The atmospheric fluctuations associated with sea surface temperature (SST) anomalies in the northern North Atlantic and their subsequent effects on river runoff variations over Siberia are studied in this thesis. A data analysis is performed in Part I of the thesis using 50 years (1930-79) of records of SST sea level pressure, precipitation and runoff. It is found from this analysis that the winter SST anomalies in the northern North Atlantic are significantly correlated with the winter and the following summer runoff fluctuations of the Ob and Yenisey rivers in western Siberia. / A modelling study is conducted in Part II of the thesis to investigate the variability of winter atmospheric responses to the SST anomalies in the northwest Atlantic. Experiments are performed with an atmospheric global spectral model using both November and January conditions. The atmospheric responses to the prescribed positive SST anomalies obtained are found to be completely different in the November and January simulations. Over the Atlantic, a positive (negative) geopotential height anomaly center is observed downstream of the warm SST area in the November (January) case. A comprehensive diagnostic analysis of the heat advection in the experiments is carried out to obtain a deeper understanding of the physical maintenance of the two responses.

The nature of balance schemes and the concept of the slow manifold are discussed using Lorenz's (1980) model. It has been found that the Bear-Tribbia's (1977) scheme leads to a divergent series in locating the slow manifold of the model. The optimal asymptotic approximation and Pade approximant are use to "sum" the divergent series. / The optimal asymptotic approximation gives reasonable approximation to the full solutions of the model and provides the optimal balance relations which are quite accurate when the forcing is small. The accuracy seems to degrade rather quickly as the forcing increases. The "imbalance", which is the difference between the actual flow and the optimal balance state, is found to consist of nearly monochromatic inertial-gravity waves. It is shown that the optimal asymptotic approximation fails to give a reasonable estimate of the level of inertial-gravity wave activity from the Rossby modes. / For the case considered in this thesis, the Pade approximant does not seem to work well and hence offers no significant advantage over the optimal asymptotic approximation. The slow manifold, fuzzy slow manifold and effects of the scaling on the balance are also discussed in this thesis.

Early scaling stochastic models of cloud and rain fields were designed to obey the simple scale invariance symmetry. The linear nature of these additive processes is intrinsically related to their single fractal dimension which is in sharp contrast with the non-linear nature of the dynamical processes within the atmosphere and with the observed multiple scaling of rain and cloud fields. We consider stochastic models corresponding to coupled cascade processes, non linearly conserving the fluxes of energy and concentration variance. Multiplicative processes, previously based on discrete cascade procedures, are generalized to their continuous limit using a dynamical generator of the cascade characterized by only two parameters which determine the full multifractal spectrum of dimensions. We show how to numerically simulate such multifractal processes with both gaussian and Levy generators, and how to perform a scale invariant "zooming" procedure in the case of clouds passively advected by a turbulent velocity field.

This study investigates the potential usefulness of a commercial laser ceilometer, an instrument used operationally at most airports for monitoring cloud base, for providing an inexpensive, continuous indication of air quality. Using lidar principles, a method is presented in which the ceilometer can provide information on the aerosol optical properties of the atmosphere. Further insight into the ceilometer measurements is obtained by comparing observations with aircraft, lidar, and a RASS-equipped profiler. The results of this work show that the ceilometer can provide estimates of aerosol extinction in the lower troposphere under sufficiently hazy conditions.

In this thesis, a series of (48-60 h) numerical simulations of a family of frontal cyclogenesis events that occurred over western Atlantic Ocean during 13-15 March 1992 are conducted using a nested-grid version of the PSU/NCAR mesoscale model (MM4) with a fine-mesh grid size of 30 km. It is shown that MM4 captures very well the genesis, track and intensity of three secondary cyclones, their associated thermal structure and precipitation pattern as well as their surface circulations. / It is found that an upper-level potential vorticity (PV) ring plays an important role in determining the initiation and track of the frontal cyclones. The cyclones appear to form as a consequence of the superposition of upper-level PV anomalies on the low-level intense baroclinicity in the cold sector behind the slow moving primary cold front, and then they propagate into colder air towards the parent cyclone's center. It is also found that as the MFC intensifies, a mesoscale trough is induced in the low-to-middle troposphere, creating a favorable phase lag between the new pressure trough and a slow moving thermal wave. This phase lag provides a baroclinic conversion mechanism by which the system's kinetic energy could increase rapidly at the expense of available potential energy. / Diagnosis of sensitivity experiments reveals (i) dry dynamics determines the initiation and track of the frontal cyclones, accounting for about 59% of the final intensity of the MFC; (ii) the low-level baroclinicity and the upper-level PV anomalies are near-equally important in the genesis of the dry systems; (iii) the Ekman spin-down tends to slow substantially the development of the frontal cyclones; and (iv) surface heat and moisture fluxes could produce a significant impact (i.e., 59%) on the final intensity of the cyclones in the presence of latent heat release, but its impact is small in the dry dynamical framework. (Abstract shortened by UMI.)

A variational semi-Lagrangian wind retrieval algorithm from single-Doppler radar data is studied both analytically and numerically, in order to understand its responses as well as limitations. The theoretical background on which the algorithm is based, is described in detail. A new approximation of the conservation equation of reflectivity for wind retrieval purposes is derived. It is shown that, in general, a unique wind field may not be available for the entire domain. However, some ways of minimizing this inconvenience are presented. The existence of a spurious solution when the conservation equation of a tracer is discretized is found analytically. A real case (a storm characterized by a mesocyclonic circulation) is used in order to illustrate the problem of the non-uniqueness of the solution. This is carried out through the study of the sensitivity of the algorithm to the moving frame and the initial guess. In addition, the influence of each one of the constraints used (conservation of reflectivity, continuity equation, smoothness constraint) in the retrieved wind field are analyzed. It is concluded that in spite of the sources of error described, the results may be meaningful.

The North Atlantic Oscillation (NAO) refers to a low-frequency mode of atmospheric variability with two centres of action in the pressure field. The present study examines various atmospheric fields associated with the NAO. The NCEP/NCAR reanalysis data set from 1958--1998 is used. We find that the NAO-positive (NAO-negative) phase induces: (i) anomalously high (low) winter mean sea-level pressure (SLP) in the Azores High region and anomalously low (high) SLP in the Icelandic Low region; (ii) anomalously high (low) winter mean temperatures over northern Eurasia and anomalously low (high) temperatures over northwestern Africa, Greenland, and segments of northeastern North America (i.e., primarily northern Quebec, Labrador, and Baffin Island); and (iii) anomalously high (low) winter mean preapitation centred over the Norwegian Sea/Barents Sea region and anomalously low (high) precipitation over southern Europe, western Greenland, and the above-mentioned region in northeastern North America. Also, it is shown that the variability in storm track position and intensity due to the NAO is largely responsible for the European NAO-induced precipitation anomalies.

Paleo-evidence suggests that the mean state of the atmosphere in the early Holocene resembles the positive phase of the modern North Atlantic/Arctic Oscillation (NAO). To test this theory we use a coupled ice-ocean model to study the effects of atmospheric and ocean forcing from the positive phase of the NAO and altered surface radiation on sea-ice concentration and thickness in the Arctic during the early Holocene. The resulting sea-ice concentrations are compatible with reconstructions based on dinoflagellate cysts assemblages. Sensitivity studies show that local winds dominate the changes in sea-ice concentration from the East Siberian Sea, whereas ocean currents and sea surface and air temperatures all contribute significantly to anomalous sea-ice concentrations in the Barents Sea. Patterns in sea-ice thickness anomalies show similar results but are spread out over larger areas. The net radiation is shown to have little effect on the Arctic sea-ice cover during the early Holocene. / Les données paléocéanographiques suggèrent un régime atmosphérique semblable à la phase positive de l'oscillation de l'Arctique/Atlantique Nord (NAO) moderne durant l'Holocène inférieur (9.5 ka). Pour examiner cette hypothèse, on utilise un modèle dynamique-thermodynamique de la glace de mer, couplé avec un modèle de circulation de l'océan, afin d'analyser les effets atmosphériques et océaniques de la phase positive de la NAO et du bilan radiatif modifié sur la concentration et l'épaisseur de la glace de mer durant l'Holocène inférieur dans l'Arctique. La concentration de la glace de mer simulée est compatible avec les reconstructions de glace de mer derivée des assemblages des kystes de dinoflagellés. Des analyses de sensibilité montrent que les vents locaux dominent les changements de glace dans la mer de Sibérie Orientale mais les courants marins et les températures de surface océaniques et atmosphériques contribuent aux anomalies dans la mer de Barents. Le patron des anomalies d'épaisseur de la glace de mer sont semblables, mais plus étalés dans l'espace. La rayonement net est un facteur négligeable dans les changements de la couverture de la glace de mer durant l'Holocène inférieur.

The McGill University S-Band radar has been upgraded in 1999 to dual-polarization capability. The transmission and reception of horizontally and vertically polarized electromagnetic waves provide new parameters that are sensitive to the hydrometeor shapes and other characteristics in the radar resolution volume. As a result, the retrieval of microphysical properties of weather events can be improved. / In this work, we studied the reliability of the polarization data obtained by the fast scanning radar. Measurements from different precipitation systems are analyzed to study how this new information can be exploited for operational purposes. The work done shows a great potential for particle typing and microphysical retrievals. However, the possible use of orthogonal polarization for measuring precipitation remains ambiguous.

The objective of this study is to detect zones of supercooled drizzle using data from a vertically pointing radar (VPR). This radar was used in the AIRS (Alliance Icing Research Study) experiment held at Mirabel Airport from November 1999 to February 2000. / For cases where conditions favor the formation of drizzle drops with a diameter sufficiently large, a second mode with a distinct speed and reflectivity can be separated from that received from snow. That the second mode corresponds to supercooled drizzle is verified by comparing reflectivities and speeds with aircraft data taken during AIRS. / For cases without bimodality, regions of icing may be localized by a computation of the mass flux gradient. Accretion of supercooled water on snow (riming) increases both its density and fall speed. Thus, a sudden increase in snow fall speed indicates the presence of supercooled water. / A conceptual model describing necessary conditions for the formation of supercooled drizzle is proposed. This model is based on the analysis of a synoptic situation present for two days of intense icing.