This thesis investigates the interaction between slantwise convection and large scale marine cyclogenesis. The major aspects of the work are: (1) to understand the physics of convective-cyclogenetic interaction using a two layer primitive equation model with a parameterization for slantwise convection, (2) to perform numerous sensitivity experiments with a three layer model to test the robustness of the proposed interaction, and (3) to quantify the contribution of convection to marine cyclogenesis using a high resolution model simulation. / The major conclusions are: (a) The two and three layer models produce realistic explosive cyclones with a simple parameterization for slantwise convection. (b) The explosive deepening coincides with the formation of the "bent-back" warm front. (c) The frontogenetic calculations and quasi-Lagrangian heat and vorticity budget calculations performed with the two layer model indicate the following sequence of events: Convection leads to rapid warm frontogenesis through "tilting" and the formation of the bent-back warm front. The sudden surge of cold advection in the regions of the bent-back warm front then forces the upper level heights over the cyclone center to fall in a rather dramatic way. Increased upper level vorticity advection interacts with the low-level system leading to explosive cyclogenesis. (d) The sensitivity study performed with the three layer model indicates that the formation of the bent-back warm front and the proposed interaction are quite robust for a range of parameters. However, there are quantitative differences when the low-level cloud mass fraction, surface drag force, the air-sea thermodynamic disequilibrium, vertical shear and static stability are varied. Shallow convection is insignificant in affecting the dynamics of cyclogenesis. (e) The high resolution simulation reveals that the descending branch of the slantwise convection may be responsible for the formation of the dry slot over the cyclone center in the early stages. The stratification in both the warm and the bent-back warm fronts exhibits slantwise neutrality after the explosively deepening period. (f) The PV (Potential Vorticity) inversion technique indicates that convection contributes to marine cyclogenesis. The inverted wind fields show a small scale cyclonic vortex and the presence of cold advection in the bent-back warm frontal region. (g) The PV diagnostics further reveal that the cold advection in the bent-back warm front decreases the surface potential temperature anoma

By combining numerical simulations with different diagnostic tools, this thesis examines the various aspects of two explosively deepening cyclones--the superstorm of March 12-14 1993 and a storm that occurred during the Intensive Observation Period 14 (IOP-14) of the Canadian Atlantic Storm Program (CASP). Using conventional observations, the general aspects of the storms are documented and the dynamical and physical mechanisms are discussed. Then the life cycles are simulated with the Canadian Regional Finite-Element model. To improve the model initial conditions, a methodology is proposed on the basis of potential vorticity thinking, and is tested to be successful in the simulation of the March 1993 superstorm. Using the successful simulations as control runs, a series of numerical sensitivity experiments are conducted to study the impacts of model physics on the development of the two rapidly deepening cyclones. / The deepening mechanisms of both storms are examined within the context of PV thinking, i.e., using piecewise potential vorticity inversion diagnostics. In both cases, the upper-level PV anomalies contribute the most to the surface cyclone, followed by the lower-level thermal anomalies and diabatic heating related moist PV anomaly. It is found that a favorable phase tilt between the upper- and lower-level PV anomalies allows a mutual interaction between them, in which the circulations associated with the upper-level anomalies enhance the lower-level anomalies, which in turn feedback positively into the upper-level PV anomalies. In addition to the vertical interactions, there also exist lateral interactions between the upper-level PV anomalies for the March 1993 superstorm. The upper-level PV features (troughs) are isolated with the piecewise PV inversion. By removing or changing the intensity of the trough in the initial conditions, the RFE model is integrated to examine the impact of each trough and its interaction with the other trough on the superstorm development.

A boundary-layer wind profiler has been operated continuously in downtown Montreal as a collaborative project of McGill University and the NOAA Aeronomy Laboratory. The records contain hundreds of hours of data on the melting layer of snow, consisting of Doppler spectra in the vertical profiler beam with a typical height resolution of 100 m and a time resolution of about 1 minute. A particular case study day was selected for analysis of precipitation processes associated with the melting of snow to become rain. We have examined the Doppler spectra just above and below the melting layer for evidence of aggregation or breakup during melting. Apart from the complete spectrum, we also show that the product of the mean Doppler velocity and the reflectivity factor in snow should be equal to that in the rain if aggregation and breakup are negligible. Both approaches indicate that there are times when snow merely melts to become rain, each snowflake forming one drop, but other times when aggregation or breakup are evidently important. It appears breakup is associated with regions of higher reflectivity.

The study of radiative transfer in multifractal clouds is of great interest, an important application being to Global Climate Models. In this work we develop a formalism analogous to the multifractal singularity formalism for understanding photon scattering statistics in radiative transfer in multifractals, and test the results numerically on lognormal multifractals. Although the results are only exactly valid in the thick cloud limit, the approximation is found to be quite accurate down to optical thickness of $ tau approx1$-10, so the results may be widely applicable. Furthermore we show the possibility of "renormalizing" the multifractal by replacing it with a near equivalent homogeneous medium but with a "renormalized" optical thickness $ tau sp{1/(1+C sb1)}$ where C$ sb1$ is the codimension of the mean singularity of the cloud. We argue that this approximation is likely to continue to be valid for multiple scattering, and is also compatible with recent results for diffusion on multifractals. Finally we analyze cloud liquid water content data and estimate the universal multifractal indices. We find that the scaling is respected over the whole range 5m-330km and that the cloud can in fact be reasonably described by a lognormal multifractal.

It is now well established that aerosols, by scattering and absorbing solar radiation, affect the terrestrial heat engine. Depending on the net relfectivity and absorptivity of the haze column and on the surface albedo, the net effect could be a cooling or a warming. But better understanding of the influence of aerosols is handicapped by lack of data and by the fact that their composition and distribution is highly variable in space and time. For the same reasons, the incorporation of aerosols in climate models is usually rudimentary. / We have in-situ measurements of the vertical profile and size distribution of aerosols ($>$0.09 $ mu$m in radius) taken over several places in eastern Canada at different times of the year. These data have been incorporated in a simple multi-layer radiative transfer model to estimate the effects of aerosols on the local radiation budget. A broad range of refractive indexes has been used and the increase of the particles size with the relative humidity is included.

Analysis of high resolution observational data gathered during a mesoscale field experiment in central Alberta (LIMEX-85) has led to a conceptual model of severe convective outbreaks in Alberta. It is proposed that most severe convective events result when upper-level cooling, associated with an advancing, synoptic-scale trough, occurs in phase with strong surface heating over the Alberta foothills. The deep destabilization over the elevated topography acts to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gives rise to east-northeasterly winds which advect the moisture-rich air of the eastern plains into the lower-branch of the mountain-plain circulation. In this manner, the plains moisture is permitted to reach the convectively active foothills through underrunning of the capping lid. The end product of the synoptic-mesoscale interactions is the initiation of well-organized, severe convective storms which move eastward with the westerly component of the mid-tropospheric winds. A statistical analysis based on archived hail data provides additional evidence for the key synoptic-scale features of the conceptual model.

Automatic analysis methods based on multichannel radiance data measured by the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA7 and NOAA9 satellites are used to: (a) detect clear and cloudy areas above coastal regions; and (b) detect clouds and to estimate cloud amount above the Arctic. / Channels 1 2 and 4 of the AVHRR were employed to separate clear from cloudy radiances above coastal regions. / All AVHRR channels were used above the Arctic. A large number of orbits were analysed automatically and the analysis results were compared to manual analyses performed by an independent observer and to published analyses. The comparisons yield that the present algorithm provides a reliable method for the automatic detection of clouds in the Arctic.

Using a barotropic model, some aspects of the theory of multiple equilibria were investigated, namely, the genesis of multiple equilibria via bent resonance, and the properties of some of the linearly unstable low index solutions. / These studies were conducted using numerical models with a truncated spectral representation of the dependent variables, truncation that was determined in terms of its capacity to allow for nonlinear interactions. By means of a continuation algorithm, the stationary solutions of the potential vorticity equation were calculated for a number of regions of parameter space, allowing us to study the dependence of the steady solutions on an ensemble of parameters, including the geometry of the domain. / We can conclude that it is possible to observe the presence of more than one stable stationary solution for certain restricted regions of parameter space, but that in a majority of situations, only one solution is linearly stable. The latter, however, is not the only stable regime present since the unstable solution with the lowest zonal index is then surrounded by a stable periodic solution also of a low index type.

The thesis is mainly concerned with the initial-value problem for barotropic Rossby waves linearized around a parabolic zonal flow, U (y) = U$ sb0$ + U$ sp prime$ y + U$ sp{ prime prime}$ y$ sp2$/2. It is shown that the non-dimensional equation for the perturbation depends on only one essential parameter, a = $-$2 ($ beta -$ U$ sp{ prime prime}$)/U$ sp{ prime prime}$. The associated Rayleigh-Kuo equation has a continuous spectrum of singulars modes for all values of "a" and an infinite number of discrete Rossby waves for a $< -$.25. / In Fourier space the problem has a Lagrangian formulation which permits a simple qualitative discussion concerning the nature of the interaction between the perturbation and the zonal flow. We have found new conservation laws that give insight into the dynamics of the jet. These results show a significant difference between the dynamics of easterlies and westerlies. / Using results from the theory of Lie groups we have obtained a number of distinct separation of variables for a $ geq -$.25 in the long wave approximation. This allow the initial value problem to be completely solved and also provides a number of exact solutions which are localized in y. For certain values of "a" these have a simple analytical form that permits a comparison with previous results for a linear zonal wind. As in the uniform shear case we observe a temporary amplification for certain initial conditions, whose nature now depends on "a". / We have shown that for an arbitrary initial condition of small amplitude (of order $ varepsilon$), the linearity hypothesis is invalidated when a $ ll$ 1 after a time of the order of $ varepsilon sp{-1}$ in a critical layer of width of order $ epsilon sp{1/2}$ centered at the jet extremum. We have also obtained the governing equations for the nonlinear evolution stage, which have a striking similarity with those for a forced Rossby wave on a linear wind profile.

Dual-wavelength radars can, in principle, provide extra information to help in the estimation of precipitation. One method would be to use the differential attenuation measured between the two frequencies of the radar as an indication of the rain rate. Microwave attenuation is widely regarded as a good estimator of the intensity of precipitation. The theory of microwave attenuation is presented, as well as an estimation of the error sources involved in the measurement of attenuation with dual-wavelength radars. Two long-time datasets of disdrometer data are used to test the feasibility of tuning the radar Z - R relationship by measuring the relation between reflectivity and X-band attenuation. As an interesting fact, a surprising proportionality between these two variables is found for higher intensities of precipitation (Z > 40 dBZ). This finding limits the capabilities of dual-wavelength radars to use attenuation as a second parameter, since at higher reflectivities X-band attenuation is almost equivalent to reflectivity.