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.

Despite the fact that spiral rainbands have long been detected in radar observations, many aspects of their dynamics and their role in hurricanes still remain unresolved. In this thesis, the high-resolution PSU-NCAR nonhydrostatic mesoscale model (MM5) is used to perform three numerical simulations, namely, the control run, the dry run and the landfall simulation. The dynamics of inner spiral rainbands formed in the simulated hurricanes are studied. / It is shown that the interaction of low-level potential vorticity (PV) anomalies with boundary layer friction and the strong coupling of the PV band and the cloud band give rise to the inner spiral rainbands. The propagation properties of the PV bands are analyzed and found to be consistent with predictions of vortex Rossby wave theory. Empirical normal mode method is further applied to separate the vortex Rossby waves from the gravity waves. The former dominates the latter in terms of wave activities. The radial and vertical propagation and the associated eddy momentum and eddy heat fluxes of vortex Rossby waves are studied using the Eliassen-Palm (EP) flux. The wave-mean-flow interaction indicated by the divergence of the EP flux shows significant effects on the intensification of the hurricanes. / While diabatic heating tends to maintain a ring structure of PV by continually generating high PV in the eyewall, the vortex Rossby waves tend to redistribute the PV in the ring and to increase the inner-core PV. When diabatic heating is suppressed in the dry run and in the landfall simulation, the PV ring evolves to monopolar structure. The dynamical properties of the inner spiral rainbands and their roles on the intensity then vary as a response to the evolving mean PV structure of the hurricane. Nevertheless, the total effect of the asymmetric diabatic heating and the vortex Rossby waves tends to counteract the effect of the mean circulation.

Despite considerable research in the study of explosively deepening oceanic storms, much less attention has been paid to coastal secondary cyclogenesis that is characterized by weak baroclinicity but heavy precipitation. In this thesis, the development of one such storm, the February 1993 cyclone, is investigated using a high-resolution version of the Canadian Regional Finite-element (RFE) model in which more realistic physical representations are incorporated. It is shown that the improved RFE model reproduces extremely well the timing and location of coastal secondary cyclogenesis along the Carolina coast, the evolution of the associated surface pressure perturbations and tropospheric baroclinic waves. The model also performs remarkably well in predicting the distribution and intensity of the storm's precipitation over the New England states, Quebec and the maritime provinces. / It is found that the coastal secondary cyclogenesis occurs in response to the low-level inshore advection of warm and moist air from the maritime boundary layer and the approach of the midtropospheric short-wave trough associated with a decaying cyclone to the west. Rapid deepening of the cyclone ensues as intense precipitation falls along the warm and cold frontal zones. / Diagnosis of a series of sensitivity simulations reveals that large-scale dry dynamics, latent heat release, air-sea interaction and surface characteristics act in concert to determine the cyclone's intensification and evolution. It is found that the low-level thermal advection contributes more to the cyclogenesis during the incipient stage, whereas the upper-level trough control the cyclone's rapid deepening at later stages. Overall, latent heat release accounts for about 50% of the total deepening. It is also found that ice microphysics (i.e., melting of snow) could indeed induce a thermally indirect circulation superposed onto the mean flow in the vicinity of the rain-snow boundary, but it has a weak negative impact on the cyclogenesis.

In this study, a rapidly east-coastal cyclogenesis event that took place during 3-4 October 1987 is investigated using a nested-grid version of the Penn State/NCAR mesoscale model (MM4) with a fine mesh grid size of 25 km. It is shown that the MM4 model reproduces reasonably well the growth of the cyclone from a coastal frontal zone, its subsequent track, intensity, circulation structures and its associated precipitation. It is found that the coastal cyclogenesis occurs in a favorable large-scale environment with pronounced thermal advection in the lower troposphere and marked potential vorticity (PV) concentration aloft associated with the tropopause depression. The transport of warm and moist air from the marine boundary layer by the low-level inshore flow provides the necessary latent energy for the production of the observed heavy precipitation and a variety of weather phenomena. / A series of 24-h sensitivity simulations are performed to examine the relative importance of diabatic heating, large-scale forcing and the quality of initial conditions in the cyclogenesis. To determine why several model simulations failed in predicting correctly all of the observed scenarios, a dynamical nudging experiment is conducted to provide a "ground-truth" dataset. (Abstract shortened by UMI.)

Observations show the presence of localized regions in the atmosphere with diminished potential vorticity gradients, like in the tropical upper troposphere where outflow from deep convective regions plays an important role. The present work investigates the effect of forcing on the evolution of Rossby waves in a zero potential vorticity gradient environment. As a preliminary investigation, the barotropic case is studied, where the analogue of potential vorticity is absolute vorticity. / The analytic solution of the linearized problem shows that the streamfunction grows algebraically in time, and eventually develops a nonlinear critical layer. The numerical solution of the nonlinear problem within the critical layer shows that the nonlinearity and the forcing act together to halt the growth as coherent vortices are put in a nonlinear oscillatory regime. At long times, the critical layer solution settles to a quasi-steady state consisting of relatively large amplitude stationary vortices, with a set of small amplitude steadily-propagating vortices superimposed. These results are contrasted with the results of previous unforced problems.

The variational analysis methods are applied to retrieve the steady state wind field from single-Doppler radar data. The wind field is retrieved by fitting, in the least-squares sense, constraining model equations to observations measured during a short assimilation period (2 or 3 time sequences). The weak and strong constraint formalisms are reviewed and examined using the one-dimensional linear advection equation as a constraint. It is shown that the retrieval is not unique, but the problem can be controlled by a smoothness constraint. Variational two-dimensional and three-dimensional wind retrieval algorithms are developed and tested using actual dual-Doppler radar data. The conservation of reflectivity and the radial momentum equation are used as weak constraints in both algorithms. The anelastic form of the continuity equation is also included as a strong constraint in the three-dimensional algorithm. The two-dimensional algorithm is tested and compared to echo tracking methods using Doppler radar observations in the clear-air planetary boundary layer. The resolution at which the methods can effectively retrieve the horizontal wind field is examined in detail. The variational algorithm can properly retrieve wind structures greater than 10 km wavelength. The three-dimensional algorithm is tested using observations of a precipitating microburst. It is demonstrated that the three-dimensional wind field can be retrieved, but the method fails near the ground level. In addition, the retrieval is sensitive to the radar position relative to the observational domain due to systematic model errors. The computational efficiency of the three-dimensional wind retrieval algorithm allows its semi-operational implementation at the J. S. Marshall Radar Observatory of McGill University.

This study tests the ability of the Canadian Land Surface Scheme (CLASS) to simulate sensible and latent heat fluxes over two subarctic tundra sites in the Trail Valley Creek (TVC) drainage basin, North West Territories, Canada. CLASS simulations with and without the new organic soil parameterisation developed for peatland conditions were compared with three summer months of measurements at each site. The sites are located in a cryoturbated region and are underlain by continuous permafrost and feature mineral soil hummocks and organic soil inter-hummock zones. Results from the peatland version of CLASS showed significant improvement over the standard version although in both cases, there was an underestimation of latent heat fluxes and overestimation of sensible heat fluxes. The observed soil moisture contents are almost constant at both sites. Field photographs show the sites are located in low-lying areas, one in a local depression and the other at the bottom of a valley. Thus, it is reasonable to assume that the constant soil moisture content is maintained by lateral flow from adjacent hillslopes. This assumption could be further supported by the influence of mineral earth hummocks in TVC drainage basin. CLASS is a one-dimensional column model and it is not possible to explicitly represent lateral flow. Thus a nudging module is added to the peatland version of CLASS to reflect this horizontal water movement. Simulations were further improved with this modification. This additional module is a simple and effective way to represent the effect of lateral flow.

Measurements of the radio refractive index (or refractivity) of near-surface air using phase information from radar ground echoes can be used to provide valuable humidity information on storm-scale anomalies not adequately resolved by the surface observations. Using changes in the phase of ground targets as proxies for changes in the two-way travel times of radar pulses, fields of near-surface refractivity have been generated in real time on McGill University's Doppler S-band radar since 1996. Retrieval of meteorological information from refractivity is possible because of its strong dependence on air humidity and also density (and hence temperature). In a sensitivity study it was found that accurate high-resolution moisture information can be extracted, as the effects of temperature and pressure fluctuations are relatively small during the summer. Errors on radar-derived near-surface moisture measurements due to neglect of temperature and pressure variability were found to be smaller than those typical of point measurements made at traditional surface stations. Further, the effects of radar phase measurement errors due to refraction and precipitation are usually less significant than those that result from ignoring temperature and pressure variability. On this basis, refractivity measurements were used to study small-scale moisture signatures, in particular, a refractivity contrast associated with initiation of a convective system which, interacting with a near-surface moist region, developed into a shallow supercell hailstorm. Refractivity was found to be a useful predictor of convective storms, through detection of convergence lines and the use of near-surface moisture diagnosis to improve stability assessments.

This thesis describes the application of a Radio-Acoustic Sounding System (RASS) at an urban site, and proposes a Rank-Order Signal Processing Algorithm (ROSPA) to overcome the problems associated with that type of application. The main problem is clutter of many kinds contaminating the clear-air profiler measurements. ROSPA uses primarily order statistics, and operates in two main stages. The first stage operates on the clear-air Doppler velocity spectra by using a threshold minimum filter on the successive spectral power values at a given Doppler velocity bin for several spectra at a given altitude. The threshold minimum filter is a variant of the minimum filter. The second stage operates on the time-height mean Doppler clear-air velocity data by imposing a median filter. It is shown using theoretical models that the minimum and median filters possess the properties required to eliminate intermittent clutter, namely their insensitivity with respect to outliers. A profiler/RASS at an urban site, another at a rural site, and an airplane flying over mainly rural terrain, are used to study the urban boundary layer on the clear and convective early afternoon of June 28, 1996. The rural profiler/RASS data are free of clutter and show an initially stable rural boundary layer becoming convective in the middle of the observation period, and attaining a depth of about 1 km at the end of the period. The urban profiler/RASS data are treated with ROSPA to eliminate the severe intermittent clutter contamination and show a convective urban boundary layer over the entire observation period, with a depth increasing from 1.5 to 1.8 km. The heat flux profile of the second half of the rural RASS data agrees well with the airplane profile up to about 0.6 km. The surface heat flux estimated by airplane measurements is 146 +/- 0.77 W/m2, while the urban RASS measurements yield 523 +/- 239 W/m2. This result, along with comparisons of the vertical velocity variance profiles, is consist