Two algorithms have been used for determining the quantity and spatial distribution of supercooled water in a cloud from the change in snow characteristics as snow falls to the ground. The snow density change algorithm is based on the change in snow density due to the riming process, while the snow flux gradient algorithm is based on changes in snow flux occurring as falling snow captures supercooled droplets. The data used to run and analyze the algorithms' results were collected during the Alliance Icing Research Study (AIRS) that took place at Mirabel during the winter of 1999-2000. / For the analyzed cases, the results of the two algorithms agreed well with each other. Furthermore, these results matched radiometric and aircraft measurements overall. Both algorithms demonstrated some limitations. Possible solutions to reduce some of the observed limitations include employing an algorithm for resolving trail patterns, and an algorithm for separating the drizzle from the snow echo.

One of the goals of the Mackenzie GEWEX Study (MAGS) is to model the critical components of the water and energy cycles that affect the climate of the Mackenzie Basin. The land surface - hydrological model WATCLASS is used to simulate the energy and water transports at and below the surface. Atmospheric input to WATCLASS is provided by the output from the atmospheric model GEM. There may be significant uncertainties in the GEM incoming solar radiation due largely to difficulties in simulating clouds and their radiative properties. The question that we address is how these uncertainties affect the simulation of the energy and water budgets of this northern river watershed. / To assess this sensitivity, two series of two WATCLASS model runs are compared. Both runs are driven by atmospheric data from GEM for the 1998--99 water-year, but in the second run shortwave radiation fluxes retrieved from satellite measurements replace the GEM fluxes. Land cover differs in the two series of runs and so provides an assessment of the sensitivity to vegetation variability. Results show that the atmospheric model overestimates the incoming solar radiation field by 36%. This results in an increase in the basin annual average surface temperature of about 1°C and an overestimation in net longwave radiation, and sensible and latent heat fluxes. Snowmelt starts earlier with a decreased first snowmelt peak in runoff and discharge hydrographs. The overall consequence is an annual discharge underestimation.

Strong surface winds are an inherent aspect of the eastern Canadian Arctic climate yet few studies have focused on these features. As a result, arctic winds are often poorly predicted by current weather forecasting models. To better predict the arctic weather, we need to understand the role of the Arctic's unique geographical and meteorological features such as mountains, sea-ice and very stable atmosphericstratification. In this study, we hypothesize that these features have an impact on high wind events in the eastern Canadian Arctic. To test this, we examine the long-term data records of six meteorological stations across southern Nunavut. We also develop a severity index to characterize high wind events, based on duration, maximum gusts and mean wind speed observed. By studying the five most severehigh wind events and the motion of storm systems generating strong winds, we identified how the Arctic's geographical and meteorological features enhanced strong surface winds. High wind events are usually associated with intense cyclones located over Hudson Bay or Labrador Sea, particularly in wintertime. Under this large scale setting, blocking and channeling due to the stable boundary-layer stratificationtypically occur at Clyde River and Iqaluit, enhancing the surface wind speed. Over flatter terrain, high wind events typically occur when the station is situated in a region of strong synoptic pressure gradient between an anticyclone and a cyclone. / Malgré leur rôle prépondérant au sein du climat arctique canadien méridional, peu d'études se sont intéressées aux vents violents de surface. Conséquemment, les prévisions météorologiques à leur sujet sont souvent imprécises. Afin d'améliorer les modèles de prévisions météorologiques dans l'Arctique, nous devons mieux doser la contribution des particularités géographiques et météorologiques inhérente à cetterégion, notamment la glace de mer, le relief accidenté et l'importante stabilité atmosphérique. Dans cette étude, nous supposons que ces particularités contribuent à la formation d'événements de vents violents dans l'Arctique canadien méridional. Afin de tester cette hypothèse, nous étudions les données météorologiques de six stations réparties dans le sud du Nunavut. Pour ce faire, nous développons un indice de sévérité basé sur trois paramètres des événements de vents violents: la durée, les rafales maximales observées et la vitesse moyenne du vent. Suite à notre étude, nous observons que les événements de vents violents sont typiquement associés à la présence d'intenses dépressions situées au-dessus de la baie d'Hudson ou de la mer du Labrador, particulièrement en hiver. Sous l'influence de ces systèmes, des phénomènes de blocage et d'effet de canal sont typiquement observés à Clyde Riveret Iqaluit respectivement, modifiant ainsi la vitesse et la direction des vents de surface. Quant auxstations en terrain plat, les événements de vents violents sont davantage provoqués par de forts gradients de pression synoptiques, lorsque la station est située entre un cyclone et un anticyclone.

A mesoscale model with a horizontal resolution of 20 km was used to perform a simulation of the rapidly deepening continental cyclone of 19--21 July 1996 that produced heavy precipitation and severe flooding in the Saguenay region of Quebec, Canada. Sensitivity experiments were conducted and diagnostics performed to investigate the interactions and relative importance of the forcing mechanisms that led to the explosive cyclogenesis and heavy precipitation. / It is found that condensational heating is integral for the establishment of a phase lock between the surface cyclone and a strong, upper-level short wave trough which steers the cyclone. A weaker trough acts to retard the progression of the stronger trough, ultimately causing the cyclone to be located in a favorable position to interact with orography. Using potential vorticity (PV) inversion diagnostics, the relative contributions to cyclogenesis from the positive anomalies from upper-level PV, condensation-generated PV and surface potential temperature are quantified. The contribution to the precipitation from orographic forcing due to upslope flow is also quantified through sensitivity experiments.

A research version of the Canadian regional finite-element (RFE) model is used to evaluate the capability of the operational model in reproducing the meso-$ beta$-scale structure and evolution of three different types of midlatitude squall-line systems, and to advance our understanding on the development of these features. / In this thesis, we use the well-documented 10-11 June 1985 squall line as a test bed to examine the appropriate incorporation of various physical representations and their coupling with RFE's model components. It is demonstrated through a series of sensitivity studies that the operational prediction of squall lines can be improved if more realistic model physics, reasonable initial conditions, and high resolution are used. It is shown that subgrid-scale moist convection and grid-scale moist physics must be adequately treated in order to reproduce the internal structures of the squall line. / Then, the improved version of the RFE model is used to study the role of gravity waves in the development of a prefrontal squall line associated with the 14 July 1987 Montreal flood. It is found that the gravity waves and convection propagate in a "phase-locked" manner and that the wave-CISK mechanism accounts for the maintenance and intensification of the system. It is also found that frontogenetical processes and release of conditional symmetric instability are responsible for the development of a trailing stratiform rainband associated with the July 1987 Montreal flood. Numerous sensitivity experiments are conducted, and they show that the meso-$ beta$-scale structures and the wave-convection system are very sensitive to the interaction of the parameterized convection with grid-scale physical processes. / In the last part of the thesis, the along-line variability 26-27 June 1985 squall line during PRE-STORM is examined. It is found that the three-dimensional structures of the squall's circulations are determined by both a large-scale trough and convectively generated disturbances. In particular, it is shown that rear inflows in the stratiform region tend to be more intense to the south of the mesolow and neat the base of the large-scale trough.

The dynamics of low-frequency fluctuations (periods between 10 days and a season) is investigated. While those fluctuations are known to be forced, at least in part, by the underlying surface, the emphasis in this thesis is placed on processes taking place within the atmosphere. / The thermal interaction between the high-frequency (periods 2 to 10 days) and the low-frequency flow is first investigated. The temporal and spatial relationship between the heat flux convergence by the synoptic-scale eddies and the low-frequency temperature field is identified. It is shown that the low-frequency temperature fluctuations are negatively correlated with the heat flux convergence by the synoptic-scale eddies, implying the damping effect of high frequency eddy heat flux on the slowly varying temperature field. The damping effect is not homogeneous in space, however, and different temperature patterns have different damping rates. The low-frequency temperature patterns over the North American and Siberian inland areas, where the strongest low-frequency temperature variances are observed, are associated with weak damping from the high-frequency eddies. A stronger dissipation is found for the low-frequency temperature variations in the storm track regions. / We then examine the El Nino-related interannual variations of the transient eddy activity and the associated multiscale interactions. A dataset from the U.S. National Meteorological Center (NMC) of 24 years is used. The purpose is to determine when the seasonal mean flow is less dependent on processes internal to the atmosphere and therefore more dependent on external forcing and thus more predictable. The results show that during El Nino winters the low-frequency eddy activity is reduced over the North Pacific and the high-frequency baroclinic waves are shifted south-eastward of their normal position in the Pacific. Over the North Pacific less kinetic energy is supplied to the low-frequency eddies both from the large-scale seasonal mean flow and from the synoptic-scale eddies. Thus the atmosphere in that region is more "stable" during El Nino winters, and its state depends more on the external forcings. / Finally, the atmospheric predictability is studied explicitly. A large number of numerical experiments are performed to determine whether the forecast skill is dependent on the weather regimes. The predictions are made with a T21 three-level quasi-geostrophic model. The relationship between the forecast behaviour and the "interannual" variation of the Pacific/North American (PNA) anomaly is investigated. Comparison of the error growth for the forecasts made during the positive and negative PNA phases indicates that little differences of error growth can be realized before about a week. After that period the forecast error grows faster during the negative PNA phases. The forecast skill for the medium- and long-range predictions over the North Pacific, the North American and the North Atlantic regions is higher during the positive PNA phase than that during the negative PNA phase. A global signal of this relationship is also observed. The physical mechanism for the difference of error growth is discussed.

Two classes of warm-season meteorological events in Southern Quebec have been studied. (1) thunderstorms and (2) mesocyclones detected by radar. / Characterization of the synoptic-scale flow accompanying warm-season mesoscale vortices and isolation of the moisture flow are documented. A comparison of both 500-hPa height and dynamic tropopause potential temperature composite fields between each set is completed. Precipitable water content and mixing ratio composites of the two classes are diagnosed. / The storm sample associated with mesocyclones show an organized trough-ridge couplet 3 to 4 days before the onset time. Strong precursors of coherent dynamic tropopause forcing seen several days in advance for the mesoscyclones are not as apparent in the thunderstorm sample. During several days prior to the events, the Atlantic Ocean, the Gulf of Mexico and the Great Lakes form a wider and more active area feeding moisture into the mesocyclonic systems than into the thunderstorms.

The Northern Aerosol Regional Climate Model (NARCM) is being developed in order to better understand how aerosols affect the Canadian climate. NARCM uses microphysical and chemical parameterizations that enable it to predict the evolution of the aerosol spectrum using the concentration and composition in 12 size-segregated bins as prognostic variables. In this study, two aerosol species are considered as prognostic variables within NARCM: sulphate and sea-salt. The sulphur species concentrations and aerosol distributions simulated by NARCM are evaluated against clear-sky in-situ aircraft measurement taken off the coast of Nova Scotia during the North Atlantic Regional Experiment (HARE). NARCM demonstrates skill at predicting the column burdens of the sulphur species concentrations at the NARE site throughout the NARE period. Simulations of the average column burdens of sulphur dioxide and sulphate showed differences of 57% and 28% respectively from the observed values. The accumulation mode in the aerosol distributions simulated by NARCM has a tendency to be smaller than the measured mode. There is evidence that the absence of other aerosol species may account for the too small sizes of the simulated aerosol when sulphate volumes are small.

In this thesis, methods based on the wavelet transform are used to extend spectral methods for studying turbulent mixing. These methods are then applied to momentum and heat flux for fast response measurements taken above and within a deciduous forest at Camp Borden, Ontario. / Multiscale distributions of flux event intensities are derived, presented and interpreted. Results show intensification of mixing efficiency near the canopy top, dominated by turbulent structures near a dominant scale, as reported in previous studies. There is no indication of a distinct, clearly separable population of intense 'coherent structures', as is often assumed, but rather an overall increase in intermittency near a particular scale. / The multiscale flux event distributions are simplified to provide component cospectra for down-gradient and counter-gradient fluxes. Dimensional arguments are used to explain observed scaling, and differences between upward and downward cospectra of momentum above and within the forest are used to understand the influence of different terms in the Reynolds stress budget. / A multiscale parameterization of the Reynolds stress budget is developed using the wavelet results for momentum flux. The physical meaning of the scale-dependent parameters is examined. Empirical values derived from the Camp Borden observations show considerable consistency. Changes in parameter values within the canopy are consistent with the effect of neglected Reynolds stress budget terms which are known to become significant within the forest.

The Empirical Orthogonal Function (EOF) analysis technique has proven to be one of the most powerful methods to analyze data in meteorology and many other fields. However, this method is statistical only and has no physical basis. Brunet (1994) has introduced Held's (1985) concept of conservation of wave activity and orthogonal functions into the EOF analysis and called it the "Empirical Normal Mode" (ENM) analysis technique. This new method uses both statistical concepts from the classical EOF analysis method and a dynamical constraint from the generalized Eliassen-Palm theorem to ensure that the functions that we obtained are orthogonal to each other and are the solutions of linearized dynamical equations. / In this thesis, we use the ENM analysis to analyze data from both a (2D) shallow water model integration and from 3-D atmospheric observations, with an emphasis on stratospheric sudden warming events. / For the shallow water model case, the results of the ENM analysis are evaluated by testing against the theoretical (numerical) normal mode solutions provided by Longuet-Higgins (1968). It is shown that the ENM analysis can recover the spatial structures and the frequencies of the normal modes with a great degree of accuracy if the temporal record is sufficiently long. The average errors in the periods for 2000 and 100 day time series are found to be 1% and 4.6%, respectively. From the eigenvalues (percentage of the total variance) and sharp frequency peaks associated with normal modes, the ENM analysis shows that the model generates only a few modes with monochromatic frequencies. The method can be used to test a new or modified shallow water model integration or to study other Hough modes generated by different kinds of forcings. / Having shown the value of the ENM technique in a barotropic context, we advance further by performing an ENM analysis on an 11 year atmospheric data set. In this study, we focus on stratospheric warming events. The winter (DJF) data set is partitioned into warming and non-warming periods in order to characterize the flow differences between the regimes. The stratospheric quasi-potential vorticity or wave activity structure in the warming period is found to be much stronger, as expected, than in the non-warming periods. The ENM analysis clearly shows the tropospheric difference between the two periods, e.g., a higher wave activity in the main tropospheric structure as well as in the tropospheric polar regions in the warming periods. The analysis also reveals that there is a higher level of stratospheric wave activity during the warming periods in the second normal mode of zonal wave number 1 but the tropospheric structures of the quasi-potential vorticity are the same as during non-warming periods. This suggests that there is/are (a) mechanism(s) associated with the stratospheric warming other than the upward wave propagation. All the common features of the stratospheric warming event are captured by the first two normal modes of zonal wave numbers 1 and 2, such as wave-mean flow interaction leading to the deceleration of the zonal mean wind, the polar vortex being displaced by the northward movement of the Aleutian High, as well as wave amplitude enhancement/reduction during the growing/decaying stages.