Cloud Formation Mechanisms Research Articles

Nucleation studies in the Martian atmosphere

We developed models for unary nucleation of water and carbon dioxide in the Martian atmosphere. Both homogeneous and heterogeneous nucleation on dust particles were studied. Our models are based on classical theory. We compare results of different adsorption approaches. Heterogeneous nucleation on the abundant dust particles seems to be the primary mechanism of both H2O and CO2 cloud formation in the Martian atmosphere. Heterogeneous nucleation is obtained at a saturation ratio of about 1.18 for H2O and 1.32 for CO2. Homogeneous nucleation is not likely to occur since it would require high supersaturations. We use our models to study nucleation as a function of height at different locations on Mars where ice fog or clouds have been observed. H2O ice nucleation results are in good agreement with surface fog observations and previous model studies. CO2 ice nucleation simulations in the polar hood cloud areas suggest that negative temperature perturbations caused by, e.g., adiabatic cooling in orographic waves or in convective plumes are required for the formation of CO2 clouds.

The APE-THESEO Tropical Campaign: An Overview

The APE-THESEO campaign was held from 15 February to 15 March 1999 from the Seychelles in the western Indian Ocean. APE-THESEO stands for Airborne Platform for Earth observation — (contribution to) the Third European Stratospheric Experiment on Ozone. The campaign aimed to study processes controlling the low water content of the stratosphere, including the mechanisms of cloud formation in the tropical tropopause region, and transport processes, studied using measurements of long-lived trace gases and ozone. Two aircraft were used: the high-altitude research aircraft, M-55 Geophysica, which can fly up to 21 km, and the Falcon of the Deutsches Zentrum fur Luft- und Raumfahrt, a tropospheric aircraft. Seven flights were performed, including the first simultaneous in situ and remote sensing of sub-visible cirrus clouds, the first interception of sub-visible cirrus using in-flight guidance from a path-finding aircraft, and guided descent of a high-altitude research aircraft into the anvil cloud at the top of a tropical cyclone. In this paper we describe the payload used, the objectives for each flight, and the meteorological conditions encountered. First results show that a new type of cloud, which we have dubbed ultra-thin tropical tropopause cloud (UTTC), has been observed frequently, and observed to cover areas of 105 km2. The frequent coincidence of the tropopause and hygropause implies that the western Indian Ocean played an important role in the dehydration of the lower tropical stratosphere during the season of the mission. UTTCs, sub-visible cirrus, and visible cirrus, have all been implicated in the observed dehydration. Tracer measurements indicate little mixing of stratospheric air into the upper tropical troposphere.

Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity

Abstract. Factors controlling the microphysical link between distributions of relative humidity above ice saturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with the help of microphysical trajectory simulations. Our findings are related to results from aircraft measurements and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with concomitant measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing the crucial importance of homogeneous freezing. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale atmospheric models, in particular chemistry-transport and weather forecast models.

Microphysical properties of continental clouds from in situ measurements

AbstractIt is important to gain knowledge about the microphysical characteristics of continental clouds in order to properly understand their formation, their radiative properties and their ability to produce precipitation. Satellites to remotely sense cloud properties, and cloud interactions with aircraft that fly through them require knowledge of cloud microphysics. Moreover, numerical simulations of global climate are sensitive to small systematic changes in cloud optical properties, but very few large datasets are available that document the characteristics of continental clouds. This paper summarizes 9 x 104 km of in‐cloud measurements made by the Central Aerological Observatory over the former USSR during 1977‐84. Statistical characteristics of total water content (W), extinction coefficient (β), effective diameter (Deff) and effective concentration (Neff) are summarized as functions of temperature and cloud type, for measurements archived at 700 m horizontal resolution. The effect of threshold sensitivity, or cloud definition, on the statistical distributions is discussed. Decreases of W, β and Neff with colder temperatures are consistent with our general knowledge of cloud formation. For all temperature intervals and cloud types, correlations between pairs of W, β and Deff are too small thereby barring any hope of simple linear parametrizations. Differences in W, β, Deff and Neff for the various cloud types indicate changes in cloud formation mechanisms. These data can assist in verification studies of cloud parametrization schemes in general‐circulation models, numerical weather‐prediction models, and cloud‐resolving models. The problems of cloud type, scale averaging, variability over a model grid cell, distribution of water between the liquid and ice phase, and the lumping together of precipitating and non‐precipitating clouds must be considered in these studies.

Chemistry and microphysics of polar stratospheric clouds and cirrus clouds.

Ice particles found within polar stratospheric clouds (PSCs) and upper tropospheric cirrus clouds can dramatically impact the chemistry and climate of the Earth's atmosphere. The formation of PSCs and the subsequent chemical reactions that occur on their surfaces are key components of the massive ozone hole observed each spring over Antarctica. Cirrus clouds also provide surfaces for heterogeneous reactions and significantly modify the Earth's climate by changing the visible and infrared radiation fluxes. Although the role of ice particles in climate and chemistry is well recognized, the exact mechanisms of cloud formation are still unknown, and thus it is difficult to predict how anthropogenic activities will change cloud abundances in the future. This article focuses on the nucleation, chemistry, and microphysical properties of ice particles composing PSCs and cirrus clouds. A general overview of the current state of research is presented along with some unresolved issues facing scientists in the future.

Water activity and equilibrium freezing temperatures of aqueous NH4HSO4 solutions from −30 to 25°C

One mechanism of cirrus cloud formation is homogeneous ice nucleation within ammoniated sulfuric acid particles. An important constraint on ice formation is the (NH4)2SO4‐H2SO4‐H2O phase diagram. In the completed laboratory work, the NH4HSO4‐H2O phase diagram is determined over the domain relevant to the troposphere. The H2O vapor pressures of aqueous NH4HSO4 solutions are measured from −30 to 25°C, and the compositions of the aqueous phase in equilibrium with a solid phase (i.e., ice or letovicite) are determined at several temperatures. Below 25°C and above 42 wt% NH4HSO4(aq), the first solid to form from supersaturated NH4HSO4(aq) is (NH4)3H(SO4)2(S), not NH4HSO4(s). Supercooling of nearly 20 K is required to initiate letovicite formation for 100 mL samples. The results of this study are in good agreement with the calculations of Clegg et al. [1998].

A theoretical study of the microphysical structure of mixed stratiform frontal clouds and their precipitation

One-dimensional numerical models with a detailed description of the evolution of cloud particles (drops, crystals, cloud nuclei, snowflakes, etc.) are used to study the microphysical processes in supercooled winter frontal clouds. A set of equations is used to simulate the evolution of the processes of condensation, nucleation, freezing, sedimentation, accretion, collection, aggregation, etc. The radius–spectrum of liquid drops is divided into two parts: the diameter either smaller or greater than 20 μm. The spectrum of cloud droplets is formed from the condensation, turbulent diffusion and motion of drops. The size distribution function of the ice particles is assumed to form due to sublimation, turbulent diffusion, motion, riming, accretion and aggregation. Simulations of mixed supercooled clouds are examined to see what extent the different microphysical processes (such as collection, aggregation, freezing, accretion, riming, etc.) and thermodynamical conditions (such as surface temperature and updrafts) can impact on the development of cloud and precipitation. They have shown that the liquid, as well as solid, precipitation from supercooled mixed clouds may be significant, especially at surface temperatures greater than 0°C. The influence of updraft values on liquid precipitation is significant, while the surface temperature affects liquid phase precipitation slightly. The opposite holds for solid precipitation; the temperature is a principal factor, while the updrafts affect solid precipitation only at temperatures above 0°C. A study of the different mechanisms of cloud and precipitation formation shows that all such mechanisms are important. If one is absent, others compensate and can form the precipitation successfully. The obtained spectra of cloud droplets and ice crystals conform to a γ-distribution and spectra of the raindrops correspond to the power distribution. Inclusion of new mechanisms of CCN originating within the cloud changes the droplets spectrum noticeably. Numerical experiments confirm that spectra of aggregates conform to an exponential law and indices of the distributions decrease with an increase of precipitation formation intensity and size of particles.

Phase transformations of the ternary system (NH4)2SO4‐H2SO4‐H2O and the implications for cirrus cloud formation

Homogeneous and heterogeneous growth mechanisms for cirrus cloud formation are considered in light of recent measurements showing the presence of NH4+ ions in the aerosol of the upper troposphere. Low‐temperature ternary phase diagrams of (NH4)2SO4‐H2SO4‐H2O are presented, and the thermodynamic and the kinetic data of the phase transitions are employed to consider the implications of NH4+ ions on the mechanisms of cirrus cloud formation. The analysis concludes that changes in the degree of ammoniation can lead to a crossover in the cloud growth system from homogeneous to heterogeneous nucleation. A corresponding change in the cloud's particle distribution alters the cloud's radiative properties, persistence, and surface area available for heterogeneous chemistry.

エアロゾルと雲の放射がアジアモンスーンに及ぼす影響とその長期予報における意義

The influence of aerosol and cloud radiations upon the Asian summer monsoon was studied by using the global numerical weather prediction (NWP) model at the Japan Meteorological Agency (JMA). We parameterize both the direct and indirect effects of aerosols on radiative processes. In addition, criteria of cloud diagnosis are modified to enhance the cloud cover over the land in contrast to that over the ocean, considering that clouds form more easily over the land due to surface inhomogeneity and stay longer in the atmosphere due to smaller droplet size over the land than over the ocean. We confirm that the prediction of Asian monsoon activity is very sensitive to inclusions of aerosols and land-cloud enhancement. The control model, which uses the same cloud diagnosis scheme both over the land and over the ocean and does not include any effects of aerosols, systematically overpredicts the absorbed solar radiation (ASR) over land. The test model with these processes fairly reduces the systematic errors of land-ocean contrast of ASR. Over the Eurasian continent, the test model reduces ASR and lowers low-level temperature through land-atmosphere interactions. It suppresses monsoonal circulations in Southeast Asia and significantly delays northward migration of the typical fronts around East Asia, such as the Meiyu, Changma and Baiu fronts. The impact of land-cloud enhancement on one-month forecasts of the Asian monsoon seems to be similar to that of aerosols. Although the reduction of systematic errors of ASR indicates the relevance of the total effects of the two parameterization schemes, relative magnitudes of their impacts in nature are still uncertain. Further studies are required on distributions of aerosols and their optical properties, and cloud formation mechanisms particularly associated with the land-ocean difference, including aerosol-cloud interactions.

A Numerical Microphysical Model of the Condensational Venus Cloud

A numerical microphysical model that treats one-dimensional vertical transport of sulfuric acid/water solution aerosols, their nucleation, growth/evaporation, coagulation, and sedimentation, is applied to the condensational middle and lower Venus clouds. The model predicts the vertical profiles of sulfuric acid gas and water vapor, cloud microphysical properties, and the acid concentration of the aerosol particles. Results of model simulations agree favorably with Pioneer Venus particle size spectrometer data, extinction and backscattering coefficients measured by Pioneer Venus and Venera probes,in situand remote observations of cloud optical depth, and sulfuric acid vapor abundances retrieved from Magellan radio occultation data. It is found that the lower Venus cloud is formed as a consequence of a large upward flux of sulfuric acid vapor from the evaporation region below the cloud base. The mechanism of lower cloud formation is probably heterogeneous nucleation of sulfuric acid and water vapors on soluble nuclei. Model altitude profiles of the abundances of water vapor and gaseous sulfuric acid demonstrate the control of these vapors by the condensational cloud. Cloud processes account for the observed decline in concentration of water vapor with altitude, from ≈30 ppm below the clouds to ≈10 ppm at 60-km altitude. The structure of the lower and middle clouds is strongly dependent on the eddy diffusion coefficient. Hence, changes in transport rates may explain the variations in cloud properties revealed by the Pioneer Venus and Venera probein situmeasurements as well as by Earth-based and Galileo near-infrared observations of the lower part of the Venus cloud.

On interstellar cloud formation due to thermal instability

Abstract The condensation mode of thermal instability is studied as a possible mechanism of interstellar cloud formation. It is shown that perturbations with characteristic dynamical time smaller than the cooling time evolve to develop hydrodynamical motions corresponding in general to the acoustic mode which prevents perturbations to condense quietly into clouds. The condensation mode is shown to be driven by thermal instability for a special choice of initial conditions for hydrodynamic variables. Perturbations of sufficiently large wavelength are shown to evolve to a dynamical state similar to the condensation mode independently of the form of the initial perturbations. Possible applications to the interstellar gas and the gas of cooling flows are indicated.

Formation of polar stratospheric clouds on preactivated background aerosols

Results of laboratory simulations of the growth of nitric acid trihydrate, HNO3;•3H2O, on sulfuric acid tetrahydrate, H2SO4•4H2O, are presented. The observations reveal that under typical stratospheric conditions uptake of HNO3 on a H2SO4•4H2O substrate results in a surface coverage of approximately one monolayer or less, and that initial HNO3•3H2O nucleation requires a large supersaturation. We also observe that a H2SO4•4H2O substrate, onto which a HNO3•3H2O film has been deposited and subsequently evaporated, exhibits a remarkable enhancement in its nucleation ability for this nitric acid hydrate. In the stratosphere, PSC particles may experience repeated cycles of evaporation and condensation of HNO3 on preexisting background frozen sulfate aerosols. Hence, growth of HNO3•3H2O on preactivated aerosols provides one important mechanism for polar stratospheric cloud formation and denitrification.

The Kleiner Feldberg Cloud Experiment 1990. An overview

An overview is given of the Kleiner Feldberg cloud experiment performed from 27 October until 13 November 1990. The experiment was carried out by numerous European research groups as a joint effort within the EUROTRAC-GCE project in order to study the interaction of cloud droplets with atmospheric trace constituents. After a description of the observational site and the measurements which were performed, the general cloud formation mechanisms encountered during the experiment are discussed. Special attention is given here to the process of moist adiabatic lifting. Furthermore, an overview is given regarding the pollutant levels in the gas phase, the particulate and the liquid phase, and some major findings are presented with respect to the experimental objectives. Finally, a first comparison attempts to put the results obtained during this campaign into perspective with the previous GCE field campaign in the Po Valley.

Observations of condensation nuclei in the Airborne Antarctic Ozone Experiment: Implications for new particle formation and polar stratospheric cloud formation

The ER‐2 Condensation Nucleus Counter (ER‐2 CNC) was operated in the Airborne Antarctic Ozone Experiment in August, September, and October 1987. The ER‐2 CNC measures the mixing ratio of particles, CN, with diameters from approximately 0.02 μm to approximately 1 μm. Comparisons of CN and other aerosol measurements with N2O show that the vertical profile of the sulfate aerosol was probably displaced downward by the subsidence and mixing processes that determined the inclination of the N2O isopleths. At altitudes above the minimum in the CN mixing ratio profile, CN mixing ratios correlated negatively with that of N2O, demonstrating new particle production. The region of particle production was above and south of the 160 ppbv N2O isopleth. The relationship between CN and N2O did not change noticeably when moving from air containing normal levels of reactive nitrogen compounds (NOy) to air depleted in those compounds. This suggests that the removal of NOy was accompanied by the removal of a small minority of the CN. Concentrations of CN are compared with those of larger particles to study cloud formation mechanisms. In some cases condensation of water or nitric acid trihydrate following rapid cooling caused most of the CN to grow to diameters larger than 0.81 μm. Models published elsewhere predict that these conditions should result in a large fraction of particles growing to these sizes. Episodes of this type probably did not remove reactive nitrogen compounds through particle sedimentation from the chemically perturbed region. Small numbers of large particles were observed in other instances. The composition of the particles and their origins are unclear.

Nucleation and particle formation in the upper atmosphere

In the high latitudes of the summer mesopause, temperatures can be sufficiently cold that the region becomes strongly supersaturated in water at ambient vapor concentrations around a part per million by volume. With supersaturation, water vapor can be removed from the vapor phase to form and grow cloud particles. Meteoric dust and ions are suspected to be the dominant sources of nuclei upon which water can collect. The development of ice particles from cores of meteoric dust involves a process known as heterogeneous nucleation. Particle formation promoted by ionic nuclei is called ion‐induced nucleation. The formalisms for these two competitive mechanisms and their application to conditions of the summer polar mesopause are reviewed. Results suggest that both processes are feasible mechanisms for initiating noctilucent cloud formation. However, because calculated nucleation rates are highly uncertain, the predominance of one or the other nucleation mechanism for noctilucent cloud formation cannot be unambiguously established.

Red Giant Winds as Emission Line Clouds (Broad or Narrow) in Active Galactic Nuclei

It is proposed that the clouds thought responsible for the line emission in AGN are not of uniform density but stratified. Such a stratification may be a result either of their self-gravity or of gas dynamics associated with each cloud (e.g. winds). To fix ideas we assume the latter possibility, we examine the consequences and compare with the observations and the phenomenology of emission line clouds.(For the general properties of these clouds see review by Netzer in this volume). Given the successes of the standard model the reader may wonder why is there any need for revisions. The reasons are given in detail elsewhere (see also Scoville and Norman this volume). In brief these are the drag of clouds though the confining medium, the excessive accretion rates implied by the standard model, the response of the line radiation to changes in the continuum, and the lack of a dynamical mechanism for cloud formation. Finally, in the “standard” picture the narrow line clouds are a distinct population with separate dynamics and origin.

Molecules in galaxies. V - CO observations of flocculent and grand-design spirals

New carbon monoxide (1-0) observations of 29 spiral galaxies are presented and combined with previously existing data for 74 galaxies in order to study the dependence of the average CO surface brightness on the spiral-arm class. The total sample shows no significant correlation between the CO surface brightness and the presence or absence of grand-design spiral structure. The observations imply that most stellar density waves do not significantly influence the average surface brightness of CO. This suggests that, if density waves trigger molecular cloud formation, then they either inhibit the other mechanisms of cloud formation to keep the total formation rate constant or they increase the visible disk area of the Galaxy to keep the average surface density of molecules approximately constant. If density waves do not significantly trigger molecular cloud formation, then the giant CO complexes observed in density-wave spiral arms are temporary conglomerates of smaller molecular clouds that formed independently of the wave. 55 references.

The effect of deforestation on the water cycle in the amazon basin: an attempt reformulate the problem

If widespread deforestation in Amazon results in reduced evaporative water flux, then either a decrease in evaporation is compensated locally by reduced rainfall,or else changed moisture balance expresses itself downwind in the yet undisturbed forest. The question of where rain will occur is crucial. It is suggested that the appearance of clouds and the occurrence of rainout is governed primarily by the interplay of local meteorologic and physical geography parameters with the atmospheric stability structure except for a few well-defined periods when rain is dominated by large scale atmospheric instability. This means that the study of these phenomena (local heat balances,studies on cloud formation mechanism, vertical atmospheric stability, etc.) must be made on the scale of the cloud size, a few tens of kilometers at most.

The size and mass distribution of Galactic molecular clouds

By combining observational results on small and large molecular clouds, the spectra of average cloud sizes and masses are obtained, resembling log-normal distributions. The shape of these distributions should place constraints on the mechanism of cloud formation.

Impurity clouds in silicon and germanium

Pure crystals of silicon and germanium contain inhomogeneities of typical size a ∼ 6 μm revealed by low-angle scattering of a laser beam. Photo-excitation and the effect of heat treatment on the scattered intensity and size a leads to the conclusion that the inhomogeneities are “impurity clouds”, i.e. crystal regions with enhanced concentrations of charged impurity centres. The established mechanism of cloud formation is dissolution of oxide inclusions at high temperature (120°C below the melting point), subsequent oxygen diffusion (giving rise to “oxygen clouds”) and finally interaction of oxygen with fast-diffusing impurities at low temperature. The so-called swirl-pattern found in etched dislocation-free silicon (and attributed previously to structural microdefects) is shown to originate from impurity clouds.