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.

Clouds and aerosols impact on climate, both independently and through their interaction with one another. Thus, in climate models, it is important to make cloud schemes and in-cloud chemistry as realistic as possible. To this end, the UQAM sub-grid scale cloud scheme was added to LCM11, a one-dimensional version of the Northern Aerosol Regional Climate Model (NARCM). The goal of this project is to assess whether it is necessary to complete parameterization of sub-grid scale processes in LCM11 by implementing sub-grid scale parcel chemistry. / Parallel runs of LCM 11 with and without parcel chemistry were performed under various chemical scenarios. The production of sulphate, the main anthropogenic constituent of tropospheric aerosols, was compared within parallel runs. Sulphate production was not significantly changed by the implementation of parcel chemistry. Thus, for the scenarios assayed in this project, the addition of sub-grid scale parcel chemistry to LCM11 is deemed unnecessary.

Supercooled cloud liquid water (SCW), which causes aircraft icing, can be detected indirectly by observing characteristics of the bright band with a vertically pointing radar. Accretion of supercooled water droplets on precipitating snow (riming) increases the density and fall speed of snow and decreases the intensity of the bright band. For stratiform precipitation with a melting layer, a small peak-to-rain reflectivity difference and a high snow-to-rain velocity ratio indicate high-density snow. A numerical model of the bright band gives a semi-quantitative relationship between snow density and bright band parameters. SCW content is computed theoretically and depends on the gradient of mass flux with height above the melting layer. A large mass flux gradient is an indication of growth by riming.

Montreal's heavy precipitation event of 8--9 November 1996 was noteworthy for its all-time record-breaking 24-hour accumulation of 134.0 mm. We analyse this case through a study of the synoptic-scale and mesoscale states. / Searches for the best analogues to this event are conducted. Fields of the best mass analogue, for 1977, are compared to those of the 1996 event. / It is found that, in the Montreal area, the 1996 event had persistently larger precipitable water content and synoptic-scale ascent values. Averaging the storm-total precipitation values of five randomly-chosen Montreal-area stations yields a value of 74.8 mm for this event. The analogue case, despite its excellent synoptic-scale similarity, was characterised by an average value of only 20.2 mm.

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.