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.

This thesis investigates the characteristics of the corrections to the initial state of the atmosphere. The technique employed is the key analysis error algorithm, recently developed to estimate the initial state errors responsible for poor short-range to medium-range numerical weather prediction (NWP) forecasts. The main goal of this work is to determine to which extent the initial corrections obtained with this method can be associated with analysis errors. A secondary goal is to understand their dynamics in improving the forecast. / In the first part of the thesis, we examine the realism of the initial corrections obtained from the key analysis error algorithm in terms of dynamical balance and closeness to the observations. The result showed that the initial corrections are strongly out of balance and systematically increase the departure between the control analysis and the observations suggesting that the key analysis error algorithm produced initial corrections that represent more than analysis errors. Significant artificial correction to the initial state seems to be present. / The second part of this work examines a few approaches to isolate the balanced component of the initial corrections from the key analysis error method. The best results were obtained with the nonlinear balance potential vorticity (PV) inversion technique. The removal of the imbalance part of the initial corrections makes the corrected analysis slightly closer to the observations, but remains systematically further away as compared to the control analysis. Thus the balanced part of the key analysis errors cannot justifiably be associated with analysis errors. In light of the results presented, some recommendations to improve the key analysis error algorithm were proposed. / In the third and last part of the thesis, a diagnosis of the evolution of the initial corrections from the key analysis error method is presented using a PV approach. The initial corrections tend to grow rapidly in time and can thus modify significantly the trajectory of a forecast over a relatively short period of time. The results shed light on different mechanisms about the evolution of small and fast growing initial perturbations.

Winter storms affect all Canadians and many of their impacts are associated with precipitation. This precipitation can occur as rain, snow, freezing rain or ice pellets. Some research has been conducted on all of these types of precipitation but the least attention has been paid to ice pellets. The atmospheric environment conducive to ice pellets is known in general but the detailed nature of the actual particles is not. / To begin to address this issue, a high resolution digital camera was used to photograph ice pellets for 4 hours during a winter storm at Mirabel, Quebec in November 2003. A total of 1023 images were analyzed to determine the morphology, shapes, and size distributions of the ice pellets. Some ice pellets were opaque, others were clear, and some had bands of varying opacity. At most, 7% of the particles were spherical. Many particles exhibited bulges, fractures, and spicules. The occurrence of such features suggests that much or all of the initial freezing occurred on the surface as opposed to the drop interior. Approximately 9% of the particles observed were aggregates of 2-5 smaller particles. The ice pellets ranged up to 5 mm in diameter for aggregate particles and up to 3 mm in diameter for individual particles. The average diameter of all particles was 1 mm. A range of different particle characteristics were observed to be occurring simultaneously throughout the storm. Collectively, such observations as well as process model results imply that different freezing mechanisms were occurring simultaneously, and that collisions between semi-frozen ice pellets must have been occurring to produce aggregates.

Thesis (Ph.D.)--Colorado State University, 2006. / (UMI)AAI3233358. Adviser: Graeme Stephens. Source: Dissertation Abstracts International, Volume: 67-09, Section: B, page: 4928.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 2858. Adviser: Greg M. McFarquhar. Includes bibliographical references (leaves 220-228) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 2856. Adviser: Donald J. Wuebbles. Includes bibliographical references (leaves 119-131) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 2857. Adviser: Robert Wilhelmson. Includes bibliographical references (leaves 233-247) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--Georgia Institute of Technology, 2007. / (UMI)AAI3271488. Source: Dissertation Abstracts International, Volume: 68-07, Section: B, page: 4336. Adviser: Yuhang Wang.

Thesis (Ph.D.)--University of Colorado at Boulder, 2007. / (UMI)AAI3284476. Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7190. Adviser: John J. Cassano.

Thesis (Ph.D.)--University of Washington, 2007. / (UMI)AAI3290603. Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7191. Adviser: Stephen G. Warren.