This study's goal is to quantify the influence of large-scale forcing on surface cyclogenesis. This is accomplished by considering seventy surface cyclones, whose maximum deepening occurs in the western North Pacific Ocean. Cyclone classification is based on this pressure change, ranging from +10 to $-$40 hPa/24 h. Composite surface, 850 and 500 hPa analyses for each class, constructed from NMC gridded data are used to diagnose quasi-geostrophic omega and frontogenetical forcing and static stability. Explosive cyclones are distinguished by strong downstream 850 and 500 hPa frontogenesis, suggesting deep baroclinicity's importance during the maximum deepening phase. Cyclogenesis, embedded in a deep, conditionally unstable environment increases with composite forcing. However, regression analyses, relating forcing and intensification, show explained variance to be generally small, owing to large case to case variability within classes. This variability is caused by exclusion of exponential surface vorticity enhancement and static stability effects, coarse-resolution data and analysis errors.

The presence of the Gulf Stream in the cold season, and a cold air outbreak following a previous cyclone passage are two important precursors of a case of rapid cyclogenesis on 13-14 December 1988. The study showed that a large area of conditional instability in the lower troposphere, and an area of symmetric instability primarily over the sea, coinciding with small values of isentropic potential vorticity below the 290$ sp circ$K level were associated with strong surface heat fluxes. / During the time prior to the onset of this cyclogenesis, the surface frontogenetical forcing was dominated by diabatic frontogenesis, and later, by geostrophic frontogenesis, though the diabatic heating still contributed substantially to this forcing. The cyclone formed and continued to travel along a prominent axis of frontogenetic forcing. Kinematically-computed vertical motions, derived from both conventional and special ERICA soundings, show good agreement with analyses and diagnostics in the domain.

The standard 2D/3D picture of atmospheric dynamics of two distinct isotropic regimes separated by a "meso-scale gap" has been seriously questioned in recent years. Using satellite cloud images and the formalism of generalized scale invariance (GSI), we test the contrary hypothesis that cloud radiance fields are scaling in the range 1-1000 km. / Using a two-dimensional representation of GSI and three new analysis techniques, we test the following relation for each picture: $ langle vert F( lambda sp{ tilde G} vec k) vert sp2 rangle = lambda sp{-s} langle vert F( vec k) vert sp2 rangle$, where $F( vec k)$ is the Fourier amplitude at wavenumber $ vec k$, $ lambda$ is the scale ratio and $ tilde G$ is the generator of the semi-group of scale changes in Fourier space. Since we test only the linear approximation to GSI, $ tilde G$ is approximated here as a matrix. / For the three texturally--and meteorologically--very different images analyzed, we find three different generators that generally well reproduce the Fourier space anisotropy. These results show that linear GSI is a workable approximation for studying the atmosphere and that GSI can be used for cloud classification and modeling over this important mesoscale range.

No description available.

Severe D-1 directional Fourier truncation of the equations for D-dimensional (D = 2, 3) incompressible flow leads to a small number of coupled PDE's in one space dimension with the same inviscid quadratic invariants as the original set. Numerically generated equilibria for inviscid truncated versions of the reduced systems agree with predicted energy-enstrophy (D = 2) and energy (D = 3) equipartition spectra based on statistical mechanical equilibrium. Viscous calculations for forced and decaying turbulence at high Reynolds number are consistent with k$ sp{-5/3}$ inverse energy and k$ sp{-3}$ direct enstrophy cascading inertial ranges in two dimensions and with a k$ sp{-5/3}$ direct energy cascading inertial range in three dimensions. The power-law behaviour is clear in the two-dimensional system, but less convincing in three dimensions due to the persistence of a shallow spectral range at low wavenumbers. / Dissipation-range intermittency consisting of spatially intermittent vorticity gradients (D = 2) and vorticity (D = 3) is observed. In contrast to full numerical simulations, there is no tendency for isolated coherent structures to emerge in two dimensions. The model consequently mimics some, but not all, of the properties of the full set.

Clouds play a major role in the production of acids in the atmosphere. One of the most studied in-cloud processes is the aqueous-phase oxidation of sulfur IV into sulfate. From previous studies, large observed concentration of sulfate species resulting from sulfur IV oxidation in the atmosphere during the late fall and winter are still unexplained. Ariya found that ozonolysis of alkenes can be an additional source of oxidant in the gasphase. / In this project, we assessed the effect of ozonolysis of alkenes on in-cloud sulfur chemistry. We used a tropospheric chemistry box-model called MOCCA (Model of Chemistry Considering Aerosols) coupled with a simple cloud droplet model in the presence and absence of ozonolysis. The results obtained reinforce the hypothesis that ozonolysis of alkenes can play a role in the sulfate production in cloud droplets. In fact, we observed that ozonolysis could, in a situation of a high pH, slow down the in-cloud oxidation and can possibly lead to an overall negative impact on the in-cloud sulfate production over an average cloud lifetime. (Abstract shortened by UMI.)

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.