The study of rain and clouds, even after centuries of research, is still a subject whose theoretical basis is obscure. A major cause of this problem is the extreme variability. The multiplicative cascade models employed in the study of turbulence lead to fields presenting the desired variability over wide range of scales. The fields produced with these models havae scale invariant properties expressed by a function specifying the way each statistical moment is transformed from one scale to the other; they are multifractal. It was proposed to consider rain and clouds as turbulent fields, thus providing a physical basis to statistical modeling of these fields. During this work, we wanted to empirically establish the applicability of these models. We established the range of scales where scale invariance is observed, we determined the transformation functions and established the limits of the model for various fields related to atmospheric water. / This verification is further complicated by the inhomogeneity of the measuring networks employed to gather data. In fact, the positions of landmasses, topography and economic constraints have resulted in networks which are not distributed on regular grids (as it might seem desirable) but on the contrary which presents holes at all scales. In fact, it has been shown that such networks are fractals. Rather, we will consider the station density as a multifractal. Multifractal fields analyzed by multifractal networks, this brings us to review the problem of removing the effect of the network from the measured data (the problem of Objective Analysis). The method that we propose (Multifractal Objective Analysis) replaces the homogeneity and regularity hypothesis more or less implicit in usual methods like Kriging by inhomogeneity and scaling hypothesis. It is then possible to develop corrections which allow us to study the multifractal properties of the analyzed field from the measured field.

A diagnostic study using the CASP (Canadian Atlantic Storm Program) dataset and available precipitation data is performed to examine the thermodynamics of melting-induced mesoscale circulations. Various terms of the thermodynamic equation were evaluated. The horizontal and vertical advection terms were calculated using the rawinsonde data while the diabatic term due to melting was parameterized using surface precipitation observations. The effects of evaporation and dissipation were neglected in the calculation of the diabatic terms. The importance of the melting term was clearly shown although direct evidence of a melting-induced circulation was not found due to the relatively low resolution of the datasets used.

The medium scale planetary wave regime, consisting largely of zonal wavenumbers 5-7, frequently dominate the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hovmoller diagrams, space-time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium scale wave regime. / The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with observations. When compared to the observed summer 1979 distributions, some quantitative differences remain: the vertical structure of the heat transport is too baroclinic, while the momentum transport tends to be too weak. The baroclinic and barotropic conversions all show a medium scale wave signal. The time evolution of the Richardson number of the mean flow suggests that the medium scale wave is due to a finite amplitude baroclinic instability.

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This study addresses the problems associated with each step of the process of retrieving radiation budgets at the top of atmosphere (TOA) and at the surface from satellite measurements in the Arctic. The data used are from the Earth Radiation Budget Experiment (ERBE) radiometer and Advanced Very High Resolution Radiometer (AVHRR), together with radiative transfer calculations. / The limitations of the ERBE scene identification algorithm when applied to observations in the Arctic are investigated by comparing the scenes identified by ERBE with simultaneous and colocated AVHRR-based scenes. Considerable discrepancies are found, especially the misidentification by the ERBE scene identification method of clear fractional ice as being partly cloudy skies over open water. / More accurate TOA fluxes and cloud forcing in the Arctic are derived by taking advantage of the broadband radiance measurements made by the ERBE radiometer and the more reliable scene identification deduced from narrowband multispectral measurements made by the AVHRR. Comparisons of the cloud forcing determined from the two scene identification methods show differences as large as 50 W m$ sp{-2}$ in some regions of the Arctic. / The validity of the ERBE angular dependence models (ADMs) is also evaluated in the Arctic. It is found that the ADM for clear ice/snow is not reliable when applied in the Arctic during summer. The ADM for overcast skies contains systematic errors when applied to overcast conditions over ice/snow surfaces. The systematic error is removed by modifying the ADM. / To estimate the surface absorbed flux from reflected flux at the TOA, a simple parameterized model is developed based on comprehensive radiative simulations. The model is independent of cloud optical thickness, surface type, and has only a moderate dependence on the presence or absence of cloud, cloud type and water vapour, but a strong dependence on solar zenith angle (SZA). Surface absorbed fluxes determined by radiative transfer calculations may generally be estimated to within 10 W m$ sp{-2}$ by the simple model from TOA reflected fluxes with knowledge of the SZA and precipitable water. / Finally, narrowband-to-broadband reflectance conversion is carried out with careful attention paid to a particular statistical problem, namely, regression analysis with spatially autocorrelated satellite measurements. Both scene-dependent and scene-independent regressional models are developed to estimate broadband reflectance from the narrowband reflectances in channels 1 and 2 of AVHRR. The RMS errors in the percentage reflectances from the scene-dependent models are 1.0, 1.8, 2.0 and 3.1 for the ocean, land, ice/snow and cloud, respectively. Except for land, the scene-independent model does virtually as good a job as the scene-dependent models.

In the vicinity of the point of minimum critical shear of a quasi-geostrophic two-level model on the $ beta$-plane, the weakly nonlinear dynamics of developing baroclinic vortices can be described in terms of a nonlinear critical layer problem which, in the inviscid case, can be solved analytically. When the supercritical shear $ delta$ is such that 0 $<$ $ delta$ $ ll$ 1 and the initial conditions are sufficiently small, finite amplitude equilibration occurs even though the potential vorticity field in the bottom layer Q(X,Y,t) remains transient, the potential enstrophy being transferred to smaller and smaller scales. It is shown that the inviscid equilibrium amplitude of the unstable wave is larger by a factor of $ surd$2 than the one found by Pedlosky (1982-b) in the limit of small dissipation. This indicates that the limits t $ to$ $ infty$ and r $ to$ 0 are not interchangeable. Inviscid equilibration occurs when the mixing in the lowest layer results in the streamwise homogenization of the coarse-grained average (Q) of the potential vorticity which means that (Q) $ to$ f($ psi$), $ psi$ being the streamfunction. When $ delta$ and the initial conditions are equally important, depending on the nature of the latter, periodic solutions and finite equilibration are both possible. An example is given of a periodic case when $ delta$ = 0. The potential vorticity field then reversibly wraps and un-wraps around the streamlines and mixing does not occur. Finally, these exact solutions are used to judge the reliability of numerical results obtained from truncated spectral models. For cases where finite equilibration occurs, the resolution of a truncated model is only adequate for a finite period of time while for periodic cases, a model with sufficient resolution can represent correctly the exact solution for any length of time.

A 915 MHz Doppler radar was able under certain conditions to provide Doppler spectra containing separate and distinct modes due to scattering by rain and by the clear air. The rain spectra are broadened and shifted on the velocity axis due to turbulent air motions. Information in the clear air mode is used to correct for these effects, leaving in the rain spectra the reflectivity-weighted distribution of drop fall speed, from which the drop-size distribution can be computed. / The fast Fourier transform and two iterative methods of deconvolution were applied to simulated Doppler spectra. We concluded that the iterative methods of deconvolution are more appropriate than the fast Fourier transform. Of the two iterative methods, Jansson's method of deconvolution, including prefiltering of the data, was found to give better results than Van Cittert's method. / Jansson's method was applied to biomodal spectra obtained from the HARP project of 1990 in Hawaii and to spectra obtained from a similar radar operated in Montreal. Evident is the different characteristics of rainfall in both regions and the appropriateness of the same deconvolution method to different classes of spectra.

This thesis is concerned with the numerical simulations of explosive cyclones using a very high resolution non-hydrostatic model with explicit precipitation physics. Two major aspects of the work are: (1) to explore the importance of various model parameters which lead to the rapid intensification of the surface cyclone through sensitivity tests, and (2) to unravel the cause-effect relations responsible for rapid surface pressure falls through a study of the heat budget, geostrophic vorticity budget, and a Lagrangian trajectory analysis of the simulated cyclone. / The results from six experiments are presented. The main conclusions are: (a) By using analytic initial conditions based on typical wintertime conditions prior to explosive cyclogenesis, it is possible to obtain very realistic simulations of rapid cyclogenesis. (b) The use of high horizontal resolution is important in simulating the mesoscale features of rapidly deepening cyclones. In particular, the structure of the intense warm front observed ahead of the cyclone is very sensitive to changes in horizontal resolution. (c) The destabilization of the lower troposphere prior to the period of rapid deepening is essential in the formation of an extremely intense frontal structure, which in turn is instrumental in the rapid spinup of the storm. (d) In the presence of strong surface fluxes, the development of the simulated cyclone is affected by the depth of the planetary boundary layer. / A detailed diagnosis of the control case is then presented. Quasi-Lagrangian heat budgets are calculated for several vertical columns about the vortex center. Very large variations in the heating were found to occur over a relatively small horizontal distance of 180 km. The reason for the mesoscale variations was traced to the intense warm front ahead of the cyclone and the bent-back warm front trailing behind the surface vortex. / The results of the heat budget were then used in the interpretation of the geostrophic vorticity budget. It was found that the primary mechanism for geostrophic vorticity growth at the surface is through horizontal variations in the horizontal potential temperature advection. In particular, the timing of the most rapid surface geostrophic vorticity growth coincided with the formation of the bent-back warm front. Although the forcing from diabatic heating is large, it is nearly cancelled by the forcing from vertical advection of potential temperature. Instantaneous surface fluxes of sensible heat provide very little direct generation of geostrophic vorticity. / The airflow in the vicinity of the intense warm front was diagnosed by a fully Lagrangian trajectory analysis. A schematic flow diagram is presented which illustrates the overturning of the air in the warm front. Calculations on the evolution of the circulation for closed loops of air parcels indicate that the warm front is responsible for the generation of large vorticity values at the surface.

Interactions among stationary planetary waves in the winter stratosphere are studied using steady-state quasi-geostrophic models from both a theoretical and numerical point of view. / A triad of small amplitude waves is examined analytically using a constant zonal wind $ beta$-plane model in which dissipation is required for the waves to interact. The nature of the modifications to the linear solutions is found to depend both on the propagation characteristics and the zonal wavenumber of the modes. / Numerical solutions are determined using a climatological basic state and boundary forcing. The changes to the linear structure are relatively weak. Zonal wavenumber 1 is the most affected, experiencing primarily an increase in amplitude in the vicinity of 65$ sp circ$N, 26 km. As a direct consequence of the almost linear relationship found to exist between the zonal streamfunction and the zonal potential vorticity in middle and northerly latitudes, the wave-wave interactions are, to a first approximation, dissipation-induced. The presence of weak dissipation in this region implies only weak interactions, which explains the quasi-linear structure of the solutions.

A case study of hurricane Floyd (1999) is performed using the Penn State/NCAR MM5 model. Hurricane Floyd was the third most costly hurricane to have hit the United States. / To predict accurately the track and evolution of the hurricane, a vortex bogusing technique has been devised. A more realistic initial vortex was specified and introduced into the large-scale analysis for model initialization. The technique used follows closely that described by Zhu et al. (2002) where Advanced Microwave Sounding Unit (AMSU) data are employed to retrieve the temperature of the hurricane vortex. An algorithm is then applied to compute the sea level pressure, geopotential heights, winds and moisture content. Three experiments initialized with three different data sets were performed, using respectively the original Canadian Meteorological Centre (CMC) analysis, the bogus-vortex modified CMC analysis with the original CMC sea surface temperature (SST) field, and a bogus-vortex modified CMC analysis with a spatially-constant SST of 28°C. (Abstract shortened by UMI.)