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

The parameterization of isopycnal mixing is examined in an ocean general circulation model. The results are compared to those obtained with lateral mixing. / In the equilibrium experiments, the isopycnal case has more gyre kinetic energy, a less intense thermohaline circulation, and less interior downwelling than the lateral case. Convection is replaced by enhanced vertical diffusion in the isopycnal case. / In the time dependent passive tracer experiments, the isopycnal case has smaller depth penetration of a surface released tracer. This is likely due to induced recirculation rather than numerical limitations. / The active tracer experiments examine the long term asymmetric behaviour of warm and cold surface anomalies introduced in an abrupt and gradual fashion for the lateral and isopycnal models. The thermal anomalies produce asymmetric transient responses. The abrupt and gradual changes produce the same equilibrium but different transient responses. The isopycnal case responds more rapidly and energetically than the lateral case.

Cyclogenesis is examined based on the principle that development can be viewed as an interaction between upper and lower cyclonic disturbances, and that the surface response is related to the magnitude of both disturbances. It is shown that variability in successive operational model forecasts results from variations in the representation of the features at both levels, but that particular predictions can be sufficiently realistic for diagnostic purposes. The variability in two successive forecasts of a specific case is explained as a moist baroclinic response modulated by the growth of low-level cyclonic vorticity prior to the period of rapid deepening. This antecedent vorticity growth came about through a self-development mechanism involving an interaction between quasigeostrophic processes, surface energy fluxes and warm-frontal latent heat release. In order to define the preconditioning for this case, it is necessary to resolve these processes, interacting on both meso and synoptic scales.

A comparative diagnostic study of upper-air persistent atmospheric events, as simulated by a general circulation model (GCM) and as observed, is presented. We start with an overview of the several theories that attempt to explain such phenomena. Particular emphasis is put on the model approach of Shutts (1983). We next show that the spatial distributions of persistent events is qualitatively similar in the GCM and observational data. The North-Atlantic events are extracted and a rotated empirical orthogonal function (REOF) analysis is done on the resulting data sets. The two REOF sets that are thus obtained are shown to greatly resemble one another. Both explain roughly 50% of their original data's variance. The relationships between the modes within a set are presented, so as to understand their probable combined evolution. The fourth chapter contains an evaluation of Shutt's theory. There, the third chapter's results are used to isolate a particular class of events, namely the strong +ATL2 dipoles. The time-tendencies associated to short time-scale synoptic waves are evaluated, using an E-vectors approach, taking care to distinguish between the onset, mature and demise phases of the events. It seems that these synoptic waves have a significant impact of the average life-cycle of this +ATL2 type of events, whether they be simulated by a GCM or obtained from a NMC set of analyses.

We identify physical processes and localized atmospheric structures that modulate the global-scale atmospheric available potential energy (APE). A 1979--95 time series of Northern Hemisphere APE is derived from the National Centers for Environmental Prediction (NCEP) reanalysis. We discern an average three-day cycle in the APE depletion rate dA/dt with a time series of intraseasonal (1.6--180-day) fluctuations. Synoptic-scale APE depletion events are falls and subsequent rises in dA/dt, and are classified as APE collapses when exceeding climatologically-derived thresholds of APE fall and dA/dt. Three associated regional baroclinic waves are observed. West Pacific warm surges (Type A) accompany cyclogenesis over Japan, anticyclogenesis over the west-central North Pacific, and shallow East Asian cold surges. Bering warm surges (Type B) comprise intense southerly flow across the Bering Strait, cyclogenesis near the Kamchatka Peninsula, and intense anticyclogenesis over Alaska. Atlantic Canada warm surges (Type C) include onshore warm flow and eastern North American storm tracks. / January/February 1989 brought the largest vacillation- and synoptic-scale APE collapses of the study period. A pronounced Madden-Julian Oscillation (MJO) modulated equatorial convection over the Indian Ocean during the APE build-up. The initial APE collapse coincided with Type A development and intensifying MJO convection shifting to the West Pacific warm pool, which was prominent because of the 1988--89 La Nina. Extensive large-scale ascent amplified the thermally direct Hadley cell circulation, with descent strong over East Asia. The second collapse was associated with Type B development characterized by a record-breaking Alaska anticyclone. / Factors explaining lacking precision in energy budget calculations include missing o values above 100 hPa, limited representation of subgrid processes, and limited temporal resolution of diurnal convection over tropical landmasses. / Correlation analysis shows baroclinic conversion (diabatic generation) accounting for 80% (20%) of vacillation-scale dA/dt fluctuations. Baroclinic conversion contributes considerably (69%) to the synoptic variability only during synoptic-scale APE collapses (23 January to 4 February), illustrating the prominence of resolvable baroclinic processes during these periods.

By defining cyclone growth as a logarithmic change in absolute vorticity of the surface center, we investigate growth rates of a ten year sample of cold-season (1 October through 31 March) extratropical cyclones in the North Pacific Ocean basin. Strong growth is frequent in coastal regions near Japan and in the East China Sea. In contrast to the conventional deepening rate intensification parameter, frequency distributions of maximum 24-h growth are found to be more Gaussian in character. / To investigate possible physical processes acting to enhance growth, composite sea surface temperature (SST) anomalies are computed for groups of cyclones which deepen maximally in the vicinity of the Gulf Stream and Kuroshio Currents. Comparison between groups of similar deepening revels stronger growth is generally associated with warm SST anomalies and with weaker upper-level support for intensification. Composite latent and sensible heat flux fields computed from observations support this finding.

In this work, we explore the possibility of combining information from the VAD technique, with data from a wind profiler, to obtain the kinematic properties of a linear 2-D wind field. The idea is first tested on an artificial test-bed wind field, and the results show that the procedure is limited by the degree of non-linearity of the wind field over the wind profiler. To overcome this problem, some modifications are made to our procedure, which is then applied to real cases of stratiform precipitation. We find that the non-linearities are important enough such that the linear approximation is only valid out to about 40 km from the radar. These non-linearities also affect the retrieval of the vorticity to the extent that we had to establish a maximum limit on the uncertainty of the vorticity. These results bring to question the assumptions that the non-linearities are relatively unimportant in stratiform cases.

NCEP reanalyses have been used to compute the climatological isentropic potential vorticity (IPV) budget at the 315K level for 39 winters and for those winters with a North Atlantic Oscillation (NAO) as well as winters with a Pacific/North American (PNA) pattern. / The climatology shows two main IPV sources, each being upstream of the two main centers of positive PV on the east coasts of North America and Asia. The results for the winters with a NAO (PNA) anomaly show, in particular, that the mean-winter IPV anomalies associated with these patterns also have upstream sources. The importance of the latter is not as clear as that of the continential sources that maintain the climatological centers. / The mean-winter IPV advection that balances the IPV sources/sinks is composed of the advection by the time-mean flow and by the transient eddies (decomposed into high- and low-frequency components), where the former is the dominant component. The latter are found to produce a negative feedback in that they act to reduce the amplitude of the IPV anomaly. For the NAO anomaly, low-frequency transient advection is more important, while high-frequency transient advection is more statistically significant for the PNA anomaly. Both the high and low-frequency advection have comparable contributions in maintaining the climatological distribution of the stationary eddy IPV.

Using three cameras and two high powered flash lamps, the HYdrometeor Detection and Ranging using stereO-Photography (HYDROP) experiment, we made the first determination of the sizes and positions of raindrops (diameters >0.2 mm) in a significant volume (approx. 10 m3). Analyzing six three dimensional reconstructions---with 11000--22000 raindrops each and with nominal rain rates in the range of 8--15 mm/hr.---we performed the first direct investigation of the continuum limit of a large number of particles in cloud/rain physics. Since the mean interparticle distance was 7--8 cm and the largest sphere has a radius of the order 2 m, our range of scales is only roughly a factor of 10. Nevertheless, we were able to show clear evidence that a non-trivial multiscaling limit was obtained at least for one rain event out of the two studied; the q th moment of the liquid water density in a radius r was found to scale with a nonlinear (but convex) exponent K(q), characteristic of multifractals. In contrast, when the particle positions were randomized, clear convergence to the classical (homogeneous) continuum was observed with K(q) = 0. By raising the raindrop volumes to a power n = 1/3, in order to improve convergence towards the continuum limit, the universal multifractal parameters of this quantity derived from the field of liquid water content were estimated to be alpha = 1.95 +/- 0.1, C1 = 0.05 +/- 0.01 and H = -0.01 +/- 0.01.