This study examines the dynamics and predictability of the mesoscale convective vortex (MCV) event of 10-13 June 2003 which occurred during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The MCV formed from a preexisting upper-level disturbance over the southwest United States on 10 June and matured as it traveled northeastward. The BAMEX field campaign provided a relatively dense collection of upper air observations through dropsondes on 11 June during the mature stage of the vortex. While several previous studies have focused on analysis of the dynamics and thermodynamics of observed and simulated vortices, few have addressed the ability to predict MCVs using numerical models. This event is of particular interest to the study of MCV dynamics and predictability given the anomalously strong and long-lived nature of the circulation and the dense data set. The first part of this study explores the dynamics of this MCV through an in-depth analysis of data from the profiler network and BAMEX dropsonde observations, in addition to the conventional surface and sounding observations as well as radar and satellite images. Next, issues relating to model performance are addressed through anevaluation of two state-of-the-art mesoscale models with varying resolutions. It is determined that the ability of a forecast model to accurately predict this MCV event is directly related to its ability to simulate convection. It is also shown that the convective-resolving Weather Research and Forecast (WRF) model with horizontal grid increments of 4 km displays superior performance in its simulation of this MCV event. Finally, an ensemble of 20 forecasts using mesoscale model MM5 with horizontal grid increments of 10 km are employed to evaluate probabilistically the dynamics and predictability of the MCV through the examination of the ensemble spread as well as the correlations between different forecast variables among ensemble members. It is shown that after MCV development, the ensemble mean performs poorly while individual ensemble members with good forecasts of convection at all stages of the MCV also forecast the midlevel vortex well. Furthermore, correlations among ensemble members generally support the findings in the observational analysis and in previous literature.

The purpose of following project is to study the effect of dust aerosols on the radiative forcing which is directly related to the surface temperature. A single column radiative convective model is used for simulation purpose. A series of simulations have been performed by varying the amount of dust aerosols present in the atmosphere to study the trends in ground temperature, heating rate and radiative forcing for both its longwave and shortwave components. A case study for dust storm is also performed as dust storms are common in Arabian Peninsula. A sensitivity analyses is also performed to study the relationship of surface temperature minimum and maximum against aerosol concentration, single scattering albedo and asymmetry factor. These analyses are performed to get more insight into the role of dust aerosols on radiative forcing.

The presence of a blind adjacent to a window affects the natural convective air flow over the window and natural convective heat transfer from the window to the room. Most numerical studies of convective heat transfer between a window-blind system and a room are based on the assumption that the flow remains laminar. However, in the case of larger windows it is to be expected that transition to turbulent flow will occur in the flow over the window. The aim of the present study was to numerically determine the effect of Venetian blind on laminar-to-turbulent transition in the flow over a simple recessed window and on the convective heat transfer from the window. An approximate model of a recessed window that is covered by a venetian blind has been considered. The fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being dealt with using the Boussinesq approach. Radiant and conductive heat transfer effects have been neglected. However in the present study the case where there is a constant heat generation rate in the blind slats, as the result of solar radiation absorbed by the slats of the blind, has been considered. The k-epsilon turbulence model with the full effects of the buoyancy forces being accounted for has been used in obtaining the solution. The turbulent, steady and two dimensional governing equations have been solved using the commercial finite-volume based CFD code FLUENT. Results are generated for different blind slat angles, for different distances of the pivot point of the slats from the window and for different constant heat generation rates in the slats. The results show that over a wide range of Rayleigh number, the distance of the blind to the window has a stronger effect on the convective heat transfer from the window and also on the laminar to turbulent transition in the flow over the window than the blind slat angle. Heat generation in the slats increases the Mean Nusselt number and this effect increases as the Rayleigh number decreases. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2011-09-22 21:17:31.777

The primary focus of the research is to extend the principles of Nanofiltration(NF) to non-aqueous systems using solvent-resistant NF membranes. Several differentlevels of interaction are introduced when organic solvents are used with polymericmembranes and thus quantification of polymer-solvent interactions is critical. Puresolvent permeation studies were conducted to understand the mechanism of solventtransport through polymeric membranes. Different membrane materials (hydrophilic andhydrophobic) as well as different solvents (polar and non-polar) were used for the study.For example, hexane flux at 13 bar through a hydrophobic silicone based NF membranewas ~ 0.6 x 10-4 cm3/cm2. s. and that through a hydrophilic aromatic polyamide based NFmembrane was ~ 6 x 10-4 cm3/cm2. s. A simple model based on a solution-diffusionapproach which uses solvent physical properties (molar volume, viscosity) andmembrane properties (surface energy, etc) is used for correlating the pure solventpermeation through hydrophobic polymeric membranes.Solute transport studies were performed using organic dyes and triglycerides inpolar and non-polar solvents. For example, the rejection of Sudan IV (384 MW organicdye) in n-hexane medium is about 25 % at 15 bar and that in methanol is about –10 % atabout 20 bar for a hydrophobic (PDMS-based) membrane. However, for a hydrophilicpolyamide based NF membrane, the direction of separation is reversed (86 % in methanoland 43 % in n-hexane). From our experimental data with two types of membranes it isclear that coupling of the solute and solvent fluxes cannot be neglected. Two traditionaltransport theories (Spiegler-Kedem and Surface Force-Pore Flow model) that considercoupling were evaluated with literature and our experimental solute permeation data. Amodel based on a fundamental chemical potential gradient approach has been proposedfor explaining solute separation. The model uses solute, solvent and membrane physicalproperties and uses the Flory-Huggins and UNIFAC theories as activity coefficientmodels. This model has been used to obtain a correlation for the diffusion coefficients ofsolutes in hexane through a hydrophobic membrane. This correlation along withconvective coupling can be used to predict separation behavior for different solutes and atdifferent temperatures.

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In this work high-resolution numerical simulation (Large-Eddie Simulation, LES) has been used to study the characteristic factors causing and influencing the development of moist convective clouds. Through this work a 1D cloud-model was derived from first principles to represent the vertical profile of individual convective clouds. A microphysics framework was implemented to ensure identical behaviour in LES and cloud-model integration where the microphysical processes represented are numerically integrated using a novel adaptive step microphysics integration which uses the physical speed at which a process takes place to adjust the integration step size (in space and time). This work also introduces a simple representation of cloud-droplet formation which allows for super-saturation to exist in-cloud and through this provide more physical representation of the in-cloud state. Together with high-resolution simulation of isolated individual and interacting multiple clouds in environmental conditions leading to shallow convection, the 1D cloud-model was used to infer that the principal influence on moist convective clouds is the entrainment of air from a cloud’s immediate environment which is significantly different from the environmental mean state. This suggests that convection parameterisations must represent the influence of moist convective downdrafts to properly predict the vertical structure of convective clouds so as to correctly predict the cloud-top height and vertical transport. Finally it was found that cloud-base radius is not in itself adequate as a means of classification for defining cloud-types as clouds with the same cloud-base radius showed large variation (≈ 600m) in cloud-top height. Based on simulations of individual convective clouds it was found that 3D simulations are necessary to capture the full dynamic behaviour of convective clouds (2D axisymmetric simulations have too little entrainment) and that agreement with the 1D cloud-model could only be found when entrainment was diagnosed from simulation instead of being parameterised by the traditional Morton-Turner model and only for 2D axisymmetric simulations, suggesting that the 1D cloud-model will require further extension or the diagnosis of entrainment improved.

L'évaluation de l'incertitude associée à la prévision numérique du temps à haute résolution, et en particulier l'estimation de la prévisibilité des événements de fortes précipitations en région méditerranéenne, sont les objectifs de ce travail de thèse. Nous avons procédé à l'étude de quatre sources d'incertitude contrôlant la prévisibilité de ces événements : la description des conditions d'échelle synoptique, la représentation des conditions atmosphériques à méso-échelle (notamment le flux de basses couches alimentant le système convectif), le rôle de processus physiques complexes tels que l'établissement d'une plage froide sous orage, et enfin la définition des conditions de surface. Pour quantifier l'impact de ces différentes sources d'incertitude, nous avons opté pour la méthode des prévisions d'ensemble avec le modèle AROME. Chaque source d'incertitude est étudiée individuellement à travers la génération de perturbations pertinentes, et les ensembles ainsi obtenus sont évalués dans un premier temps pour des cas de fortes précipitations. Nous avons aussi procédé à une évaluation statistique du comportement des prévisions d'ensemble réalisées sur des périodes de prévision longues de deux à quatre semaines. Cette évaluation, ainsi que celle de systèmes de prévision d'ensemble échantillonnant plusieurs sources d'incertitude simultanément, permettent d'établir une hiérarchisation de ces sources d'incertitude et enfin quelques recommandations en vue de la mise en place d'un système de prévision d'ensemble à échelle convective opérationnel à Météo-France / This PhD thesis aims at quantifying the uncertainty of convection-permitting numerical weather forecasts, with a particular interest in the predictability of Mediterranean heavy precipitating events. Four uncertainty sources, which impact the predictability of these events, were investigated : the description of the synoptic-scale circulation, the representation of meso-scale atmospheric conditions (especially the low-level jet feeding the convective systems with moist and unstable air), the impact of complex physical processes such as the setting up of a cold pool, and the definition of surface conditions. To quantify the impact of these four uncertainty sources, the ensemble forecasting technique was chosen, using the AROME model. Each uncertainty source is studied separately through the definition of dedicated perturbations, and the resulting ensembles are first evaluated over heavy precipitation case studies. We then proceed to a statistical evaluation of the ensembles for 2- and 4-week long forecast periods. This evaluation, completed with the design of ensembles sampling several uncertainty sources together, allows us to draw some practical tips for the design of an operational convective scale ensemble forecasting system at Météo-France

The quality of convective precipitation and storm prediction is dependent on the level of knowledge of the convective environment. Convective environment characteristics describe various conditions in the atmosphere which are suitable for the convective storm formation and evolution. The characteristics CAPE, wind shear S01, S03 and S06 were selected for the discussion of their properties and predictive ability. This master thesis deals with the CAPE and S0X values analysis in the Czech Republic during the period May-August 2005-2011. Furthermore it analyzes the distribution of the combined characteristics CAPE × S0X and index NDSEV, especially in the critical interval of values which indicate a storm occurrence. It also deals with the estimation of relationship between NDSEV index and the heavy precipitation occurrence in the Czech Republic. The verification criterion CSI was applied to the evaluation of various threshold values for the NDSEV index.

Les écoulements diphasiques liquide-gaz et liquide-vapeur sont présents dans de nombreuses problématiques industrielles. De fait, ils sont rencontrés dans des configurations diverses, tant en termes de dimensions que d’orientation et présentent des caractéristiques variées. Pour autant, il peut être distingué dans la littérature deux cas limites de modélisation de l’écoulement diphasique liées à son confinement : l’échelle micro et l’échelle macro. Cependant, la caractérisation de la transition entre ces deux échelles reste un enjeu majeur de compréhension de l’écoulement diphasique. De plus, les comportements thermiques en ébullition convective sont impactés par le phénomène de nucléation, fortement influencé par la géométrie de l’application et les conditions de saturations. De ce fait, l’objectif principal de la thèse est de contribuer à la compréhension des écoulements diphasiques adiabatiques et non adiabatiques à une échelle intermédiaire entre les échelles micro et macro, où les comportements tant dynamiques que thermiques sont mal définis. Pour ce faire, dans le travail présent, les effets du confinement et de l’inclinaison de l’écoulement diphasique sur les régimes d’écoulement, les chutes de pression et les échanges thermiques sont étudiés et comparés avec les observations et modèles de la littérature. Une section test inclinable de 1,6 mm de diamètre intérieur a été installée sur le banc d’essais de Charnay (2014), permettant l’étude de l’ébullition convective du R-245fa. Celle-ci permet de visualiser l’écoulement à l’intérieur de l’évaporateur ainsi que d’acquérir simultanément les chutes de pression et le champ de température de paroi extérieure. Pour ce faire, un tube en saphir chauffé par un dépôt d’ITO a été employé. Le champ de température a été obtenu par une procédure d’acquisition et de post-traitement infrarouge. Au préalable, une campagne d’essais a été menée en conditions adiabatiques sur la section de Charnay (2014). La section test présente, quant à elle, a permis deux campagnes d’essais en conditions adiabatiques et en ébullition convective. Des études menées en conditions adiabatiques est apparu un fort effet de l’inclinaison et du confinement sur les régimes d’écoulement ainsi que sur les chutes de pression. L’effet de l’inclinaison s’apparente à ce qui a pu être observé en macro-canal dans la littérature. De plus, l’inclinaison n’affecte que peu les échanges thermiques au regard des incertitudes inhérentes au dispositif infrarouge. Enfin, il est observé un effet de l’inclinaison sur le régime d’assèchement. Dans cette configuration, l’évolution temporelle du champ de température de paroi extérieure apparaît synchrone avec l’évolution dynamique de l’écoulement diphasique. / Liquid-gas and liquid-vapour two-phase flows are encountered in a large range of industrial applications. They are observed in multiple configurations, in terms of dimension and orientation and then have various characteristics. In the literature, it can be distinguished two limit cases of the two-phase flow linked to its confinement: micro and macro-scale. Nonetheless, characterisation of the micro-to-macro scale transition is still a main issue for the comprehension of two-phase flow. In addition, thermal behaviours of flow boiling are affected by the nucleation phenomenon, which is strongly influenced by the geometry of the application and the saturation conditions. The main objective of the thesis is to contribute to the comprehension of adiabatic and diabatic two-phase flows in mini-scale, where the general behaviours are not well definite. In the present study, the effects of confinement and orientation of two-phase flows on flow patterns, pressure drops and heat exchanges are studied and compared with observations and models of the literature. An 1.6 mm inner diameter inclinable test section was installed on the test bench of Charnay (2014), which enabled to study R-245fa flow boiling. This test section enables to visualize the flow directly in the evaporator and the simultaneous acquisition of the pressure drops and the outside wall temperature field. A sapphire tube, heated by a transparent ITO coating, is unemployed to insure the transparency. The temperature field is obtained by an IR image processing. Previously, tests were led on the section of Charnay (2014) in adiabatic conditions. The present test section was used for two series of tests, led in both adiabatic and diabatic conditions. It appeared a strong effect of confinement and orientation on the flow patterns and pressure drops. The effect of the orientation is likely the same that one observed in macro-scale. In addition, the orientation slightly affects heat exchanges in comparison with the uncertainties of the IR dispositive. Finally, it is observed an effect of orientation on dryout flow pattern. In this configuration, the temperature field evolution with time is synchronized with the dynamic evolution of the two-phase flow.