ice formation budget in mixed-phase clouds.
18
Vergara, Temprado Jesus. "Global modelling of ice-nucleating particles and impacts on mixed-phase clouds." Thesis, University of Leeds, 2017. http://etheses.whiterose.ac.uk/19602/.
Анотація:
The process of cloud glaciation strongly alters the properties of mixed-phase clouds. Between 0C to about -37C, cloud liquid droplets can either exist in the liquid phase in metastable state known as supercooling, or they can be composed of solid ice crystals. For a liquid droplet to freeze at these temperatures, the action of an external agent, known as ice-nucleating particle (INP) is needed. The atmospheric distribution of ice-nucleating particles was simulated in past studies as a function of the aerosol concentration, however, new experimental information about the ice- nucleating ability of different aerosol species and several new atmospheric measurements of INP are now available to be used in models. In this thesis, I use this new information to develop a global atmospheric model of the distribution of ice-nucleating particles to assess the relative importance of mineral dust, marine organic aerosols and black carbon for contributing to atmospheric concentrations of INPs. The model is evaluated against several datasets of INP concentrations measured in the atmosphere to test its realism and locate regions of the world where additional currently missing sources of INP could be important. The results show that feldspar aerosols dominate the atmospheric INP concentration for most parts of the globe, whereas marine organic aerosols are more relevant in the remote Southern Ocean. Black carbon particles, in contrast, seem not to play a substantial role when new estimates of its ice-nucleating ability are used. With the information obtained by this model, I explore whether the representation of ice-nucleating particles in climate models plays a role in the Southern Ocean radiative bias. This bias is related to modelled clouds reflecting too-little solar radiation, causing large errors in sea-surface temperatures and atmospheric circulations. I combine cloud-resolving simulations over regions of 1000 km with the new estimates of the INP concentration in remote regions to show that the simulated clouds reflect much more solar radiation than predicted by a global climate model, agreeing much better with satellite observations in both magnitude and frequency. Overall, these results will improve our understanding of the role, distribution and importance of ice-nucleating particles in the atmosphere and provide the scientific community new points of view to understand model biases.
19
Barrett, Paul Alan. "Turbulence and Ice nucleation in mixed-phase altocumulus clouds in the mid-latitudes." Thesis, University of Leeds, 2017. http://etheses.whiterose.ac.uk/16746/.
Анотація:
Mixed-phase layer clouds are common throughout the globe, from the tropics where detrainment from convection forms long-lived altocumulus layers, to the mid-latitudes where humidity is brought to the mid-troposphere by cyclonic activity, and in the Arctic regions where low-level mixed-phase stratocumulus clouds persist. Supercooled water is common in these clouds and so they have a strong impact on the radiative balance of the planet. Global climate and numerical weather prediction models fail to predict sufficient mid-level cloud and this leads to deficiencies in the representation of incoming solar radiation at the surface and thus large biases in surface temperature, notably in the Southern Ocean. High resolution and cloud resolving models do not perform significantly better, and in part this is due to large uncertainties in the nature of ice nucleation and the phase transition from liquid to ice. This study exploits new observations of mixed-phase cloud to attempt to better understand the processes that control their evolution. Observations of altocumulus clouds from an instrumented aircraft are presented that probe the nature of liquid and ice cloud particles, and the underlying aerosol population. The performance of cloud microphysics probes in measuring ice particles smaller than 100 microns when liquid cloud drops are present is assessed. New characterisation of SID2 (Small Ice Detector 2) and CIP15 (Cloud Imaging Probe, 15 microns) is presented. Calculations are performed that assess the ice nucleating particle budget in altocumulus, and the ice production rate in mixed-phase altocumulus and cumulus clouds.
20
Herbert, Ross James. "Modelling studies on the impact of heterogeneous ice nucleation on mixed-phase clouds." Thesis, University of Leeds, 2014. http://etheses.whiterose.ac.uk/7291/.
Анотація:
Clouds are a critical component of Earth’s climate and hydrological cycle. The formation of ice in the atmosphere, especially at lower altitudes, can substantially impact the evolution of clouds and their radiative properties, and represents the initiation of the cold rain precipitation process. In mixed-phase clouds quantitatively understanding the interactions between ice and liquid, and the subsequent impact on the cloud development, is fundamentally dependent on the process of ice formation and its representation within cloud models. Experiments show that ice nucleating particles (INPs) exhibit variability in both freezing efficiency and time-dependent behaviour. The variability in freezing efficiency is currently well characterised and represented, but variability in time-dependence is poorly characterised and rarely represented in models and parameterisations. The primary aim of this thesis is to understand the role that time-dependence plays in the freezing behaviour of droplets, and secondly to examine the sensitivity of mixed-phase clouds to time-dependence in immersion mode freezing. It is initially found that CNT-based models are unable to reproduce the observed time-dependent behaviour. A new model is therefore presented that uniquely incorporates the variability in both freezing efficiency and time-dependent behaviour; this is applied to experimental data to understand the manifestation of time-dependence in experiments. The model is then used to derive a new theoretical framework for use in experimental analysis and cloud modelling studies. The framework is underpinned by the finding that the temperature dependence (named λ) of the nucleation rate coefficient solely determines the time-dependent behaviour observed in droplet freezing experiments. New and existing experimental data is used to demonstrate the ability for the framework to reconcile data obtained on different timescales with different experimental methods. Finally, an efficient and representative parameterisation is used to explore the sensitivity of mixed-phase clouds to time-dependence. Using a series of increasingly complex models (0D to 2D) it is shown that the inclusion of time-dependence impacts cloud properties in regimes where the updraught speed is relatively low.
21
Dearden, Christopher. "Exploring the effects of microphysical complexity in numerical simulations of liquid and mixed-phase clouds." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/exploring-the-effects-of-microphysical-complexity-in-numerical-simulations-of-liquid-and-mixedphase-clouds(abe4b249-a608-4a42-819c-962e2114d1ba).html.
Анотація:
This thesis forms a NERC funded CASE studentship with the Met Office, whose aim is to investigate the treatment of cloud microphysical processes in numerical models, with a particular focus on exploring the impacts and possible benefits of microphysical complexity for the purpose of simulating clouds and precipitation. The issue of complexity is an important one in numerical modelling in order to maintain computational efficiency, particularly in the case of operational models. The latest numerical modelling tools are utilised to perform simulations of cloud types including idealised trade wind cumulus, orographic wave cloud and wintertime shallow convective cloud. Where appropriate, the modelling results are also validated against observations from recent field campaigns. The Factorial Method is employed as the main analysis tool to quantify the effect of microphysical variables in terms of their impact on a chosen metric. Ultimately it is expected that the techniques and results from this thesis will be used to help inform the future development of cloud microphysics schemes for use in both cloud resolving and operational models. This is timely given the current plans to upgrade the microphysics options available for use within the Met Office Unified Model. For an idealised warm cloud, it is shown that different bin microphysics schemes can produce different results, and therefore additional microphysical complexity does not necessarily ensure a more consistent simulation. An intercomparison of bin microphysics schemes in a 1-D column framework is recommended to isolate the origin of the discrepancies. In relation to the mixed-phase wave cloud, model simulations based on an adaptive treatment of ice density and habit struggled to reproduce the observed ice crystal growth rates, highlighting the need for further laboratory work to improve the parameterization of ice growth by diffusion within the sampled temperature regime. The simulations were also found to be largely insensitive to values of the deposition coefficient within the range of 0.1 to 1.0. Results from a mesoscale modelling study of shallow wintertime convection demonstrate the importance of the representation of dynamical factors that control cloud macrostructure, and how this has the potential to overshadow any concerns of microphysical complexity. Collectively, the results of this thesis place emphasis on the need to encourage more synergy between the dynamics and microphysics research communities in order to improve the future performance of numerical models, and to help optimise the balance between model complexity and computational efficiency.
22
Schlenczek, Oliver [Verfasser]. "Airborne and ground-based holographic measurement of hydrometeors in liquid-phase, mixed-phase and ice clouds / Oliver Schlenczek." Mainz : Universitätsbibliothek Mainz, 2018. http://d-nb.info/115115590X/34.
23
Hoose, Corinna. "Aerosol processing and its effect on mixed-phase clouds in a global climate model." Zürich, 2008. http://e-collection.ethbib.ethz.ch/view/eth:30446.
24
Loewe, Katharina [Verfasser]. "Arctic mixed-phase clouds : Macro- and microphysical insights with a numerical model / Katharina Loewe." Karlsruhe : KIT Scientific Publishing, 2017. http://www.ksp.kit.edu.
25
Bühl, Johannes. "Combined lidar and radar observations of vertical motions and heterogeneous ice formation in mixed-phase layered clouds." Doctoral thesis, Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-171998.
Анотація:
Im Rahmen der Arbeit wurden Lidar- und Wolkenradarmessungen von troposphärischen Schichtwolken durchgeführt und ausgewertet, um den Zusammenhang zwischen Vertikalwinden und Eisbildung in diesen Wolken zu untersuchen. Der Eis- und Flüssigwassergehalt von Schichtwolken wurde mit einer Kombination aus Raman-Lidar und Wolkenradar untersucht. Die vertikalen Windbewegungen an der Wolkenunterkante wurden mit einem Doppler-Lidar aufgezeichnet. Durch die Auswertung vorangegangener Messkampagnen konnte die Vertikalwindstatistik in mittelhohen Schichtwolken zwischen den Standorten Leipzig und Praia (Kap Verde) verglichen werden. Messverfahren für die Vertikalwindmessung mit Doppler-Lidar wurden im Rahmen dieser Arbeit weiterentwickelt. In Zusammenarbeit mit dem Deutschen Wetterdienst wurde außerdem die Kombination von Doppler-Lidar, Wolkenradar und Wind-Profiler getestet.
Die Eisbildungseffizienz in der Troposphäre wurde im Temperaturbereich zwischen 0 und -40°C für den Standort Leipzig untersucht und sowohl mit vorangegangenen Lidarmessungen, als auch mit aktuellen Satellitenmessungen verglichen. Zum ersten Mal wurde außerdem die statistische Verteilung von Vertikalwinden an der Basis von Mischphasenwolken dargestellt.
Es wurde festgestellt, dass sich bei einer Temperatur von (-9 +/- 3)°C bereits in 50% der Schichtwolken über Leipzig Eis bildet. Zwischen -15 und 0°C wurden Verhältnisse zwischen Eis- und Flüssigwasserpfad zwischen 0,1 und 0,0001 abgeschätzt. Im Rahmen der Messgenauigkeit wurden zwischen den Standorten Leipzig und Praia keine Unterschiede in der Vertikalwindstatistik festgestellt.
26
Mülmenstädt, Johannes, Odran Sourdeval, Julien Delanoë, and Johannes Quaas. "Frequency of occurrence of rain from liquid-, mixed-, and ice-phase clouds derived from A-Train satellite retrievals." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-202461.
Анотація:
A climatology of thermodynamic phase of precipitating cloud is presented derived from global—land and ocean—, retrievals from Cloudsat, CALIPSO, and Moderate Resolution Imaging Spectroradiometer. Like precipitation rate, precipitation frequency is dominated by warm rain, defined as
rain produced via the liquid phase only, over the tropical oceans outside the Intertropical Convergence Zone and by cold rain, produced via the ice phase, over the midlatitude oceans and continents. Warm rain is very infrequent over the continents, with significant warm rain found only in onshore flow in the tropics, and over India, China, and Indochina. Comparison of the properties of precipitating and nonprecipitating
warm clouds shows that the scarcity of warm rain over land can be explained by smaller effective radii in continental clouds that delay the onset of precipitation. The results highlight the importance of ice-phase
processes for the global hydrological cycle and may lead to an improved parameterization of precipitation in general circulation models.
27
Lampert, Astrid. "Airborne lidar observations of tropospheric arctic clouds." Phd thesis, Universität Potsdam, 2009. http://opus.kobv.de/ubp/volltexte/2010/4121/.
Анотація:
Due to the unique environmental conditions and different feedback mechanisms, the Arctic region is especially sensitive to climate changes. The influence of clouds on the radiation budget is substantial, but difficult to quantify and parameterize in models.
In the framework of the PhD, elastic backscatter and depolarization lidar observations of Arctic clouds were performed during the international Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) from Svalbard in March and April 2007.
Clouds were probed above the inaccessible Arctic Ocean with a combination of airborne instruments: The Airborne Mobile Aerosol Lidar (AMALi) of the Alfred Wegener Institute for Polar and Marine Research provided information on the vertical and horizontal extent of clouds along the flight track, optical properties (backscatter coefficient), and cloud thermodynamic phase. From the data obtained by the spectral albedometer (University of Mainz), the cloud phase and cloud optical thickness was deduced. Furthermore, in situ observations with the Polar Nephelometer, Cloud Particle Imager and Forward Scattering Spectrometer Probe (Laboratoire de Météorologie Physique, France) provided information on the microphysical properties, cloud particle size and shape, concentration, extinction, liquid and ice water content. In the thesis, a data set of four flights is analyzed and interpreted.
The lidar observations served to detect atmospheric structures of interest, which were then probed by in situ technique. With this method, an optically subvisible ice cloud was characterized by the ensemble of instruments (10 April 2007). Radiative transfer simulations based on the lidar, radiation and in situ measurements allowed the calculation of the cloud forcing, amounting to -0.4 W m-2. This slight surface cooling is negligible on a local scale. However, thin Arctic clouds have been reported more frequently in winter time, when the clouds' effect on longwave radiation (a surface warming of 2.8 W m-2) is not balanced by the reduced shortwave radiation (surface cooling).
Boundary layer mixed-phase clouds were analyzed for two days (8 and 9 April 2007). The typical structure consisting of a predominantly liquid water layer on cloud top and ice crystals below were confirmed by all instruments. The lidar observations were compared to European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological analyses. A change of air masses along the flight track was evidenced in the airborne data by a small completely glaciated cloud part within the mixed-phase cloud system. This indicates that the updraft necessary for the formation of new cloud droplets at cloud top is disturbed by the mixing processes.
The measurements served to quantify the shortcomings of the ECMWF model to describe mixed-phase clouds. As the partitioning of cloud condensate into liquid and ice water is done by a diagnostic equation based on temperature, the cloud structures consisting of a liquid cloud top layer and ice below could not be reproduced correctly. A small amount of liquid water was calculated for the lowest (and warmest) part of the cloud only. Further, the liquid water content was underestimated by an order of magnitude compared to in situ observations.
The airborne lidar observations of 9 April 2007 were compared to space borne lidar data on board of the satellite Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). The systems agreed about the increase of cloud top height along the same flight track. However, during the time delay of 1 h between the lidar measurements, advection and cloud processing took place, and a detailed comparison of small-scale cloud structures was not possible.
A double layer cloud at an altitude of 4 km was observed with lidar at the West coast in the direct vicinity of Svalbard (14 April 2007). The cloud system consisted of two geometrically thin liquid cloud layers (each 150 m thick) with ice below each layer. While the upper one was possibly formed by orographic lifting under the influence of westerly winds, or by the vertical wind shear shown by ECMWF analyses, the lower one might be the result of evaporating precipitation out of the upper layer. The existence of ice precipitation between the two layers supports the hypothesis that humidity released from evaporating precipitation was cooled and consequently condensed as it experienced the radiative cooling from the upper layer.
In summary, a unique data set characterizing tropospheric Arctic clouds was collected with lidar, in situ and radiation instruments. The joint evaluation with meteorological analyses allowed a detailed insight in cloud properties, cloud evolution processes and radiative effects.Die Arktis mit ihren speziellen Umweltbedingungen ist besonders empfindlich gegenüber Klimaveränderungen. Dabei spielen Wolken eine große Rolle im Strahlungsgleichgewicht, die aber nur schwer genau bestimmt und in Klimamodellen dargestellt werden kann. Die Daten für die Promotionsarbeit wurden im Frühjahr 2007 bei Flugzeug-Messungen von Wolken über dem Arktischen Ozean von Spitzbergen aus erhoben. Das dafür verwendete Lidar (Licht-Radar) des Alfred-Wegener-Instituts lieferte ein höhenaufgelöstes Bild der Wolkenstrukturen und ihrer Streu-Eigenschaften, andere Messgeräte ergänzten optische sowie mikrophysikalische Eigenschaften der Wolkenteilchen (Extinktion, Größenverteilung, Form, Konzentration, Flüssigwasser- und Eisgehalt, Messgeräte vom Laboratoire de Météorologie Physique, France) und Strahlungsmessungen (Uni Mainz). Während der Messkampagne herrschte Nordwind vor. Die untersuchten Luftmassen mit Ursprung fern von menschlichen Verschmutzungsquellen war daher sehr sauber. Beim Überströmen der kalten Luft über den offenen warmen Arktischen Ozean bildeten sich in der Grenzschicht (ca. 0-1500 m Höhe) Mischphasenwolken, die aus unterkühlten Wassertröpfchen im oberen Bereich und Eis im unteren Bereich der Wolken bestehen. Mit den Flugzeug-Messungen und numerischen Simulationen des Strahlungstransports wurde der Effekt einer dünnen Eiswolke auf den Strahlungshaushalt bestimmt. Die Wolke hatte lokal eine geringe Abkühlung der Erdoberfläche zur Folge. Ähnliche Wolken würden jedoch im Winter, wenn keine Sonnenstrahlung die Arktis erreicht, durch den Treibhauseffekt eine nicht vernachlässigbare Erwärmung der Oberfläche verursachen. Die Messungen der Mischphasenwolken wurden mit einem Wettervorhersagemodell (ECMWF) verglichen. Für die ständig neue Bildung von flüssigen Wassertropfen im oberen Teil der Wolke ist das Aufsteigen von feuchten Luftpaketen nötig. Während einer Messung wurden entlang der Flugstrecke verschiedene Luftmassen durchflogen. An der Luftmassengrenze wurde eine reine Eiswolke inmitten eines Mischphasen-Systems beobachtet. Die Messungen zeigen, dass das Mischen von Luftmassen den Nachschub an feuchter Luft blockiert, was unmittelbare Auswirkungen auf die thermodynamische Phase des Wolkenwassers hat. Weiterhin wurde bestimmt, wie groß die Abweichungen der Modellrechnungen von den Messungen bezüglich Wassergehalt und der Verteilung von Flüssigwasser und Eis waren. Durch die vereinfachte Wolken-Parameterisierung wurde die typische vertikale Struktur von Mischphasenwolken im Modell nicht wiedergegeben. Die flugzeuggetragenen Lidar-Messungen vom 9. April 2007 wurden mit Lidar-Messungen an Bord des Satelliten CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) verglichen. Die Messungen zeigten beide eine ansteigende Wolkenobergrenze entlang desselben Flugwegs. Da die Messungen jedoch nicht genau gleichzeitig durchgeführt wurden, war wegen Advektion und Prozessen in den Wolken kein genauer Vergleich der kleinskaligen Wolkenstrukturen möglich. Außerdem wurde eine doppelte Wolkenschicht in der freien Troposphäre (4 km Höhe) analysiert. Die Wolke bestand aus zwei separaten dünnen Schichten aus flüssigem Wasser (je 150 m dick) mit jeweils Eis darunter. Die untere Schicht entstand wahrscheinlich aus verdunstetem Eis-Niederschlag. Diese feuchte Schicht wurde durch die Abstrahlung der oberen Wolkenschicht gekühlt, so dass sie wieder kondensierte. Solche Wolkenformationen sind in der Arktis bisher vor allem in der Grenzschicht bekannt. Ein einzigartiger Datensatz von arktischen Wolken wurde mit einer Kombination verschiedener Flugzeug-Messgeräte erhoben. Zusammen mit meteorologischen Analysen konnten für verschiedene Fallstudien Wolkeneigenschaften, Entwicklungsprozesse und Auswirkungen auf den Strahlungshaushalt bestimmt werden.
28
Mülmenstädt, Johannes, Odran Sourdeval, Julien Delanoë, and Johannes Quaas. "Frequency of occurrence of rain from liquid-, mixed-, and ice-phase clouds derived from A-Train satellite retrievals." Geophysical research letters (2015), 42, S. 6502-6509, 2015. https://ul.qucosa.de/id/qucosa%3A14690.
Анотація:
A climatology of thermodynamic phase of precipitating cloud is presented derived from global—land and ocean—, retrievals from Cloudsat, CALIPSO, and Moderate Resolution Imaging Spectroradiometer. Like precipitation rate, precipitation frequency is dominated by warm rain, defined as
rain produced via the liquid phase only, over the tropical oceans outside the Intertropical Convergence Zone and by cold rain, produced via the ice phase, over the midlatitude oceans and continents. Warm rain is very infrequent over the continents, with significant warm rain found only in onshore flow in the tropics, and over India, China, and Indochina. Comparison of the properties of precipitating and nonprecipitating
warm clouds shows that the scarcity of warm rain over land can be explained by smaller effective radii in continental clouds that delay the onset of precipitation. The results highlight the importance of ice-phase
processes for the global hydrological cycle and may lead to an improved parameterization of precipitation in general circulation models.
29
Shupe, Matthew David. "An intricate balance of liquid and ice: The properties, processes, and significance of Arctic stratiform mixed-phase clouds." Connect to online resource, 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3284464.
30
Beydoun, Hassan. "Advances in Heterogeneous Ice Nucleation Research: Theoretical Modeling and Measurements." Research Showcase @ CMU, 2017. http://repository.cmu.edu/dissertations/793.
Анотація:
In the atmosphere, cloud droplets can remain in a supercooled liquid phase at temperatures as low as -40 °C. Above this temperature, cloud droplets freeze via heterogeneous ice nucleation whereby a rare and poorly understood subset of atmospheric particles catalyze the ice phase transition. As the phase state of clouds is critical in determining their radiative properties and lifetime, deficiencies in our understanding of heterogeneous ice nucleation poses a large uncertainty on our efforts to predict human induced global climate change. Experimental challenges in properly simulating particle-induced freezing processes under atmospherically relevant conditions have largely contributed to the absence of a well-established model and parameterizations that accurately predict heterogeneous ice nucleation. Conversely, the sparsity of reliable measurement techniques available struggle to be interpreted by a single consistent theoretical or empirical framework, which results in layers of uncertainty when attempting to extrapolate useful information regarding ice nucleation for use in atmospheric cloud models. In this dissertation a new framework for describing heterogeneous ice nucleation is developed. Starting from classical nucleation theory, the surface of an ice nucleating particle is treated as a continuum of heterogeneous ice nucleating activity and a particle specific distribution of this activity g is derived. It is hypothesized that an individual particle species exhibits a critical surface area. Above this critical area the ice nucleating activity of a particle species can be described by one g distribution, 𝑔, while below it 𝑔 expresses itself expresses externally resulting in particle to particle variability in ice nucleating activity. The framework is supported by cold plate droplet freezing measurements for dust and biological particles in which the total surface area of particle material available is varied. Freezing spectra above a certain surface area are shown to be successfully fitted with 𝑔 while a process of random sampling from 𝑔 can predict the freezing behavior below the identified critical surface area threshold. The framework is then extended to account for droplets composed of multiple particle species and successfully applied to predict the freezing spectra of a mixed proxy for an atmospheric dust-biological particle system. The contact freezing mode of ice nucleation, whereby a particle induces freezing upon collision with a droplet, is thought to be more efficient than particle initiated immersion freezing from within the droplet bulk. However, it has been a decades’ long challenge to accurately measure this ice nucleation mode, since it necessitates reliably measuring the rate at which particles hit a droplet surface combined with direct determination of freezing onset. In an effort to remedy this longstanding deficiency a temperature controlled chilled aerosol optical tweezers capable of stably isolating water droplets in air at subzero temperatures has been designed and implemented. The new temperature controlled system retains the powerful capabilities of traditional aerosol optical tweezers: retrieval of a cavity enhanced Raman spectrum which could be used to accurately determine the size and refractive index of a trapped droplet. With these capabilities, it is estimated that the design can achieve ice supersaturation conditions at the droplet surface. It was also found that a KCl aqueous droplet simultaneously cooling and evaporating exhibited a significantly higher measured refractive index at its surface than when it was held at a steady state temperature. This implies the potential of a “salting out” process. Sensitivity of the cavity enhanced Raman spectrum as well as the visual image of a trapped droplet to dust particle collisions is shown, an important step in measuring collision frequencies of dust particles with a trapped droplet. These results may pave the way for future experiments of the exceptionally poorly understood contact freezing mode of ice nucleation.
31
Bühl, Johannes [Verfasser], Andreas [Akademischer Betreuer] Macke, Andreas [Gutachter] Macke, and Anthony [Gutachter] Illingworth. "Combined lidar and radar observations of vertical motions and heterogeneous ice formation in mixed-phase layered clouds : Field studies and long-term monitoring / Johannes Bühl ; Gutachter: Andreas Macke, Anthony Illingworth ; Betreuer: Andreas Macke." Leipzig : Universitätsbibliothek Leipzig, 2015. http://d-nb.info/1239566808/34.
32
Myagkov, Alexander. "Shape-temperature relationship of ice crystals in mixed-phase clouds based on observations with polarimetric cloud radar: Shape-temperature relationship of ice crystals in mixed-phase cloudsbased on observations with polarimetric cloud radar." Doctoral thesis, 2016. https://ul.qucosa.de/id/qucosa%3A15204.
Анотація:
This thesis is devoted to the experimental quantitative characterization of the shape and orientation distribution of ice particles in clouds. The characterization is based on measured and modeled elevation dependencies of the polarimetric parameters differential reflectivity and correlation coefficient. The polarimetric data is obtained using a newly developed 35-GHz cloud radar MIRA-35 with hybrid polarimetric configuration and scanning capabilities. The full procedure chain of the technical implementation and the realization of the setup of the hybrid-mode cloud radar for the shape determination are presented. This includes the description of phase adjustments in the transmitting paths, the introduction of the general data processing scheme, correction of the data for the differences of amplifications and electrical path lengths in the transmitting and receiving channels, the rotation of the polarization basis by 45°, the correction of antenna effects on polarimetric measurements, the determination of spectral polarimetric variables, and the formulation of a scheme to increase the signal-to-noise ratio. Modeling of the polarimetric variables is based on existing backscattering models assuming the spheroidal representation of cloud scatterers. The parameters retrieved from the model are polarizability ratio and degree of orientation, which can be assigned to certain particle orientations and shapes.
In the thesis the first quantitative estimations of ice particle shape at the top of liquid-topped clouds are presented. Analyzed ice particles were formed in the presence of supercooled water and in the temperature range from -20 °C to -3 °C. The estimation is based on polarizability ratios of ice particles measured by the MIRA-35 with hybrid polarimetric configuration, manufactured by METEK GmbH. For the study, 22 cases observed during the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign were used. Polarizability ratios retrieved for cloud layers with cloud-top temperatures of about -5, -8, -15, and -20 °C were 1.6, 0.9, 0.6, and 0.9, respectively. Such values correspond to prolate, quasi-isotropic, oblate, and quasi-isotropic particles, respectively. Data from a free-fall chamber were used for the comparison. A good agreement of detected shapes with well-known shape{temperature dependencies observed in laboratories was found.:1 Introduction
2 Formation and development of ice particles: Laboratory studies and remote observations
2.1 Heterogeneous ice formation in the atmosphere
2.2 Laboratory investigations of ice crystal development
2.3 Polarimetric radar observations of ice microphysics
2.3.1 Polarimetry in weather radar networks
2.3.2 Polarimetry in cloud radars
2.3.3 Polarization coupling
2.4 Aims and scientific questions
3 Effects of antenna patterns on cloud radar polarimetric measurements
3.1 Measurements of complex antenna patterns
3.1.1 Problem definition
3.1.2 Measurement description
3.1.3 Results of antenna pattern measurements
3.2 Correction of LDR measurements
3.3 Discrimination between insects and clouds
4 Cloud radar MIRA-35 with hybrid mode
4.1 Implementation and phase adjustment
4.2 Processing of the coherency matrix
4.3 Correction of the coherency matrix for differences of channels
4.4 The coherency matrix in the slanted basis
4.5 Correction for the antenna coupling
4.6 Spectral polarimetric variables
4.7 Sensitivity issue
5 Shape and orientation retrieval
5.1 Backscattering model
5.2 Retrieval technique
5.3 Case study
6 Shape-temperature relationship of pristine ice crystals
6.1 Instrumentation and data set
6.2 Examples of the shape retrieval
6.2.1 Case 1: 12 October 2014, 15:00-16:00 UTC
6.2.2 Case 2: 18 October 2014, 01:00-02:00 UTC
6.2.3 Case 3: 20 October 2014, 18:00-19:00 UTC
6.2.4 Case 4: 10 November 2014, 02:00-03:00 UTC
6.2.5 Case 5: 7 November 2014, 20:00-21:00 UTC
6.3 Comparison of shape with laboratory studies
6.4 Orientation of pristine ice crystals
7 Summary and outlook
Bibliography
List of Abbreviations
List of Symbols
33
Avramov, Alexander Elkov Harrington Jerry Y. "Simulations of aerosol, microphysical and coastal influences on Arctic mixed-phase clouds." 2009. http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-3672/index.html.
34
Lee, Joonsuk. "Mixed-phase clouds, thin cirrus clouds, and OLR over the tropics: observations, retrievals, and radiative impacts." 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1897.
Анотація:
The tropics is a very important region in terms of earth’s radiation budget
because the net radiative heating is largest in the tropics and that surplus energy is
redistributed by the circulations of oceans and atmospheres. Moreover, a large number
of clouds are formed by deep convection and convergence of water vapor. Thus, it is
very important to understand the radiative energy balance of the tropics and the effect of
clouds on the radiation field.
For mixed-phase clouds, error analyses pertaining to the inference of effective
particle sizes and optical thicknesses are performed. Errors are calculated with respect to
the assumption of a cloud containing solely liquid or ice phase particles. The analyses
suggest that the effective particle size inferred for a mixed-phase cloud can be
underestimated (or overestimated) if a pure liquid phase (or pure ice phase) is assumed
for the cloud, whereas the corresponding cloud optical thickness can be overestimated
(or underestimated). The analyses of optical depth and fraction of occurrence for thin cirrus clouds
showed that about 40% of pixels flagged as clear-sky contain detectible thin cirrus
clouds. The regions of high occurrence and large optical depth located around deep
convection showed seasonal variations. The thin cirrus clouds occur more frequently
with larger optical depth in the northern (southern) hemisphere during spring and
summer (autumn and winter). The net cloud radiative forcing by thin cirrus clouds is
positive at the top of atmosphere and is negative at the bottom of atmosphere.
The difference in OLR between measurement and model is 4.2 Wm-2 for
September 2005. The difference is smaller in moist regions and larger in drier regions.
OLR increases with increasing surface temperatures up to 300 K but decreases at surface
temperatures larger than 300 K due to the strong absorption of increased water vapor. In
summary, if the surface temperature is lower than the threshold of convection (300 K),
temperature is a dominant factor in OLR and if the surface temperature is larger than 300
K, OLR is strongly influenced by water vapor.
35
De, Boer Gijs. "An improved understanding of the lifecycle of mixed-phase stratiform clouds through oberservations and simulation." 2009. http://www.library.wisc.edu/databases/connect/dissertations.html.
36
Turner, David D. "Microphysical properties of single and mixed-phase arctic clouds derived from ground-based AERI observations /." 2003. http://www.library.wisc.edu/databases/connect/dissertations.html.
37
Ruiz-Donoso, Elena. "Small-scale structure of thermodynamic phase in Arctic mixed-phase clouds observed with airborne remote sensing during the ACLOUD campaign." 2020. https://ul.qucosa.de/id/qucosa%3A74833.
Анотація:
This thesis evaluates the limitations of passive airborne remote sensing methods to retrieve optical and microphysical properties of Arctic mixed-phase clouds. These limitations are circumvented using a synergy of passive and active remote sensing techniques, and large eddy simulations. Using this synergetic approach, the three-dimensional spatial distribution of the thermodynamic phase of two cloud case studies is characterized. The findings are subsequently applied to a statistical analysis of the cloud properties measured during the Arctic Cloud Observations Using airborne measurements during polar Day (ACLOUD) campaign.
38
Stachlewska, Iwona Sylwia [Verfasser]. "Investigation of tropospheric arctic aerosol and mixed-phase clouds using airborne lidar technique / von Iwona Sylwia Stachlewska." 2006. http://d-nb.info/979804183/34.
39
Ehrlich, André [Verfasser]. "The impact of ice crystals on radiative forcing and remote sensing of arctic boundary-layer mixed-phase clouds / vorgelegt von André Ehrlich." 2009. http://d-nb.info/994470355/34.
40
Bühl, Johannes. "Combined lidar and radar observations of vertical motions and heterogeneous ice formation in mixed-phase layered clouds: Field studies and long-term monitoring." Doctoral thesis, 2014. https://ul.qucosa.de/id/qucosa%3A13366.
Анотація:
Im Rahmen der Arbeit wurden Lidar- und Wolkenradarmessungen von troposphärischen Schichtwolken durchgeführt und ausgewertet, um den Zusammenhang zwischen Vertikalwinden und Eisbildung in diesen Wolken zu untersuchen. Der Eis- und Flüssigwassergehalt von Schichtwolken wurde mit einer Kombination aus Raman-Lidar und Wolkenradar untersucht. Die vertikalen Windbewegungen an der Wolkenunterkante wurden mit einem Doppler-Lidar aufgezeichnet. Durch die Auswertung vorangegangener Messkampagnen konnte die Vertikalwindstatistik in mittelhohen Schichtwolken zwischen den Standorten Leipzig und Praia (Kap Verde) verglichen werden. Messverfahren für die Vertikalwindmessung mit Doppler-Lidar wurden im Rahmen dieser Arbeit weiterentwickelt. In Zusammenarbeit mit dem Deutschen Wetterdienst wurde außerdem die Kombination von Doppler-Lidar, Wolkenradar und Wind-Profiler getestet.
Die Eisbildungseffizienz in der Troposphäre wurde im Temperaturbereich zwischen 0 und -40°C für den Standort Leipzig untersucht und sowohl mit vorangegangenen Lidarmessungen, als auch mit aktuellen Satellitenmessungen verglichen. Zum ersten Mal wurde außerdem die statistische Verteilung von Vertikalwinden an der Basis von Mischphasenwolken dargestellt.
Es wurde festgestellt, dass sich bei einer Temperatur von (-9 +/- 3)°C bereits in 50% der Schichtwolken über Leipzig Eis bildet. Zwischen -15 und 0°C wurden Verhältnisse zwischen Eis- und Flüssigwasserpfad zwischen 0,1 und 0,0001 abgeschätzt. Im Rahmen der Messgenauigkeit wurden zwischen den Standorten Leipzig und Praia keine Unterschiede in der Vertikalwindstatistik festgestellt.