Ground-based and Voyager observations in and out of methane and ammonia bands are analyzed to search for longitudinal variations in Saturn's equatorial region. A model with reflecting layer at 2.1 bars, an extended haze to 170 mb and an overlying thin stratospheric haze is adopted. Two sets of data are analyzed, a set of ground based observations covering the 6000-6600 A spectral region and a set of Voyager 1 images obtained with the orange and methane filters. The spectral variations are not consistent with a variation in the height of the reflecting layer. They are modeled by variations in the single scattering albedo of the haze and in the specific abundance of gas in the haze. The ground based spectra, having a spatial resolution of 21,000 km, are consistent with a specific abundance of gas in the haze of 14 $\pm$ 1 km-amagats per mean free path and a haze single scattering albedo in the continuum at 6055 A of 0.990 ($\pm$.006) with a longitudinal variation of $\pm$0.003. The single scattering albedo derived from the ground based observations at 6475 A is 0.993 ($\pm$.006) with a longitudinal variation of $\pm$.003. The Voyager data, having a spatial resolution of $\sim$500 km, are consistent with a specific abundance of gas in the haze between 10b and 24 km-amagats. The larger variation in the specific abundance derived from the Voyager data set is due to the larger relative uncertainty in the intensity (2%) for Voyager as compared to 1% for the ground based spectra. We derive a methane mixing ratio of 2.2 $(\sbsp{-0.2}{+0.8})$ $\times$ 10$\sp{-3},$ representing a C/H ratio which is enhanced by a factor of 2.3 over the solar value. Our estimate of the ammonia mixing ration, 4.5 $\times$ 10$\sp{-4},$ is a lower limit due to our assumption that ammonia exists at its saturation vapor pressure everywhere above the reflecting layer. There is no conclusive evidence that there are longitudinal variations in the structure of the Saturnian atmosphere in the Equatorial Region on a scale greater than 600 km.

A two-dimensional numerical model is created to calculate the electric field and current that flow from a thunderstorm source into the global electrical circuit. The model includes a hemisphere in which the thunderstorm is located, an equalization layer, and a passive magnetic conjugate hemisphere. To maintain the fair weather electric field, the output current from the thunderstorm is allowed to spread out in the ionosphere or flow along the magnetic field lines into the conjugate hemisphere. The vertical current is constant up to $\sim$65 km, decays and is redirected horizontally in the ionosphere. Approximately half of the current that reaches the ionosphere flows along magnetic field lines into the conjugate hemisphere while the rest is spread out in the ionosphere and redirected to the fair weather portion of the storm hemisphere. Our results show that it is important to include a realistic model of the equalization layer to evaluate the role of thunderstorm charging of the global circuit. The mapping of thunderstorm electric fields at middle and subauroral latitudes into the magnetic equatorial plane is studied. The geomagnetic field lines are assumed to be dipolar above $\sim$150 km. The horizontal electric field computed in the ionosphere by our model is of sufficient size and shape for the formation of electron density irregularities in the magnetosphere. The mechanism involves a localized convection of ionization tubes by E x B drift. It is shown that the horizontal range of the electric field disturbance in the ionosphere must be within $\sim$160 km to produce density irregularities necessary for the formation of whistler ducts, well within the predicted size of thunderstorm produced electric fields. Although the electric field strength at ionospheric heights depends sensitively on the conductivity profile, the results presented show that whistler duct formation is possible by thunderstorm generated electric fields.

The climate system exhibits behavior on a wide range of time and spatial scales. Computer models that simulate climate must be faithful to these matters of scale in order to achieve meaningful results. This study involves the consolidation of two models: a model that simulates the seasonal fluctuations of the atmosphere and ocean with a time constant of one day and a continental ice sheet model designed to simulate the behavior of ice sheets over tens of thousands of years. These models are asynchronously coupled in both time and space because computational requirements limit the minimum grid size and time step size. Experiments are presented which examine the long time constant of the continental ice sheet and the fast approach to quasi-equilibrium of the seasonal model. Simulations of present day conditions from the combined model are compared with climate observations and simulations of other climate models.

Considerable progress has been made in the past decades on the life cycle of rapidly deepening winter cyclones. However, little attention has been paid to the roles of mesoscale convective systems (MCSs) in extratropical cyclogenesis that occurs within weak baroclinic environments. In this thesis, the impact of an MCS on the subsequent surface cyclogenesis is investigated, using a 36-h three-dimensional, high-resolution simulations of the famous 10-12 June squall line that occurred during PRE-STORM. The model simulates remarkably well the initiation of the squall line, numerous mesoscale surface pressure perturbations and midlevel circulation structures during the mature stage, and the subsequent surface cyclogenesis after the dissipation of the system. / It is found that the squall line is initiated ahead of a weak surface front with the aid of baroclinic forcing. Once initiated, the squall system is more or less driven by the interaction of convectively generated circulations with the potential unstable environment ahead. The baroclinic forcing only provides a favorable environment for the evolution of the squall system. As the squall system rapidly intensifies and accelerates eastward, it enhances the larger-scale baroclinicity and produces a phase-lag between the pressure and thermal waves so that the baroclinic environment is more favorable for surface cyclogenesis. / To isolate the roles of moist convection in the surface cyclogenesis, a "moist" and a "dry" simulation are compared. It is found that in the absence of moist convection the model could also produce a surface cyclone, but with much weaker intensity, much smaller extent and slower displacement. The effects of moist convection are shown not only to increase the upper-level and decrease the lower-level height of isobaric surfaces, but also to condition the baroclinic environment by increasing the phase lag between the pressure and thermal waves and enhancing the large-scale baroclinicity.

The barotropic interaction in the upper troposphere between the synoptic-scale eddies and the Pacific/North American (PNA) pattern is investigated. The study uses NMC data for the years 1965-89 and examines separately the positive and negative phases of the monthly mean PNA anomalies. Northern Hemisphere maps of the eddy kinetic energy, the barotropic conversion between the eddies and the monthly mean flow and the eddy vorticity forcing are presented for both PNA phases. The eddy vorticity forcing of the PNA anomalies by the eddies is presented in terms of monthly mean geopotential tendencies. / It is found that the geopotential tendencies generated by the synoptic-scale eddies are spatially in phase with the PNA anomalies at the 250 hPa level. The characteristic time scale of the forcing in winter is about 6 to 10 days. The strongest anomaly in the eddy forcing is collocated with the largest amplitude PNA centre over the northern Pacific. / During periods of strong PNA patterns, the synoptic-scale eddy kinetic energy and its transfer into the monthly mean flow is most important in the mean jet exits. The maxima in eddy activity and kinetic energy transfer for the positive PNA phase are situated over the eastern Pacific and are significantly greater than the corresponding maxima for the negative phase, which are located over the central Pacific.

Winter storms produce major problems for society and their varying types of precipitation are often the key factor responsible. The objective of this study is to better understand the formation of winter precipitation types (freezing rain, ice pellets, snow, slush, wet snow and refrozen wet snow) within the varying, and interacting, environmental conditions experienced within many winter storms. A one dimensional cloud model utilizing a double-moment microphysics scheme has been developed to address this issue. Temperature and moisture profiles favorable for the formation of different winter precipitation types were varied in a systematic manner in an environment in which snow is falling continuously from above a temperature inversion. The ensuing precipitation evolved due to atmospheric temperature and moisture variations arising from phase changes such as melting and freezing. Average soundings associated with various surface precipitation types were consequently defined as well as the atmospheric conditions leading to the longest duration of various precipitation events. In addition, the reduction of visibility by precipitation was calculated to determine the conditions leading to the greatest reduction and those leading to its most rapid changes.

A potential vorticity (PV) analysis is performed using a 60-h simulation of a family of mesoscale frontal cyclones which occurred over the Western Atlantic ocean from 0000 UTC 13 to 1200 UTC 15 March 1992. The frontal cyclogenesis events were characterized by an upper-level ring of PV anomalies of stratospheric origin, located on the cyclonic side of the jet stream. The shape of the "PV ring" resembled the tracks of the surface frontal cyclones, implying its role in determining the initiation and the tracks of the surface cyclones. Piecewise PV inversions indicate that low-level baroclinicity contributed most to the intensity of two of the frontal cyclones, followed by low-level latent heating, and upper-level PV anomalies (trough/ridge system). The upper-level PV anomalies contributed most to the depth of a third frontal cyclone. The major frontal cyclone (MFC), underwent explosive deepening (27 hPa in 24 h) and eventually absorbed the low-level circulation of the parent cyclone. The explosive deepening of the MFC can be attributed to the phase-locking between the upper-level and low-level PV anomalies. / A series of full physics sensitivity experiments is performed to test the impact of removing upper-level PV anomalies on the development of the MFC. It is found that the delay in the genesis of the MFC, as well as weakening in its final intensity, is directly related to the proximity of the removed upstream PV anomaly. In another series of "dry" experiment with the removal of upper-level positive PV anomalies, the model fails to generate the MFC when the upstream PV anomaly closest to the pre-MFC is removed. From the results of piecewise PV inversion and sensitivity experiments, low-level thermal advection induced by the flow of the upper-level PV ring appears to initiate and sustain the development of the MFC. (Abstract shortened by UMI.)

The absorption and albedo of low, thin, marine stratus clouds are investigated using data collected by two aircraft during the Radiation, Aerosol and Cloud Experiment, conducted off the coast of Nova Scotia, Canada. Measured values of absorption and albedo are compared to calculations using a plane parallel radiative transfer model and drop size distributions determined from cloud profiles conducted close in time to the radiation measurements. The calculated values of albedo and absorption were consistent with the measurements. Uncertainties in absorption measurements due to instrument error were high. The model is used to determine cloud susceptibility, the sensitivity of cloud albedo to changes in droplet concentration. The susceptibilities determined from these calculations agree well with previous studies. The measured changes in albedo are consistent with those predicted by the model.

Daily 1200 UTC sea-level pressure, 850-hPa height and temperature, and 500-hPa height fields are used in a compositing analysis to document the synoptic evolution of wintertime precipitation events over Montreal. Since there has been a bias in past studies towards analyzing extreme events, four precipitation intensity categories are defined. A summary of the findings are as follows: (i) Heavier precipitation events are associated with a ridge-trough couplet as opposed to a trough for lighter events. The magnitude of the downstream ridge is influenced by the strength of warm advection at the 850-hPa level. (ii) Differences in the intensity of precipitation can be related to preferred climatological storm tracks and possible moisture inflow from the Atlantic Ocean, the Gulf of Mexico, and the Great Lakes. (iii) A negative PNA pattern is associated with winter events lightest in magnitude.

A comprehensive DMS oxidation reaction mechanism was added to a chemical box model (Model of Chemistry Considering Aerosols - MOCCA) to investigate how atmospheric sulfate levels are affected by pollution in the marine boundary layer. The interaction of a remote air mass with a polluted coastal air mass containing elevated NOx, alkane, and alkene levels was simulated for a mid-latitude region of the Northern Hemisphere. Model runs from different settings were compared with respect to species concentrations, branching ratios, and product yields. Results showed moderate decreases in gas phase DMS, DMSO, DMSO2, and MSA concentrations, and increases of approximately 7% and 100% for SO2 and H2SO4 , respectively. A sensitivity analysis was completed to determine how variations in temperature, DMS emission, NO emission, aerosol characterization, accommodation coefficients, PAN reactions, and a background SO2 emission influence the results.