A 915 MHz Doppler radar was able under certain conditions to provide Doppler spectra containing separate and distinct modes due to scattering by rain and by the clear air. The rain spectra are broadened and shifted on the velocity axis due to turbulent air motions. Information in the clear air mode is used to correct for these effects, leaving in the rain spectra the reflectivity-weighted distribution of drop fall speed, from which the drop-size distribution can be computed. / The fast Fourier transform and two iterative methods of deconvolution were applied to simulated Doppler spectra. We concluded that the iterative methods of deconvolution are more appropriate than the fast Fourier transform. Of the two iterative methods, Jansson's method of deconvolution, including prefiltering of the data, was found to give better results than Van Cittert's method. / Jansson's method was applied to biomodal spectra obtained from the HARP project of 1990 in Hawaii and to spectra obtained from a similar radar operated in Montreal. Evident is the different characteristics of rainfall in both regions and the appropriateness of the same deconvolution method to different classes of spectra.

This thesis is concerned with the numerical simulations of explosive cyclones using a very high resolution non-hydrostatic model with explicit precipitation physics. Two major aspects of the work are: (1) to explore the importance of various model parameters which lead to the rapid intensification of the surface cyclone through sensitivity tests, and (2) to unravel the cause-effect relations responsible for rapid surface pressure falls through a study of the heat budget, geostrophic vorticity budget, and a Lagrangian trajectory analysis of the simulated cyclone. / The results from six experiments are presented. The main conclusions are: (a) By using analytic initial conditions based on typical wintertime conditions prior to explosive cyclogenesis, it is possible to obtain very realistic simulations of rapid cyclogenesis. (b) The use of high horizontal resolution is important in simulating the mesoscale features of rapidly deepening cyclones. In particular, the structure of the intense warm front observed ahead of the cyclone is very sensitive to changes in horizontal resolution. (c) The destabilization of the lower troposphere prior to the period of rapid deepening is essential in the formation of an extremely intense frontal structure, which in turn is instrumental in the rapid spinup of the storm. (d) In the presence of strong surface fluxes, the development of the simulated cyclone is affected by the depth of the planetary boundary layer. / A detailed diagnosis of the control case is then presented. Quasi-Lagrangian heat budgets are calculated for several vertical columns about the vortex center. Very large variations in the heating were found to occur over a relatively small horizontal distance of 180 km. The reason for the mesoscale variations was traced to the intense warm front ahead of the cyclone and the bent-back warm front trailing behind the surface vortex. / The results of the heat budget were then used in the interpretation of the geostrophic vorticity budget. It was found that the primary mechanism for geostrophic vorticity growth at the surface is through horizontal variations in the horizontal potential temperature advection. In particular, the timing of the most rapid surface geostrophic vorticity growth coincided with the formation of the bent-back warm front. Although the forcing from diabatic heating is large, it is nearly cancelled by the forcing from vertical advection of potential temperature. Instantaneous surface fluxes of sensible heat provide very little direct generation of geostrophic vorticity. / The airflow in the vicinity of the intense warm front was diagnosed by a fully Lagrangian trajectory analysis. A schematic flow diagram is presented which illustrates the overturning of the air in the warm front. Calculations on the evolution of the circulation for closed loops of air parcels indicate that the warm front is responsible for the generation of large vorticity values at the surface.

Interactions among stationary planetary waves in the winter stratosphere are studied using steady-state quasi-geostrophic models from both a theoretical and numerical point of view. / A triad of small amplitude waves is examined analytically using a constant zonal wind $ beta$-plane model in which dissipation is required for the waves to interact. The nature of the modifications to the linear solutions is found to depend both on the propagation characteristics and the zonal wavenumber of the modes. / Numerical solutions are determined using a climatological basic state and boundary forcing. The changes to the linear structure are relatively weak. Zonal wavenumber 1 is the most affected, experiencing primarily an increase in amplitude in the vicinity of 65$ sp circ$N, 26 km. As a direct consequence of the almost linear relationship found to exist between the zonal streamfunction and the zonal potential vorticity in middle and northerly latitudes, the wave-wave interactions are, to a first approximation, dissipation-induced. The presence of weak dissipation in this region implies only weak interactions, which explains the quasi-linear structure of the solutions.

A case study of hurricane Floyd (1999) is performed using the Penn State/NCAR MM5 model. Hurricane Floyd was the third most costly hurricane to have hit the United States. / To predict accurately the track and evolution of the hurricane, a vortex bogusing technique has been devised. A more realistic initial vortex was specified and introduced into the large-scale analysis for model initialization. The technique used follows closely that described by Zhu et al. (2002) where Advanced Microwave Sounding Unit (AMSU) data are employed to retrieve the temperature of the hurricane vortex. An algorithm is then applied to compute the sea level pressure, geopotential heights, winds and moisture content. Three experiments initialized with three different data sets were performed, using respectively the original Canadian Meteorological Centre (CMC) analysis, the bogus-vortex modified CMC analysis with the original CMC sea surface temperature (SST) field, and a bogus-vortex modified CMC analysis with a spatially-constant SST of 28°C. (Abstract shortened by UMI.)

The McGill Paleoclimate Model (MPM) was used to simulate the past and future glacial inceptions. This model of intermediate complexity was first run between 122 and 80 kyr BP (Before Present). After some parameter tuning, the MPM simulated the last glacial inception at 119 kyr BP. The recent addition of a vegetation component in the model led to an improvement of the results, especially for the ice sheet distribution over Eurasia. / The MPM was then run to simulate projections of the climate for the next 100 kyr and possibly the next glacial inception. When forced by a constant atmospheric CO2 concentration, the model predicted three possible evolutions for the ice volume: an imminent glacial inception (low CO2 levels), a glacial inception in 50 kyr (intermediate CO2 levels) or no glacial inception during the next 100 kyr (CO2 levels of 370 ppm and higher). This is mainly due to the exceptional configuration of the future variations of the summer insolation at high northern latitudes. The MPM also responded realistically to rapid CO2 changes. If a global warming episode was included at the beginning of the 100-kyr run, the evolution of the climate was slightly different and the threshold over which no glacial inception occurred was lower (300 ppm).

A ground-based radar refractivity mapping technique is used to measure water vapor near the surface during the International H2O Project in May and June, 2002 (IHOP_2002). Radar-measured refractivity is compared with refractivity estimated from surface station observations during this field experiment. Bias in radar and station refractivity is found to occur often when humidity is high. Possible reasons for this difference between radar and station observations are discussed. Most of the biases were associated either with inaccurate humidity observations by stations or with the small height difference of the two measurements. With confirming this last observation further during these wet ground conditions, radar refractivity shows much better agreement with radiosonde sounding refractivity just above the surface than with station refractivity. / In addition, columnar water vapor is computed using the mixing ratios retrieved from radar and station refractivity and using the observed height of the convective boundary layer from a FM-CW radar. Surface moisture fluxes are computed as a residual of the columnar water vapor and compared with observations from flux-towers, which compute this using the eddy-covariance technique. Although the results show that the radar-based measurements may have some skill over longer time periods, the technique completely fails to reproduce observations over scales smaller than 1 hour.

Wildfires are a major part of the Mackenzie River Basin climate system, and most wildfires within the basin are lightning-caused. Thus, a study has been conducted that identifies the atmospheric circulation patterns associated with extreme lightning events. In addition, events are stratified according to the number of resultant large wildfires to determine why some extreme lightning events induce a large quantity of wildfires within the basin, and why others do not. / Extreme lightning events in the MRB are characterized by a strong, persistent upper-tropospheric ridge dominating the basin until event onset. The triggering mechanism of the majority of extreme lightning events is cyclogenesis in lee of the Rocky and Mackenzie Mountains. A Haines Index analysis has been used to show that lack of lower-tropospheric moisture and ample lower-tropospheric instability in advance of an extreme lightning event increase the probability of several resultant large wildfires. Furthermore, extreme lightning events with several induced large wildfires are characterized by a lack of moisture transport into the MRB from the northeast Pacific Ocean.

Thesis (Ph.D.)--The Ohio State University, 2007. / (UMI)AAI3247957. Source: Dissertation Abstracts International, Volume: 68-01, Section: B, page: 0342. Adviser: David H. Bromwich.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3849. Adviser: Larry Di Girolamo. Includes bibliographical references (leaves 109-116) Available on microfilm from Pro Quest Information and Learning.

<p> The work presented in this thesis is a topological approach for understanding the formation of structures from the liquid state. The strong difference in the thermal transport properties of non- crystalline solid states as compared to crystalline counterparts is considered within this topological framework. Herein, orientational order in undercooled atomic liquids, and derivative solid states, is identified with a quaternion order parameter. </p><p> In light of the four-dimensional nature of quaternion numbers, spontaneous symmetry breaking from a symmetric high-temperature phase to a low-temperature phase that is globally orientationally ordered by a quaternion order parameter is forbidden in three- and four-dimensions. This is a higher-dimensional realization of the Mermin-Wagner theorem, which states that continuous symmetries cannot be spontaneously broken at finite temperatures in two- and one-dimensions. </p><p> Understanding the possible low-temperature ordered states that may exist in these scenarios (of restricted dimensions) has remained an important problem in condensed matter physics. In approaching a topological description of solidification in three-dimensions, as characterized by a quaternion orientational order parameter, it is instructive to first consider the process of quaternion orientational ordering in four-dimensions. This 4D system is a direct higher-dimensional analogue to planar models of complex <i>n</i>&ndash;vector (<i> n</i> = 2) ordered systems, known as Josephson junction arrays. </p><p> Just as Josephson junction arrays may be described mathematically using a lattice quantum rotor model with <i>O</i>(2) symmetry, so too can 4D quaternion <i>n</i>&ndash;vector (<i>n</i> = 4) ordered systems be modeled using a lattice quantum rotor model with <i> O</i>(4) symmetry. <i>O</i>(<i>n</i>) quantum rotor models (that apply to <i>n</i>&ndash;vector ordered systems that exist in restricted dimensions) include kinetic and potential energy terms. It is the inclusion of the kinetic energy term that leads to the possible realization of two distinct ground states, because the potential and kinetic energy terms cannot be minimized simultaneously. </p><p> The potential energy term is minimized by the total alignment of <i> O</i>(n) rotors in the ground state, such that it is perfectly orientationally ordered and free of topological defects. On the other hand, minimization of the kinetic energy term favors a low-temperature state in which rotors throughout the system are maximally orientationally disordered. </p><p> In four-dimensions, the <i>O</i>(4) quantum rotor model may be used to describe a 4D plastic crystal that forms below the melting temperature. A plastic crystal is a mesomorphic state of matter between the liquid and solid states. The realization of distinct low-temperature states in four-dimensions, that are orientationally-ordered and orientationally-disordered, is compared with the realization of phase-coherent and phase-incoherent low-temperature states of <i>O</i>(2) Josephson junction arrays. Such planar arrays have been studied extensively as systems that demonstrate a topological ordering transition, of the Berezinskii-Kosterlitz-Thouless (BKT) type, that allows for the development of a low-temperature phase-coherent state. </p><p> In <i>O</i>(2) Josephson junction arrays, this topological ordering transition occurs within a gas of misorientational fluctuations in the form of topological point defects that belong to the fundamental homotopy group of the complex order parameter manifold (<i>S¹</i>). In this thesis, the role that an analogous topological ordering transition of third homotopy group point defects in a four-dimensional <i>O</i>(4) quantum rotor model plays in solidification is investigated. Numerical Monte-Carlo simulations, of the four-dimensional <i>O</i>(4) quantum rotor model, provide evidence for the existence of this novel topological ordering transition of third homotopy group point defects. </p><p> A non-thermal transition between crystalline and non-crystalline solid ground states is considered to exist as the ratio of importance of kinetic and potential energy terms of the <i>O</i>(4) Hamiltonian is varied. In the range of dominant potential energy, with finite kinetic energy effects, topologically close-packed crystalline phases develop for which geometrical frustration forces a periodic arrangement of topological defects into the ground state (major skeleton network). In contrast, in the range of dominant kinetic energy, orientational disorder is frozen in at the glass transition temperature such that frustration induced topological defects are not well-ordered in the solid state. </p><p> Ultimately, the inverse temperature dependence of the thermal conductivity of crystalline and non-crystalline solid states that form from the undercooled atomic liquid is considered to be a con- sequence of the existence of a singularity at the point at which the potential and kinetic energy terms become comparable. This material transport property is viewed in analogue to the electrical transport properties of charged <i>O</i>(2) Josephson junction arrays, which likewise exhibit a singularity at a non-thermal phase transition between phase-coherent and phase-incoherent ground states. </p><p>