The responses to positive and negative sea surface temperature (SST) anomalies in the North Pacific are computed with a time-dependent, quasi-geostrophic, global spectral model with a T21 horizontal resolution and three levels in the vertical. The simplicity of the model allows a large number of cases starting from different initial conditions to be run. The model produces on average a ridge (low) downstream of the warm (cold) SST anomaly, but the average response to the warm anomaly is much weaker and statistically less significant than that to the cold anomaly. In the case of the warm SST anomaly, the storm track is displaced northward into the high-pressure atmospheric anomaly, whereas in the case of the cold SST anomaly, the storm track is moved southward, away from the atmospheric low-pressure anomaly. The higher level of atmospheric nonlinearity in the warm cases leads to more case-to-case variability in the model response to the SST anomaly than for the cold SST anomaly. The results are compared with those of previous work in the literature where the response of a GCM to a warm SST anomaly was found to be weaker and statistically less significant than that to a cold anomaly.

The transformation of 45 tropical cyclones into extratropical cyclones over the western Noah Atlantic Ocean between 1963 and 1996 is studied. Cases are selected from the National Hurricane Center's "best track" archive. National Centers, for Environmental Prediction (NCEP) reanalyses of geopotential height data are used to construct a synoptic-dynamic climatology of extratropical transition, or "ET". The Forecast Systems Laboratory (FSL) upper-air archive of six near-track stations is used to produce sounding composites. / Primary results of the study follow. (1) A statistically-significant 1000--500-hPa warm anomaly (with respect to the 1963--96 climatology) persists for the one-week period prior to the passage of the tropical systems into the Canadian Maritime provinces. (2) A northwestward extension of the surface subtropical anticyclone exists over the Canadian Atlantic Provinces during the two-day period prior to the arrival of the cyclone. (3) The tropical cyclone's warm core and conditionally-unstable tropical airmass are maintained after transition. (4) The presence of quasi-geostrophic forcing for ascent, typically seen in extratropical cyclones, is observed during periods in which the systems are still classified as tropical cyclones. This forcing for ascent continues during the extratropical transformation, and typically occurs ahead and to the left of the storm track.

Despite advances in mesometeorology and computer technology, high-resolution numerical simulations of small-scale severe weather events remain extremely challenging. This is primarily due to insufficient initial conditions and inadequate convective parameterization schemes (CPSs). This thesis serves to illustrate how these difficulties may be overcome in a real-data simulation of the Garden City, Kansas, tornadic thunderstorm, which occurred during the VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment) field experiment of 1995. / Using a sophisticated mesoscale model at 18 km horizontal resolution, a successful reproduction of the environment over southwestern Kansas is obtained. However, modifications to the CPS are required to trigger the Garden City storm at the correct time and location. Utilizing severe weather parameters, it is found that the simulated atmosphere is susceptible to tornadic supercells. The results of a sensitivity study also indicate that a neighbouring storm may have influenced tornadogenesis in the Garden City supercell.

The McGill/Oklahoma University Bistatic Radar Network provides Doppler velocity measurements from three different points of view. This, in principle, allows a three-dimensional wind field retrieval. Because the receivers are looking at low elevations, the vertical component of the wind is poorly sampled and is mainly obtained by integration of the continuity equation with great associated uncertainty. The retrieval can be improved with measurements taken by a vertically pointing Doppler radar. / In this work, we study the impact on the reconstruction of the wind field when UHF wind profiler information is used as an additional constraint along with bistatic measurements. Experiments were done with a synthetic wind field to study how the information from the profiler can be integrated and how it is propagated. These experiments show significant improvement of the retrieved vertical motion. Consequently, the algorithm for the retrieval of the 3-D wind has been modified to combine the UHF information with the bistatic network and its impact is studied for the case of a shallow supercell.

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.)