Dissertations / Theses on the topic 'Atmospheric cloud'

Anthropogenic atmospheric aerosols (suspended particulate matter) can modify the radiative balance (and climate) of the Earth by altering the properties and global distribution of clouds. Current climate models however cannot adequately account for many important aspects of these aerosol-cloud interactions, ultimately leading to a large uncertainty in the estimation of the magnitude of the effect of aerosols on climate. This thesis focuses on the development of physically-based descriptions of aerosol-cloud processes in climate models that help to address some of such predictive uncertainty. It includes the formulation of a new analytical parameterization for the formation of ice clouds, and the inclusion of the effects of mixing and kinetic limitations in existing liquid cloud parameterizations. The parameterizations are analytical solutions to the cloud ice and water particle nucleation problem, developed within a framework that considers the mass and energy balances associated with the freezing and droplet activation of aerosol particles. The new frameworks explicitly account for the impact of cloud formation dynamics, the aerosol size and composition, and the dominant freezing mechanism (homogeneous vs. heterogeneous) on the ice crystal and droplet concentration and size distribution. Application of the new parameterizations is demonstrated in the NASA Global Modeling Initiative atmospheric and chemical and transport model to study the effect of aerosol emissions on the global distribution of ice crystal concentration, and, the effect of entrainment during cloud droplet activation on the global cloud radiative properties. The ice cloud formation framework is also used within a parcel ensemble model to understand the microphysical structure of cirrus clouds at very low temperature. The frameworks developed in this work provide an efficient, yet rigorous, representation of cloud formation processes from precursor aerosol. They are suitable for the study of the effect of anthropogenic aerosol emissions on cloud formation, and can contribute to the improvement of the predictive ability of atmospheric models and to the understanding of the impact of human activities on climate.

Aerosol-cloud interactions, the mechanisms by which aerosols impact clouds and precipitation and clouds impact aerosols as they are released upon droplet evaporation, are investigated by means of explicit high-resolution (3 km) numerical simulations with the Mesoscale Compressible Community (MC2) model. This model, which is non-hydrostatic and compressible, was extended by including separate continuity equations for dry and activated multi-modal aerosol, and for chemical species. The sources and sinks include: particle activation, solute transfer between drops, generation of extra soluble material in clouds via oxidation of dissolved SO2, and particle regeneration. The cloud processes are represented by an advanced double-moment bulk microphysical parameterization.
Three summertime cases have been evaluated: a marine stratus and a cold frontal system over the Bay of Fundy near Nova Scotia, formed on 1 Sep 1995 and extensively sampled as a part of the Radiation, Aerosol, and Cloud Experiment (RACE); and a continental stratocumulus, formed over the southern coast of Lake Erie on 11 July 2001. The marine stratus and the frontal system have been examined for the effects of aerosol on cloud properties and thoroughly evaluated against the available observations. The frontal system and the continental stratocumulus have been evaluated for the effects of cloud processing on the aerosol spectrum.
The marine stratus simulations suggest a significant impact of the aerosol on cloud properties. A simulation with mechanistic activation and a uni-modal aerosol showed the best agreement with observations in regards to cloud-base and cloud-top height, droplet concentration, and liquid water content. A simulation with a simple activation parameterization failed to simulate essential bulk cloud properties: droplet concentration was significantly underpredicted and the vertical structure of the cloud was inconsistent with the observations. A simulation with a mechanistic parameterization and a bi-modal aerosol, including a coarse mode observed in particle spectra below cloud, showed high sensitivity of droplet concentration to the inclusion of the coarse mode. There was a significant reduction in droplet number relative to the simulation without the coarse mode. A similar change occurred in the precipitating system preceding the stratus formation, resulting in an enhancement of precipitation in the weaker (upstream) part of the system while the precipitation in the more vigorous (downstream) part of the system remained almost unaffected.
Aerosol processing via collision-coalescence and aqueous chemistry in the non-drizzling stratocumulus case suggests that impact of the two mechanisms is of similar magnitude and can be as large as a 3-5 % increase in particle mean radius. A more detailed analysis reveals that the impact of chemical processing is oxidant-limited; beyond times when the oxidant (H 2O2) is depleted (∼ 40 minutes), the extent of processing is determined by supply of fresh oxidant from large-scale advection (fresh gaseous emissions are not considered). Aerosol processing via drop collision-coalescence alone suggests, as expected, sensitivity to the strength of the collection process in clouds. Larger particle growth, up to 5-10 %, is observed in the case of the frontal clouds, which exhibit stronger drop collection compared to that in the stratocumulus case. The processed aerosol exerted a measurable impact on droplet concentrations and precipitation production in the frontal clouds. For the case modeled here, contrary to expectations, the processed spectrum (via physical processing) produced higher droplet concentration than the unprocessed spectrum. The reasons explaining this phenomenon and the resulting impact on precipitation production are discussed.
The current work illustrates the complexity of the coupled system at the cloud system scales, revealed earlier at much smaller large eddy scales. If future parameterizations of the regional effect of aerosols on clouds are to be developed, careful consideration is required of the many of feedbacks in the boundary layer.

The study of radiative transfer in multifractal clouds is of great interest, an important application being to Global Climate Models. In this work we develop a formalism analogous to the multifractal singularity formalism for understanding photon scattering statistics in radiative transfer in multifractals, and test the results numerically on lognormal multifractals. Although the results are only exactly valid in the thick cloud limit, the approximation is found to be quite accurate down to optical thickness of $ tau approx1$-10, so the results may be widely applicable. Furthermore we show the possibility of "renormalizing" the multifractal by replacing it with a near equivalent homogeneous medium but with a "renormalized" optical thickness $ tau sp{1/(1+C sb1)}$ where C$ sb1$ is the codimension of the mean singularity of the cloud. We argue that this approximation is likely to continue to be valid for multiple scattering, and is also compatible with recent results for diffusion on multifractals. Finally we analyze cloud liquid water content data and estimate the universal multifractal indices. We find that the scaling is respected over the whole range 5m-330km and that the cloud can in fact be reasonably described by a lognormal multifractal.

Orographically-enhanced clouds are essential for global hydrological cycles. To better understand the structure and microphysics of orographically-enhanced clouds, an airborne study, the Colorado Airborne Mixed-Phase Cloud Study (CAMPS), and a ground-based field campaign, the Storm Peak Lab (SPL) Cloud Property Validation Experiment (StormVEx) were conducted in the Park Range of the Colorado Rockies. The CAMPS study utilized the University of Wyoming King Air (UWKA) to provide airborne cloud microphysical and meteorological data on 29 flights totaling 98 flight hours over the Park Range from December 15, 2010 to February 28, 2011. The UWKA was equipped with instruments that measured cloud droplet and ice crystal size distributions, liquid water content, and 3-dimensional wind speed and direction. The Wyoming Cloud Radar and LiDAR were also deployed during the campaign. These measurements are used to characterize cloud structure upwind and above the Park Range. StormVEx measured temperature and cloud droplet and ice crystal size distributions at SPL. The observations from SPL are used to determine mountain top cloud microphysical properties at elevations lower than the UWKA was able to sample in-situ. To assess terrain flow effects on cloud microphysics and structure, vertical profiles of temperature, humidity and wind were obtained from balloon borne soundings and verified with high resolution modeling. Comparisons showed that cloud microphysics aloft and at the surface were consistent with respect to snow growth processes and previous studies on terrain flow effects. Small ice crystal concentrations were routinely higher at the surface and a relationship between small ice crystal concentrations, large cloud droplet concentrations and temperature was observed, suggesting liquid-dependent ice nucleation near cloud base.

Satellite cloud remote sensing provides us the opportunity to study the spatial and temporal distributions of marine boundary layer clouds, as well as their connections with environments on a global scale. However, cloud remote sensing is not without difficulties; retrievals require numerous simplifying assumptions, placing limits on our understanding of cloud processes. Passive remote sensing retrievals often assume that clouds are homogeneous slabs, when in reality, these clouds often have complex inhomogeneous vertical and horizontal structures. Enhancing our understanding of how cloud inhomogeneity influences passive cloud remote sensing requires comparison between cloud retrievals and the underlying cloud properties. In observational data-sets this can become problematic, as it is difficult to compare satellite and airborne measurements because they have both different observed spatial scales and sensitivities to cloud properties. To avoid these complications, this work is based on a satellite retrieval simulator – a Large-Eddy Simulation (LES) cloud model coupled to radiative transfer and retrieval algorithms. The LES-satellite simulator can be used to study the source of retrieval biases. It provides the underlying realistic cloud structure as a reference, informing conclusions about its impact on various cloud retrieval methods. In the first step we focus on cloud vertical profile, finding that the selection of appropriate vertical profile assumptions for the retrieval of cloud liquid water path. Confirming previous studies, drizzle and cloud top entrainment of dry air are identified as physical features that bias liquid water path retrievals away from adiabatic and toward homogeneous profile assumptions. The mean bias induced by drizzle-influenced profiles was shown to be on the order of 5–10 grams per meter squared. In contrast, the influence of cloud top entrainment was found to be smaller by about a factor of 2. A theoretical framework is also developed to explain variability in LWP retrievals by introducing modifications to the adiabatic effective radius profile. The second step focuses on horizontal inhomogeneity and exploring a comparison of both the bispectral and polarimetric cloud retrieval techniques. Using the satellite retrieval simulator we are able to verify that at high spatial resolution (50 meters) the bispectral and polarimetric retrievals are indeed highly correlated with one another. The small differences at high spatial resolution can be attributed to different sensitivity limitations of the two retrievals. In contrast, a systematic difference between the two effective radius retrievals emerges at coarser resolution. This bias largely stems from differences related to sensitivity of the two retrievals to unresolved inhomogeneities in effective variance and optical thickness. The influence of coarse angular resolution is found to increase uncertainty in the polarimetric effective radius retrieval, but generally maintains a constant mean value. The third study focuses on 3-D radiative effects influencing both total and polarized reflectances and retrievals. Comparisons between the 1-D and 3-D reflectances are made in order to study horizontal photon transfer and radiative smoothing. We find noticeable differences between the total and polarized reflectance 3-D effects, with radiative smoothing and roughening occurring at different scales as well as viewing geometry dependence. Despite these apparently strong 3-D effects on polarized reflectances, the polarimetric retrieval is robust to the influence of 3-D effects – with only sub-micron biases in the retrieval of effective radius.

Microphysical and dynamical interactions between aerosols and clouds are associated with some of the largest uncertainties in projections of future climate. Many possible aerosol effects on clouds have been suggested, but large uncertainties remain. In order to improve model projections of future climate, it is essential that we improve our quantitative understanding of anthropogenic aerosol effects. Several studies investigating interactions between satellite-observed aerosol and cloud properties have been published in recent years. However, the observed relationships are not necessarily due to aerosol effects on clouds. They may be due to cloud and precipitation effects on aerosol, meteorological covariation, observational data errors or methodological errors. An analysis of methodological errors arising through climatological spatial gradients is performed. For region sizes larger than 4°×4°, commonly used in the literature, spurious spatial variations in retrieved cloud and aerosol properties are found to introduce widespread significant errors to calculations of aerosol-cloud relationships. Small scale analysis prior to error-weighted aggregation to larger region sizes is recommended. Appropriate ways of quantifying relationships between aerosol optical depth (τ) and cloud properties are considered, and results are presented for three satellite datasets. There is much disagreement in observed relationships between τ and liquid cloud droplet number concentration and between τ and liquid cloud droplet effective radius, particularly over land. However, all three satellite datasets are in agreement about strong positive relationships between τ and cloud top height and between τ and cloud fraction (f_c). Using reanalysis τ data, which are less affected by retrieval artifacts, it is suggested that a large part of the observed f_c-τ signal may be due to cloud contamination of τ. General circulation model simulations further demonstrate that positive f_c-τ relationships may primarily arise due to covariation with relative humidity, and that negative f_c-τ relationships may arise due to scavenging of aerosol by precipitation. A new method of investigating the contribution of meteorological covariation to the observed relationships is introduced. Extratropical cyclone storm-centric composites of retrieved aerosol and cloud properties are investigated. A storm-centric description of the synoptics is found to be capable of explaining spurious f_c-τ relationships, although the spurious relationships explained are considerably smaller than observed relationships.

Entrainment, extreme inhomogeneous mixing, in the presence of wind shear, and their effect on cloud droplet spectra are investigated. A dynamical model in conjunction with a microphysical model designed to predict evolution of cloud droplet spectra, is employed to perform a two-dimensional simulation of a small nonprecipitating cumulus cloud in the presence of wind shear.
Results show that vortex circulations and penetrative downdrafts are responsible for entrainment of clear air into the cloud structure. Entrainment and mixing are more severe on the downshear side of the cloud leading to a more fragmented structure and often to total dissipation of cloudy air rather than partial dilution as is the case on the upshear side. Mixing followed by uplifting leads to fresh activation of cloud droplets and results in multimodal spectra. In areas where mixing has occurred, the spectra exhibit smaller average radius and larger standard deviation.

Optical depths at visible and infrared wavelengths obtained in Tucson, Arizona before and after the Pinatubo eruption in June 1991 have been used to investigate the characteristics of the stratospheric aerosols due to the Pinatubo eruption. The intrusion of the Pinatubo aerosols over Tucson first occurred on July 26, 1991 when the spectral optical depth values rose to two to four times their normal values. In general, there was a pattern of increase between June 1991 and April 1992, and a gradual decrease after April 1992. The stratospheric Pinatubo aerosol in April 1992 was characterized by a typical columnar total number density on the order of 8.78 x 106 in the size range of 0.2-0.7 μm. The total number density decreased to the order of 9.28 x 105 by April 1994. Simulations of the size distribution using a simple polydisperse coagulation and fallout model showed that both of the processes played a very important role in the evolution and transport of the particles in the interval from April 1992 to March 1993. A strong seasonal variation was observed in the aerosol optical depth data. The values are higher in the winter and spring and lower in the summer and fall. This variation is explained by more effective transport of particles from the tropics poleward in the winter and spring than in the summer and fall. We also observed that there was a reduction in stratospheric ozone associated with the Pinatubo aerosols, possibly because of the extra sites available for heterogeneous chemical reactions. The reduction was more noticeable in the spring and summer than in other seasons. The magnitude of the ozone reduction was in a good agreement with other studies.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 49-52).
The Arctic atmosphere is especially sensitive to changes in climate forcing; however, Arctic processes and feedbacks are not understood well enough to accurately predict how the Arctic environment might change under anthropogenic forcing. Further study of the basic atmospheric processes is needed, especially due to uncertainties in modeling cloud feedbacks. August and September are the months when the Arctic sea surfaces begin to freeze; clouds play an important role in determining when this process begins. In this study, the radiative properties of Arctic stratocumulus are studied by comparing measurements for two days in August 2008 during the Arctic Surface Cloud Ocean Study (ASCOS) with simulations using the Rapid Radiative Transfer Model (RRTM). Cloud radiative forcing for both days is examined, and the modeled radiative fluxes were found to compare well to observations. Sensitivity studies are conducted on single and multi-level stratocumulus clouds to study their radiative interactions with each other. Cloud-top cooling in upper clouds is found to radiatively turn off cloud-top cooling in clouds below it. The RRTM and the surface radiative observations are used together to constrain estimates of liquid droplet radius; constraining these radii shows the sensitivity of shortwave cloud radiative forcing and the insensitivity of long wave cloud forcing to changes in drop size.
by Elizabeth A. Maroon.
S.B.

The feedbacks among aerosols, clouds, and radiation are important components for understanding Earth's climate system and quantifying human-induced climate change, yet the magnitude of these feedbacks remain highly uncertain. Since every cloud droplet in the atmosphere begins with water condensing on a pre-existing aerosol particle, characterizing the ability of aerosols to uptake water vapor and form cloud condensation nuclei (CCN) are key to understanding the microphysics behind cloud formation, as well as assess the impact aerosols have on the Earth system. Through a combination of controlled laboratory experiments and field measurements, this thesis characterizes the ability of atmospheric aerosols to uptake water vapor and become CCN at controlled levels of water vapor supersaturation. The origin of the particle water uptake, termed hygroscopicity, is also explored, being from either the presence of deliquescent soluble material and/or adsorption onto insoluble surfaces. The data collected and presented is comprehensive and includes (1) ground samples of volcanic ash, collected from six recent eruptions re-suspended in the laboratory for analysis, (2) laboratory chamber and flow-tube studies on the oxidation and uptake of surface active organic compounds, and (3) in-situ aircraft measurements of aerosols from the Arctic background, Canadian boreal forests, fresh and aged biomass burning, anthropogenic industrial pollution, and from within tropical cyclones in the Atlantic basin. Having a more thorough understanding of aerosol water uptake will enable more accurate representation of cloud droplet number concentrations in global models, which can have important implications on reducing the uncertainty of aerosol-cloud-climate interactions, as well as additional uncertainties in aerosol transport, atmospheric lifetime, and impact on storm dynamics.

A parameterization that describes in-cloud oxidation of S(IV) by hydrogen peroxide and ozone, has been developed for use in large scale models. This parameterization, which is based on the reaction rate equations and basic cloud characteristics, is an explicit function of the concentration of ambient chemical species and some gross cloud parameters. Comparisons of the parameterization scheme with a well-established three-dimensional cloud chemistry model, and also with the cloud chemistry module of a regional model have been used to formulate and test this parameterization scheme. Results show that the parameterization agrees with the 3-D chemistry model very well and that the parameterization holds considerable potential for application in large-scale models.
Preliminary application in a regional climate model confirms that the parameterization is able to improve the agreement of the mass budget and spectrum distribution of sulphate aerosol with observations.

Cloud top height plays an important roll in the energy budget and is also important for aviation. This thesis concerns the quality control of cloud top height (CTH) retrievals. The approach for quality controlling retrieved CTH has been using the forward simulating software RTTOV. An error estimation function has been developed as well as an investigation to what simplifications can be done regarding the forward simulations for CTH purposes at SMHI. The purpose of the error estimation function is to validate CTH output from CTH retrieval algorithms by giving a rough error estimate of the retrieved CTH compared to what forward simulations predict. For simplifying the forward simulations the most promising results have been shown for lower clouds. Further testing is still of interest and for future work suggestions are provided regarding the error estimation function as well as for simplifying the forward simulations.

Water vapor, clouds and aerosols are three major components in the atmosphere that largely influence the Earth's climate and weather systems. However, there is still a lack of understanding on the distribution and interaction of these components. Large uncertainties still remain in estimating the magnitude and direction of the aerosol indirect effect on cloud radiative forcing, which potentially can either double or cancel out all anthropogenic greenhouse gas effect. In particular, a small variation in water vapor mixing ratio and cloud distribution in the upper troposphere and lower stratosphere (UT/LS) can generate large impacts on the Earth's surface temperature. Yet the understanding of water vapor and clouds in the UT/LS is still limited due to difficulties in observations. To improve our understanding of these components, observations are needed from the microscale (~100 m) to the global scale. The first part of my PhD work is to provide quality-controlled, high resolution (~200 m), in situ water vapor observations using an open-path, aircraft-based laser hygrometer. The laboratory calibrations of the laser hygrometer were conducted using complementary experimental systems. The second part is to compare the NASA AIRS/AMSU-A water vapor and temperature retrievals with aircraft-based observations from the surface to the UT/LS at 87°N-67°S in order to understand the accuracy and uncertainties in remote sensing measurements. The third part of my research analyzes the spatial characteristics and formation condition of ice supersaturation (ISS), the birthplace of cirrus clouds, and shows that water vapor horizontal heterogeneities play a key role in determining the spatial distribution of ISS. The fourth part is to understand the formation and evolution of ice crystal regions (ICRs) in a quasi-Lagrangian view. Finally, to help estimate the hemispheric differences in ice nucleation, the ISS distribution and ICR evolution are compared between the two hemispheres. Overall, these analyses provided a microphysical scale yet global perspective of the formation of ISS and cirrus clouds. Ultimately, these efforts will help to improve the understanding of human activities' influences on clouds, water vapor and relative humidity in the UT/LS and provide more accurate representations of these components in future climate prediction.

The standard 2D/3D picture of atmospheric dynamics of two distinct isotropic regimes separated by a "meso-scale gap" has been seriously questioned in recent years. Using satellite cloud images and the formalism of generalized scale invariance (GSI), we test the contrary hypothesis that cloud radiance fields are scaling in the range 1-1000 km.
Using a two-dimensional representation of GSI and three new analysis techniques, we test the following relation for each picture: $ langle vert F( lambda sp{ tilde G} vec k) vert sp2 rangle = lambda sp{-s} langle vert F( vec k) vert sp2 rangle$, where $F( vec k)$ is the Fourier amplitude at wavenumber $ vec k$, $ lambda$ is the scale ratio and $ tilde G$ is the generator of the semi-group of scale changes in Fourier space. Since we test only the linear approximation to GSI, $ tilde G$ is approximated here as a matrix.
For the three texturally--and meteorologically--very different images analyzed, we find three different generators that generally well reproduce the Fourier space anisotropy. These results show that linear GSI is a workable approximation for studying the atmosphere and that GSI can be used for cloud classification and modeling over this important mesoscale range.

The Acid Deposition and Oxidant Model (ADOM) is an Eulerian long-range transport and deposition model. One of the most highly parametrized and least well established parts of the model is the cloud module that describes cloud formation, pollutant scavenging, aqueous-phase chemistry and wet deposition. As a means of gaining insight into the cloud module, results from simulations with the module are compared with the results of simulations for equivalent conditions with a three-dimensional dynamic cloud chemistry model.
Comparisons of results for a variety of initial conditions show that wet-deposition of sulphate, nitrate and ammonium ions tend to be underpredicted by the cloud module and that the pH of the rain is overpredicted. However, the differences are for the most part not large. Concentrations of hydrogen peroxide deposited at the surface are significantly smaller in the ADOM module than in the cloud chemistry model. The results of the cloud module do seem to be sensitive to the model cloud top height.

Les nuages sont essentiels dans le cycle de l'eau et dans le budget climatique mais certains aspects de leur formation sont encore incompris. La théorie de Köhler prédit que les surfactants devraient favoriser l'activation des particules en goutte de nuage alors que les modèles actuels les considèrent comme négligeables. Au début de ce travail de thèse, quelques études commençaient à démontrer le contraire mais des preuves du rôle de ces composés dans l'atmosphère étaient encore manquantes, d'où l'objectif de ce travail de thèse. Le développement d'une méthode pour déterminer la concentration en surfactants dans les aérosols a conduit aux premières courbes de tension de surface de surfactants atmosphériques dans des aérosols PM2.5 côtiers (Suède), et à l'identification du ratio C/CMC comme paramètre clé contrôlant l'efficacité des aérosols à former des nuages. Une seconde étude a révélé des corrélations fortes entre la présence de nuages et les propriétés intrinsèques des surfactants dans des aérosols PM1 boréaux (Finlande), démontrant pour la première fois le rôle des surfactants dans la formation des gouttes de nuage à partir d'observations directes dans l'atmosphère. Les résultats prédisent un nombre de noyaux de condensation en moyenne quatre fois plus important que lorsque les effets des surfactants étaient négligés, montrant l‘importance d'inclure l'effet des surfactants dans les modèles prédictifs. Cette importance a été confirmée en laboratoire par des expériences sur des gouttes individuelles microniques par l'augmentation de leur taille en présence de surfactants. Enfin, les observations à partir des différentes études indiquent une origine biologique des surfactants dans les aérosols atmosphériques
Clouds are essential components of the Earth’s hydrological system and climate but some aspects of their formation are still not completely understood. In particular, although Köhler theory predicts that surfactants should enhance cloud droplet activation, current models consider this role negligible. At the time of this PhD work, a few studies had started to demonstrate the contrary but atmospheric evidence for the role of these compounds was still missing and very little was known about their atmospheric concentrations, sources, and mechanism of action. The objective of this PhD work was to investigate these aspects. A method was developed to quantify surfactant concentrations in aerosols. Its application led to the first absolute atmospheric surfactants’ surface tension curves, in coastal PM2.5 aerosols in Sweden, and to the identification of the ratio C/CMC as the key parameter controlling the cloud-forming efficiency of aerosols. A second study revealed strong correlations between cloud occurrence and intrinsic surfactant properties in boreal PM1 aerosols in Finland, demonstrating for the first time the role of surfactants in cloud formation from direct atmospheric observations. The results predicted Cloud Condensation Nuclei numbers four times larger on average than when neglecting surfactant effects, showing the importance of including surfactant effects in cloud predictions models. The role of surfactants inferred from macroscopic measurements was confirmed by laboratory experiments on individual micron-sized droplets showing an increase of droplet growth in the presence of surfactants. Finally, observations from the different field studies indicated a biological origin for the surfactants present in atmospheric aerosols

Cirrus clouds are thought to have a significant role in atmospheric processes: specifically; their heating/cooling contribution to the Earth's radiative balance, and the consumption of water substance due to their formation. Their presence in the upper troposphere I lower stratosphere (UTLS) also provides a surface for heterogeneous chemistry. The SLIM CAT-Cirrus model is developed to provide a tool to investigate aspects of these properties. SLIM CAT-Cirrus is based upon the existing SLIM CAT chemistry transport model and a parameterisation of the formation of cirrus ice by homogeneous nucleation. The advantages and drawbacks of the use of legacy models are discussed especially issues regarding the loss of the underlying decision-making regarding design approach, approximations, and assumptions. Techniques adopted and adapted from the software engineering and QA disciplines are used to mitigate these problems and maintain future traceabilty; this takes the form of examples of practical measures that small groups or individuals researchers can use. The difficulty in validating a complex global model in the absence of a definitive reference has been addressed by using diverse measurement data sources, and a suite of statistical merries. Model verification testing is also used to characterise processes that are difficult [0 validate. Validation of the modelled frequency of cirrus occurrence against satellite data showed an initial under-prognosis by the model. To address this a statistical scheme has been devised to reproduce some of the effects of phenomena such as gravity waves that are not resolved by the model grid. The modelled effects of the formation of cirrus on the water budget in the UTLS are comparable with measurements from the HALOE (HALogen Occultation Experiment), and are also in-line with the drying effect cirrus are thought to have on air entering the stratosphere. The radiative effects of cirrus have been represented using specific cirrus radiative parameterisations. The cirrus heating shows positive feedback into vertical transport causing meso-scale uplift of the kind thought to be responsible for part of the BrewerDobson atmospheric circulation.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2003.
Vita.
Includes bibliographical references (leaves 134-148).
Clouds play an extremely important role in our atmosphere, from controlling the local weather, air pollution and chemical balance in the atmosphere to affecting long-term climatic changes at local, regional and global scales. The mechanisms through which tropospheric clouds form are still not fully understood, leading to gross uncertainties in understanding the effect of atmospheric aerosols on the environment. Using laboratory measurements, microphysical properties of typical micro-meter size atmospheric aerosols are investigated in this study. Upper tropospheric ice clouds (cirrus) form when ice is nucleated either homogeneously or heterogeneously in aqueous aerosols. We have investigated the homogeneous and heterogeneous ice nucleation in aqueous particles. Our results for homogeneous nucleation in aqueous ammonium nitrate particles show that the current thermodynamic models do not correctly predict water activities in these particles under super-saturated conditions. High super-saturations are required for ice to nucleate homogeneously in aqueous ammonium nitrate particles. We have also investigated the role of crystallized salt cores, such as solid ammonium sulfate and letovicite, in the heterogeneous nucleation of ice in saturated aqueous ammonium sulfate particles. Our results show that the surface morphology and defects on microcrystals could result in the creation of active sites, leaving the crystallized salt cores as potent ice nuclei under certain conditions. We have also investigated the role of mineral dust and soot, major components of insoluble particulates in the atmosphere, as ice-nuclei. We have found mineral dust to be an effective ice nuclei but both fresh and aged soot do not promote ice nucleation in aqueous particles.
(cont.) Soot is the most ubiquitous aerosol in the atmosphere. The lifetime and microphysics of nano-porous soot has a large impact on earth's radiative budget, heterogeneous chemistry, urban and regional air pollution and human health. We have investigated the hydrophilic properties of both fresh and aged soot as a function of relative humidity. Our results show that fresh hydrophobic soot oxidized (aged) by OH/0₃/UV in the presence of water vapor or by exposure to concentrated HNO₃ becomes hydrophilic and exhibits a greater affinity for water. Due to this increased hydrophilicity, aged soot can be easily entrained inside existing liquid cloud droplets, and even activate as cloud condensation nuclei at high super-saturations, thus influencing its heterogeneous chemistry, radiative properties and atmospheric lifetime.
by Bilal Zuberi.
Ph.D.

A four dimensional variational (4D-Var) formulation is developed for the assimilation of radar reflectivity, Doppler velocities, and near-surface refractivity index data into a non-hydrostatic fully compressible limited-area atmospheric model coupled with a simplified warm microphysics scheme. The cloud-model is used as a weak constraint so the model error is explicit in the 4D-Var formulation. The ultimate goal is to provide initial conditions to a high-resolution numerical weather prediction model. The environmental flow around storms is modelled by a linear wind in a moving frame using Doppler velocity measurements over a given assimilation window. A three-stage procedure is established to solve the assimilation problem. The background-, observationand model-error statistics are adaptively estimated by comparison with a posteriori residuals and they converge after only a few minimizations. During the adaptive procedure, a smoothing constraint is applied to the analysis variables. The smoothing constraint diminishes towards zero for the last minimization while still leading to a smooth analysis.
Experiments with synthetic data from model outputs at 1 km horizontal resolution show that the method is able to retrieve unobserved variables. An assimilation period of 10 minutes is shown to be optimal for the analysis of clouds. All the other variables of the model are rather insensitive to the assimilation period. Here the model time step has been varied from 1 to 5 minutes. Most of the a posteriori residual distributions have a high kurtosis while the velocity and the near-surface refractivity index residual distributions are nearly Gaussian. The data assimilation for the case of a shallow hailstorm suggests that the model used for the assimilation is able to forecast the system for 30 minutes within the estimated observational errors. Application of the method to the initialization of the MC2 model leads to a better forecast of a convective system over 40 minutes than the nowcasting technique based on Lagrangian persistence.

Early scaling stochastic models of cloud and rain fields were designed to obey the simple scale invariance symmetry. The linear nature of these additive processes is intrinsically related to their single fractal dimension which is in sharp contrast with the non-linear nature of the dynamical processes within the atmosphere and with the observed multiple scaling of rain and cloud fields. We consider stochastic models corresponding to coupled cascade processes, non linearly conserving the fluxes of energy and concentration variance. Multiplicative processes, previously based on discrete cascade procedures, are generalized to their continuous limit using a dynamical generator of the cascade characterized by only two parameters which determine the full multifractal spectrum of dimensions. We show how to numerically simulate such multifractal processes with both gaussian and Levy generators, and how to perform a scale invariant "zooming" procedure in the case of clouds passively advected by a turbulent velocity field.

The effects of longwave radiation on the development of a small cumulus cloud were investigated by a combination of three-dimensional radiative transfer model as well as slab-symmetric and axially symmetric cloud dynamics models.
A longwave radiation model was developed. The model was first applied to the case of an isolated cumuliform cloud in the form of a cylinder, the results of which illustrate the importance of cloud side cooling. We proposed that the radiatively forced sinking of air at the sides of cumuliform clouds would lead to lower-level convergence and thereby enhances the upward motion in the interior of the cloud.
A study of cloud top cooling rate in stratiform clouds with positive (lifting) or negative (sinking), axially symmetric perturbations shows that a lifting of the cloud top does not increase cooling rate, whereas the peak cooling rate decreases rapidly in depressions. For small perturbation, $ approx$10 m, this may tend to inhibit the growth of negative perturbations. For larger perturbations, $ approx$100 m, changes to the radiative cooling rates within the positive perturbations and next to the negative perturbations may act to sustain the perturbation and promote its growth.
Using a three-dimensional longwave radiation model and a slab-symmetric cloud dynamics model, we demonstrated that longwave radiative cooling substantially enhances the maximum cloud water content. The maximum increase reaches 96%. The total cloud water was also increased somewhat (maximum 20%).
In the initial stage of the development, the augmentation of cloud water content near the cloud top and sides is traced mainly to the direct effect of longwave radiative cooling on cloud microphysics (i.e. radiative cooling reduces the local temperature and hence the saturation water vapor pressure, which leads to additional condensation).
In the mature stage of cloud, the increase of total cloud water content comes from a combination of the effects of radiation on microphysics and dynamics. The cooling from radiation and evaporation produces additional downward motion at the sides of the cloud. The enhanced low-level convergence invigorates the updraft promoting further cloud development.
In the decaying stage, the negative buoyancy produced by cloud top radiative cooling and a higher liquid water load speeds up the decay process in the LW run.
In a sheared environment, shear suppresses convection. In conjunction with horizontal momentum transport, radiative cooling also results in a more negative temperature perturbation and a stronger downdraft on the downshear flank relative to the upshear side.
In an axially symmetric simulation, longwave cooling produces a weaker updraft in the cloud core. This phenomena was attributed to the shorter lifetime of the axially symmetric cloud, and the fact that the cloud side sinking motion can spread around the cloud core instead of being confined to a vertical plane. As a result, a weaker secondary circulation develops which is offset by the negative effects of the cloud top radiative cooling and the higher liquid water load.

Little is known about clouds during drought. In 1999-2005 the Canadian Prairies experienced one of the most severe and prolonged droughts in its climatological record. The objective of this study is to characterize and better understand clouds and their associated radiative properties during drought in the Canadian Prairie Provinces with a particular focus on this recent drought. Drought severity was determined using the Standardized Precipitation Index. Cloud properties from NASA/GEWEX's Surface Radiation Budget data base were used to examine overall cloud amount, optical thickness, and top-of-the-atmosphere albedo. It was found that, although cloud amounts differ slightly from dry to wet conditions (approximately a 10% increase in cloud cover fraction from 63% when severely dry to 73% when severely wet), the correlation with precipitation is weak. The occurrence of thin clouds increased as drought severity increased, whereas medium thickness clouds and thick clouds decreased. Similar trends were found to pertain at smaller scales.
Très peu est connu a propos des nuages durant une sècheresse. Entre 1999 et 2005, les Prairies canadiennes ont vécu une des sècheresses les plus sévères et les plus longues dans ses registres climatologiques. L'objectif de cette étude est de caractériser et de mieux comprendre les nuages et leurs propriétés radiatives durant les sècheresses dans les provinces des Prairies canadiennes. Une attention particulière a été donnée à cette récente sècheresse. La sévérité de la sècheresse a été déterminée avec l'index standardise de précipitations. Les propriétés des nuages obtenues de la base de données du Budget Radiatif de Surface de NASA/GEWEX ont été utilisées pour examiner l'ensemble de la quantité de nuage, l'épaisseur optique et l'albédo du haut de l'atmosphère. Les résultats ont démontré que malgré une légère différence dans la quantité de nuage entre des périodes sèches et humide (une augmentation d'environ 10% dans la couverture nuageuse de 63% durant des conditions extrêmement sèches a 73% pendant des périodes extrêmement humide), la corrélation avec la précipitation est faible. Les nuages minces augmentent lorsque la sévérité de la sècheresse augmente, alors que les nuages d'épaisseur moyenne ou épaisse diminuent. Les résultats ont aussi démontré que des tendances similaires se rapportent à plus petite échelle.

Aerosols affect the radiation budget of the earth-atmosphere system by directly reflecting or absorbing solar radiation and also indirectly, by altering the cloud albedo through changes in the cloud condensation nuclei concentration (CCN). Increases in CCN concentrations result in an increase in the cloud droplet number concentration (N). Assuming the cloud liquid water content (LWC) stays the same, this will result in smaller cloud droplet sizes. Thus, this will increase cloud reflectance and cloud lifetime as cloud cover also increases. An accurate quantification of the aerosol forcing effect is still not possible due to the complexity involved in understanding aerosol processes and their effects on climate. There has also been a lack of a coordinated effort toward linking surface and in situ observations, as well climate model results and satellite data. Due to the spatial and temporal heterogeneity in aerosol forcing, regional effects are important. In this dissertation, the direct and indirect radiative forcing effects of aerosols - primarily sulfates and to lesser extent soot aerosols at a site located in the southeastern U.S. are investigated by means of surface observations, modeling results and satellite data.During the summers of 1993-96, field experiments were conducted at Mt. Mitchell, North Carolina, at a site representative of the southeastern U.S. to determine the effect of pollutants on the cloud microphysical and optical properties. Analyses of the results from empirical relationships are used to obtain an estimate of the contribution of sulfates to indirect radiative forcing. Concurrent measurements of size resolved chemical concentrations, light scattering and absorption coefficients, aerosol size distribution and optical depth measurements were obtained during the winter of 1997 for cloud-free skies. Data from these measurements are used to investigate the chemical-physical-optical interaction between aerosols and to determine the direct forcing effect of aerosols by means of a column forcing model. Cloud water sulfate concentration is used as a measure of anthropogenic pollution. Back-trajectory analysis is used to identify the source of the air masses classified as polluted continental, continental and marine. The effect of anthropogenic pollution on cloud microphysical properties such as LWC, N, effective radii (Reff), CCN activation spectrum, cloud optical depth and reflectivity are investigated. The relationship between Reff and sulfate for different air masses, as well as the N-sulfate mass relationship, suggests that the counteracting effect of sulfates on greenhouse warming for the southeastern U.S. would be of a magnitude greater than -4.0 W m-2 obtained by previous modeling studies. Acidity variations between cloud droplets of different sizes indicated that on an average, smaller drops are enriched in sulfates, nitrates and ammonium, whereas, larger droplets have higher concentrations of sodium, calcium and magnesium. As part of a closure experiment cloud albedo calculated from in situ measurements was compared to that retrieved from the Advanced Very High Resolution Radiometer data for four years (1993-96). The nonlinear relationships between the cloud microphysical/optical properties and the sulfate content imply the existence of an optimum level for the sulfate concentration that would affect cloud albedo. In terms of the direct forcing effect, wintertime forcing obtained for an internal mixture of sulfate and soot aerosols is much lower than that obtained during summer, due to reduced sulfate concentrations in winter. A quantitative measure of the direct forcing indicates higher magnitudes both for summer and winter than is obtained from previous modeling results. Analyses of the direct and indirect radiative forcing effect of sulfates for the southeastern U.S. indicate that the negative forcing effect is of greater magnitude than is predicted by modeling results. Thus, reduction in sulfate emissions would have a significant impact on climate for the southeastern U.S.

To evaluate the precision of a three dimensional cloud chemistry model, the ion concentrations of simulated clouds are compared with the chemical analysis of cloud water samples collected in the Muskoka, Ontario area. Five summer case studies are presented.
In general, the simulated concentrations of NO$ sbsp{3}{-},$ NH$ sbsp{4}{+},$H$ sp+$, H$ sb2O sb2$ and the concentrations of SO$ sbsp{4}{2-}$ calculated with the perfect nucleation condition concur with the observed concentrations. For the four polluted cases, more than 80% of the cloud water SO$ sbsp{4}{2-}$ comes from nucleation, whereas for the clean case, it is mostly created by SO$ sb2$ oxidation. The relative importance of $ rm H sb2O sb2$ and O$ sb3$ in SO$ sb2$ oxidation, as well as the relative importance of nucleation and NH$ sb3$ absorption on the cloud water NH$ sbsp{4}{+}$ budget vary according to the chemical state of the atmosphere.

Previous studies, such as van der Veen (2012) and White et al. (2017), have demonstrated the potential of using measurement-based cloud data to improve Numerical Weather Prediction (NWP) based cloud forecasts. This can be done through cloud initialization; a process of injecting cloud data after the regular data assimilation in an NWP model. The purpose of this study was to use cloud data from the Mesoscale Analysis system MESAN to investigate cloud initialization in the HARMONIE-AROME model system for improving short-range cloud forecasts. The cloud initialization method that was used was similar to a method used by van der Veen (2012), where specific humidities, temperatures, and hydrometeor concentrations were altered using information on cloud fractions, cloud base heights and cloud top heights. MESAN input data analyses as well as cloud initialization investigations were carried out. MESAN input data analyses revealed significant differences in cloud fractions between MESAN and the background model field in MESAN. Overestimations of cloud fractions in MESAN over sea were caused by satellite data, particularly due to the inclusion of the fractional cloud category. Underestimations of cloud fractions over land were caused by limitations of the synoptic weather (SYNOP) stations in measuring clouds. Furthermore, larger differences between MESAN and SYNOP were found over Sweden and Finland compared to Norway, which may be tied to Norway having mostly manual SYNOP stations, and Sweden and Finland having mostly automatic stations. Shortcomings were found in the investigated cloud initialization method. Such shortcomings involved a limit check on the specific humidity change, the cloud initialization being repeated for an unnecessarily large amount of iterations, and the use of a sub-optimal profile of critical relative humidity. Using a one-dimensional vertical column version of HARMONIE-AROME, named MUSC, to integrate forward in time revealed a large sensitivity to the use of forcing profiles and forcing time scales in MUSC. Alterations made through cloud initialization were found to last over 12 h, with varying effects depending on the investigated height. A reasonably good agreement between MUSC results and results from the three-dimensional version of HARMONIE-AROME was found. Findings in this thesis point at potential to further enhance the HARMONIE-AROME cloud initialization technique. These enhancements concern a revised MESAN cloud product and taking care of some flaws in the cloud initialization method.
I en operationell vädermodell inkluderas olika mätdata, såsom temperatur och atmosfärstryck, i ett regelbundet intervall. Molnighet är inte vanligtvis en del av dessa cykler; istället bildas molnen av modellen utifrån balanser i de andra fysikaliska fälten. Detta projekt gick ut på att direkt införa molnmätningar från väderanalyssystemet MESAN i vädermodellsystemet HARMONIE-AROME genom en metod som kallas molninitialisering. Specifikt förbättringar för korttidsprognoser var av i ntresse. MESAN är ett system vars produkter är en sammanslagning av ett bakgrundsfält från en vädermodellkörning med olika mätdata. I MESAN kommer molndata från tre källor: bakgrundsfältet, satellitdata och synoptisk väderstationsdata (SYNOP-data). Undersökningar av indata till MESAN samt molninitialiseringsmetoden har utförts. Analyser av indata till MESAN visade på överskattningar av moln i satellitdata över hav och underskattningar av moln i SYNOP-data över land. För satellitdatat berodde detta på medtagande av moln på liten skala eller väldigt tunna moln, medan det för SYNOP berodde på begränsningar i mätmetoderna. Det fanns även en skillnad i kvalitet i SYNOP-data i Sverige och Finland gentemot Norge, vilket kan bero på att de flesta mätstationer i Norge är manuella medan de flesta i Sverige och Finland är automatiska. Molninitialiseringsmetoden bestod i att extrahera data om molnbashöjd och molntopphöjd från MESAN, och sedan modifiera fuktighet, temperatur och hydrometeorer (såsom molndroppar och iskristaller) i HARMONIE-AROME utifrån molnens position. Brister i metoden hittades. Initialiseringsprocessen upprepades ett suboptimalt antal gånger. En begränsning i hur mycket fuktigheten tillåts modifieras förändras under initialiseringsprocessen och fungerade inte som avsett. Dessutom, jämförese med radiosonddata pekar på att relativa fuktighetsgränserna för villket moln bildas inledningsvis inte ansattes korrekt. Effekterna av metoden kunde vara i över 12 timmar, men denna studie pekar på ytterligare troliga förbättringsmöjligheter i HARMONIE-AROME genom införande av reviderad version av metoden samt förbättrade satellitprodukter.

Microorganisms were discovered in clouds over 100 years ago but detailed information on community structure and function is severely limited. Clouds may be a niche within which bacteria could thrive and influence dynamic cloud processes using ice nucleating and cloud condensing abilities. Gaining an understanding of the bacterial communities and their possible role in these processes might introduce another discipline into meteorology and climate modelling. Cloud and rain samples were collected in 2003 from Bowbeat Windfarm in the Scottish Moorfoot Hills and two mountains in the Outer Hebrides. Community composition was determined using a combination of amplified 16S ribosomal DNA restriction analysis and sequencing. 100 clones from the Bowbeat sample revealed ten OTUs of which three contained more than two clones. 256 clones from the Hebrides samples revealed 111 OTUs of which 33 contained two or more clones. In all the cloud samples the largest OTUs were identified as fluorescent Pseudomonas sp. To investigate bacterial metabolic activity in clouds a further four cloud samples were collected from Bowbeat in 2006. Reverse transcriptase and quantitative PCR did not definitively reveal metabolic activity in cloud bacteria, however the methodology requires further testing. Heterogeneous nucleation is central to the Bergeron-Findeisen process of raindrop formation. Several bacterial species act as heterogeneous nuclei by producing an ice nucleation (IN) protein. PCR targeting the IN gene of Pseudomonas fluorescens (InaW) in Pseudomonas isolates and cloud DNA did not amplify the IN gene. Freezing cultures using differential scanning calorimetry also failed to reveal the IN phenotype. A finding which evolved from the research was all the fluorescent Pseudomonas cloud isolates displayed biosurfactant activity. Surfactants are very important in the process of activating aerosols into cloud condensation nuclei (CCN). It is also known that surfactants influence cloud droplet size and increase cloud lifetime and albedo. Some bacteria are known to act as CCN and so it is conceivable that these fluorescent pseudomonads could be using surfactants to facilitate their activation from aerosols into CCN. This might allow water scavenging, counter desiccation and aid their dispersal.

Two algorithms have been used for determining the quantity and spatial distribution of supercooled water in a cloud from the change in snow characteristics as snow falls to the ground. The snow density change algorithm is based on the change in snow density due to the riming process, while the snow flux gradient algorithm is based on changes in snow flux occurring as falling snow captures supercooled droplets. The data used to run and analyze the algorithms' results were collected during the Alliance Icing Research Study (AIRS) that took place at Mirabel during the winter of 1999-2000.
For the analyzed cases, the results of the two algorithms agreed well with each other. Furthermore, these results matched radiometric and aircraft measurements overall. Both algorithms demonstrated some limitations. Possible solutions to reduce some of the observed limitations include employing an algorithm for resolving trail patterns, and an algorithm for separating the drizzle from the snow echo.

In this thesis, we aim to systematically understand the relationship between cloud spatial structure and its radiation imprints, i.e., three-dimensional (3D) cloud effects, with the ultimate goal of deriving accurate radiative energy budget estimates from space, aircraft, or ground-based observations under spatially inhomogeneous conditions. By studying the full spectral information in the measured and modeled shortwave radiation fields of heterogeneous cloud scenes sampled during aircraft field experiments, we find evidence that cloud spatial structure reveals itself through spectral signatures in the associated irradiance and radiance fields in the near-ultraviolet and visible spectral range.

The spectral signature of 3D cloud effects in irradiances is apparent as a domain- wide, consistent correlation between the magnitude and spectral dependence of net horizontal photon transport. The physical mechanism of this phenomenon is molecular scattering in conjunction with cloud heterogeneity. A simple parameterization with a single parameter ϵ is developed, which holds for individual pixels and the domain as a whole. We then investigate the impact of scene parameters on the discovered correlation and find that it is upheld for a wide range of scene conditions, although the value of ϵ varies from scene to scene.

The spectral signature of 3D cloud effects in radiances manifests itself as a distinct relationship between the magnitude and spectral dependence of reflectance, which cannot be reproduced in the one-dimensional (1D) radiative transfer framework. Using the spectral signature in radiances and irradiances, it is possible to infer information on net horizontal photon transport from spectral radiance perturbations on the basis of pixel populations in sub-domains of a cloud scene.

We show that two different biases need to be considered when attempting radiative closure between measured and modeled irradiance fields below inhomogeneous cloud fields: the remote sensing bias (affecting cloud radiances and thus retrieved properties of the inhomogeneous scene) and the irradiance bias (ignoring 3D effects in the calculation of irradiance fields from imagery-based cloud retrievals). The newly established relationships between spatial and spectral structure lay the foundation for first-order corrections for these 3D biases within a 1D framework, once the correlations are explored on a more statistical basis.

Generic predictions for the acoustic wavenumber at low frequencies in the condensational cloud layers of Venus are presented, based on and adapted from the terrestrial model of Baudoin et al. (J. Acoust. Soc. Am. 130. 1142 (2011)). While the general thermodynamics of Earth clouds is well understood, that of Venusian clouds is still a matter of debate. Venus’ clouds are primarily formed of H₂O and H₂SO₄ vapors and aqueous sulfuric acid droplets, the fluxes of which are not fully constrained due to the few in situ observations. Inside the clouds, the Navier-Stokes-Fourier equations of continuum fluid mechanics are used for the gaseous (dry + vapor) and liquid phases of H ₂O andH₂SO₄, combined with equations describing the evaporation/condensation processes; the gaseous phase is treated as an ideal gas and the liquid droplets are considered polydisperse. Thermophysical parameters are interpolated at the ambient conditions pertaining to an altitude of 50 km, a level where balloon platforms (e.g., European Space Agency’s EVE) and manned airships (e.g., NASA’s HAVOC) may be deployed in the future. At low frequencies, the dominant source of absorption is caused by the evaporation/condensation of the liquid phase. At higher frequencies, absorption is dominated by momentum transfers between the wave and the ambient gas and liquid droplets. The intrinsic dispersion is negligible. Sensitivity studies of the attenuation coefficient and the sound speed on the cloud physical parameters is performed, namely, the mean cloud particle size and the cloud density. The attenuation coefficient is sensitive to changes in both mean cloud particle size and cloud density, while the intrinsic dispersion changes negligibly.

Thesis: S.M. in Atmospheric Sciences, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 42-48).
Ice clouds scatter and absorb solar radiation, affecting atmospheric and surface temperatures (Gettelman et al., 2012). On Mars, where ice contained in clouds makes up a large portion of total atmospheric water vapor, ice clouds also alter the planetary water budget (Maltagliati et al., 2011; Rafkin et al., 2013). Thus, it is important for climate models to be able to accurately predict the conditions under which ice clouds can form. Typical Martian temperatures at cloud-formation height range from -150-200 K (Trainer, Toon, & Tolbert, 2009). Heterogeneous deposition nucleation is thought to be the dominant freezing mechanism on Mars due to the abundance of mineral dust to serve as ice nuclei (IN) (Mdittanen et al., 2005). The parameters for such nucleation are not well characterized at such low temperatures (Trainer et al., 2009). Previous experimental studies have investigated the relative humidity required for deposition nucleation within the Martian temperature range. However, most studies took place on bulk aerosol samples, did not use mineral dusts analogous to Martian dust, or were constrained by particle lifetime and temperature limits. In this project, we re-purpose a single-particle instrument and set it up to perform experiments for more precise ice nucleation data under Martian atmospheric conditions. We use an electrodynamic balance (EDB) to levitate individual particles with diameters around 10 pm. We calculate the size of the particle and changes in size based on the holding voltages. The system can be cooled to 200 K in its current configuration, and the relative humidity and atmospheric constituents can be controlled by adding gas. To test the EDB, we perform validation experiments. We investigate deliquescence and efflorescence on salts at room temperature and 0 'C. We modify the cooling system, thermocouples, and relative humidity sensors and begin freezing experiments with Arizona Test Dust (ATD) and with Mojave Mars Simulant (MMS) dust. We investigate water uptake on MMS particles and find it to be non-hygroscopic but wettable, uptaking monolayers of water between 65-95% relative humidity. From 200 K to 220 K, MMS does not nucleate up to 115% RHice, suggesting that higher supersaturations are needed for ice clouds to form; some Martian cloud modelers should revisit the critical supersaturation parameterization. Future work will improve the EDB and use it to examine phase functions and light scattering.
by Shaena R. Berlin.
S.M. in Atmospheric Sciences

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 171-184).
Increased industrialization and human activity modified the atmospheric aerosol composition and size-distribution during the last several decades. This has affected the structure and evolution of clouds, and precipitation from them. The processes and mechanisms by which clouds and precipitation are modified by changes in aerosol composition and size-distribution are very intricate. The objective of this thesis is to improve the understanding of the processes and mechanisms through which the changes in aerosol concentrations impact the evolution of deep convective clouds and precipitation formation. We develop a new coupled model in which a very detailed model of aerosol activation is coupled to a three-dimensional cloud resolving model. This coupled model can accurately represent different kinds of aerosol populations. This coupled model is used to investigate the impact of changing aerosol concentrations on the dynamics, microphysical evolution and precipitation formation in deep convective clouds. We examine the theories of aerosol activation, and the representation of aerosol activation in cloud models. The limitations of the extant methods of representation of aerosol activation in cloud models are evaluated. Then we descibe the components of the coupled model - Modified Eulerian and Lagrangian Aerosol Model (MELAM) and the Cloud Resolving Model (CRM). The features of these two component models with respect to aersol activation and cloud formation are discussed. The evaluation of the coupled model by simulation of a deep convertive event observed during the INDian Ocean EXperiment (INDOEX) by statistcal comparison of observed and simulated cloud fields shows that the coupled model can simulate deep convective events reasonably well. We present a study of the senstivity of the model to initial thermodynamic conditions (CAPE). Different initial thermodynamic conditons sampled during the INDOEX are used to initialize the coupled model and, the structure and evolution of the deep convective event are discussed. The study sheds new light on the respone of deep convection to CAPE. It is found that when the atmosphere has moderate CAPE, the precipitation forming processes are very active and when the CAPE is (cont.) low or high, they are comparatively less efficient.
As the most important part of our study, we examine the response of deep convection to changing initial aerosol concentration. Different aerosol concentrations from those representing pristine to polluted atmospheres are considered. We look at the buoyancy of the cloud and the microphysical evolution. It is found that the dynamics and microphysics are tightly coupled and we infer that to understand aerosol-cloud interactions in deep convective clouds, both - dynamics and microphysics - and their interaction have to be taken into consideration. Our results show that the response of a deep convective cloud to changing aerosol concentration is very different from the much well understood reponse of shallow clouds or small cumulus clouds. In general, increase in aerosol concentratin is seen to invigorate convection and lead to greater condensate. Although the cloud droplet size decreases, collision-coalescence is not completely inefficient. The precipitation in high aerosol regime is seen to occure in short spells of intense rain. A very interesting anomalous response of deep convection to initial aerosol concentration is observed at intermediate aerosol concentrations. The cloud lifetime, and precipitation are seen to increase in this regime. A possible mechanism to explain this anomalous behavior is proposed and the available circumstantial support for the mechanism from extant observations is presented. It is proposed that the efficient collection of rain and cloud droplets by ice and graupel particles in the middle troposphere is primarily responsible for this increased cloud lifetime and precipitation.
by Udaya Bhaskar Gunturu.
Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2008.
Cataloged from PDF version of thesis.
Includes bibliographical references (page 24).
In this thesis, spontaneous tropical cyclogenesis occurring in a cloud-resolving numerical model is studied. The model environment is one of radiative convective-equilibrium on an f-plane with doubly periodic boundary conditions and constant sea surface temperature. While a variety of initial conditions may exhibit spontaneous tropical cyclogenesis, this study focuses on one. Using assumptions of axisymmetry for the growing disturbance and focusing on the large scale processes, fields were created for a number of thermodynamic variables along constant height surfaces and as azimuthal means plotted against height. The tropical cyclone is hypothesized to develop in three steps. First, convective aggregation creates regions of high moist static energy, and regions of cold dry air. Importantly, a deep moist column is created which provides a perfect environment the developing storm. In the second step, mid-level cyclone intensification, a mid-level cold core cyclone develops in the deep moist region, and benefits from moist static energy and potential vorticity fluxes from the upper troposphere. Exhibiting anticyclonic convergent flow, the upper troposphere is an unlikely source for the mid-level disturbance, while convective downdrafts and divergent surface flow hinder energy transport from the ocean to the growing system. In fact, a cold surface anticyclone exists near the center for much of the second step. It is not until potential vorticity anomalies advect down to the surface that the final step, low-level cyclone intensification, creates a classical hurricane structure. Potential vorticity advection stimulates cyclonic flow at the surface, extinguishing the surface anticyclone, and thereby linking the mid-level disturbance to the oceanic energy source. While like some cold core cyclones previously studied, the anticyclone as an energy source is unique to this spontaneous case.
by Paul M. Hoffman.
S.B.

The effects of aerosol in the atmosphere account for some of the largest uncertainties in estimates of the human impact on climate. These effects depend not only on the total mass of aerosol, but also its size distribution, mixing state and vertical profile. Previous studies have suggested that both the size distribution and mixing state of aerosol may be strongly influenced by repeated cycling through non-precipitating cloud. The extent of this process is assessed in the HadGEM3–UKCA model; although fewer cycles are seen for all aerosol than in previous studies, the figure varies considerably between aerosol types. The role of scavenging by precipitating cloud is also considered, and several approaches to increasing the physical realism of its representation are considered. In particular, coupling convective scavenging into the convective transport scheme is shown to provide significant benefits over an operator-split approach (which underestimates removal and allows excess aerosol to reach the upper troposphere and be transported to remote regions). To evaluate the alternative convective scavenging schemes, a method is developed for carrying out a pointwise evaluation against vertically-resolved in-situ observations from large-scale aircraft campaigns, based on nudging and flight-track sampling in the model. It is demonstrated that this approach can help to constrain the choice between different model configurations with a degree of statistical confidence. Finally, the processes controlling the vertical profile of aerosol are investigated using a series of model-based sensitivity tests, along with the extent to which these processes can account for the large diversity in vertical profiles seen amongst current models. For mass profiles and number profiles of large particles (greater than about 100nm dry diameter), removal and secondary production processes are shown to be most important; for number profiles of smaller particles, microphysical processes are shown to become increasingly dominant.

The role of turbulence in the process of collision and coalescence of small cloud droplets is still an outstanding problem in the area of cloud physics. In particular, the growth of droplets in the radius range for 10 to 15 $ mu$m is not well understood. The present research has been motivated by the curiosity whether or not turbulence affects the growth rate of such small drops.
We developed a method to calculate collision rates of small hydrodynamically interacting drops embedded in an external flow field; we call it the flux method. Then, the method was tested for simple cases of laminar flows such as linear shear and a two-dimensional deformation field. The tests were designed not only to validate the method but also to examine the mechanisms associated with the simplified types of external flows which may be equally important for real turbulent flows.
In order to obtain estimates of collision rates for turbulent flows, the flux method was used in conjunction with a probabilistic approach. Numerous simulations of trajectories of two hydrodynamically interacting droplets in a turbulent field were carried out. The ratio of the number of collisions to the total number of simulations gave the probability of collision for different relative positions of the drops. Because the Reynolds number of the flow around droplets (based on the drop radius and terminal velocity) is small, the trajectories were calculated with the help of a model based on the linear Stokes hydrodynamics. Turbulence was modelled in the form of random Fourier modes with both the space and the time spectrum prescribed. Both spectra were characterized by Kolmogorov scaling. The space spectrum was modelled in the inertial and dissipation subranges. On the basis of scale analysis, only small scale time variations were allowed, and, the so called Eulerian-Lagrangrian time spectrum was applied.
The results show that most collision rates increase moderately in a turbulent flow characterized by a rate of energy dissipation of the order of 1, 10, and 100 cm$ sp2$ sec$ sp{-3}.$ The estimated increase in collision efficiencies, however, is not uniform, and a rather complicated relation between the increase in the collision efficiency and the parameters--the drop radii, and the rate of energy dissipation--can be observed.

Despite their importance in Earth's radiation budget and atmospheric models, Arctic clouds remain poorly documented and understood. The deployment of a cloud radar and a high spectral resolution lidar at Eureka (80°N) in August 2005 offers a unique data set for the study of Arctic clouds. In this project, synergetic retrievals were developed and applied to two years of data in order to provide a first climatology of the clouds and their microphysics at this remote location. Results show an annual cycle in cloud coverage. They are mostly detected in the low levels or in single-layer, especially in winter due to a temperature inversion and cloud top radiative cooling. An analysis of the winds also demonstrated that different wind directions relate to different cloudiness conditions, while a strong channelling from the topography is present in the low levels. Moreover, liquid phase particles were detected all year round, with a minimum occurrence in winter due to colder temperatures. Turbulence and high relative humidity seem to maintain supercooled liquid, especially when ice crystals were also present. Precipitation was mostly identified during summer months, often in the form of virga, although falling snow might have been missed due to the difficulty to distinguish it from glaciated clouds. Finally, results show that satellite validation is possible using Eureka's data, but only under homogeneous conditions and when the instruments characteristics (like the sampling and sensitivity) are taken into account.
Malgré leur importance dans le budget radiatif terrestre et les modèles atmosphériques, les nuages arctiques restent mal documentés et incompris. Le déploiement d'un radar millimétrique et d'un lidar à haute résolution spectrale à Eureka (80°N) en août 2005 offre un ensemble unique de données pour l'étude des nuages arctiques. Dans ce projet, des algorithmes synergétiques furent développés et appliqués à deux ans de données pour fournir une première climatologie des nuages et de leur microphysique à cet emplacement éloigné. Les résultats montrent un cycle annuel dans l'étendue des nuages. Ils sont surtout détectés dans les bas niveaux ou en une couche, particulièrement en hiver à cause de l'inversion thermique et du refroidissement radiatif du haut des nuages. Une analyse des vents a démontré que différentes directions sont reliées à différentes conditions nuageuses, alors qu'une forte canalisation des vents due à la topographie est présente dans les bas niveaux. De plus, la phase liquide fut détectée à l'année longue, avec une occurrence minimale en hiver causée par des températures plus froides. De la turbulence et un haut taux d'humidité semblent maintenir les particules liquides surfondues, particulièrement quand des cristaux de glace sont aussi présents. Les précipitations furent principalement identifiées durant l'été, surtout sous forme de virga, bien que la difficulté à distinguer la neige des nuages glacés a pu influencer les résultats. Finalement, la validation d'un satellite est possible grâce aux données d'Eureka, mais seulement sous des conditions homogènes et si les caractéristiques instrumentales

Aerosols are thought to have a large effect on the climate, especially through their interactions with clouds. The magnitude and in some cases the sign of aerosol effects on cloud and precipitation are highly uncertain. Part of the uncertainty comes from the multiple competing effects that aerosols have been proposed to have on cloud properties. In addition, covariation of clouds and aerosol properties with changing meteorological conditions has the ability to generate spurious correlations between cloud and aerosol properties. This work presents a new way to investigate aerosol-cloud-precipitation interactions while accounting for the influence of meteorology on cloud and aerosol. The clouds are separated into cloud regimes, which have similar retrieved cloud properties, to investigate the regime dependence of aerosol-cloud-precipitation interactions. The strong aerosol optical depth (AOD)- cloud fraction (CF) correlation is shown to have the ability to generate spurious correlations. The AOD-CF correlation is accounted for by investigating the frequency of transitions between cloud regimes in different aerosol environments. This time-dependent analysis is also extended to investigate the development of precipitation from each of the regimes as a function of their aerosol environment. A modification of the regime transition frequencies consistent with an increase in stratocumulus persistence over ocean is found with increasing AI (aerosol index). Increases in transitions into the deep convective regime and in the precipitation rate consistent with an aerosol invigoration effect are also found over land. Comparisons to model output suggest that a large fraction of the observed effect on the stratocumulus persistence may be due to aerosol indirect effects. The model is not able to reproduce the observed effects on convective cloud, most likely due to the lack of parametrised effects of aerosol on convection. The magnitude of these effects is considerably smaller than correlations found by previous studies, emphasising the importance of meteorological covariation on observed aerosol-cloud-precipitation interactions.

Most infrared-based techniques of satellite rainfall estimation contain substantial uncertainties due to the indirect relationship between precipitation particles and space-borne infrared observations of clouds. Generally, these uncertainties include (1) IR temperature threshold defining cold clouds; (2) inclusion of no-rain clouds; (3) exclusion of warm rain clouds; and (4) the coefficients between rain rate and cloud-top properties. To address these uncertainties, a methodology, Cloud Patch Analysis, was developed to estimate rainfall by removing large portion of no-rain clouds from IR cloud imagery. Seven cloud features, including physical, geometric and textural, were defined, and ID3, an inductive decision tree, was used to identify no-rain clouds. Particularly, textural characteristics were extended from square images to irregular cloud patches to extract cloud features related to rainfall. In addition, the method adopted a mechanism to adjust IR temperature threshold according to locations and seasons, and this adjustment can be made by the combination of microwave observations by polar-orbiting satellites with infrared observations by geostationary satellites. The application of the adjusted IR threshold to GPI algorithm showed significant improvement for monthly rainfall estimation. The method was applied to the Japanese Islands and surrounding oceanic regions in June and July/August 1989 and to the Florida region in June and August 1996. The monthly rainfall estimates by the proposed method showed significant and consistent improvements over those by GPI.

Low-level stratiform clouds remain one of the wildcards in future climate simulations. Despite their important role in the earth's radiation budget and the large number of dedicated field campaigns, several cloud-scale processes in marine stratocumulus clouds remain misrepresented. The 19-month-long deployment of the Atmospheric Radiation Measurement Program Mobile Facility in the Azores provided the longest and most comprehensive ground-based observational dataset of marine boundary layer clouds to date. The first objective of this project was the documentation of the frequency of occurrence of different cloud and precipitation systems in the Azores using a combination of passive and active measurements. The analysis indicates that, even though clouds were often observed (close to 80 % of the time), especially in the boundary layer (~50 %), a single-layer stratocumulus coverage rarely persisted more than a day. In fact, many stratocumulus clouds were observed to have cumulus clouds underneath them. This is linked to the nearly constant decoupled state of the boundary layer in the Azores, contrary to what has been observed in the Pacific decks. 35 cases of mostly single-layer persisting stratocumulus coverage were selected for further analysis. Results include similarities with other studies (e.g., maximum coverage at night, thicker clouds needed to drizzle, and importance of cloud-top radiative cooling at night), as well as differences (e.g., coherent structures account for a smaller fraction of the updraft mass flux). The second objective of this project was to revisit the detection of drizzle-size particles in stratocumulus clouds using radar observations. First, the cloud and drizzle size distributions are related theoretically to the radar measurements, including the effects of the dynamics. Then, a forward radar Doppler spectra model was developed to test the sensitivity of the radar measurements to modifications of the drizzle contribution. Finally, a simple 1-D steady-state model was exploited to simulate drizzle growth as it falls in a cloud, using the forward model to link the output back to the radar observations. Using that combination of models, some observed features of the drizzle evolution inside continental and maritime stratocumulus clouds were successfully investigated. Overall, it was found that the skewness of a radar Doppler spectrum is a good indicator of the presence of early drizzle droplets, while a reflectivity or Doppler velocity threshold indicates the change in dominance in the Doppler spectrum occurring when drizzle is well developed. The third and final objective of this project was to revisit another long-standing challenge: the retrieval of cloud microphysical properties using a combination of radar-radiometer measurements. A new technique was developed to retrieve the cloud particle size distribution in stratocumulus clouds, adding a microphysical condensational model under steady-state supersaturation conditions to a common retrieval method. The results appear reasonable in two nondrizzling marine stratocumulus clouds, and the derived cloud optical depth compares well with the one derived independently with another instrument. The errors of the retrievals were also estimated, demonstrating the added value of the new technique.
Les nuages stratiformes de basse altitude restent un des facteurs imprévisibles dans les simulations du climat futur. Malgré leur rôle important dans le budget radiatif terrestre et le grand nombre de campagnes de terrain dédiées, plusieurs procédés à l'échelle nuageuse dans les stratocumulus marins demeurent mal représentés. Le déploiement dans les Açores d'un laboratoire mobile du programme « Atmospheric Radiation Measurement » pendant 19 mois a fourni l'ensemble de données d'observation au sol sur les nuages de couche limite marine le plus long et le plus complet à ce jour. Le premier objectif de ce projet fut la documentation de la fréquence d'apparition de différents systèmes de nuages et de précipitations dans les Açores en utilisant une combinaison de mesures passives et actives. L'analyse indique que, même si des nuages étaient souvent observés (près de 80 % du temps), en particulier dans la couche limite (~50 %), une couverture de stratocumulus seul persistait rarement plus d'une journée. En fait, de nombreux stratocumulus furent observés avec des cumulus en dessous. Ceci est lié à l'état découplé de la couche limite quasi-constant dans les Açores, contrairement à ce qui a été observé dans les stratocumulus du Pacifique. 35 cas de couverture d'un stratocumulus persistant principalement seul furent sélectionnés pour une analyse approfondie. Les résultats incluent des similarités avec d'autres études (par exemple, une couverture maximale durant la nuit, un besoin de nuages plus épais pour bruiner et l'importance du refroidissement radiatif au haut des nuages durant la nuit), ainsi que des différences (par exemple, les structures cohérentes représentent une plus petite fraction du flux ascendant de masse). Le deuxième objectif de ce projet fut de revisiter la détection des particules de bruine dans les stratocumulus en utilisant les observations radar. Tout d'abord, les distributions de tailles des gouttelettes de nuage et de bruine sont liées théoriquement aux mesures radar, en incluant les effects dynamiques. Ensuite, un modèle direct de spectres Doppler radar fut développé pour tester la sensibilité des mesures radar à des modifications de la contribution de la bruine. Finalement, un simple modèle 1-D à l'état d'équilibre fut exploité pour simuler la croissance de la bruine pendant sa descente dans un nuage, en utilisant le modèle direct pour relier de nouveau les données sortantes avec les observations radar. En utilisant cette combinaison de modèles, quelques caractéristiques de l'évolution de la bruine observées à l'intérieur de nuages stratocumulus continental et maritime furent examinées avec succès. Dans l'ensemble, il fut déterminé que l'asymétrie d'un spectre Doppler radar est un bon indicateur de la présence de jeunes gouttelettes de bruine, alors qu'un seuil de réflectivité ou de vitesse Doppler indique le changement de domination dans le spectre Doppler se produisant quand la bruine est bien développée. Le troisième et final objectif de ce projet fut de revisiter un autre défi de longue date : le recouvrement de propriétés microphysiques des nuages en utilisant une combinaison de mesures radar et radiométriques. Une nouvelle technique fut développée pour retrouver la distribution de tailles des particules nuageuses, en ajoutant un modèle microphysique de condensation dans des conditions de supersaturation en équilibre à une méthode populaire de recouvrement. Les résultats semblent raisonnables pour deux stratocumulus marins ne bruinant pas et la profondeur optique dérivée pour ces nuages se compare bien avec celle dérivée indépendamment avec un autre instrument. Les erreurs de recouvrement furent également estimées, démontrant la valeur ajoutée de la nouvelle technique.

Airborne aerosols are crucial atmospheric constituents that are involved in global climate change and human life qualities. Understanding the nature and magnitude of aerosol-cloud-precipitation interactions is critical in model predictions for atmospheric radiation budget and the water cycle. The interactions depend on a variety of factors including aerosol physicochemical complexity, cloud types, meteorological and thermodynamic regimes and data processing techniques. This PhD work is an effort to quantify the relationships among aerosol, clouds, and precipitation on both global and regional scales by using satellite retrievals and aircraft measurements. The first study examines spatial distributions of conversion rate of cloud water to rainwater in warm maritime clouds over the globe by using NASA A-Train satellite data. This study compares the time scale of the onset of precipitation with different aerosol categories defined by values of aerosol optical depth, fine mode fraction, and Ångstrom Exponent. The results indicate that conversion time scales are actually quite sensitive to lower tropospheric static stability (LTSS) and cloud liquid water path (LWP), in addition to aerosol type. Analysis shows that tropical Pacific Ocean is dominated by the highest average conversion rate while subtropical warm cloud regions (far northeastern Pacific Ocean, far southeastern Pacific Ocean, Western Africa coastal area) exhibit the opposite result. Conversion times are mostly shorter for lower LTSS regimes. When LTSS condition is fixed, higher conversion rates coincide with higher LWP and lower aerosol index categories. After a general global view of physical property quantifications, the rest of the presented PhD studies is focused on regional airborne observations, especially bulk cloud water chemistry and aerosol aqueous-phase reactions during the summertime off the California coast. Local air mass origins are categorized into three distinct types (ocean, ships, and land) with their influences on cloud water composition examined and implications of wet deposition discussed. Chemical analysis of cloud water samples indicates a wide pH range between 2.92 and 7.58, with an average as 4.46. The highest pH values were observed north of San Francisco, coincident with the strongest land mass influence (e.g. Si, B, and Cs). Conversely, the lowest pH values were observed south of San Francisco where there is heavy ship traffic, resulting in the highest concentrations of sulfate, nitrate, V, Fe, Al, P, Cd, Ti, Sb, P, and Mn. The acidic cloud environment with influences from various air mass types can affect the California coastal aquatic ecosystem since it can promote the conversion of micronutrients to more soluble forms. Beyond characterization of how regional air mass sources affect cloud water composition, aircraft cloud water collection provides precious information on tracking cloud processing with specific species such as oxalic acid, which is the most abundant dicarboxylic acid in tropospheric aerosols. Particular attention is given to explore relationship between detected metals with oxalate aqueous-phase production mechanisms. A number of case flights show that oxalate concentrations drop by nearly an order of magnitude relative to samples in the same vicinity with similar environmental and cloud physical conditions. Such a unique feature was consistent with an inverse relationship between oxalate and Fe. In order to examine the hypothesis that oxalate decreasing is potentially related to existing of Fe, chemistry box model simulations were conducted. The prediction results show that the loss of oxalate due to the photolysis of iron oxalato complexes is likely a significant oxalate sink in the study region due to the ubiquity of oxalate precursors, clouds, and metal emissions from ships, the ocean, and continental sources.

This thesis addresses a series of questions related to the problem of achieving reliable and physically consistent representations of aerosol-cloud interaction in global circulation models (GCM). In-situ data and modeling tools are used to develop and evaluate novel parameterization schemes for the process of aerosol activation for applications in GCM simulations. Atmospheric models of different complexity were utilized, ranging from detailed Lagrangian parcel model simulations of the condensational growth of droplets, to one-dimensional single column model with aerosol and cloud microphysics, and finally GCM simulations performed with the Community Atmosphere Model (CAM). A scheme for mapping the sub-grid scale variability of cloud droplet number concentrations (CDNC) to a number of microphysical process rates in a GCM was tested, finding that neglecting this impact can have substantial influences in the integrated cloud properties. A comprehensive comparison and evaluation of two widely used, physically-based activation parameterizations was performed in the framework of CAM5.1. This was achieved by utilizing a numerical adjoint sensitivity approach to comprehensively investigate their response under the wide range of aerosol and dynamical conditions encountered in GCM simulations. As a result of this, the specific variables responsible for the observed differences in the physical response across parameterizations are encountered, leading to further parameterization improvement.