The evolution of a long-lived mesovortex and its transformation into a tropical storm are studied by a three dimensional mesoscale model. The major aspects of the work are: (1) to demonstrate the mesoscale predictability of a long-lived mesoscale convective system (MCS) from a mid-level vortex over land to a tropical storm over ocean, (2) to understand the processes involved in the transformation of a mid-level continental vortex into a tropical storm, and (3) to perform sensitivity experiments to assess the impact of model physics on an idealized simulation. / The major conclusions are: The 90-h real-data simulation shows that the mesoscale model reproduces very well much of the meso-beta-structures and the evolution of the long-lived MCS. These include the development and dissipation of the continental mesovortex, the initiation of a new MCS both in time and in space, the genesis of a surface mesolow over the warm Gulf Stream water, the track and the deepening of the surface cyclone into a "tropical storm", the maintenance of a mid-level mesovortex system, and the propagation of a large-scale cold front with respect to the surface cyclone. / The simulation also shows that the mid-level mesovortex provides persistent convergence at its southern periphery for the continued convective development, whereas the convectively enhanced low-level flow increases surface energy fluxes over the warm water causing further conditional instability. Such feedback processes lead to the rapid deepening of the "tropical storm". / An idealized simulation was performed to eliminate the possible influence of the cold frontal system on the genesis of the tropical storm. The initial conditions resemble the basic structure of the subtropical high in the real-data case, but without the embedded frontal system. The simulation reproduces almost all of the essential features in the real-data simulation. In particular, the eye-like warm core structure of the tropical storm is well simulated. By decomposing the vertical relative vorticity into the curvature vorticity and the shear vorticity, it is shown that the amplification of a low-level vortex after 36 h arises mainly from the increase of curvature vorticity. The quasi-Lagrangian theta budget calculation shows that the descending motion in the center of the surface cyclone contributes to the formation of the warm core at 800 hPa. / The results of sensitivity experiments demonstrate the impact of model physics on the idealized simulation. Using the Betts-Miller scheme, we showed that the model fails to simulate the multiple-episodes of convective activity which are present in observational studies of tropical cyclogenesis and in the simulation using the Kain-Fritsch scheme. We further found that the surface latent heat fluxes represent a dominant factor in the production of CAPE to maintain the persistent convection. Finally, we discussed the cumulative cooling effect by long-wave radiation on the destabilization of the environment of the storm.

This thesis explores the relationships between multifractal measures, multiplicative cascades and correlations. A review of fractal geometry, multifractal formalism and multiplicative cascades is offered. The importance of the Legendre transformation in multifractal formalism is highlighted, especially for multifractal spectrums which are not convex or twice differentiable. By reconsidering the scaling assumption $N sb{p}( alpha$) d$ alpha$ = $ sigma( alpha) rho sp{-f( alpha)} d alpha$, we show that subsets of a measure may offer a different multifractal spectrum. A gradation of self-similarity and scaling vis-a-vis canonicity is offered. Localized and generalized correlations are introduced, and two-point correlations are revisited for multiplicative cascades. A three-point correlation function is presented and discussed. The presence of an integral scale is shown to produce a more involved correlation scaling behaviour.

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.

Data collected from zenith pointing radar is used to study the range dependence of some meteorological radar errors associated with different sampling methods between radar and gauges in stratiform precipitation. Errors due to the vertical variation of reflectivity such as those related with the bright band or with snow growth cause a much larger bias in radar estimates than those due to beam filling or gradients. The maximum useful range varies with the bright band height and the elevation angle program used, a CAPPI giving superior results especially for snow. The sudden changes in bright band height over short distances and the large scatter of its thickness limit the accuracy of current corrections for the vertical variation of reflectivity based on scanning radar data. The possibility of using a zenith pointing radar to obtain this correction is discussed.

A thorough literature review of cloud algorithm validation strategies is presented, with particular emphasis on the problems of detecting cirrus clouds. To further advance our cirrus detection capabilities, a new cloud detection technique is proposed for the Multi-angle Imaging Spectro-Radiometer (MISR), which is scheduled to be on the first platform of the Earth Observing System. Radiative Transfer simulations have been used to develop a Band-Differenced Angular Signature technique. This new technique takes the difference between two solar spectral reflectances as a function of view angle. The resulting angular signature is used to discriminate between high- and low-level clouds and surface reflectance anomalies. This technique, coupled with a Predetermined Clear Sky Threshold developed in this study for MISR, can detect cirrus clouds of visible optical thickness $>$0.5 without any a priori knowledge of atmospheric conditions. Other techniques that can be used with MISR (i.e. stereo) are also discussed.

Hurricane Hazel struck North America on 15-16 October 1954, leaving a pattern of heavy rainfall and flooding in its wake. A complete analysis of the synoptic-scale conditions associated with the transformation of Hazel from its tropical phase into an extratropical cyclone was first undertaken to discern the dynamic and thermodynamic elements crucial to the intensification of this storm. / An analogue search was then conducted for Hazel using linear correlations of anomaly sea level pressure and 1000-500 hPa thickness. Three cases were found in 1985, 1995 and 1999. A comparison of these analogues to Hazel yielded the conclusion that Hazel is a unique event in recent meteorological history, as none of the analogues produces the extreme precipitation values in Hazel. The lack of significant hurricane circulations in all of the analogues is the important difference, as Hazel provides important moisture and latent heating that are absent in the analogues. / Finally a mesoscale modeling study was carried out to test the sensitivity of Hazel to improved surface vortex structure and increased horizontal resolution. Specification of the vortex led to a dramatic improvement in the simulation results, as precipitation and track closely mimicked the observed values. Enhancing the horizontal resolution to 12 km did not improve upon the 36 km specified vortex simulation. The movement of the storm slowed considerably as the development of an upper tropospheric cutoff circulation was diminished in the 12 km run. The parameterizations governing the interaction between the diabatic outflow from Hazel and dynamics of the midlatitude trough are poorly modeled in this situation, and lead to the severe time lag in the path of Hazel.

Ensemble of seasonal forecasts done with four atmospheric models over 26 winters and springs (1969-1994) are used to investigate the Northern Hemispheric model responses to SST anomalies and to compare them with observations. The response, for winter (DJF) and spring (MAM), is obtained as the regression of the 500hPa geopotential heights against the main modes of sea surface temperature (SST) variability over two different regions of the world oceans. The regression against the main mode of the equatorial Pacific Ocean SST, takes the expected form of the Pacific-North-America (PNA) pattern both in models and observations. The response to the North Atlantic SST is an AO/NAO-like signal for the observations and also, to varying degrees, for the models. The similarity of the regression to the third mode of equatorial Pacific SST and to the first mode of North Atlantic SST during the spring season is discussed, along with the nonlinear features of the ENSO influence.

This thesis addresses the problem of understanding and quantifying the variability of the atmosphere over wide ranges of space-time scales. We present empirical tests of a scaling model, the 23/9D model, which is an intermittent and anisotropic generalization of classical laws of turbulence - such as the Kolmogorov law - which describes how the statistical properties of atmospheric fields vary with spatial scales. We first address this problem for vertical sections for which there is still an ongoing debate about the nature of atmospheric dynamics: whether it is isotropic with a break in the horizontal scaling or anisotropic, but in a scaling manner. We make the first direct estimate of the joint horizontal-vertical structure function using wind velocity data measured on 14500 aircraft flights segments and demonstrate that it strongly supports the 23/9D model. We also study the consequences of this spatial anisotropy for the full horizontal space-time statistics. By considering that small structures are advected by larger turbulent ones and by considering averages over the latter, we theoretically obtain scale functions (which are generalizations of the notion of scale) which allow us to estimate structure functions and spectra. We test these predictions using geostationary satellite infrared data over the range 5 km to ~10000 km, 1 hour to 2 months. We found that our model could accurately reproduce the 3D (kx, ky, ω) spectral density over the range 1 hour to ~ 4 days and 30 km to ~5000 km. To obtain a more complete description of the statistics, we also found that our model accurately describes atmospheric radiance turbulent flux statistics (including infrared and passive microwave imagery over scale ranges of 100 km to 20000 km, 1 day to 1 year) with only small deviations at small and large scales. We finally show how to include atmospheric waves considered as strongly nonlinear phenomenon driven by turbulent fluxes and constrained by scaling symmetry, thus extending the 23/9D model. The theoretical development proposed is based on an effective turbulence - wave propagator which corresponds to a fractional and anisotropic extension of the classical wave equation propagator with dispersion relation similar to those of inertial gravity waves (and Kelvin waves) yet with anomalous (fractional) order Hwav/2. Using geostationary MTSAT IR radiances, we estimated the parameters finding that Hwav ≈0.17 (the classical value =2). / Cette thèse s'intéresse au problème de comprendre et de quantifier la variabilité de l'atmosphère sur de grandes gammes d'échelles spatio-temporelles. Nous présentons des tests empiriques d'un modèle ''scaling'', le modèle 23/9D, qui est une généralisation intermittente et anisotrope des lois classiques de la turbulence - telle la loi de Kolmogorov - qui décrivent comment les propriétés statistiques des champs atmosphériques varient avec l'échelle spatiale. Nous abordons d'abord le problème pour des sections verticales par rapport auxquelles un débat sur la nature de la dynamique atmosphérique continue toujours: l'atmosphère est-il isotrope avec une brisure de symetrie d'échelle dans l'horizontal ou est-il décrit par une symétrie d'échelle, mais anisotrope ? Nous présentons le premier estimé direct de la fonction structure horizontale-verticale sur des vitesses de vent mesurées sur 14500 segments de vols d'avions et démontrons qu'elle supporte fortement le modele 23/9D. Nous étudions également les conséquences de cette symétrie d'échelle anisotrope sur les statistiques dans l'espace horizontal-temporel. En considérant que les plus petites structures turbulentes sont "advectées" par les plus grosses et en considérant un moyennage sur ces dernières, nous obtenons une forme théorique pour la fonction d'échelle (qui est une généralisation de la notion d'échelle), ce qui nous permet d'estimer les fonctions structure et les spectres. Nous testons ensuite ces prédictions à l'aide de données de rayonnement infrarouge prises par des satellites geostationnaires sur l'intervalle 5 km ~ 10000 km, 1 h ~2 mois. Nous avons trouvé que notre modèle pouvait précisement reproduire la densité spectrale 3D (kx, ky, ω) sur l'intervalle 1 h à ~ 4 jours et 30 km à ~5000 km. Afin d'obtenir une description statistique plus complète, nous avons également trouvé que notre modèle décrit précisement les statistiques des flux turbulents de rayonnements atmosphériques (dans l'infrarouge et pour les micro-ondes passives sur la gamme d'échelles 100 km a 20000 km, 1 jour a 1 an) avec seulement de légères déviations à petites et grandes échelles. Finalement, nous démontrons comment inclure les ondes atmosphériques, considerées ici comme un phénomène fortement non-linéaire dirigé par les flux turbulents et contraint par la symétrie d'échelle; étendant donc le modèle 23/9D. Les développements théoriques présentes sont basés sur un propagateur effectif turbulence-ondes qui correspond a une extension fractionnaire et anisotrope du propagateur de l'équation d'ondes classique avec une relation de dispersion similaire à celle des ondes gravitationnelles inertielles (et des onde sde Kelvin), mais d'ordre anormal (fractionnaire) = Hwav/2 . À l'aide de données infrarouge du satellite géostationnaire MTSAT, nous avons estimé les paramètres du modèle, trouvant Hwav ≈0.17, (la valeur classique étant =2).

Acceleration of ions transverse to the local magnetic field is ultimately necessary for gravitationally bound ionospheric plasma to escape out to the magnetosphere. Transversely accelerated ions (TAI's), which take the form of upflowing "conical" ion distributions, are not completely understood and are the subject of this thesis. TIDE and TIMAS are highly sensitive ion mass spectrometers carried by NASA's Polar spacecraft and capable of exceptional temporal, spatial, and energy resolution. Polar's elliptical orbit allows these instruments to sample auroral outflows at low altitudes near common source regions of transverse energization and also at high altitudes where conics are rarely reported. Four conic events observed by both TIDE and TIMAS are compared with several possible acceleration mechanisms. Low altitude conics are found to be the result of resonant wave-particle heating, while high altitude transverse distributions appear to arise from acceleration through small-scale electromagnetic structures.

The spatial distribution of water in all its three phases is an important factor in determining the climate. The interactions among temperature, water vapor, infrared emission and solar radiation form a series of feedback mechanisms, which play a very important role in the climate system. In order to trace moisture flow through the climate system and examine its impact on climate, a parameterization for the computation of moisture transport and precipitation is developed, one that will eventually be incorporated into a coupled energy balance climate-thermodynamic sea ice model (the CCSI model). This parameterization is tested by comparing computed energy transports and precipitation rates with available observations and by evaluating its sensitivity to variations in the values of specified parameters. The results of these studies indicate that the moisture parameterization is somewhat sensitive to variations in wind speed, surface air temperature and moisture flux, while it is relatively insensitive to changes in relative humidity. In general this parameterization does a good job in simulating the seasonal cycle and latitudinal distribution of the wind speed, moisture transport and precipitation when compared to the observed data and general circulation model (GCM) results.