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The role played by fluctuations of supersaturation in the growth of cloud droplets is examined in this study. The stochastic condensation framework and the three regimes of activation of cloud droplets— namely, mean dominant, fluctuation influenced, and fluctuation dominant—are used for analyzing the data from high-resolution large-eddy simulations of the Pi convection-cloud chamber. Based on a detailed budget analysis the significance of all the terms in the evolution of the droplet size distribution equation is evaluated in all three regimes. The analysis indicates that the mean-growth rate is a dominant process in shaping the droplet size distribution in all three regimes. Turbulence introduces two sources of stochasticity, turbulent transport and particle lifetime, and supersaturation fluctuations. The transport of cloud droplets plays an important role in all three regimes, whereas the direct effect of supersaturation fluctuations is primarily related to the activation and growth of the small droplets in the fluctuation-influenced and fluctuation-dominant regimes. We compare our results against the previous studies (experimental and theory) of the Pi chamber, and discuss the limitations of the existing models based on the stochastic condensation framework. Furthermore, we extend the discussion of our results to atmospheric clouds, and in particular focus on recent adiabatic turbulent cloud parcel simulations based on the stochastic condensation framework, and emphasize the importance of entrainment/mixing and turbulent transport in shaping the droplet size distribution.

The prediction of ice cloud formation in the atmosphere remains challenging. Free tropospheric aerosols can act as ice nucleating particles, affecting cloud properties and precipitation. The physic...

Microphysical processes are important for the development of clouds and thus Earth's climate. For example, turbulent fluctuations in the water vapor mixing ratio, r, and temperature, T, cause fluctuations in the saturation ratio, S. Because S is the driving factor in the condensational growth of droplets, fluctuations may broaden the cloud droplet size distribution due to individual droplets experiencing different growth rates. The small-scale turbulent fluctuations in the atmosphere that are relevant to cloud droplets are difficult to quantify through field measurements. We investigate these processes in the laboratory using Michigan Tech's Π Chamber. The Π Chamber utilizes Rayleigh–Bénard convection (RBC) to create the turbulent conditions inherent in clouds. In RBC it is common for a large-scale circulation (LSC) to form. As a consequence of the LSC, the temperature field of the chamber is not spatially uniform. In this paper, we characterize the LSC in the Π Chamber and show how it affects the shape of the distributions of r, T, and S. The LSC was found to follow a single roll with an updraft and downdraft along opposing walls of the chamber. Near the updraft (downdraft), the distributions of T and r were positively (negatively) skewed. At each measuring position, S consistently had a negatively skewed distribution, with the downdraft being the most negative.

Heterogeneous ice nucleation is a crucial phenomenon in various fields of fundamental and applied science. We investigate the effect of surface cations on freezing of water on muscovite mica. Mica is unique in that the exposed ion on its surface can be readily and easily exchanged without affecting other properties such as surface roughness. We investigate freezing on natural (K+) mica and mica in which we have exchanged K+ for Al3+, Mg2+, Ca2+, and Sr2+. We find that liquid water freezes at higher temperatures when ions of higher valency are present on the surface, thus exposing more of the underlying silica layer. Our data also show that the size of the ion affects the characteristic freezing temperature. Using molecular dynamics simulations, we investigate the effects that the ion valency and exposed silica layer have on the behavior of water on the surface. The results indicate that multivalent cations enhance the probability of forming large clusters of hydrogen bonded water molecules that are anchored by the hydration shells of the cations. These clusters also have a large fraction of free water that can reorient to take ice-like configurations, which are promoted by the regions on mica devoid of the ions. Thus, these clusters could serve as seedbeds for ice nuclei. The combined experimental and simulation studies shed new light on the influence of surface ions on heterogeneous ice nucleation.

The Pi Cloud Chamber offers a unique opportunity to study aerosol‐cloud microphysics interactions in a steady‐state, turbulent environment. In this work, an atmospheric large‐eddy simulation (LES) model with spectral bin microphysics is scaled down to simulate these interactions, allowing comparison with experimental results. A simple scalar flux budget model is developed and used to explore the effect of sidewalls on the bulk mixing temperature, water vapor mixing ratio, and supersaturation. The scaled simulation and the simple scalar flux budget model produce comparable bulk mixing scalar values. The LES dynamics results are compared with particle image velocimetry measurements of turbulent kinetic energy, energy dissipation rates, and large‐scale oscillation frequencies from the cloud chamber. These simulated results match quantitatively to experimental results. Finally, with the bin microphysics included the LES is able to simulate steady‐state cloud conditions and broadening of the cloud droplet size distributions with decreasing droplet number concentration, as observed in the experiments. The results further suggest that collision‐coalescence does not contribute significantly to this broadening. This opens a path for further detailed intercomparison of laboratory and simulation results for model validation and exploration of specific physical processes.

Water vapor supersaturation in the atmosphere is produced in a variety of ways, including the lifting of a parcel or via isobaric mixing of parcels. However, irrespective of the mechanism of production, the water vapor supersaturation in the atmosphere has typically been modeled as a Gaussian distribution. In the current theoretical and numerical study, the nature of supersaturation produced by mixing processes is explored. The results from large eddy simulation and a Gaussian mixing model reveal the distribution of supersaturations produced by mixing to be negatively skewed. Further, the causes of skewness are explored using large eddy simulations (LES) and the Gaussian mixing model (GMM). The correlation in forcing of temperature and water vapor fields is recognized as playing a key role.

The National Science Foundation (NSF) provided the funding support for this workshop through grant number AGS-1945681. We are grateful to the following: Susannah Burrows (Pacific Northwest National Laboratory), Hermann Gerber (Gerber Scientific), and Fan Yang (Brookhaven National Laboratory) for looking over the meeting summary for accuracy; Jessica Brassard of Michigan Technological University for invaluable support and advising on the strategy, organization, and logistics of the workshop, as well as for the artistic musings shown in Figure 1; Peter Sullivan and Wojciech Grabowski for facilitating the coordination and co-sponsorship by the Geophysical Turbulence Program; Kris Marwitz of NCAR for administrative and logistical support; And to the more than 60 attendees who actively engaged in brainstorms and discussions, without whom this meeting summary would not have been possible Clouds and aerosols, ubiquitously embedded in turbulent flows, are central to the prediction of weather and climate. The purpose of the workshop described here was to explore scientific questions and set priorities for a large-scale aerosol-cloud-turbulence laboratory facility. (Here, “facility” denotes one or more aerosol/cloud chambers and the associated instrumentation and technical/scientific staff.) Specifically, at the workshop we attempted to gauge community interest and to obtain a sense of priorities for the scientific challenges likely to be amenable to laboratory investigation.

Aerosol indirect effects are one of the leading contributors to cloud radiative properties relevant to climate. Aerosol particles become cloud droplets when the ambient relative humidity (saturation ratio) exceeds a critical value, which depends on the particle size and chemical composition. In the traditional formulation of this problem, only average, uniform saturation ratios are considered. Using experiments and theory, we examine the effects of fluctuations, produced by turbulence. Our measurements, from a multiphase, turbulent cloud chamber, show a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. The laboratory measurements demonstrate cloud formation in mean-subsaturated conditions (i.e., relative humidity

Cloud droplet relative dispersion, defined as the standard deviation over the mean cloud droplet size, is of central importance in determining and understanding aerosol indirect effects. In recent work, it was found that cloud droplet size distributions become broader as a result of supersaturation variability and that the sensitivity of this effect is inversely related to cloud droplet number density. The subject is investigated in further detail using an extensive dataset from a laboratory cloud chamber capable of producing steady-state turbulence. An extended stochastic theory is found to successfully describe properties of the droplet size distribution, including an analytical expression for the relative dispersion. The latter is found to depend on the cloud droplet removal time, which in turn increases with the cloud droplet number density. The results show that relative dispersion decreases monotonically with increasing droplet number density, consistent with some recent atmospheric observations. Experiments spanning fast to slow microphysics regimes are reported. The observed dispersion is used to estimate time scales for autoconversion, demonstrating the important role of the turbulence-induced broadening effect on precipitation development. An initial effort is made to extend the stochastic theory to an atmospheric context with a steady updraft, for which autoconversion time is the controlling factor for droplet lifetime. As in the cloud chamber, relative dispersion is found to increase with decreasing cloud droplet number density.

Diffusional growth of droplets by stochastic condensation and a resulting broadening of the size distribution has been considered as a mechanism for bridging the cloud droplet growth gap between condensation and collision–coalescence. Recent studies have shown that supersaturation fluctuations can lead to a broadening of the droplet size distribution at the condensational stage of droplet growth. However, most studies using stochastic models assume the phase relaxation time of a cloud parcel to be constant. In this paper, two questions are asked: how variability in droplet number concentration and radius influence the phase relaxation time and what effect it has on the droplet size distributions. To answer these questions, steady-state cloud conditions are created in the laboratory and digital inline holography is used to directly observe the variations in local number concentration and droplet size distribution and, thereby, the integral radius. Stochastic equations are also extended to account for fluctuations in integral radius and obtain new terms that are compared with the laboratory observations. It is found that the variability in integral radius is primarily driven by variations in the droplet number concentration and not the droplet radius. This variability does not contribute significantly to the mean droplet growth rate but does contribute significantly to the rate of increase of the size distribution width.

Prior observations have documented the process of cloud cleansing, through which cloudy, polluted air from a continent is slowly transformed into cloudy, clean air typical of a maritime environment. During that process, cloud albedo changes gradually, followed by a sudden reduction in cloud fraction and albedo as drizzle forms and convection changes from closed to open cellular. Experiments in a cloud chamber that generates a turbulent environment show a similar cloud cleansing process followed by rapid cloud collapse. Observations of 1) cloud droplet size distribution, 2) interstitial aerosol size distribution, 3) cloud droplet residual size distribution, and 4) water vapor supersaturation are all consistent with the hypothesis that turbulent fluctuations of supersaturation accelerate the cloud cleansing process and eventual cloud collapse. Decay of the interstitial aerosol concentration occurs slowly at first then more rapidly. The accelerated cleansing occurs when the cloud phase relaxation time exceeds the turbulence correlation time.

Moist Rayleigh–Benard convection with water saturated boundaries is explored using a One-Dimensional Turbulence model. The system involves both temperature and water vapour pressure as driving scalars. The emphasis of the work is on a supersaturation , a nonlinear combination of and that is crucial to cloud formation. Its mean as well as fluctuation statistics determine cloud droplet growth and therefore precipitation formation and cloud optical properties. To explore the role of relative scalar diffusivities for temperature ( ) and water vapour ( ), three different regimes are considered: , and . Scalar fluxes (Nusselt number, and Sherwood number, ) and their scalings with moist Rayleigh number are consistent with previous studies of one-component convection. Moreover, variances of the scalars in the bulk region increase with their diffusivities and also reasonably follow derived scaling expressions. Eulerian properties plotted in coordinates have a different slope compared to an idealized mixing process. Additionally, the scalars are highly correlated, even in the cases of high relative diffusivities (factor of four) and . Based on the above fact and the scaling relation of the scalars, the supersaturation variance is found to vary approximately as , in agreement with numerical results. Finally, the supersaturation profile in the boundary layer is explored and compares well with scalar boundary layer models. A sharp peak appears in the boundary-layer-supersaturation profile, not only in the variance but also in the mean profile, due to relative diffusivities of the scalars.

A detailed understanding of interactions of aerosols, cloud droplets/ice crystals, and trace gases within the atmosphere is of prime importance for an accurate understanding of Earth’s weather and climate. One aspect that remains especially vexing is that clouds are ubiquitously turbulent, and therefore thermodynamic and compositional variables, such as water vapor supersaturation, fluctuate in space and time. With these problems in mind, a multiphase, turbulent reaction chamber—called the Π chamber because of the internal volume of 3.14 m3 with the cylindrical insert installed—has been developed. It is capable of pressures ranging from 1,000 to –60 hPa and can sustain temperatures of –55° to 55°C, thereby spanning much of the range of tropospheric clouds. To control the relative humidity in the chamber, it can be operated with a stable, unstable, or neutral temperature difference between the top and bottom surfaces, with or without expansion. A negative temperature difference induces turbulent Rayleigh–Bénard convection and associated supersaturation generation through isobaric mixing. Supporting instrumentation includes a suite of aerosol generation and characterization techniques; temperature, pressure, and humidity sensors; and a phase Doppler interferometer. Initial characterization experiments demonstrate the ability to sustain steady-state turbulent cloud conditions for times greater than 1 day, with droplet diameters typically in the range of 5–40 µm. Typical turbulence has root-mean-square velocity fluctuations on the order of 10 cm s–1 and kinetic energy dissipation rates of 1 × 10–3 W kg–1.

Optical propagation is strongly influenced b y t he n umber concentration, size distribution, thermodynamic phase, and spatial distribution of particles in atmospheric clouds. These properties have been investigated in the field using an airborne digital holographic instrument. A laboratory facility has also been developed, in which optical propagation is being investigated in steady-state turbulent-cloud conditions.

Cloud optical properties are determined not only by the number density n(d) and mean radius [Formula: see text] of cloud droplets but also by the shape of the droplet size distribution. The change in cloud optical depth with changing n(d), due to the change in distribution shape, is known as the dispersion effect. Droplet relative dispersion is defined as [Formula: see text]. For the first time, a commonly used effective radius parameterization is tested in a controlled laboratory environment by creating a turbulent cloud. Stochastic condensation growth suggests d independent of n(d) for a nonprecipitating cloud, hence nearly zero albedo susceptibility due to the dispersion effect. However, for size-dependent removal, such as in a laboratory cloud or highly clean atmospheric conditions, stochastic condensation produces a weak dispersion effect. The albedo susceptibility due to turbulence broadening has the same sign as the Twomey effect and augments it by order 10%.

Mineral dust particles in the atmosphere are often aspherical and their shape is important for a multitude of processes. Yet, measurements of the true shape in three dimensions are rare, even in a laboratory setting. Here, we employ atomic force microscopy to determine the 3D shape and surface roughness for two dusts commonly used in laboratory experiments, Arizona test dust (ATD) and kaolinite. Our major finding is that both are thin and remarkably smooth. An oblate spheroidal description is an excellent fit to these data. We use correlation analysis to further examine the surface properties of the dusts and find essentially no features at horizontal spatial scales between ∼10 and 100 nm. This surprising result is supported by the agreement between our specific surface area and BET surface area measurements. The thin, smooth, spheroidal approximation has many implications for physical processes in the atmosphere, including dust transport and radiative transfer.Copyright 2015 American Association for Aero...

Laboratory measurements of freezing by aerosol particles in contact mode are presented. The fraction of particles catalyzing freezing is quantified for three mineral dusts and three strains of bacteria. This is the most comprehensive such dataset to date for temperatures greater than −20°C, relevant for warm, mixed-phase clouds. For Arizona Test Dust, feldspar, or rhyolitic ash, more than 103 particles are required to initiate a freezing event at −20°C in the contact mode. At −15°C, more than 105 particles are required. An ice-negative strain of Pseudomonas fluorescens is an order of magnitude more effective than the mineral dusts at every temperature tested. To the best of the authors’ knowledge, this is the first measurement of contact-mode freezing by an ice-negative bacterium. An ice-positive strain of Pseudomonas syringae reaches its maximum nucleating efficiency, E = 0.1, 12°C higher than does Pseudomonas fluorescens. This is consistent with the behavior of ice-negative and ice-positive bacteria in the immersion mode, as discovered 40 years ago. Surprisingly, cells of the ice-positive strain Pseudomonas syringae CC94 that do not express the ice nucleation active gene showed no contact-freezing activity, whereas the cells of the ice-negative strain of Pseudomonas fluorescens showed significant activity.

This study presents a new technique to study ice nucleation by aerosols in the contact mode. Contact freezing depends upon the interaction of a supercooled droplet of water and an aerosol particle, with the caveat that the particle must be at the air–water interface. To measure nucleation catalyzed in this mode, the technique employs water droplets that are supercooled via a temperature-controlled copper stage, then pulls aerosol-laden air past them. Particles deposit out of the airstream and come into contact with the surface of the droplet. The probability that a particle–droplet collision initiates a freezing event, necessitating knowledge of the total number of particles that collide with the droplet, is reported. In tests of the technique, ice nucleation by the bacteria Pseudomonas syringae is found to be more efficient in the contact mode than in the immersion mode by two orders of magnitude at −3°C with the difference diminishing by −8°C.

The reason why ice nucleation is more efficient by contact nucleation than by immersion nucleation has been elusive for over half a century. Six proposed mechanisms are summarized in this study. Among them, the pressure perturbation hypothesis, which arose from recent experiments, can qualitatively explain nearly all existing results relevant to contact nucleation. To explore the plausibility of this hypothesis in a more quantitative fashion and to guide future investigations, this study assessed the magnitude of pressure perturbation needed to cause contact nucleation and the associated spatial scales. The pressure perturbations needed were estimated using measured contact nucleation efficiencies for illite and kaolinite, obtained from previous experiments, and immersion freezing temperatures, obtained from well-established parameterizations. Pressure perturbations were obtained by assuming a constant pressure perturbation or a Gaussian distribution of the pressure perturbation. The magnitudes of the pressure perturbations needed were found to be physically reasonable, being achievable through possible mechanisms, including bubble formation and breakup, Laplace pressure arising from the distorted contact line, and shear. The pressure perturbation hypothesis provides a physically based and experimentally constrainable foundation for parameterizing contact nucleation that may be useful in future cloud-resolving models.

Radiative transfer through particle-laden media such as clouds can be impacted by variations in particle spatial distributions. Due to mixing and inertial effects of droplets suspended in the almost always turbulent atmosphere, cloud particles are often spatially-correlated. The correlations result in clusters and voids within the droplet field that, even when smaller than the photon mean free path, can lead to deviations from the exponential extinction law. Prior work has numerically investigated these departures from exponential attenuation in absorptive media; this work extends those results for a scattering medium. The problem is explored with a Monte Carlo Ray Tracing (MCRT) program capable of tracking light attenuation through both perfectly random (uncorrelated) and spatially correlated collections of scatterers and/or absorbers. The MCRT program is favorably compared to two-stream flux equations, and numerical exploration of the pure-absorption case is used to determine the sampling statistics necessary to characterize radiative transmission within the numerical simulation. Light transmission through fields of spatially-correlated, non-absorbing, scattering particles is explored. Particles are distributed following a Matern Point Process, which allows cluster strength and size, as well as the usual variables of particle scattering cross section and number density to be varied. The results show that the degree of non-exponential attenuation is determined by the magnitude and shape of the radial distribution function, which describes correlations in discrete (non-continuous) particle distributions. Parametric studies revealed that the number of clusters and cluster radius, factors in the Matern radial distribution function, impact direct, diffuse and backward radiative transfer. The Matern RDF is shown to be consistent with a previous “cloudlet” approach, providing a bridge between the analytical cloudlet model and continuous correlation function approaches.

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The influence of aerosol concentration on the cloud-droplet size distribution is investigated in a laboratory chamber that enables turbulent cloud formation through moist convection. The experiments allow steady-state microphysics to be achieved, with aerosol input balanced by cloud-droplet growth and fallout. As aerosol concentration is increased, the cloud-droplet mean diameter decreases, as expected, but the width of the size distribution also decreases sharply. The aerosol input allows for cloud generation in the limiting regimes of fast microphysics ([Formula: see text]) for high aerosol concentration, and slow microphysics ([Formula: see text]) for low aerosol concentration; here, [Formula: see text] is the phase-relaxation time and [Formula: see text] is the turbulence-correlation time. The increase in the width of the droplet size distribution for the low aerosol limit is consistent with larger variability of supersaturation due to the slow microphysical response. A stochastic differential equation for supersaturation predicts that the standard deviation of the squared droplet radius should increase linearly with a system time scale defined as [Formula: see text], and the measurements are in excellent agreement with this finding. The result underscores the importance of droplet size dispersion for aerosol indirect effects: increasing aerosol concentration changes the albedo and suppresses precipitation formation not only through reduction of the mean droplet diameter but also by narrowing of the droplet size distribution due to reduced supersaturation fluctuations. Supersaturation fluctuations in the low aerosol/slow microphysics limit are likely of leading importance for precipitation formation.

Earth’s atmosphere can significantly impact the propagation of electromagnetic radiation, degrading the performance of imaging systems. Deleterious effects of the atmosphere include turbulence, absorption and scattering by particulates. Turbulence leads to blurring, while absorption attenuates the energy that reaches imaging sensors. The optical properties of aerosols and clouds also impact radiation propagation via scattering, resulting in decorrelation from unscattered light. Models have been proposed for calculating a point spread function (PSF) for aerosol scattering, providing a method for simulating the contrast and spatial detail expected when imaging through atmospheres with significant aerosol optical depth. However, these synthetic images and their predicating theory would benefit from comparison with measurements in a controlled environment. Recently, Michigan Technological University (MTU) has designed a novel laboratory cloud chamber. This multiphase, turbulent “Pi Chamber” is capable of pressures down to 100 hPa and temperatures from -55 to +55°C. Additionally, humidity and aerosol concentrations are controllable. These boundary conditions can be combined to form and sustain clouds in an instrumented laboratory setting for measuring the impact of clouds on radiation propagation. This paper describes an experiment to generate mixing and expansion clouds in supersaturated conditions with salt aerosols, and an example of measured imagery viewed through the generated cloud is shown. Aerosol and cloud droplet distributions measured during the experiment are used to predict scattering PSF and MTF curves, and a methodology for validating existing theory is detailed. Measured atmospheric inputs will be used to simulate aerosol-induced image degradation for comparison with measured imagery taken through actual cloud conditions. The aerosol MTF will be experimentally calculated and compared to theoretical expressions. The key result of this study is the proposal of a closure experiment for verification of theoretical aerosol effects using actual clouds in a controlled laboratory setting.

Unlike reversible phase transitions, the amount of heat released upon freezing of a metastable supercooled liquid depends on the degree of supercooling. Although terrestrial supercooled water is ubiquitous and has implications for cloud dynamics and nucleation, measurements of its heat of freezing are scarce. We have performed calorimetric measurements of the heat released by freezing water at atmospheric pressure as a function of supercooling. Our measurements show that the heat of freezing can be considerably below one predicted from a reversible hydrostatic process. Our measurements also indicate that the state of the resulting ice is not fully specified by the final pressure and temperature; the ice is likely to be strained on a variety of scales, implying a higher vapor pressure. This would reduce the vapor gradient between supercooled water and ice in mixed phase atmospheric clouds.

Deleterious effects of the atmosphere on remotely acquired images includes absorption and scattering of light by aerosol particulates, which not only attenuates the signal but can potentially cause blurring due to forward-scattered light accepted by the imaging system. Proposed aerosol scattering models (e.g., Ishimaru) provide a method for simulating the contrast and spatial detail expected when imaging through atmospheres with significant aerosol optical depth. This work explores closure between modulation transfer functions (MTFs) obtained from directly measured images and MTFs calculated from theory using measured cloud properties. The closure experiments are performed in a laboratory cloud chamber in which cloud droplet number density and size distribution are directly measured. Images of a binary knife-edge target were taken with an optical detector on the other side of a water cloud generated through reduction of pressure in the humidified chamber. The key results of this closure experiment are: the theoretical expression for the aerosol MTF is likely overly simplistic and does not account for broad particle size distributions. The significance of optical blurring from light scattering by aerosol particles depends sensitively on the properties of both the particles and the imaging system.

Measurements are presented of the vapor pressure of supercooled water utilizing infrared spectroscopy, which enables unambiguous verification that the authors’ data correspond to the vapor pressure of liquid water, not a mixture of liquid water and ice. Values of the vapor pressure are in agreement with previous work. Below −13°C, the water film that is monitored to determine coexistence of liquid water (at one temperature) and ice (at another, higher, temperature) de-wets from the hydrophilic silicon prism employed in the authors’ apparatus. The de-wetting transition indicates a quantitative change in the structure of the supercooled liquid.

The freezing of supercooled water droplets in the atmosphere, with an emphasis on the entropic aspects of the problem, is examined. Supercooled water is a metastable state and, therefore, the associated phase transition must be irreversible. Temperature-dependent heat capacities of supercooled water and ice are used to calculate the entropy difference. That difference is then used to establish a lower bound on the amount of latent heat that can be liberated by the freezing droplets. The calculation is compared with tabulated values of the latent heat of fusion with surprising results. Based on a novel physical picture of the freezing process, the authors suggest a simple estimate for the effective latent heat that is suitable for heat budget calculations of glaciating clouds. In addition, the authors arrive at a quadratic dependence on supercooling, (ΔT)2, for the irreversible contribution to heat exchange during the freezing process. The proportionality factor is estimated as −0.3 J mol−1 K−2.

Water is unlikely to crystallize homogeneously at temperatures greater than -34 °C. Freezing at higher temperatures is heterogeneous-catalyzed by the presence of a second substance. If that substance is at an air-water interface, then the mode is called contact freezing, and it typically will trigger nucleation at a higher temperature than if the substance were wholly immersed within the liquid. We find that the impact of salt particles initiates freezing in experiments using water droplets at supercoolings of 9 to 16 °C. These results show that contact freezing nuclei need not be effective as immersion mode nuclei. We discuss our results in the context of proposed mechanisms of contact freezing. Finally, we use the time scales for diffusion of heat and of ions and the propagation of a sound wave through the droplet to estimate that contact freezing occurs within 10 ns of impact.

he transport and transformations of water sub-stance in the global water cycle are of fundamental and primary importance for understanding the ecosystems, the land surface hydrology that supports them, and the management of water resources for humans. Th e water cycle is also critical for predicting weather, and the long-term climate balance of the planet. Despite the importance of water and a tremendous body of work researching its processes, critical questions re-garding the global water cycle remain. Th ese questions range from fundamental details of the spectroscopy of water, to the forecasting of precipitation, to closing the budget of the hydrological cycle on many scales.Discussion of these and other topics relevant to contemporary water-cycle research led us to outline some themes, questions, and proposed opportunities. Many of these, and broader, issues have been brought to the forefront of scientific discourse through a number of strategic documents that have recently been published (see “For Further Reading” below). To minimize duplication with earlier eff orts, we focus on integrating themes and strategic opportunities identifi ed at the workshop.

Ice in the troposphere affects a variety of processes, including the formation of precipitation, and cloud lifetime, albedo, dynamics, and electrification. A lack of understanding of the ways in which ice is created and multiplied hampers progress in understanding all of these processes. We survey the state of knowledge, starting with homogeneous nucleation, where current formulations for freezing from both pure water and solutions have considerable predictive power. However, debate still exists on the underlying mechanisms of nucleation. Using the concepts and framework that homogeneous nucleation provides, heterogeneous nucleation, where neither a commonly agreed upon theory nor even standard measurement technique exists, is considered. Investigators have established the ice-nucleating characteristics of broad classes of substances, such as mineral dust and soot, which are important ice nuclei in t he atmosphere, but a coherent theory of why these substances act as they do has yet to emerge. All ice in ...

Most approaches for evaluating rates of fundamental processes in aerosol science depend upon the implicit assumption that aerosol particles are independently and identically distributed in space. The validity of this assumption has not been examined in several decades, despite the fact that the presence of correlations can be shown to alter theoretical expressions significantly for such phenomena as attenuation of electromagnetic radiation and coagulation rate. Here, we provide evidence that the classification of the positions of aerosol particles as a Poisson process--even under stationary conditions--is often in error. In particular, small-scale aerosol clustering is experimentally demonstrated. Some consequences of these findings are discussed.

We present a spectroscopic technique that is a hybrid between reflection-absorption and attenuated total reflection (ATR) spectroscopy. The method, which we call attenuated internal reflection (AIR), is suitable for the investigation of thin film liquids on non-or weakly absorbing substrates. In AIR the interrogating infrared beam is not exposed to vapor that may be associated with the film, but unlike ATR there are few geometric constraints on the substrate. We show the theoretical basis for the method and results from experiments demonstrating its use.

We have studied deliquescence on the (001) face of single crystals of NaCl and find that it is a nucleated phenomenon. The phase transition occurs only after the relative humidity exceeds that found over the saturated solution by at least 5%. The contrast between our observations and previous measurements using salt powders or crystallites leads us to the conclusion that deliquescence is controlled by the differences in energy required to solvate ions from the smooth (001) face and from the defect-rich surfaces of particulate samples.

Predicting the cloud condensation nuclei (CCN) supersaturation spectrum from aerosol properties is a fairly straightforward matter, as long as those properties are simple. During the Indian Ocean Experiment we measured CCN spectra, size-resolved aerosol chemical composition, and aerosol number distributions and attempted to reconcile them using a modified form of Köhler theory. We obtained general agreement between our measured and modeled CCN spectra. However, the agreement was not as good during a time period when organic carbon comprised a quarter of the total mass of the aerosol in the submicron size range. The modeled concentrations overpredict those actually measured during that time period. This suggests that some component, presumably organic material, can inhibit the uptake of water by the electrolytic fraction of the mass.

We have shown that water adsorbed to the (001) plane of muscovite mica has an infrared spectrum consistent with a bonding network that is more structured than that found in bulk water. Isotherms taken at temperatures ranging from near the ice melting point to room temperature suggest that water wets mica incompletely. Additionally, we find that the enthalpy and entropy of the adsorbed water molecules imply a strongly bound first layer. Both enthalpy and entropy approach the bulk values as the thickness of the adsorbed layer increases.

This communique describes a new and novel instrument for measuring concentrations of natural cloud condensation nuclei (CCN) from the free atmosphere. It simultaneously provides the size spectrum of nucleated particles, a useful parameter for gaining insight into the composition of the CCN. The supersaturation spectrum N(S), where N is the number concentration of droplets nucleated at supersaturation S, is readily obtained. This parameter is one of the keys to evaluating aerosol influence on cloud microphysics and on the possible modulations of cloud albedo. Laboratory evaluations show that the instrument has good measurement accuracy and precision, even at low CCN concentrations found in the clean troposphere. The instrument is rugged and reliable.