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Les noyaux de congélation de l'atmosphère
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... The origins and the natures of atmospheric ice nuclei have been pursued by many workers on the basis of observations of ice nculeus concentration (e.g. Schaefer, 1954;Isono et aI., 1959aIsono et aI., , 1959bSoulage, 1957;Bigg & Miles, 1964) and examination of center nuclei of snow crystals with electon-microscope (Kumai, 1951(Kumai, , 1961Isono, 1955Isono, , 1959Kumai & Francis, 1962). According to the results of these studies, particles of clay and other minerals have been found to constitute a great part of the atmospheric ice nuclei. ...
Simultaneous collections of ice nuclei in the air were made with instruments of the same type at four sites in the rim of the North Pacific: College, Alaska; Blue Glacier, Washington; Mauna Loa, Hawaii; and Nagoya, Japan. Ice nuclei were collected on filter paper for counting of their number and also sheet meshes for examination with electron microscope. During the period of the collection from the beginning of February through the beginning of March, 1968, three marked maxima of the concentrations of ice nuclei effective at ?15°C appeared at each of the sites except Mauna Loa. The peak values were the largest at Nagoya (5.3 nuclei/litre) followed by College (2.7 nuclei/litre) and Blue Glacier (1.3 nuclei/litre). At the Mauna Loa Observatory, no marked peak was observed. Neither a diurnal variation nor any other variations with a specific period have been detected. The result of identification of materials of ice nuclei collected at the four sites shows that clay and other mineral particles constitute the main part of the ice nuclei. The results of the studies on the features of ice nucleus concentration, the trajectories of air masses and the examination of ice nuclei suggest that the ice nuclei detected originated from arid and semi-arid regions of the Asian Continent. DOI: 10.1111/j.2153-3490.1971.tb00545.x
... Since the middle of the last century, there has been a substantial body of work showing the importance of bioaerosols as ice-nucleating particles (INPs) in mixed-phased and ice clouds, ultimately affecting the local, regional and global climate and precipitation [1][2][3][4][5][6][7] . With the increasing importance of microbiological meteorology, scientists in disparate fields have led a resurgent attempt to understand the mechanisms of biological IN in which supercooled water droplets are transformed into ice 8 . ...
It has been known for several decades that some bioaerosols, such as ice-nucleation-active (INA) bacteria, especially Pseudomonas syringae strains, may play a critical potential role in the formation of clouds and precipitation. We investigated bacterial and fungal ice nuclei (IN) in rainwater samples collected from the Hulunber temperate grasslands in North China. The median freezing temperatures (T50) for three years’ worth of unprocessed rain samples were greater than −10 °C based on immersion freezing testing. The heat and filtration treatments inactivated 7–54% and 2–89%, respectively, of the IN activity at temperatures warmer than −10 °C. We also determined the composition of the microbial community. The majority of observed Pseudomonas strains were distantly related to the verified ice-nucleating Pseudomonas strains, as revealed by phylogenetic analysis. Here, we show that there are submicron INA particles <220 nm in rainwater that are not identifiable as the known species of high-INA bacteria and fungi and there may be a new potential type of efficient submicroscale or nanoscale ice nucleator in the regional rainwater samplers. Our results suggest the need for a reinterpretation of the source of high-INA material in the formation of precipitation and contribute to the search for new methods of weather modification.
... Examples of bioaerosol include bacteria, fungi, viruses, pollen, cell debris and bio-films that range in size between 10 nm and 100 mm. [51] The role of bacteria in ice nucleation has been known for decades and is fairly well understood.525354 However, much less is known about the potential role of airborne microorganisms in the formation of both water and ice clouds, along with the production of snow. ...
A combined field and laboratory study was conducted to improve our understanding of the chemical and hygroscopic properties of organic compounds in aerosols sampled in the background continental atmosphere. PM2.5 (particles with aerodynamic diameters smaller than 2.5 µm) aerosols were collected from 24 June to 28 July 2010 at Storm Peak Laboratory (SPL) in the Park Range of northwestern Colorado. New particle formation (NPF) was frequent at SPL during this campaign, and the samples were not influenced by regional dust storms. Filter samples were analyzed for organic carbon (OC) and elemental carbon (EC), water soluble OC (WSOC), major inorganic ions, and detailed organic speciation. WSOC was isolated from inorganic ions using solid phase absorbents. Hygroscopic growth factors (GFs) and cloud condensation nucleus (CCN) activity of the WSOC were measured in the laboratory. Organic compounds compose the majority (average of 64% with a standard deviation (SD) of 9%) of the mass of measured species and WSOC accounted for an average of 89% (with a SD of 21%) of OC mass. Daily samples were composited according to back trajectories. On average, organic acids, sugars, and sugar alcohols accounted for 12.5 ± 6.2% (average ± SD) of WSOC. Based on the composition of these compounds and that of high molecular weight compounds identified using ultra high resolution mass spectrometry, the organic mass to OC ratio of the WSOC is estimated to be 2.04. The average hygroscopic GFs at RH = 80% (GF80) were 1.10 ± 0.03 for particles derived from isolated WSOC and 1.27 ± 0.03 for particles derived from the total water‐soluble material (WSM). CCN activity followed a similar pattern. The critical diameters at a super‐saturation of 0.35% were 0.072 ± 0.009 and 0.094 ± 0.006 µm for particles derived from WSM and isolated WSOC, respectively. These GF results compare favorably with estimates from thermodynamic models, which explicitly relate the water activity (RH) to concentration for the total soluble material identified in this study.
... Examples of bioaerosol include bacteria, fungi, viruses, pollen, cell debris and bio-films that range in size between 10 nm and 100 mm. [51] The role of bacteria in ice nucleation has been known for decades and is fairly well understood.525354 However, much less is known about the potential role of airborne microorganisms in the formation of both water and ice clouds, along with the production of snow. ...
Water-soluble organic constituents of PM2.5 aerosol (particulate matter with an aerodynamic diameter ≤2.5 µm) have not been well characterised so far. The goal of this work was to perform quantitative analysis of individual water-soluble organic species in aerosol samples collected in July of 2010 at the Storm Peak Laboratory (3210 m above sea level) located in the Colorado Park Range (Steamboat Springs, CO, USA). Aqueous extracts were combined into six composites and analysed for organic carbon (OC), water-soluble organic carbon (WSOC), water-insoluble OC, inorganic ions, organic acids, lignin derivatives, sugar-alcohols, sugars and sugar-anhydrates. Analysis of higher molecular weight water-soluble organics was done using ultrahigh resolution mass spectrometry. Approximately 2400 positive and 4000 negative ions were detected and assigned to monoisotopic molecular formulae in the mass range of 100–800 Da. The higher number of negative ions reflects a predominant presence of highly oxidised organic compounds. Individual identified organic species represented up to 30 % of the water-soluble organic mass (WSOM). The WSOM fractions of the low molecular weight organic acids, sugars and sugar alcohols were 3–12 %, 1.0–16 % and 0.4–1.9 %. Significant amounts of arabitol, mannitol and oxalic acid are most likely associated with airborne fungal spores and conidia that were observed on the filters using high resolution electron microscopy. Overall, higher concentrations of sugars (glucose, sucrose, fructose etc.) in comparison with biomass burning tracer levoglucosan indicate that a significant mass fraction of WSOC is related to airborne biological species.
... The origins and the natures of atmospheric ice nuclei have been pursued by many workers on the basis of observations of ice nculeus concentration (e.g. Schaefer, 1954; Isono et aI., 1959a Isono et aI., , 1959b Soulage, 1957; Bigg & Miles, 1964) and examination of center nuclei of snow crystals with electon-microscope (Kumai, 1951Kumai, , 1961 Isono, 1955 Isono, , 1959 Kumai & Francis, 1962). According to the results of these studies, particles of clay and other minerals have been found to constitute a great part of the atmospheric ice nuclei. ...
Simultaneous collections of ice nuclei in the air were made with instruments of the same type at four sites in the rim of the North Pacific: College, Alaska; Blue Glacier, Washington; Mauna Loa, Hawaii; and Nagoya, Japan. Ice nuclei were collected on filter paper for counting of their number and also sheet meshes for examination with electron microscope.During the period of the collection from the beginning of February through the beginning of March, 1968, three marked maxima of the concentrations of ice nuclei effective at −15°C appeared at each of the sites except Mauna Loa. The peak values were the largest at Nagoya (5.3 nuclei/litre) followed by College (2.7 nuclei/litre) and Blue Glacier (1.3 nuclei/litre). At the Mauna Loa Observatory, no marked peak was observed. Neither a diurnal variation nor any other variations with a specific period have been detected. The result of identification of materials of ice nuclei collected at the four sites shows that clay and other mineral particles constitute the main part of the ice nuclei.The results of the studies on the features of ice nucleus concentration, the trajectories of air masses and the examination of ice nuclei suggest that the ice nuclei detected originated from arid and semi-arid regions of the Asian Continent.
... (d) In some cases, activation of a particle is not reproducible, i.e., a particle that acted as an ice nucleus cannot be activated in further cycles. A similar behaviour was observed by Soulage (1957) for natural mineral dust particles. ...
Heterogeneous ice nucleation on synthetic silver iodide, natural kaolinite and montmorillonite particles via condensation, freezing and deposition modes was studied by environmental scanning electron microscopy (ESEM) in the temperature range of 250–270 K. By increasing the H2O pressure in the sample chamber at constant temperature, ice formation can be studied in situ and can be related to the chemical composition of the particles that can be determined simultaneously. For silver iodide and kaolinite, supersaturation values of first ice formation are in good agreement (1–2% absolute) with diffusion chamber experiments. For both substances, threshold temperatures for the condensation, freezing and deposition modes are also in good agreement (within 2 K) with previous literature data. For montmorillonite, ESEM results for the supersaturation value of first ice formation and for threshold temperatures of condensation freezing and deposition mode lie within the large range reported in the literature.
Knowledge of the small-scale processes that occur within clouds leading to the production of rainfall is required to determine how the release or absorption of latent heat drives atmospheric motion and circulation, to better predict the distribution and phase of precipitation at the Earth’s surface, and to understand how precipitation distributions will vary in a changing climate. This chapter reviews the microphysical processes occurring within and beneath cloud that produce rainfall at the Earth’s surface, highlighting discoveries made in the last 100 years that have led to the development of existing theories. The chapter is divided into sections describing warm rain and cold rain process. For warm rain, topics covered include the nucleation of aerosols into cloud particles, growth of cloud droplets by condensation, production of drizzle drops by collision and coalescence, and evolution of raindrop size distributions by collision-induced breakup below cloud. For cold rain, the topics are primary nucleation mechanisms for ice, growth of ice particles by vapor deposition, accretion of supercooled water and aggregation, their evolution to rain by melting, and the enhancement of ice crystal numbers by secondary ice crystal production processes. Outstanding problems in the understanding of microphysical processes are highlighted throughout the chapter.
It is often difficult to go back through time to find who did the pioneer work in a given area of research. This is not the case for the sea-salt aerosol. Throughout the early years of this century a few papers on the subject appeared in the literature, but they contained little quantitative information. But all this changed in the late 1940’s with the appearance of the first of several pioneer papers by Woodcock. His first paper (Woodcock and Gifford, 1949) described in detail the technique of obtaining the sea-salt particle size distribution. In a later paper Woodcock (1952) presented his ideas on the role of the giant sea-salt particles in the formation of raindrops, and in 1953 he published a paper that for the first time showed the salt particle distribution as a function of wind speed. Though numerous investigators have followed in Woodcock’s footsteps, his 1953 paper is still relevant and extensively quoted today.
Recent studies showed that living microorganisms, including bacteria, fungi and yeasts, are present in the atmospheric water phase (fog and clouds) and their role in chemical processes may have been underestimated. At the interface between atmospheric science and microbiology, information about this field of science suffers from the fact that not all recent findings are efficiently conveyed to both scientific communities. The purpose of this paper is therefore to provide a short overview of recent work linked to living organisms in the atmospheric water phase, from their activation to cloud droplets and ice crystal, to their potential impact on atmospheric chemical processes. This paper is focused on the microorganisms present in clouds and on the role they could play in atmospheric chemistry and nucleation processes. First, the life cycle of microorganisms via the atmosphere is examined, including their aerosolization from sources, their integration into clouds and their wet deposition on the ground. Second, special attention is paid to the possible impacts of microorganisms on liquid and ice nucleation processes. Third, a short description of the microorganisms that have been found in clouds and their variability in numbers and diversity is presented, emphasizing some specific characteristics that could favour their occurrence in cloud droplets. In the last section, the potential role of microbial activity as an alternative route to photochemical reaction pathways in cloud chemistry is discussed.
This essay reviews the multidisciplinary science of bioprecipitation, using it as a lens through which to envision integrative options for land use and water resource management in a new light. Bioprecipitation is the hypothesis that microbial ice nucleators, including Pseudomonas syringae, may be highly adapted causal agents of rain and snow. To the extent that land use policies, including pathogenic eradication campaigns, may inhibit the local production of biotic ice nucleators, they may be responsible for ‘killing’ a generative source of rain. Such possibilities should invite major interest in this gathering field of research. Assuming that it contributes to a richer comprehension of the hydrological cycle's dependence on circulatory biota, these findings should help to stimulate assimilative, integrated reformulations of land use and water management policies and norms.
Sauf au voisinage immédiat de quelques sources de noyaux glaçogènes, le rôle de ces noyaux dans l'absorption de la lumière est toujours très faible parce qu'ils sont peu nombreux. Cependant, il peut exister une relation de proportionnalité entre le pouvoir glaçogène et l'opacité de l'air lorsque la concentration des noyaux de congélation présente un rapport constant avec la concentration des particules responsables de l'absorption de la lumière. La théorie deMie nous apprend que, pour un aérosol normal et homogène, celles-ci sont de «grosses» particules (0.1 μ<r<1 μ). En pratique on peut espérer rencontrer une relation constante dans deux cas: 1) loin de sources de noyaux glaçogènes, lorsqu'après de multiples brassages l'atmosphère devient homogène et que chaque particule acquiert une probabilité d'action comme noyau glaçogène proportionnelle à sa surface; 2) sous le vent d'une source de pollution assez importante pour constituer l'élément perturbateur principal de la visibilité et productrice d'un nombre de noyaux glaçogènes proportionnel aux «grosses» particules inactives émises en même temps qu'eux. Dans tous les autres cas, c'est-à-dire dans la majorité, il ne peut pas exister de relation constante entre les valeurs instantanées du pouvoir glaçogène et de la visibilité.
De nombreuses mesures du pouvoir glaçogène effectuées par l'auteur dans des points de pollutions différentes permettent de vérifier expérimentalement le second cas de relation et montrent que le premier est peu fréquent. Elles confirment qu'il n'existe pas de relation constante dans les autres cas; cependant,en moyenne, de bas pouvoirs glaçogènes y sont fréquemment associés à de bonnes visibilités et inversement. Les mesures fournissent enfin quelques renseignements sur le rôle respectif de la concentration des poussières et de l'humidité de l'air dans la relation pouvoir glaçogène-visibilité.
Air samples collected at several heights above the Arctic Ocean contained at least two types of bacteria and five different fungal spores which could elevate the freezing temperature of water droplets. It was observed that the freezing temperature increased logarithmically with the number of cells per drop up to a maximum temperature dependent upon species. Bacteria were more effective than fungal spores but, unlike the spores, they lost their activity under laboratory growth conditions.
Maximum concentration of nearly 10 microbial cells per liter occurs in clouds and 1 per liter in air outside clouds. However, numbers and varieties of microbial cells in excess of those found near sea ice surface level were observed up to 7 km.
Two series of aerosol collections were made at ground level in several sites in Ivory Coast, in January and June 1986. These two periods are respectively characteristic of the dry and rainy seasons in Western Africa.Oxalate-containing particles are detected with an average concentration of 99.5 ppt. only during the dry period. The dry season aerosol is composed of a mixture of materials produced by the Saharan terrigenous sources and burning vegetation. The oxalate is most likely produced by the combustion of vegetal biomass.The coincidence of the presence of oxalate in the dry season aerosol, and the ice-nucleating properties of this same aerosol shown by Bertrand (1977), suggests that the oxalate is in the form of calcium oxalate, which is an active ice nucleus at −7°C. We present this as a possible source of ice nuclei for the formation of the ice phase in the convective clouds.RésuméDeux séries de prélèvements d'aérosols ont été effectuées, au niveau du sol, dans plusieurs sites de Côte d'Ivoire en janvier et juin 1986, périodes caractéristiques de la saison sèche et de la saison des pluies en Afrique de l'Ouest.L'oxalate est détectè, avec une moyenne de 99,5 ppt, seulement en période sèche quand l'aérosol est constitué d'un mélange produit par la source terrigène saharienne et par la source des feux de végétation. L'oxalate est certainement produit par la combustion de la biomasse végétale.La concordance entre la présence d'oxalate dans l'aérosol de saison sèche et les propriétés fortement glaçogènes de l'aérosol de saison sèche mise en évidence par Bertrand, nous conduit à proposer un processus de formation de l'oxalate de calcium, substance glaçogène dès la température de −7°C, et à souligner la présence d'une nouvelle source possible de noyaux glaçogènes pour la formation de la phase glace dans les nuages convectifs.
To develop theories and numerical models of the formation and
microstructure of clouds and precipitation, it is necessary to identify
the potential sources of ice nuclei in the atmosphere. However, the
subject remains an area of debate. According to the most accepted
theory, the great majority of atmospheric ice nuclei constitute soil
mineral particles. But some evidence appears to favor the hypothesis of
a nonnegligible contribution to the population of effective ice nuclei
made by biogenic material, living or dead. Moreover, some specific human
activities have been identified as prolific sources of particles on
which ice crystals can be generated. In contrast, it has also been
suggested that some anthropogenic effluents deactivate nuclei naturally
occurring in the atmosphere. This paper summarizes present knowledge
about the biogenic and anthropogenic sources of atmospheric ice nuclei.
Recent research reveals an increasingly greater variety of sources and
activities of ice nuclei. However intriguing and potentially significant
these findings are, the overall picture emerging from the review is one
of inconclusive, and sometimes contradictory, results. A standarization
of measurement techniques and a more coordinated and systematic effort
in the search for a general theory of heterogeneous ice nucleation are
needed to answer the fundamental questions, what is the origin of
atmospheric ice nuclei, and what is their activity spectrum?
The actual methods for counting ice nuclei are compared. It appears that the mixing chamber method is most valid.Parmi les mthodes actuelles de mesure du pouvoir glaogne de l''air, la mthode des chambres mlange semble la plus valable.
Recent studies showed that living microorganisms, including bacteria, fungi and yeasts, are present in the atmospheric water phase (fog and clouds) and their role in chemical processes may have been underestimated. At the interface between atmospheric science and microbiology, information about this field of science suffers from the fact that not all recent findings are efficiently conveyed to both scientific communities. The purpose of this paper is therefore to provide a short overview of recent work linked to living organisms in the atmospheric water phase, from their activation to cloud droplets and ice crystal, to their potential impact on atmospheric chemical processes. This paper is focused on the microorganisms present in clouds and on the role they could play in atmospheric chemistry and nucleation processes. First, the life cycle of microorganisms via the atmosphere is examined, including their aerosolization from sources, their integration into clouds and their wet deposition on the ground. Second, special attention is paid to the possible impacts of microorganisms on liquid and ice nucleation processes. Third, a short description of the microorganisms that have been found in clouds and their variability in numbers and diversity is presented, emphasizing some specific characteristics that could favour their occurrence in cloud droplets. In the last section, the potential role of microbial activity as an alternative route to photochemical reaction pathways in cloud chemistry is discussed.
Pseudomonas syringae, l'une des 50 espèces bactériennes phytopathogènes, est caractérisée par une grande variabilité génétique, physiologique et biologique qui s'exprime au travers des 52 pathovars qui la composent. L'espèce Pseudomonas syringae, décrite sur près de 400 espèces végétales (le seul pathovar syringae a été signalé sur 177 hôtes), possède 3 propriétés importantes : un pouvoir pathogène, un pouvoir glaçogène et une aptitude à la vie épiphyte. Dans une première partie, les principales caractéristiques de l'espèce, hormis les caractères génétiques non significativement développés ici, sont présentées. Dans la seconde partie, les 3 propriétés remarquables sont analysées en détail. L'aptitude épiphyte de Pseudomonas syringae s'exprime par une capacité à coloniser la surface des organes aériens des plantes, à s'y multiplier de façon importante. Elle conduit à l'existence de niveaux de population élevés au printemps, et en automne aussi pour les plantes pérennes; ces populations épiphytes constituent un réservoir d'inoculum et sont essentielles pour l'initiation de l'infection. La distribution de ces populations épiphytes, en général abondantes, n'est pas homogène ni entre plantes, ni entre organes d'une même plante ; leur répartition à la surface de la feuille qui constitue leur support essentiel est aussi très hétérogène. La nature de l'interaction entre l'hôte et les bactéries épiphytes n'est pas connue ; l'existence d'un lien entre spécificité parasitaire et la spécificité épiphyte au sein de l'espèce syringae est suspectée. Le pouvoir glaçogène de l'espèce syringae s'exprimant par une capacité à induire une rupture précoce de la surfusion de l'eau ne concerne qu'une fraction de pathovars, essentiellement le pathovar syringae. On a montré qu'il était impliqué dans la gélivité des plantes au printemps (vigne, arbres fruitiers, tomate, pomme de terre...) et qu'il est l'un des facteurs favorisant les premières phases de l'infection bactérienne. Le pouvoir pathogène de Pseudomonas syringae montre une spécificité d'hôte liée au pathovar considéré, sauf pour le pathovar syringae dont la gamme d'hôtes est actuellement contestée par plusieurs auteurs. Le développement de l'infection est conditionné par l'existence de voies de pénétration naturelles (elles sont limitées) ou artificielles (blessures) offertes à l'inoculum ; les conditions climatiques pendant les phases initiales de l'infection et durant l'incubation apparaissent capables de moduler l'intensité de l'infection. La connaissance de l'ensemble de ces données permet une esquisse générale de cycle biologique à partir duquel des stratégies de lutte peuvent être élaborées ; différents exemples sont alors présentés en appui. Diverses perspectives de recherches sur cette espèce bactérienne sont discutées. Pseudomonas syringae, an epiphytic ice nucleation active and phytopathogenic bacterium. Pseudomonas syringae, one of the 50 phytopathogenic bacterial species, is the most frequently occurring bacterium under temperate climatic conditions. P syringae has been isolated from = 400 different plant hosts (177 hosts for pv syringae) and is characterized by high genetic, physiological and biological variability expressed through 52 pathovars. Three important properties are associated with this species: pathogenicity, ice nucleation activity and ability for epiphytic survival. The general characteristics of P syringae, apart from the genetic aspects, which have not been examined here, are presented in the first part of this review. In the second part, the 3 remarkable properties of the species have been analyzed. Epiphytic survival, expressed by a capacity to colonize aerial parts of plants or by a significant epiphytic multiplication characterizes most pathovars. High levels of epiphytic populations can be recovered in the spring and also in autumn for perennial plants. These epiphytic populations constitute an inoculum source and are essential to the development of infection. The generally abundant distribution of epiphytic populations is not homogeneous, either between or within plants; their distribution on the surface of the leaf which constitutes their main support is also very heterogeneous. The nature of the interaction between epiphytic bacteria and their host is not yet known, but a relation between host specificity for pathogenicity and host specificity for epiphytic capability within P syringae has been hypothesized. The ice nucleation activity of P syringae, expressed as the ability to induce an early nucleation of water, concerns only some pathovars, mainly the pathovar syringae. It has been shown to be involved in the ice nucleation of plants and frost damage in spring (eg grapevine, tomato, potato) and to be one of the favorable factors in the first stages of some bacterial infections. The pathogenicity of Pseudomonas syringae is dependent on the pathovar except for pv syringae which exhibits a rather wide host range. Some authors have contested this host range for the latter pathovar. The development of infection caused by Pseudomonas syringae is conditioned by the existence of natural (these are limited) or artificial (wounds) means of penetration into the plant tissues and by the inoculum level. Climatic conditions during the first stages of infection or during the incubation period appear to be involved in the modulation of disease intensity. These biological data are useful in understanding the life cycle of P syringae and in elaborating an efficient strategy for disease control; different examples are then presented for illustration. Finally, further developments of research on this phytopathogenic bacterium have been discussed.
This study focuses on ice initiation and ice multiplication processes in warm-based precipitating shallow cumulus clouds. The five principal components of the investigation are: (1) development and application of a two-cylinder cloud and aerosol interaction model which allows sensitivity tests on the microphysical processes; (2) analysis of the role of perturbation pressure in the evolution of cloud drop spectra; (3) analysis of the impacts of cloud drop spectra on ice formation; (4) evaluation of the impacts of concentration of cloud condensation nuclei on the ice formation and the dynamics of warm-based precipitating shallow cumulus clouds; and (5) analysis of the influences of ice-active bioaerosols on ice bursts. A time-dependent cloud and aerosol interaction two-cylinder model is formulated which incorporates the explicit microphysical processes of cloud condensation nuclei (CCN) and ice nuclei (IN). Perturbation pressure is determined explicitly by a Fourier Bessel series expansion. The aerosol masses of CCN and IN in hydrometeors are calculated explicitly in the warm rain formation and the ice crystal riming processes. Simulation results show that the updraughts induced by the gradient force of the dynamic pressure result in the new activation of cloud droplets at the cloud-clear air interface. The broadening of the droplet spectra at the cloud top results in a continuous feeding process of small cloud droplets. This feeding process can accelerate the speed of warm rain formation due to large differences in gravitational settling velocities between the small-sized cloud drops originally activated at the cloud top and large-sized cloud drops activated at the cloud base. The existence of the cloud drop activation process at the interface of cloud and clear air at the cloud summit allows the occurrence of the condensation freezing process. On the other hand, the processes of immersion freezing and contact freezing become significant when precipitation-sized water drops appear in the interior cloud. Both of these ice nucleation modes contribute to ice crystal bursts when the small rain drops reach subfreezing heights. This finding indicates that the Hallett-Mossop mechanism can explain ice crystal multiplication in warm-based precipitating shallow cumulus clouds. An increase in concentration of CCN can strengthen the ice formation and the cloud development. The different ice-nucleating efficiencies of bioaerosols can lead to the different times of ice initiation and ice production speeds. Cette étude s'intéresse à la nucléation de la glace ainsi qu'a sa multiplication durant les processus chaud de formation de précipitation dans les cumulus. Les cinq principaux aspects de cette étude sont (1) le développement et l'application d'un modèle de nuages à deux cylindres incluant l'interaction des nuages et des aérosols. Ce modèle permet de faire des tests de sensibilité des processus microphysiques; (2) L'analyse de l'effet du changement de pression durant l'évolution du spectre de gouttelettes de nuages; (3) L'analyse de l'impact du spectre de gouttelettes de nuages sur la formation de glace; (4) L'analyse de l'impact de la concentration de noyaux de condensation de gouttelettes d'eau sur la glaciation des nuages ainsi que la dynamique des formations chaudes de précipitation dans les cumulus; (5) L'analyse de l'influence de la glace issue de bio-aérosols sur la multiplication rapide de glace. Un modèle à deux cylindres dépendant du temps reposant sur l'interaction entre les nuages et les aérosols est développé. Ce modèle inclut aussi les processus microphysiques des noyaux de condensation de nuages (CCN) et des noyaux de glace (IN). La variation de pression est explicitement représentée par un développement en série de Fourier Bessel. La masse des aérosols (CCN et IN) dans les hydrométéores est calculée explicitement dans les processus chauds de formation de la pluie ainsi que dans les processus de givrage des cristaux de glace. Le mouvement ascendant de l'air induit par le gradient de la pression dynamique entraine l'activation de gouttelettes de nuages à l'interface entre l'air et le nuage. L'élargissement du spectre de gouttelettes d'eau au sommet du nuage génère un processus d'alimentation continue de petites gouttelettes de nuages. Ce processus d'alimentation peut accélérer la vitesse de formation de pluie chaude car il existe une grande différence entre la vitesse de chute des petites gouttelettes d'eau initialement activées au sommet du nuage et les plus grosses activées à la base de celui-ci. L'existence du processus d'activation de gouttelettes de nuages à l'interface entre sommet du nuage et l'air permet la manifestation du processus de congélation par condensation. D'autre part, le processus de congélation par immersion et par contact se produit lorsque des gouttes d'eau de dimension favorable pour précipiter apparaissent à l'intérieur du nuage. Ces deux processus de congélation contribuent à la multiplication rapide de glace quand des bandes de petites gouttes de pluie atteignent une altitude ayant une température de l'air sous le point de congélation. Cette observation indique que le mécanisme Hallett-Mossop peut expliquer la multiplication des cristaux de glace dans les processus chauds de précipitation dans les cumulus. De plus, l'augmentation de CCN peut renforcer la glaciation du nuage ainsi que son développement. L'efficacité des différents processus de congélation généré par des bio-aérosols peut influencer le temps d'initiation de la congélation ainsi que la vitesse de la congélation du nuage.
Twenty fungal genera, including 14 Fusarium species, were examined for ice nucleation activity at -5.0 degrees C, and this activity was found only in Fusarium acuminatum and Fusarium avenaceum. This characteristic is unique to these two species. Ice nucleation activity of F. avenaceum was compared with ice nucleation activity of a Pseudomonas sp. strain. Cumulative nucleus spectra are similar for both microorganisms, while the maximum temperatures of ice nucleation were -2.5 degrees C for F. avenaceum and -1.0 degrees C for the bacteria. Ice nucleation activity of F. avenaceum was stable at pH levels from 1 to 13 and tolerated temperature treatments up to 60 degrees C, suggesting that these ice nuclei are more similar to lichen ice nuclei than to bacterial ones. Ice nuclei of F. avenaceum, unlike bacterial ice nuclei, pass through a 0.22-mum-pore-size filter. Fusarial nuclei share some characteristics with the so-called leaf-derived nuclei with which they might be identified: they are cell free and stable up to 60 degrees C, and they are found in the same kinds of environment. Highly stable ice nuclei produced by fast-growing microorganisms have potential applications in biotechnology. This is the first report of ice nucleation activity in free-living fungi.
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